Noncentrosymmetric hybrid gallium halides built from non-polar inorganic units

Abstract

Hybrid organic–inorganic metal halide compounds are a versatile platform for developing materials for optoelectronic applications. In this report, we studied the crystal structures and optical properties of five hybrid noncentrosymmetric Ga³⁺ halides with tri- and tetraethylammonium cations. All new phases crystallize in polar space groups and contain isolated GaX₄⁻ units in their structures, which are charge-balanced by the corresponding organic cations. Different packing arrangements of structural units result in three distinct crystal systems for five new compounds: hexagonal, orthorhombic, and monoclinic. All new structures demonstrate SHG activity, varying from weak (0.13 × KDP) to strong (1.88 × KDP), which agrees with acentric structures of these Ga³⁺ phases. Although our attempts to relate the net dipole moments of the compounds with SHG response were unsuccessful, the coalignment of the GaX₄ tetrahedra is a likely structural prerequisite for the high SHG response observed in Et₄NGaCl₄. Overall, Ga hybrid halides provide a promising and flexible platform for the synthesis of new polar phases.

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Vladislav V. Klepov

Vladislav V. Klepov is an Assistant Professor in the Department of Chemistry at the University of Georgia (UGA). Prior to joining UGA, he conducted postdoctoral research at the University of South Carolina and Northwestern University. His research group studies a broad range of topics in solid state materials chemistry, including the synthesis of new quantum materials, radiation detectors, and compounds for optoelectronic applications. His primary research goal is to elucidate the relationships between the composition and structure of new materials and their electronic and physical properties.

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Introduction

The discovery of nonlinear optical (NLO) phenomena, such as optical harmonics generation, has broadened the spectral range of lasers, which is crucial for non-destructive studies, medicine, semiconductor deposition, and data transmission.^1–5 Noncentrosymmetric materials capable of frequency doubling (second harmonic generation, SHG) are usually divided into classes, depending on the operational spectral region: far IR and mid IR, near IR and visible, UV and deep UV.^6–9 NLO materials must transmit the light in the intendent region of application, which is defined by their bandgap values: for example, mid IR NLO materials need bandgap values above 3.0 eV, while UV NLO materials – above 6.2 eV.^5,9 Naturally, oxides have wider bandgaps compared to chalcogenides or halides, and the most common UV NLO materials are borates, carbonates, nitrates, and phosphates.⁸ Particular interest in borates arises from their tendency to crystallize with noncentrosymmetric structures.¹⁰

Chalcogenides with more covalent bonds compared to oxides and thus lower bandgaps show promising SHG behavior in the IR region.⁷ Halides with intermediate bandgaps and high transparency for IR radiation offer broad tunability of the compositions, structure, and properties to satisfy key requirements for mid-IR NLO materials.⁹ One specific class of materials, hybrid organic–inorganic metal halides (and specifically halide perovskites), emerged as a tremendously versatile system covering a wide range of applications spanning from solar cells^11–13 and LEDs^14–16 to photocatalysis,¹⁷ energy storage,¹⁸ and medical imaging.^19,20 The flexibility of halides compositions and properties grants them a place among prospective NLO materials.^21,22

The universal tendency of materials to crystallize in centrosymmetric space groups also applies to halides. However, hybrid halides can be endowed with noncentrosymmetric structures via chirality transfer from organic cations.^12,23 In a direct transfer, a chiral organic cation is introduced to a metal halide structure, resulting in symmetry breaking and crystallization of material in a chiral space group.²⁴ Remote chirality transfer is more peculiar: here, a chirality-inducing agent is not a part of the halide structure; however, its proximity induces chirality in an otherwise non-chiral structure.²⁵ Both approaches employ the same organic cations.²⁶ Besides SHG, chiral materials demonstrate circular dichroism and circularly polarized photoluminescence.^23,27 Both inorganic metal-halide anions and organic cations of hybrid halides influence the NLO properties of a material. For example, the (C₅H₁₂N)SnCl₃ phase contains highly noncentrosymmetric SnCl₃⁻ units in the structure, which are the main contributors to the SHG response of the phase.²⁸ On the contrary, in the (C₇H₁₀N)PbBr₃ phase with lead being octahedrally coordinated by bromide ions, an enhanced SHG signal is enabled via charge transfer from PbBr₆⁴⁻ units to the organic cation.²⁹

Chiral materials are used in enantiomer recognition and separation,³⁰ circularly polarized LEDs,³¹ optoelectronic sensors,³² spintronics,³³ and quantum optics.³⁴ Despite the compositional flexibility of halide perovskites, the practicality of their application is hindered by chemical instability issues. For example, lead halide perovskites (LHPs) decompose over time if exposed to the atmosphere and light.³⁵ Sn²⁺ and Ge²⁺ ions of lead-free halides are prone to oxidation to Sn⁴⁺ and Ge⁴⁺, respectively.³⁶ Thus, it is practical to search for new compositions containing main group elements in stable oxidation states.³⁷

In this paper, we report on the synthesis, structural, optical, and thermal characterization of five new acentric hybrid halides containing GaX₄⁻ anions (X = Cl, Br, or I) and tri- (Et₃NH⁺) or tetraethylammonium (Et₄N⁺) cations. While noncentrosymmetric structures are generally much less common compared to the centrosymmetric ones, all new phases showed a second harmonic generation signal, confirming the lack of an inversion center in their structures. As expected, the optical bandgap of a phase decreases when moving from X = Cl to I, reaching a value of 2.5 eV in Et₄NGaI₄. Interestingly, since all new phases belong to one of three polar point groups, m, 6mm, or mm2, they can be employed as pyroelectric and circular dichroism materials.

Results and discussion

Synthesis and crystal structures of new halides

We obtained all five materials as precipitates from aqueous solutions of HCl/HBr/HI containing Ga₂O₃ and respective organic cations (Table S1). All samples were obtained in phase pure form (Fig. 1), and no other phases were observed in these systems.

Fig. 1 Powder X-ray diffraction (PXRD) patterns of as-synthesized Et₃NHGaX₄ (a and b) and Et₄NGaX₄ (c–e) phases overlayed with the corresponding calculated patterns.

Et₃NHGaBr₄ and Et₃NHGaI₄ phases are isostructural and crystallize in the monoclinic polar space group Cc (Table 1). All atoms occupy the only 4a positions in the unit cells of Et₃NHGaX₄ (X = Br of I) phases. Each Ga atom in Et₃NHGaX₄ is surrounded by four halides in a slightly distorted tetrahedral manner, with all four Ga–X bonds being different in length as a result of C₁ (1) site symmetry (Tables S12 and S13). GaX₄ units are isolated from each other by organic cations, which also compensate for their negative charges (Fig. 2).

Fig. 2 GaX₄ (X = Cl, Br, or I) tetrahedra and organic cations packing in (a) Et₃NHGaBr₄, (b) Et₄NGaCl₄ and (c) Et₄NGaI₄ structures. GaX₄ tetrahedra are orange. Organic cations are disordered in (b) and (c).

Two Et₄NGaX₄ phases (X = Cl or Br) are isostructural and crystallize in the hexagonal polar space group P6₃mc (Table 1). Ga atoms are located in the 2a positions with C_3v (3m) site symmetry. Because of this, GaX₄ tetrahedra are distorted with one bond elongated compared to three others, which are equal in length (Tables S14 and S15). The nitrogen atom of the sole tetraethylammonium cation is in the 2b position with C_3v (3m) site symmetry. Since Et₄N⁺ cation is incompatible with the trifold rotation symmetry of the crystal structure, it exhibits disorder of one ethyl group. Lowering the symmetry of the structure can, in principle, resolve the disorder; however, PXRD data do not evidence lower symmetry (Fig. 1).

When a larger iodide anion is introduced to the system, the resulting Et₄NGaI₄ phase demonstrates lower symmetry compared to the chloride and bromide analogs and crystallizes in the polar orthorhombic space group Pmn2₁ (Table 1). Two independent Ga atoms are in 2a positions of the unit cell with C_s (m) site symmetry. Thus, two Ga–I bonds that are related by a mirror plane running through the GaI₄ units are equal in length, while the other two in-plane bonds have different lengths (Table S16). Tetraethylammonium cations in the structure are disordered by a mirror plane as both independent nitrogen atoms are in 2a positions in the Et₄NGaI₄ unit cell.

The average Ga–X bond length increases from 2.165(2) Å in chloride to 2.316(10) Å in bromides and 2.540(12) Å in iodides, in agreement with the increasing radius of the halide ion. These data also agree with the acentric hybrid Ga halides deposited to CCDC³⁸ with average Ga–X bond lengths being 2.17(1) Å, 2.33(2) Å, and 2.55(3) Å for chlorides, bromides, and iodides, respectively. Previously reported structures also contain only isolated tetrahedral GaX₄⁻ units. For most of the Ga atoms in the halide surroundings, the site symmetry of their position is C₁ (1) with all four Ga–X bonds in tetrahedra having different lengths. One of the factors causing elongation of some Ga–X bonds is N–H⋯X hydrogen bonds between GaX₄⁻ tetrahedra and organic cations. The C₃ (3) site symmetry is also quite common. Other variations include C_s (m), C_2v (mm2), C_3v (3m), and S₄ (4), but are rarely presented. The X–Ga–X bond angles lie in the 104–115° range, however, despite this wide range and somewhat low site symmetry for the most Ga atoms, the average X–Ga–X bond angle is 109.47(1)°, which corresponds to the near ideal tetrahedral coordination.

Optical properties

All new phases crystallize in the polar space groups and thus should demonstrate piezoelectric and NLO properties.³⁹ SHG data collected for new halides confirm that all phases are acentric (Fig. 3 and Table S22). The highest SHG response (1.88 × KDP, Fig. 3) was found in the Et₄NGaCl₄ phase, while the Et₄NGaI₄ phase demonstrates the lowest SHG intensity among the five samples (0.13 × KDP). SHG activities of three other phases are slightly higher than for KDP (Fig. 3 and Table S22).

Fig. 3 SHG signal intensity in new Ga hybrid halides and KDP standard sample at 1064 nm.

Table 1 Crystallographic parameters and single crystal X-ray diffraction experimental details of the new phases

Empirical formula	Et3NHGaBr4	Et3NHGaI4	Et4NGaCl4	Et4NGaBr4	Et4NGaI4
Formula weight	475.43	663.39	321.61	499.45	687.41
Space group	Cc	Cc	P63mc	P63mc	Pmn21
a, Å	7.553(6)	7.8828(5)	8.2244(11)	8.3651(6)	8.4479(6)
b, Å	14.467(11)	15.0869(10)	8.2244(11)	8.3651(6)	14.2031(10)
c, Å	14.113(11)	14.5970(10)	13.259(3)	13.6354(13)	15.1904(12)
β, °	102.32(2)	101.596(2)	90	90	90
Volume, Å^3	1507(2)	1700.5(2)	776.7(3)	826.31(14)	1822.6(2)
Z	4	4	2	2	4
ρcalc, gcm^−3	2.096	2.591	1.375	2.007	2.505
Independent reflections	3895 [Rint = 0.0712, RSigma = 0.0704]	4746 [Rint = 0.0564, RSigma = 0.0557]	833 [Rint = 0.1303, RSigma = 0.0510]	878 [Rint = 0.0914, RSigma = 0.0330]	5150 [Rint = 0.0973, RSigma = 0.0837]
Data/restraints/parameters	3895/15/109	4746/66/109	833/3/27	878/5/27	5150/56/117
GOOF on F^2	1.047	1.003	1.102	1.124	1.020
Final R indexes [I ≥ 2σ (I)]	R1 = 0.0504, wR2 = 0.1087	R1 = 0.0518, wR2 = 0.0969	R1 = 0.0895, wR2 = 0.2172	R1 = 0.0517, wR2 = 0.1335	R1 = 0.0536, wR2 = 0.0923
Final R indexes [all data]	R1 = 0.1432, wR2 = 0.1481	R1 = 0.1215, wR2 = 0.1166	R1 = 0.1541, wR2 = 0.2555	R1 = 0.1043, wR2 = 0.1799	R1 = 0.1595, wR2 = 0.1245
Largest diff. peak/hole, ēÅ^−3	0.59/−0.61	0.69/−1.10	0.71/−0.46	0.63/−0.49	1.03/−0.87
Flack parameter	0.011(14)	0.052(18)	0.05(14)	0.01(2)	0.38(19)

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As can be seen from the SHG data, the new phases exhibit significantly different NLO activity, which, however, does not align with the expected trends. One would anticipate that compounds with more polarizable bonds, i.e., those containing heavier halide anions, would demonstrate higher SHG response. Contrary to this expectation, Et₄NGaI₄ exhibits the lowest SHG response. One possible reason for that is the reabsorption of the double-frequency light by the material. To evaluate this effect, we collected their UV-vis spectra and calculated band gaps of the materials assuming direct bandgaps (Fig. S1). As expected, the iodides, which demonstrate darker colors, have lower bandgaps, 2.5 eV for Et₄NGaI₄ and 2.8 eV for Et₃NHGaI₄. Although the 532 nm second harmonic of the YAG:Nd laser corresponds to a slightly lower energy, ≈2.33 eV, the UV-vis spectra of both compounds exhibit absorption tails in the lower energy region, which likely arise from defect states. Due to the reabsorption, both compounds show either low (Et₄NGaI₄, 0.13 × KDP) or moderate (Et₃NHGaI₄, 1.07 × KDP) SHG responses. To avoid reabsorption effects, both iodides were also studied with 2090 nm IR laser against AgGaS₂ (AGS) standard. The activity of Et₃NHGaI₄ was moderate (1.65 × AGS), while no SHG signal was detected from the tetraethylammonium phase (Fig. S2).

The remaining three compounds with bromide and chloride anions have wider bandgaps (>4 eV, Fig. S1) according to UV-vis data and should be transparent for the double frequency SHG response. However, even isostructural Et₄NGaCl₄ and Et₄NGaBr₄ counter the expected trend of increasing SHG response upon heavier halide atom incorporation. One of the factors contributing to the difference is the alignment of the GaX₄ tetrahedra net dipole moments in the structures of new phases. To calculate the dipole moments of Ga–X bonds and the total net dipole moments, we used the approach previously employed by Choi et. al. for ZnCl₄ units.⁴⁰ For isostructural Et₄NGaX₄ (X = Cl and Br) phases, the net dipole moments are coaligned along one of the Ga–X bonds of the tetrahedra and the c axis of the unit cell (Fig. S3). The net dipole moment of a GaCl₄ tetrahedron is half that of the GaBr₄ one (Table S23), which is in agreement with the expected trend, but contradicts the higher SHG response in the Et₄NGaCl₄ phase. It is therefore likely that other factors, such as birefringence and phase-matching, play a more important role. We collected phase-matching data for Et₄NGaCl₄ that demonstrated the highest SHG response among the phases reported here (Fig. S4). We found that the SHG response significantly decreases as the particle size increases, showing that this sample is not phase matchable.

NLO materials based on acentric Ga³⁺ hybrid halides were not extensively studied to date, as these phases commonly form as side products in the synthesis of galloorganic molecules. Out of all acentric hybrid chlorides, bromides, and iodides of Ga³⁺ deposited in CCDC, SHG activity was only reported for the Me₄NGaCl₄ phase (Me = –CH₃).⁴¹ This tetramethylammonium phase demonstrates a higher activity (2.5 × KDP) compared to the ethylammonium analog. In the Me₄NGaCl₄ structure, site symmetry on Ga atoms is C_2v (mm2), which does not match the symmetry of the GaX₄⁻ units in our phases. The dipole moments of individual Ga–Cl bonds are comparable to those in the Et₄NGaCl₄ phase (≈16.7 D), however the net dipole moments of GaCl₄⁻ tetrahedra are six times higher (Table S23). Moreover, unlike in Et₃NHGaBr₄ and Et₄NGaI₄ phases with similar magnitude of net dipole moments, in Me₄NGaCl₄ they are also coaligned along the same axis, which explains the higher NLO activity of this material.

Thermal behavior

As structural units of the phases under consideration show only weak interactions, these materials can be candidates for ionic liquids. To study the thermal behavior of new phases, we collected DSC data. DSC study of the Et₄NGaI₄ sample revealed two endo effects followed by one exo effect upon the first heating cycle (Fig. 4). Further cooling and heating curves do not demonstrate any features, suggesting that Et₄NGaI₄ phase undergoes irreversible phase transition or partial decomposition. Since thermogravimetric analysis (TGA) shows no significant weight loss in the 30–150 °C temperature range, no gaseous products should form in the decomposition process (Fig. 4). Complete decomposition of the sample observed above 400 °C (Fig. S5). PXRD study of the Et₄NGaI₄ sample after DSC revealed that a new phase (or phases) formed during temperature treatment of the Et₄NGaI₄ sample (Fig. S5). The new phase was indexed from the experimental PXRD data as an orthorhombic one, Pmmm space group, a = 4.3428(5) Å, b = 7.5295(6) Å, and c = 14.3868(8) Å (Fig. S5), but no apparent matches of the refined unit cell parameters with reported structures containing the corresponding elements were found.

Fig. 4 DSC-TGA study of Et₄NGaI₄ sample. Peaks positions are shown in the figure. No significant sample weight loss was detected in TGA analysis.

Unlike Et₄NGaI₄, Et₄NGaBr₄ does not melt or undergo a phase transition up to 150 °C (Fig. S6). Et₃NHGaI₄ and Et₄NGaCl₄ melt above 100 °C and do not show notable supercooling (Fig. S6). Finally, the Et₃NHGaBr₄ phase appears to react with the aluminum DSC pan, resulting in unreliable melting temperatures for this phase. PXRD data collected from Et₃NHGaI₄, Et₄NGaCl₄, and Et₄NGaBr₄ samples after DSC confirm their stability upon heating disregarding the presence of thermal effects (Fig. S7). Moreover, unlike many hybrid phases containing Pb²⁺, Ge²⁺, or Sn²⁺, these materials showed long term stability. Samples stored for 5 months on a shelf did not show deterioration in crystallinity, nor did new peaks appear in the PXRD patterns (Fig. S8).

Conclusion

Five acentric gallium hybrid halides were synthesized and their structures, linear and nonlinear optical properties, and thermal behavior were characterized. Surprisingly, all new phases crystallized in polar space groups, even though no polarity inducing agents were introduced in the synthesis. In all the new phases, Ga atoms demonstrate exclusively tetrahedral coordination to halides. The resulting tetrahedra are distorted, as the site symmetries of Ga positions are lower than T_d. One of the phases, Et₄NGaCl₄, demonstrates promising NLO activity (1.88 × KDP) and can be easily prepared on a large scale, showing that Ga³⁺ hybrid halides can be a versatile platform for the preparation of new NLO materials. Two phases, Et₃NHGaI₄ and Et₄NGaCl₄, melt above 100 °C offering another platform for high-temperature reaction media.

Experimental

Synthesis

Initial reagents for the synthesis of new halide phases were Ga₂O₃ (99.9%), triethylamine (Et₃N, 99%), tetraethylammonium chloride (Et₄NCl, 99.4%), tetraethylammonium bromide (Et₄NBr, 99%), and concentrated HX acids (X = Cl, Br, or I). First, calculated amounts of Ga₂O₃ were weighed and dissolved in excess of the corresponding acid (Table S1). For the synthesis of iodides, small portions of H₃PO₂ were added to the reaction to prevent iodide anion oxidation. Triethylamine was added on stirring directly to the acidic solutions containing Ga³⁺ ions. Tetraethylammonium halides were dissolved in HX separately and then added to the Ga³⁺ solutions. For all systems, mixing of reagents caused immediate precipitation. Precipitates used for properties measurements were separated from solutions by vacuum filtration on a glass filter. Small portions of precipitates were used for recrystallization of target phases to obtain crystals suitable for a single crystal X-ray diffraction study (Table S1).

Characterization

Single crystal X-ray diffraction was used to determine the structures of new phases (Tables S2–S21). Room temperature data sets were collected on Bruker D8 QUEST diffractometer (Mo Kα radiation, λ = 0.71073 Å) equipped with hybrid photon counting area detector. Raw data were integrated with Saint-PLUS software.⁴² Correction for absorption was performed with SADASBS program.⁴³ Structures of all new compound were solved by the intrinsic phasing method with SHELXT software,⁴⁴ and refined against F² with SHELXL program⁴⁵ using Olex2 interface.⁴⁶

Phase purity of the samples was tested with PXRD on a Bruker D2 Phaser instrument (Cu anode) equipped with LYNXEYE energy dispersive 1D detector. Indexing of the unknown product formed upon heating of the Et₄NGaI₄ sample was done with GSAS-II.⁴⁷ LeBail fitting was performed in Rietica software.⁴⁸

A modified Kurtz–Perry system^49,50 equipped with a 1064 nm Nd:YAG and 2090 nm Ho:YAG lasers was used to collect powder SHG data. KH₂PO₄ (KDP) and AgGaS₂ (AGS) served as the references for the samples studied with a 1064 nm and 2090 nm lasers respectively.

Defuse reflectance spectra were collected on a Shimadzu UV-2450 (Kyoto, Japan) spectrometer and an Edinburgh Instruments FS5 Spectrofluorometer equipped with an integrating sphere at room temperature in the 200–800 nm wavelength range. BaSO₄ was employed as a non-absorbing reflectance reference for calibration. Iodides were ground with excess BaSO₄ before the measurements.

Thermal transitions were evaluated using a TA Discovery 250 differential scanning calorimeter (TA Instruments, New Castle, Delaware, USA) under nitrogen. The samples (10–20 mg) were enclosed in aluminum pans before testing. After equilibrating at 20 °C, samples were heated to 150 °C at a heating rate of 5 °C min⁻¹ and subsequently cooled to 20 °C at a rate of 5° min⁻¹. Two of these heating and cooling cycles were employed. Thermogravimetric analysis (TGA) of samples was conducted using a TA Instruments Discovery TGA 550. The samples (7–14 mg) were heated from 20 °C to 900 °C at 10 °C min⁻¹ rate.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: synthesis procedures, crystallographic data, SHG data, Tauc plots, dipole moments, PXRD study of the samples, DSC and TGA data. See DOI: https://doi.org/10.1039/d5tc03829d.

CCDC 2492463–2492467 contain the supplementary crystallographic data for this paper.^51a–e

Acknowledgements

This work was supported by the University of Georgia Department of Chemistry, Department of Physics and Astronomy, Franklin College of Arts and Sciences, and the Office of Provost. EG thanks the Welch Foundation (Grant E-1457) for their support.

Notes and references

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