Structural, electrical, ferroelectric and mechanical properties with Hirshfeld surface analysis of novel NLO semiorganic sodium p-nitrophenolate dihydrate piezoelectric single crystal

Abstract

Novel non-linear sodium para-nitrophenolate dihydrate (SPNPD) single crystals were grown by the controlled evaporation method. Various functional groups and chemical bonding were identified by FTIR and Raman analysis. Hirshfeld surface and fingerprint plots were drawn and analyzed to investigate the intermolecular interactions present in the crystal structure. UV-visible studies indicate the high transmittance of the crystals in the visible region with a wide band gap 2.80 eV. In the photoluminescence spectrum, a sharp broad emission peak centered at 525 nm indicates green emission. In the dielectric study a broad peak with low dielectric constant value was observed at 34 °C which may be due to a ferroelectric to paraelectric transition. Piezoelectricity was confirmed by determining the piezoelectric charge coefficient (d₃₃ = 2.24 pC N⁻¹). The hysteresis loop shows values of remnant polarization and coercive field of 2.67 μC cm⁻² and 7.43 kV cm⁻¹, respectively. Piezo-/ferroelectricity were reported for the first time in p-nitrophenolate based compounds. SHG efficiency of this material was found to be 4.15 times of that of KDP. The mechanical strength was confirmed from a Vicker’s microhardness study and void volume. Thus the promising piezoelectric, ferroelectric behavior of the material along with high SHG efficiency and low dielectric constant, established SPNPD as a potential material for transducer, optoelectronics and nonvolatile memory devices applications.

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Introduction

In recent years there has been considerable interest in nonlinear optical materials because of their applications in optical communications, optical computing, high speed information processing, electro-optic shutters and optical data storage.^1,2 Pure organic crystals have very large nonlinear susceptibilities compared to inorganic crystals but their use in device fabrication is restricted by low optical transparency, poor mechanical properties and the difficulties in the growth of large crystals.^3,4 On the other hand, inorganic materials have excellent mechanical and thermal properties but possess relatively modest optical non-linearities because of the lack of delocalization of π-electrons.⁵ Semiorganic materials combine the high optical non-linearities of organics with good thermal stability and transmittance of inorganics.^6,7

para-Nitrophenol is such an organic material which gives a variety of derivatives with alkali metal hydroxides.^8–11 In recent years, semi-organic complex products of p-nitrophenol have attracted interest for their NLO properties because of the presence of delocalized π-electrons in the organic ligand p-nitrophenol resulting in high nonlinear optical coefficients in this kind of material. Sodium p-nitrophenolate dihydrate (SPNPD·2H₂O) is such a semiorganic non linear material in which a sodium ion (Na⁺) is bonded to an organic ligand (nitrophenoxy ion). In this crystal, the π-electron cloud movement from donor to acceptor results in an intermolecular charge transfer interaction and hence makes it non-centrosymmetric. Crystals having non-centrosymmetric structure exhibit both SHG and piezoelectric effects.¹² Although, SPNPD is a well established non-linear optical material,¹³ its piezoelectricity has not been reported yet and therefore its possible applications in piezoelectric devices have not been exploited so far. This piezoelectricity has not been reported in other p-nitrophenolate based compounds either.^8–11 Furthermore, crystals exhibiting both luminescence and non linear optical properties are used in solid state laser sources.^14,15 In addition to the NLO property, the dielectric behaviour of a material plays an important role in device applications. Regarding NLO device fabrication, materials should also have stringent properties imposed by the operating frequency such as low dielectric constant and required power levels under the electric field.¹⁶ A low dielectric constant means a material supports an electrostatic field with minimal heat energy dissipation. Thus low-dielectric materials can lower the power consumption and decrease RC delay in microelectronics.¹⁷ Therefore in the grown crystals, we have carried out the study of dielectric properties i.e. dielectric constant and dielectric loss over a range of frequencies and temperatures to analyze their electrical properties. Ferroelectric materials have many optoelectronic applications such as capacitors, nonvolatile memory devices, high-performance gate insulators etc. The molecular surface of the grown crystal was computed on the basis of Hirshfeld surface analysis to understand the intermolecular interactions and hardness of the crystal was compared based on void volume estimation.^18,19 Therefore, in the present work we have reported the structural characterization, Hirshfeld surface analysis, optical properties, photoluminescence, dielectric behavior, piezoelectric charge coefficient (d₃₃ pC N⁻¹), ferroelectricity, non-linear optical and mechanical properties with calculated void volume in detail for the first time. Due to the excellent non linear optical property, strong PL emission, low dielectric constant value and good piezo-/ferroelectric behavior, the grown SPNPD crystals can have technological applications in opto-electronic and microelectronics devices.

Experimental

Crystal growth and characterization

Sodium p-nitrophenolate dihydrate (SPNPD) single crystals were grown by the slow evaporation solution technique by taking sodium hydroxide and p-nitrophenol in the molar ratio of 1 : 1. These salts were dissolved in double distilled water. The resultant solution was stirred well at 45 °C for 4 h to ensure homogeneous temperature and concentration over the entire volume of the solution. The reaction which takes place for the synthesis of sodium p-nitrophenolate dihydrate is as follows:

NO₂–C₆H₄–OH + NaOH + H₂O → NO₂–C₆H₄–ONa·2H₂O

The resulting solution was filtered twice and finally it was kept in a constant temperature oil bath at 40 °C with an accuracy of 0.01 °C. The purity of the synthesized salt was improved by performing recrystallization many times. Yellow colored transparent single crystals of size 9 × 7 × 2 mm³ were harvested from the mother solution in 2–3 weeks as shown in Fig. 1(a). The SPNPD single crystals grown have been characterized from single crystal X-ray diffraction data analysis using an Oxford Diffractometer with graphite monochromatic MoK_α radiation (λ = 0.7107 Å). The structure of the crystal grown was solved by the full-matrix least-square technique on F² using the SHELX-97 program.²⁰ Its morphology has been deduced from BFDH and modified BFDH theory.²¹ To confirm the functional groups present, the FTIR spectrum of a grown crystal was recorded in the middle infrared region of 400 to 4000 cm⁻¹ with a Perkin-Elmer spectrum BX spectrophotometer using the KBr pellets while Raman spectra were recorded with an FT-Raman spectrophotometer in the range 400–3000 cm⁻¹ using excitation radiation at 514 nm with an Ar-ion laser at room temperature. The Hirshfeld surfaces and unit cell voids were generated using the Crystal Explorer 3.1 programme.²² Fingerprint plots showing intermolecular interactions were derived from the Hirshfeld surfaces. To investigate the suitability of the crystals for optical applications, the transmission spectra were recorded using an evolution 300 spectrophotometer in the region 200–1100 nm. During this optical analysis, powdered sample was dissolved in distilled water therefore baseline corrections were performed so that only the material data required was obtained. Photoluminescence (PL) measurements were taken on a Varian Cary Eclipse fluorescence spectrophotometer at room temperature by exciting the crystal at 450 nm. Dielectric measurements were carried out using an impedance analyzer (Agilent E4980A) with sample holder (Agilent Model 16048A) for a frequency range 100 Hz to 2 MHz in the temperature range from RT to 58 °C. Crystals were poled using a DC poling unit (electric field range 0–5 kV mm⁻¹) by immersing them in silicon oil. The piezometer charge coefficient (d₃₃) measurement was performed on a PM-300 piezometer system. To investigate the NLO property, the grown crystal was subjected to a second harmonic generation (SHG) test performed by the Kurtz powder technique. In the experiment, crystals were ground into powder form with uniform particle size of 63 μm, then packed in a microcapillary tube and finally a Q-switched Nd:YAG laser beam of wavelength 1064 nm was passed through it to obtain the SHG efficiency. The second harmonic radiations generated in the sample were focused by a lens and detected by a photomultiplier tube. This SHG output was converted into an electrical signal and displayed on an oscilloscope which indicated the SHG efficiency of the sample. The emission of green light confirmed the SHG property of the material. The hysteresis loop was traced using an automatic P–E loop tracer. The mechanical characterization of the SPNPD crystals was obtained by a Vickers microhardness test performed at room temperature. The indentations on crystals were made gently by loads varying from 5 to 100 g for a dwell time of 10 s.

Fig. 1 (a) Photographs of yellow colored transparent SPNPD single crystals grown at 40 °C. This good transparency indicates high crystal quality. (b) Morphology of the grown crystal obtained using WinXMorph software.

Morphological studies

The morphological importance (MI_hkl) of the planes present in a crystal can be solved by the BFDH law which states that the rates of growth of the given hkl face are equal to the reciprocal of the interplanar distance d_hkl. The morphology of the crystals grown with crystal planes and directions is shown in Fig. 1(b) which has been obtained using WinXMorph software.²³ The BFDH law is applicable in many cases in predicting morphology, but in addition, it is also dependent on the interfacial angles with adjacent planes. Thus, the instantaneous growth rate of the hkl face can be expressed by a modified BFDH formula as given below²⁰
where R_hkl, R_h1k1l1, R_h2k2l2 are the normal growth rates of the hkl, h₁k₁l₁ and h₂k₂l₂ faces and their values are equal to the reciprocal of their interplanar distances accordingly, 1/d_hkl according to the BFDH law, α and γ are the interfacial angles for given hkl plane. The growth rates and the corresponding morphological importance (MI) of the (010), (111) and (123) faces were computed from the BFDH law and modified BFDH laws which are summarized in Table 1 and these directly reflect the corresponding area of the crystal faces according to their weighting. It has been found that the experimentally observed morphology of the crystal grown closely matches the predictions of both simple BFDH and modified BFDH law which means that the growth rates of the adjacent planes significantly contribute to the growth rate of the SPNPD crystal plane.

Table 1 Morphological importance of SPNPD crystal faces computed from BFDH and modified BFDH laws

Face (hkl)	dhkl (Å)	Calculated relative growth rate from BFDH law	Morphological importance by BFDH law	dlhkl/dt	Morphological importance by modified BFDH law
010	19.6850	1.000	1.000	0.7165	1.000
111	4.5787	4.299	0.233	0.5489	1.305
123	2.0075	9.8057	0.102	0.2692	2.661

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Results and discussion

Single crystal XRD analysis and structural description

The structural analysis of a grown crystal was carried out by single crystal X-ray diffraction analysis. For this characterization a good single crystal with dimensions 0.5 mm × 0.5 mm × 0.5 mm was chosen. The structure was deposited at the CCDC (no. 1403345).† It was observed that the grown crystal belongs to the orthorhombic system with non-centrosymmetric space group Ima2. The observed cell parameters agree well with the reported values.^24–27 The details of the crystal data and refinement parameters of the studied crystal are listed in Table 2. The molecular structure of the grown crystal with the atom numbering scheme is depicted in Fig. 2. Fig. 3 shows the packing of the molecules arranged in a net pattern. In the structure, a sodium cation is bonded with the oxygen atom (O2) of the nitro group (NO₂) and also attached to four oxygen atoms of water molecules (Fig. 2). Thus the sodium ion exists in a six fold coordinated geometry based on an octahedron. Another significant fact is that the phenolic oxygen (O1) atom has a strong interaction with the hydrogen atoms of water molecules which are attached to the sodium atom. Thus, intermolecular hydrogen bonding interactions {O⋯H(W)} take place between the non-coordinated oxygen (O1) atom and the hydrogen atoms of water molecules (Fig. 3) which minimizes energy and hence helps in stabilization of the crystal structure. Further, the atomic coordinates of the non-hydrogen atoms with their equivalent displacement parameters for SPNPD compound are listed in Table 3. Bond lengths and bond angles of atoms present in the material are presented in Tables 4 and 5. Also, the hydrogen bonds present in the crystal structure are listed in Table 6.

Table 2 Single crystal data and structure refinement of SPNPD crystal

Empirical formula	C6H8O5NNa
Formula weight	197.12
Temperature	293 K
Wavelength	0.71073 Å (MoKα radiation)
Crystal system, space group	Orthorhombic, Ima2
Cell length, a	6.8849(6) Å
Cell length, b	19.6618(1) Å
Cell length, c	6.4413(5) Å
Volume	871.95(12) Å^3
Crystal size	0.5 × 0.5 × 0.5 mm^3
Theta range for data collection	3.3–29.3°
Limiting indices	−9 ≤ h ≤ 8, −26 ≤ k ≤ 26, −8 ≤ l ≤ 8
Refinement method	Full-matrix least-squares on F^2
Data/restraints/parameters	1087/1/85
Goodness-of-fit on F^2	0.797
R[F^2 > 2σ(F^2)], wR(F^2)	0.0295, 0.0854

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Fig. 2 An ORTEP view of the SPNPD crystal structure with the atom numbering scheme. Carbon and oxygen atoms are denoted by black and red ellipsoid balls while white balls represent hydrogen atoms. The sodium cation strongly interacts with the oxygen atoms of the nitro (NO₂) group and water molecules.

Fig. 3 Crystal packing diagram in which carbon, oxygen and nitrogen atoms are represented in black, red, blue, respectively, while the bigger grey atoms represent sodium atoms. The hydrogen bonding interactions are shown by dotted green lines which take place between phenolic oxygen atoms and hydrogen atoms of water molecules.

Table 3 Atomic coordinates (×10⁴) of the non-hydrogen atoms and their equivalent isotropic displacement parameters (Ų × 10³) for SPNPD. U(eq.) is defined as one third of the trace of the orthogonalized U_ij tensor

Atom	X	Y	Z	U(eq.)
Na(1)	−2500	−7284(1)	−7079(1)	27(1)
C(5)	−2500	−5256(1)	−3716(3)	33(1)
O(1)	−2500	−3217(1)	−2456(2)	26(1)
O(2)	−2500	−6147(1)	−5989(4)	64(1)
N(1)	−2500	−5972(1)	−4151(4)	42(1)
C(3)	−2500	−4355(1)	−1264(3)	40(1)
C(2)	−2500	−3870(1)	−2853(3)	23(1)
O(3)	−2500	−6381(1)	−2730(4)	56(1)
C(1)	−2500	−4109(1)	−4911(3)	40(1)
C(6)	−2500	−4798(1)	−5326(3)	46(1)
C(4)	−2500	−5045(1)	−1674(3)	39(1)
O(4)	−69	7364(1)	−9627(1)	30(1)

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Table 4 Bond lengths for SPNPD

Atoms	Length
Na(1)–O(2)	2.3437(19)
Na(1)–O(4)#1	2.3491(10)
Na(1)–O(4)	2.3491(10)
Na(1)–O(4)#2	2.4032(10)
Na(1)–O(4)#3	2.4032(10)
Na(1)–O(3)#4	2.6567(15)
Na(1)–Na(1)#4	3.3305(4)
Na(1)–Na(1)#3	3.3305(4)
C(5)–C(6)	1.373(3)
C(5)–C(4)	1.379(3)
C(5)–N(1)	1.435(2)
O(1)–C(2)	1.3071(18)
O(2)–N(1)	1.233(3)
N(1)–O(3)	1.219(3)
C(3)–C(4)	1.381(3)
C(3)–C(2)	1.400(3)
C(2)–C(1)	1.407(2)
O(3)–Na(1)#3	2.6567(15)
C(1)–C(6)	1.381(3)
O(4)–Na(1)#4	2.4032(10)

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Table 5 Bond angles for SPNPD^a

Atoms	Angle
a Symmetry transformations used to generate equivalent atoms: #1 −x − 1/2, y, z; #2 x + 0, −y − 3/2, z + 1/2; #3 −x − 1/2, −y − 3/2, z + 1/2; #4 −x − 1/2, −y − 3/2, z − 1/2.
O(2)–Na(1)–O(4)#1	105.82(5)
O(2)–Na(1)–O(4)	105.82(5)
O(4)#1–Na(1)–O(4)	90.86(5)
O(2)–Na(1)–O(4)#2	94.48(5)
O(4)#1–Na(1)–O(4)#2	159.42(3)
O(4)–Na(1)–O(4)#2	86.79(3)
O(4)–Na(1)–O(4)#3	159.42(3)
O(4)#2–Na(1)–O(4)#3	88.28(5)
O(2)–Na(1)–O(3)#4	171.64(9)
O(4)#1–Na(1)–O(3)#4	79.88(5)
O(4)–Na(1)–O(3)#4	79.88(5)
O(4)#2–Na(1)–O(3)#4	79.58(5)
O(4)#3–Na(1)–O(3)#4	79.58(5)
O(2)–Na(1)–Na(1)#4	122.19(7)
O(4)#1–Na(1)–Na(1)#4	46.17(2)
O(4)–Na(1)–Na(1)#4	46.17(2)
O(4)#2–Na(1)–Na(1)#4	124.21(3)
O(4)#3–Na(1)–Na(1)#4	124.21(3)
O(3)#4–Na(1)–Na(1)#4	66.17(6)
O(2)–Na(1)–Na(1)#3	87.32(7)
O(4)#1–Na(1)–Na(1)#3	131.20(3)
O(4)–Na(1)–Na(1)#3	131.20(3)
O(4)#2–Na(1)–Na(1)#3	44.85(2)
O(4)#3–Na(1)–Na(1)#3	44.85(2)
O(3)#4–Na(1)–Na(1)#3	84.32(6)
Na(1)#4–Na(1)–Na(1)#3	150.49(4)
C(6)–C(5)–C(4)	121.52(15)
C(6)–C(5)–N(1)	119.70(18)
C(4)–C(5)–N(1)	118.78(18)
N(1)–O(2)–Na(1)	123.66(17)
O(3)–N(1)–O(2)	122.42(19)
O(3)–N(1)–C(5)	120.1(2)
O(2)–N(1)–C(5)	117.5(2)
C(4)–C(3)–C(2)	121.99(19)
O(1)–C(2)–C(3)	121.76(16)
O(1)–C(2)–C(1)	120.81(15)
C(3)–C(2)–C(1)	117.43(16)
N(1)–O(3)–Na(1)#3	140.42(16)
C(6)–C(1)–C(2)	120.71(18)
C(5)–C(6)–C(1)	119.79(18)
C(5)–C(4)–C(3)	118.56(18)
Na(1)–O(4)–Na(1)#4	88.98(3)

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Table 6 Hydrogen bond geometry (Å)^a

D–H⋯A	D–H	H⋯A	D⋯A	D–H⋯A
a Symmetry codes: (i) x, y, z; (ii) −x, −y − 1, +z − 1; (iii) x + 1/2, −y − 1, +z − 1; (iv) x + 1/2, +y − 1/2, +z − 1/2; (v) −x, +y − 1/2, +z − 1/2; (vi) x, −y − 1/2 − 1, +z + 1/2.
C6–H6⋯O2^i	0.93	2.394	2.687	98.04
C4–H6⋯O3^i	0.93	2.434	2.715	97.40
O4–H4A⋯O1^ii	0.786	2.033	2.785	159.98
O4–H4A⋯O1^iii	0.786	2.033	2.785	159.98
O4–H4B⋯O1^iv	0.824	2.004	2.811	166.40
O4–H4B⋯O1^v	0.824	2.004	2.811	166.40
O4–H4B⋯O1^vi	0.824	2.957	3.265	104.75

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Vibrational spectroscopy

Vibrational spectroscopy is an important tool in identifying the functional groups present in a material. The observed FTIR and Raman spectra are depicted in Fig. 4(a and b), respectively. In the FTIR spectrum the broad asymmetric band centered at 3301 cm⁻¹ can be assigned to the O–H stretching modes of the two hydrogen bonds which shows that the compound contains water molecules. The infrared bands observed at 493, 704 cm⁻¹ correspond to the torsion vibrations of the ring. The IR and Raman bands observed at 646, 1115, 1587 and 646, 1110, 1270 cm⁻¹ are due to the ring bending or stretching and stretching vibrations of C–NO₂ and C–O respectively. Out of plane symmetric deformation in NO₂ and out of plane bending in C–H vibrations are observed at 758 and 829, 853 cm⁻¹, respectively, in the IR spectrum. In the IR and Raman spectra the bands observed at 990, 1172 and 1170 cm⁻¹ respectively are due to the in-plane bending vibrations of the C–H bond. In the IR and Raman spectra, rocking vibrations of NO₂ are observed at 545 and 548 cm⁻¹ respectively, while out of plane bending vibrations of C–H are present at 835 and 860 cm⁻¹ in the Raman spectrum. Symmetric and antisymmetric bands of NO₂ are observed at 1347 and 1487 cm⁻¹ in Raman and IR spectra respectively. IR and Raman bands at 1301, 1462 and 1409, 1524, 1579 cm⁻¹ respectively are attributed to the stretching or bending vibrations of the ring. Antisymmetric vibrations of NO₂ are observed at 1469 cm⁻¹ in the Raman spectrum. Thus the vibrational frequencies observed in the FTIR and Raman spectra are found to be in good agreement with the reported values.²⁸

Fig. 4 (a) FTIR spectrum of the grown sodium p-nitrophenolate dihydrate crystal. (b) Raman spectrum of the grown crystal. These spectra identify O–H, NO₂ and benzene ring vibrations present in the material.

Hirshfeld surface analysis

Hirshfeld surfaces visualize intermolecular interactions by color-coding for short or long contacts and explores the properties of all intercontacts within the crystal structure.¹⁸ The surfaces of a molecule in the crystal are constructed based on the electron distribution which is calculated as the average sum of the spherical atoms’ electron densities.²⁹ We use the geometric function d_norm plotted onto the Hirshfeld surface. This normalized contact distance d_norm based on d_e (distance from a point on the surface to the nearest nucleus external to the surface) and d_i (distance from a point on the surface to the nearest nucleus internal to the surface) and the van der Waal radii of atoms internal or external to the surface (r^vdW_i/r^vdW_e) identifies the regions of particular importance to the intermolecular interactions which are given by the following equation:

d_norm = d_i − r^vdW_i/r^vdW_i + d_e − r^vdW_e/r^vdW_e (1)

where r^vdW_i and r^vdW_e are the van der Waals radii of two atoms internal and external to the molecular surface.

The value of d_norm is positive or negative depending upon whether the intermolecular contacts are shorter or longer than r^vdW, respectively.

Hirshfeld surfaces mapped with different properties, i.e. d_norm, curvedness and shape index create a very useful tool for visualizing intermolecular interactions and their contribution towards the crystal packing behavior of molecules.²⁹ The Hirshfeld surfaces of all the molecules mapped with the above mentioned properties are shown in Fig. 5–7.

The red regions in the Hirshfeld surface mapped over a d_norm range of −0.75 to 1.10 (Fig. 5(a)) represent the dominant interactions taking place in the crystal. The contributions of different intermolecular interactions present in the crystal structure are provided by two dimensional pictures of fingerprint plots derived from the Hirshfeld surfaces. The Hirshfeld surfaces and the associated fingerprint plots have been generated using Crystal explorer. The 2-D fingerprint plot of d_e vs. d_i as shown in Fig. 5(b) summarizes all intermolecular contacts experienced by molecules in the SPNPD crystal. Particular atom pair close contacts have been highlighted by decomposing fingerprint plots.³⁰ In Fig. 5(c and d) the O⋯H (d_i > d_e) and H⋯O (d_e > d_i) intermolecular contacts are highlighted in the red region on the d_norm surface. These interactions (O⋯H/H⋯O) appear as large spikes pointing towards the lower left in the 2-D plot comprising 8.2% and 6.3%, respectively of the total area of the Hirshfeld surface of the molecules which reveals that these interactions are around the r^vdW separation. The C⋯H contacts corresponding to the π-donor and π-acceptor contribute much less (0.9%) to the total Hirshfeld surface area.

Fig. 5 (a) Hirshfeld surface mapped with d_norm for the structure. (b) Two-dimensional fingerprint plot for the SPNPD molecule. (c and d) Front and back views of O⋯H and H⋯O intermolecular interactions in the SPNPD molecule. (e and f) Fingerprint plots of O⋯H and C⋯H interactions showing percentages of contacts contributing to the total Hirshfeld surface area of the molecules.

The shape index (S) represents the local morphology of any given surface in terms of color coded information i.e. hollow (red) and bumps (blue)³¹ and can be expressed as:

S = 2/π tan⁻¹(k₂ + k₁/k₂ − k₁)

where k₂ and k₁ are the two principal curvatures of the surface, k₁ ≤ k₂.

Curvedness (C) is defined as the root mean square curvature of the surface and is given by the following equation:³²

C = 2/π ln(k₂² + k₁²/2)

Fig. 6(a and b) illustrates how shape index and curvedness surfaces are used to identify planar stacking (π⋯π) interaction arrangements. In the same region of the shape index surface, the presence of red and blue triangles shown by the purple ellipse in Fig. 6(a) indicates that the π⋯π interaction is almost identically present in the crystal structure. Blue triangles represent the convex region which is formed due to the carbon atoms present in the benzene ring of the molecule inside the surface while the red triangle represents concave regions due to the carbon atoms of the π stacked molecule above it. This pattern of red and blue triangles in the region of both sides of the molecule determines the way in which molecules overlap and make contact with each other and also shows how adjacent molecules are related by translation. The mapping of curvedness on Hirshfeld surface (Fig. 6(b)) shows a flat green region separated by blue edges. These clearly visible flat regions as shown by purple lines on the curvedness surface are another characteristic of the π⋯π stacking interaction.

Fig. 6 (a) Hirshfeld surface of SPNPD molecule mapped with shape index. The presence of red and blue triangles is shown by the purple ellipse in which red and blue color represent the bumps and hollow regions on the shape index surfaces. (b) Hirshfeld surface mapped with curvedness to identify the planar (green) and curved (blue edge) regions in the SPNPD molecule for planar stacking interactions.

The contribution of π⋯π stacking interactions (C⋯C) was further investigated by Hirshfeld surfaces as shown in Fig. 7. These decomposed fingerprints have enabled contributions of different intermolecular interactions present in crystal structure to be separated. The C⋯C contacts contribute very less (2.9%) to the total Hirshfield surface area of molecules. H⋯H interactions are reflected in the distribution of scattered points in the 2-D fingerprint plot which have a relatively significant contribution 19.9% to the total Hirshfeld surface area of the molecules This indicates that stronger H⋯H bonds exist between the SPNPD molecules.

Fig. 7 Two dimensional fingerprint plots and Hirshfeld surfaces of H⋯H and C⋯C interactions with 19.9% and 2.9% contributions, respectively. These C⋯C interactions contribute to the total intermolecular interactions area for π⋯π stacking of the SPNPD molecules.

Optical transmission study

For any crystal, the optical absorption or transmittance window and cut off wavelength are very important parameters in providing information about its opto-electronic transitions. UV-visible studies give important structural information because absorption of UV and visible light results in the promotion of electrons in π and σ orbital from the ground state to excited energy states.³³ Fig. 8(a) shows the recorded UV-visible transmittance spectrum of the grown crystals which revealed that the ‘lower cut-off’ in the case of SPNPD samples is around 450 nm and there is very high transmittance (∼99%) in the region 480–110 nm which shows that the crystal is free of defects such as precipitates and inclusions. In general, the absorption peak of benzene is observed around 200 nm. In the case of 4-nitrophenol, absorption peaks are strongly shifted because of the presence of hydroxyl and p-substituted nitro groups on the benzene ring.³⁴ Although this crystal has a lower cut-off at 450 nm, it is still better compared to those of potassium p-nitrophenolate and guanidinium p-nitrophenolate crystals, which have lower cut-offs at 510 nm and 505 nm, respectively.^8,9 The very low absorption in the entire region of 480–110 nm enables it to be a good candidate for opto-electronic and NLO applications. Below 280 nm the absorbance rises which may be due to the electronic transition π–π* taking place in the benzene ring of the SPNPD material. The appearance of such an absorption peak in the lower wavelength region in a p-nitrophenolate based compound has been reported by another researcher as well.⁸

Fig. 8 (a) Optical transmission spectrum of SPNPD single crystal. (b) Plots of (αhν)² function against hν with evaluation of the band gap of the material.

The optical constants of the material play an important role in fabricating optical devices. The dependence of the optical absorption coefficient on the photon energy helps in determining the nature of optical transitions of electrons which take place in the material.

The optical absorption coefficient (α) was calculated using the following relation:

α = (2.303/t) × log(1/T)

where, ‘t’ is the sample thickness and ‘T’ is the transmittance.

With the occurrence of a direct band gap, the crystal under study has an optical absorption coefficient (α) which obeys the following relationship for high photon energies,

(αhν)² = A(hν − E_g)

where ‘E_g’ is the optical band gap and ‘A’ is a constant which is nearly independent of photon energy. The graph plotted between (αhν)² and photon energy (hν) is shown in Fig. 8(b) and the optical band gap was found to be 2.80 eV which has been evaluated by extrapolating the linear portion of (αhν)² in the photon energy axis. This band gap value indicates the large transparency range of material which makes it more suitable for optical applications.

Photoluminescence

In solids, photoluminescence (PL) is the phenomenon in which electronic states of a solid are excited by light of a particular energy and the excitation energy is released as photons with different energy which is related to the electronic structure of the material. Photoluminescence is a useful technique to identify the impurities and find applications in lighting technologies. Generally the photoluminescence phenomenon is expected in aromatic molecules which contain multiple conjugated bonds leading to a high degree of resonance stability. The SPNPD crystal consists of a phenoxy ion (benzene derivative) having delocalized π-electrons in the C=C bonds due to which all C=C bonds can have different energy spacing between ground and excited states. This means that there exist a large number of energy states between the ground and excited states which are responsible for radiative recombination resulting in luminescence spectrum in the grown crystal. The photoluminescence (PL) spectrum of the grown SPNPD crystal was recorded at room temperature by exciting it at 450 nm as shown in Fig. 9. A sharp broad emission peak centered at 525 nm was observed in the spectrum which indicates green emission.³⁵ This broad green emission peak is also related to deep level defect transitions i.e. there is a contribution from deep holes in the energy levels.³⁶ This broadening in emission peak is attributed to the presence of strong intermolecular hydrogen bonding interactions present in the crystal structure as evidenced in the Hirshfeld surface analysis. This hydrogen bonding is taking place between the oxygen of the OH group (phenolic oxygen) attached to the benzene ring and the hydrogens of water molecules attached to the sodium cation in the crystal structure (as discussed earlier in Fig. 3) which causes the dominance of radiationless pathways in the decay process. Thus the intermolecular interaction will have an important role in crystal packing. The asymmetric nature of the emission peak shows the difference in the interaction of the fluorescing moiety with the neighbors in the crystal. Thus the grown crystals can be used to fabricate green lasers. Such materials having a broad emission peak can be used in a new tunable laser system.³⁷

Fig. 9 Photoluminescence spectrum of an SPNPD crystal showing green emission.

Dielectric properties

The dielectric constant is one of the basic electrical properties of solids and gives information about the nature of atoms, ions and the polarization mechanism present in a crystal. Basically, dielectric materials enhance the capacitance between the plates of a condenser which can have useful applications in capacitor technology. Dielectric studies of sodium p-nitrophenolate dihydrate crystal were carried out at various frequencies and temperatures. For doing this, we selected a good transparent crystal and both surfaces of it were coated with high grade silver paste to make parallel electrodes. The dielectric constant of the material is directly proportional to the polarization (charge displacement) which further depends on the applied electric field and the responses of the different constituents (atoms, ions) of the solids. Fig. 10(a) shows the variation of dielectric constant which decreases from 111 to 30 with increasing frequency at room temperature. The higher dielectric constant at lower frequencies is due to the contribution of all polarizations i.e. electronic, ionic, dipolar and space charge polarizations. Further with increase in frequency, the contribution of space charge polarization decreases resulting in a decrease in dielectric constant value. At lower frequencies, the dipoles follow the rapidly changing electric field which results in a higher dielectric constant value, while dipoles are unable to follow it at higher frequencies and hence this leads to a reduction in the dielectric constant value.

Fig. 10 (a) Variation of dielectric constant with frequency at RT. (b) Variation of dielectric constant with temperature at various frequencies showing ferroelectric phase transition at 34 °C. Temperature dependence of dielectric loss and ac conductivity at various frequencies is shown in (c) and (d).

Now, the sample was heated from RT to 58 °C in the frequency range of 20 Hz to 2 MHz at a heating rate of 1 °C min⁻¹. Fig. 10(b) shows the variation of dielectric constant with temperature at various frequencies. It is observed from the figure that dielectric constant increases with increase of temperature and it attains a maximum value at 34 °C and then it gradually decreases with further increase in temperature. Thus a transition was observed at 34 °C which may be due to a ferroelectric to paraelectric phase transition. It is inferred that the grown crystal is a ferroelectric material. The maximum value of dielectric constant obtained at 34 °C was found to be decreased with increasing frequency while the temperature of dielectric maxima (34 °C) with different frequencies remains same showing the absence of any relaxation behavior. The value of dielectric constant was found to be low which enhances the SHG coefficient of the material in accordance with Miller’s rule.³⁸ Also, the low dielectric constant makes the material a potential candidate for microelectronics industry applications. The variation of dielectric loss with temperature at different frequencies is shown in Fig. 10(c). The dielectric loss decreases with increasing frequency which indicates that the crystal has minimal defects and it can be used for photonic and electro-optic device applications. Overall, the very low value of dielectric loss clearly reveals the usefulness of the dielectric property of the material.³⁹ After 5 kHz onwards, dielectric loss goes to a very much smaller value which reveals that the material can be used for storing a large number of charges without much energy loss.

Electrical conduction in a crystal takes place when an electron jumps from a lower energy state to a higher energy state. The variation of the ac conductivity (σ_ac) with temperature at different frequencies is plotted in Fig. 10(d). The ac conductivity (σ_ac) of the grown crystal was calculated using the relationship:

σ_ac = 2πfε_oε_rω tan δ

where ε_o is the permittivity of free space and ‘f’ is the frequency of the applied field. It is observed from the figure that the ac conductivity increases with increase in frequency which is attributed to the reduction in the space charge polarization at higher frequencies.⁴⁰ With increase in temperature, more and more defects are produced and the movements of these defects result in increased conductivity. Further, the sharp change in conductivity at 34 °C supports the phase transition as observed in the dielectric constant study (Fig. 10(b)). The variation of conductivity with temperature shows that the conduction process is thermally activated.

Poling and d₃₃ measurements

Crystal symmetry plays an important role in the piezoelectric effect which is defined as the ability of materials to produce an electrical charge on applying mechanical stress. Piezoelectric studies were carried out on the grown crystals. Firstly, the sample was poled by applying a moderate dc electric field of 1.5 kV mm⁻¹ for 25 min at room temperature because the poling process improves the net alignment of the dipoles of the material in the direction of the applied field. In general, piezoelectric coefficient (d_ij) can be defined as:

d_ij = (∂D_i/∂T_j)_E

where D, T and E are the electric displacement, stress and electric field respectively. The variation of the electric displacement ΔD with external stress T under a constant applied electric field E is directly proportional to the dipole movement. The larger the polarizability, the greater will be the piezoelectric coefficient of the material. The piezoelectric coefficients can be reported in terms of d₃₃, d₁₅ and d₃₁ etc. In the present study, piezoelectric properties were confirmed by determining the piezoelectric charge coefficient, d₃₃ pC N⁻¹ which can be defined as the generation of polarization in the c-direction when stress is applied in the same direction. This d₃₃ measurement was done along the poling direction of the crystal axis across the basal surfaces using a PM-300 Piezometer system by applying a tapping force of 0.25 N and tapping frequency of 110 Hz. The piezoelectric coefficient of the crystal was found to be 2.24 pC N⁻¹ which is comparable with that of quartz (2.3 pC N⁻¹) and triglycine sulphate crystal (TGS = 10.22 pC N⁻¹)^41,42 and hence SPNPD material can find applications in transducer devices, sensing microstructures on electronic micro-chips and electric voltage sources.^43,44 In semiorganic crystals, the hydrogen bond and π–π stacking interactions have an important role in analyzing piezoelectric properties of the crystal.⁴⁵ These π–π interactions present in the crystal can screen the H-bond from the applied electric field.⁴⁵ This means that the contribution of the H-bond is higher than the π-system along the poling direction of the crystal axis as described in Hirshfeld surface analysis (Fig. 5–7) resulting in a significant d₃₃ value. Thus the piezoelectricity reported in the grown crystal makes it a better candidate for device applications than the other reported p-nitrophenolate based compounds.^8–11

Hysteresis loop

Ferroelectricity properties in materials can be defined as the presence of spontaneous electric polarization which can be reversed by an applied electric field. The ferroelectric behavior of the grown crystal was confirmed by tracing the P–E hysteresis loop at room temperature which is being reported for the first time. The hysteresis loop of the grown SPNPD crystal is shown in Fig. 11. It is found that the grown crystals can sustain a switching ac electric field up to 10 kV cm⁻¹ beyond which cracks are developed in them. So, we cannot apply a field more than this value to achieve saturation polarization. Therefore, the grown crystal does not show saturation in the polarization curve. The ferroelectric loop has an oval shape which may be due to its T_c lying just near room temperature. From the hysteresis loop, the values of the remnant polarization (P_r) and coercive field (E_c) are found to be 2.67 μC cm⁻² and 7.43 kV cm⁻¹, respectively. We define coercive field (E_c) as the minimum applied electric field strength which is required for switching full remnant polarization. Till now ferroelectric behavior has not been reported in many other alkali p-nitrophenolate hydrate compounds.^8–11 Due to the ferroelectric behavior reported in the grown crystal, this material can have many optoelectronic applications such as capacitors, nonvolatile memory devices, high-performance gate insulators etc.

Fig. 11 P–E hysteresis loop of the grown crystal at room temperature.

Nonlinear optical studies

The efficiency of a nonlinear optical material in transferring energy from a fundamental beam to a second harmonic beam is determined by the second harmonic generation efficiency (SHG) parameter. The SHG efficiency of the grown crystals was determined using the Kurtz and Powder technique⁴⁶ which also identifies the materials as having non-centrosymmetric structures. In semiorganic crystals, due to the presence of a strong intermolecular interaction, wave functions overlap to a great extent which results in the delocalization of π-electrons.⁴⁷ The grown crystal crystallizes in the Ima2 space group, which fulfils the required symmetry condition of second harmonic generation. The SPNPD crystal basically consists of a metal ion (Na⁺) surrounded by an organic ligand (phenoxy ion) which is a benzene derivative and contains delocalized π-electrons. Therefore, these crystals are expected to exhibit high NLO properties. To check the SHG efficiency of SPNPD, a KDP crystal was taken as a reference material and was powdered to the same particle size of 63 μm. A Q-switched Nd–YAG laser beam of wavelength 1064 nm was passed through the powdered sample. The SHG property in SPNPD crystals was confirmed from the output of the laser beam having green emission of wavelength 532 nm. The SHG efficiencies of SPNPD and KDP crystals were found to be 14.1 mV and 3.4 mV respectively. Thus the SHG value of the grown crystal was found to be 4.15 times that of KDP which is a much higher value than some reported p-nitrophenolate based semiorganic compounds.^8–11 This higher SHG value in the grown crystal compared to other similar p-nitrophenolate materials is due to the much greater ease of electron delocalization arising from the lower electronegativity of the sodium atom which makes it a potential material for NLO device applications.

Microhardness analysis

Hardness testing is one of the important tools used for determining mechanical properties of solids which gives information about the strength, molecular binding, elastic constants and yield strength of the material.^48,49 Mechanical strength, including voids, plays an important role in device fabrication. In order to study the mechanical property, the Vickers microhardness measurement was carried out on flat and smooth surfaces of the grown SPNPD crystals at room temperature. Loads 5–100 g were applied on the crystals for a dwell time of 10 s which result in micro-indentations on the surfaces. Beyond 100 g of applied load, multiple cracks were observed on the crystal surface around the indenter which may occur due to the internal stress released. The length of both diagonals of indentations was measured using Quantimet software and the average of the diagonals (d) for different loads was considered.

Fig. 12 Vicker microhardness analysis: (a) variation of Vicker’s hardness H_v with load P. (b) Plot of log P vs. log d.

The Vickers microhardness number (H_v) of the crystal was calculated using the following formula:

H_v = 1.8544 × P/d²

where P is applied load in kilograms and d is the average of two indentation diagonal lengths in μm. Fig. 12(a) shows the variation of H_v with applied loads ranging from 5 g to 100 g for an SPNPD crystal. It is observed from the figure that the Vickers hardness parameter (H_v) increases with increasing applied load, suggesting a reverse indentation size effect (RISE).^50,51 In RISE the ‘relaxation’ in the material results in a release of the indentation stress along the surface away from the indentation site which may be due to crack formation or elastic deformation of the tip of the indenter.

The Meyer’s work hardening coefficient (n) was calculated from Meyer’s law⁵² which gives a relationship between load and the size of indentation as:

P = k₁dⁿ

log P = log k₁ + n log d

where k₁ is material constant and ‘n’ is the Meyer’s index.

In order to find the value of ‘n’, a graph was plotted between log P and log d as shown in Fig. 12(b) which gives a straight line after least square fitting. From the slope of the line the value of n was found to be 4.89. According to Onistch,⁵³ the value of ‘n’ should lie between 1 and 1.6 for hard materials and more than 1.6 for soft materials. So SPNPD crystal is categorized as a soft material.

The anisotropic properties of the molecular solids are determined equally by the empty spaces as well as the filled ones.¹⁹ Voids in the crystal can be defined as the region of empty space between the molecules in the crystal and this empty space is the region which lies outside the normal van der Waals surface area of molecules in the unit cell and within which no nuclei exists. Voids in the crystalline material have been visualized by constructing the (0.002 au)-isosurface of procrystal electron density. This 0.002 au electron density isosurface contains more than 98% of the electronic charge of molecules and identifies the empty space in the crystal by determining the shape and size of molecules. Fig. 13 displays the voids surface of the grown crystal. The volume of the void was computed to be 467.71 ų while from single crystal XRD analysis, the volume of the unit cell comes out to be 871.95 ų. Thus the volume occupied by voids is around 53.64% of the unit cell volume which is considerably higher than the void volume (30.7%) calculated in the case of L-alanyl-L-valine.¹⁹ This higher value of volume occupied by voids reveals that the grown crystal is a very soft crystal which has been also supported by the Vicker’s microhardness study in the form of the Meyer’s index value. Void volume, surface area and void volume as a percentage of total unit cell volume of grown crystal computed at (0.002 au)-isosurface have been summarized in Table 7.

Fig. 13 SPNPD unit cell void at (0.002 au)-isosurface.

Table 7 Different parameters of voids in the SPNPD crystal

Grown crystal	Volume/Å^3	Surface area/Å^2	% of unit cell volume
SPNPD	467.71	273.89	53.64

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Conclusion

Good quality sodium p-nitrophenolate dihydrate single crystals have been grown from aqueous solution by adjusting the growth parameters. The experimentally observed morphology of the crystal matches very well with the BDFH and modified BDFH laws. FTIR and Raman studies confirmed the formation of the desired material. The nature of the intermolecular interactions present in the crystal structure was studied by Hirshfeld surface analysis. The fingerprint plots which have been generated from the Hirshfeld surface enabled the decoding of the quantitative contribution of interactions towards the crystal packing. The mapping of shape index and curvedness on the Hirshfeld surface analysis is characteristic of π⋯π stacking interactions. Optical studies show that the grown crystals have very high optical transmittance. In the PL spectra, a broad band observed at 525 nm suggests it is suitable for applications for fabricating green lasers and a new tunable laser system. This broadening in PL emission peak is mainly caused by the presence of strong intermolecular hydrogen bonding interactions in the crystal structure which plays an important role in crystal packing. The low value of dielectric constant and dielectric loss indicates the suitability of the material for NLO applications in accordance with Miller’s rule. The piezoelectric charge coefficient (d₃₃ pC N⁻¹) was found to be 2.24 pC N⁻¹ showing the contribution of hydrogen bonding interaction along the poling direction of crystal axis and hence gives the possibility for transducer device applications. The ferroelectricity reported in the grown crystal makes it favorable for nonvolatile memory devices and high-performance gate insulator applications. Because of very high observed value of SHG (4.14 times of that of KDP) efficiency, SPNPD crystals are useful for laser and photonics devices. In a micro-hardness study Meyer’s index ‘n’ was found to be 4.89 which revealed that the SPNPD crystal belongs to the soft material category. Further, the void volume (53.63% of total unit cell volume) also confirms that the grown crystal is a very soft crystal. Hence, the semiorganic crystal SPNPD is not only a potential material for device fabrication in optoelectronics but also a promising low dielectric constant material showing piezoelectric and ferroelectric behavior.

Acknowledgements

We are thankful for the financial support received in DST project (Sanction no. SR/S2/CMP-0068/2010) and DU R&D Grant (Sanction no. RC/2014/6820). Jyoti Dalal is thankful to CSIR, India for Senior Research Fellowship. Harsh Yadav is thankful to UGC for Meritorious Scholarship.

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Footnote

† CCDC 1403345. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra10501c

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