Yuji
Masubuchi
*a,
Sayaka
Nishitani
b,
Akira
Hosono
b,
Yuuki
Kitagawa
c,
Jumpei
Ueda
c,
Setsuhisa
Tanabe
c,
Hisanori
Yamane
d,
Mikio
Higuchi
a and
Shinichi
Kikkawa
a
aFaculty of Engineering, Hokkaido University, N13 W8, Kita-ku, Sapporo 060-8628, Japan. E-mail: yuji-mas@eng.hokudai.ac.jp
bGraduate School of Chemical Science and Engineering, Hokkaido University, N13 W8, Kita-ku, Sapporo 060-8628, Japan
cGraduate School of Human and Environmental Studies, Kyoto University, Yohida-nihonmatsu-cho, Sakyo-ku, Kyoto 606-8501, Japan
dInstitute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
First published on 25th April 2018
A new polymorph of BaCN2 was obtained via a simple nitridation reaction of BaCO3 under an NH3 flow at 900 °C. The product was analyzed via single crystal X-ray diffraction and infrared spectroscopy, and it was found to have a tetragonal I4/mcm crystal structure (space group no. 140) with a = 0.60249(4) nm and c = 0.71924(5) nm. In this structure, each Ba2+ cation is situated in the square antiprism of N atoms of NCN2− anionic groups. Eu2+ doped BaCN2 can be excited by irradiation of blue and green light (from 400 to 550 nm), and it generates an intense red emission peak at 660 nm with a quantum efficiency of 42% in response to 465 nm excitation at room temperature. The peak emission wavelength varies over an extremely wide range of temperature, from 640 nm at 500 K to 680 nm at 80 K, and this red-shift with decreasing temperature is attributed to a unit cell shrinkage that results in significant crystal field splitting of the 5d energy levels of the Eu2+ ions.
Metal cyanamides and carbodiimides are interesting inorganic materials that exhibit a nitrogen-related pseudo-oxide chemistry, since NCN2− anionic groups can replace O2− anions. The triatomic anionic groups in these compounds can have varying numbers of neighboring metal cations and thus form a characteristic host lattice depending on the cation. Many transition metal cyanamides have been prepared via solution processes. As an example, CuCN2, ZnCN2 and CdCN2 can be obtained from the reaction of the associated metal chlorides and cyanamides in aqueous media.6–8 Solid-state metathesis has also resulted in the formation of MnCN2 from MnCl2 and ZnCN2,9 while SrCN2 was synthesized via a flux route based on a combination of strontium halide, alkaline cyanide, and alkaline azide precursors.10 Calcium carbide has also been used as the starting material to form CaCN2 under a pressurized N2 atmosphere,11 and the high temperature nitridation reactions of CaCO3 and SrCO3 under an ammonia flow have also been applied to synthesize the respective cyanamides.12,13 The rhombohedral structure (Rc) of BaCN2 with a = 1.5282(2) nm and c = 0.7013(2) nm has only been reported for a cyanamide prepared from the reaction of Ba3N2 and melamine under an Ar flow at temperatures between 740 °C and 850 °C.14 In this structure, six N atoms of NCN2− coordinate to each Ba2+ cation, forming a distorted octahedron. In addition, only the formation of BaCN2 has been reported in the reaction of BaCO3 with hydrogen cyanide or the nitridation reaction of BaCO3.12,15
Alkaline earth cyanamides and related compounds such as metal cyanate (MOCN) and thiocyanate (MSCN) have been applied as host materials for Eu2+ doping. A blue emission peak at 457 nm is generated by Eu2+-doped Sr(OCN)2, while Eu2+-doped Sr(SCN)2 produces a green emission at 508 nm in response to excitation at 420 nm.16,17 A longer emission wavelength of 610 nm (corresponding to an orange color) has been reported for Eu2+-doped α-SrCN2.13 However, these characteristic emissions are only evident at low temperatures and are completely quenched at room temperature. Their thermal quenching temperatures (T1/2) at which the luminescence intensities are reduced by 50% are 65 K, 157 K and 90 K for Eu2+-doped Sr(OCN)2, Sr(SCN)2 and α-SrCN2, respectively. However, a room temperature orange emission at 603 nm has been reported for Eu2+-doped α-SrCN2 obtained from the reaction of SrI2, EuI2, CsN3 and CsCN in Ta ampules.10 This emission at room temperature has been attributed to the reduced defect concentration caused by direct doping with Eu2+I2. Ba(SCN)2 was also studied as a host material with regard to doping with Eu2+.18 Ba2+ was coordinated with eight SCN− anionic groups to form distorted square antiprism polyhedra. This material was found to generate an intense bright green emission peak at 511 nm at low temperature that was completely quenched at room temperature, with a T1/2 of 181 K.
In this paper, we present the crystal structure of a new polymorph of barium carbodiimide (BaCN2) prepared by a simple nitridation reaction of BaCO3. This polymorph crystallized in a tetragonal lattice with square antiprism coordination around the Ba2+ ions, and it was studied as a novel host material for Eu2+ ions. Eu2+-doped BaCN2 was found to produce a red emission over a wide range of wavelengths depending on temperature, and these emission characteristics were investigated in relation to the crystal structure.
The total radiant flux spectra of the pump-LED source and sample luminescence under direct- and indirect- excitation of the sample were measured with an integrating sphere (Labsphere, LMS-100). The measured spectra were calibrated with a standard halogen lamp (Labsphere, CLS-600) to obtain the precise spectral power distributions, from which photon emission rate of the pump-LED under direct- and indirect- excitation, Φex_direct (λ) and Φex_indirect (λ), and that of sample luminescence under direct- and indirect excitation, Φlumin_direct (λ) and Φlumin_indirect (λ), can be calculated. Quantum efficiency (QE) values were calculated using the following equation:
(1) |
The fluorescence decay curves monitoring 650 nm luminescence were measured with a Quantaurus-Tau (Hamamatsu Photonics, C11367-01) under 365 nm picosecond LED excitation at different temperatures from 90 to 450 K. The temperature of the sample was controlled by a cryostat (Japan High Tech, 10035L). The obtained fluorescence decay curve was fitted after baseline subtraction using a single-exponential function,
(2) |
I 0 is initial intensity and τ is lifetime.
The crystal structure of the BaCN2 polymorph was analyzed by single crystal XRD. The structure was refined to give the tetragonal space group I4/mcm (no. 140) with a = 0.60249(4) nm and c = 0.71924(5) nm, and Z = 4 (Table 1). The atomic coordinates and anisotropic displacement parameters are given in Table 2. This structure is an isotype of the alkaline cyanates, AOCN (A = K, Rb, or Cs),24,25 in which each Ba2+ ion is coordinated with eight N atoms belonging to NCN2− groups (Fig. 1), and in the regular square antiprism of N. The NCN2− anion is in the square prism of Ba atoms (Fig. 1(c)). The Ba-centered square antiprism polyhedra are connected via face sharing along the c-axis and edge-sharing in the ab-plane. Along the c-axis, Ba2+ and NCN2− layers are stacked on one another along the c-axis to form a CsCl-type arrangement of both ions in conjunction with an ordered arrangement of the NCN2− groups. The C–N bond length is 0.1233(7) nm (Table 3), which corresponds to values reported for C–N double bonds in metal cyanamides.14 This value is respectively longer and shorter than the C–N triple and single bonds in PbCN2 (0.1156 nm for triple and 0.1297 nm for single).26 The Ba–N bond length in this new polymorph is 0.2928(5) nm, which is longer than the values reported for rhombohedral BaCN2 having the octahedral coordinated Ba2+ cations (0.2773–0.2867 nm).14 The face-sharing Ba2+ polyhedra along the c-axis result in a Ba–Ba distance of 0.35962(3) nm, whereas the distance in the ab-plane is 0.42602(2) nm and thus similar to that in rhombohedral BaCN2.
R 1 = Σ||Fo| − |Fc||/Σ|Fo|. wR2 = [Σw(Fo2 − Fc2)2/Σ(wFo2)2]1/2, w = 1/[σ2(Fo2) + (aP)2 + bP], where Fo is the observed structure factor, Fc is the calculated structure factor, σ is the standard derivation of Fc2, and P = (Fo2 + 2Fc2)/3. S = [Σw(Fo2 − Fc2)2/(n − p)]1/2, where n is the number of reflections and p is the total number of parameters refined. | |
---|---|
Chemical formula | BaCN2 |
Formula weight, Mr (g mol−1) | 177.37 |
Crystal form, color | Granule, translucent colorless |
Crystal size, mm3 | 0.064 × 0.052 × 0.036 |
Radiation wavelength, λ (nm) | 0.071073 |
Temperature, T (K) | 302 |
Crystal system | Tetragonal |
Space group | I4/mcm (No. 140) |
Unit-cell dimensions, a (nm) | 0.60249(4) |
c (nm) | 0.71924(5) |
Unit-cell volume, V (nm3) | 0.26108(4) |
Z | 4 |
Calculated density, Dcal (Mg m−3) | 4.512 |
Absorption coefficient, μ (mm−1) | 14.862 |
Absorption correction | MULTI-SCAN (SADABS)20 |
Limiting Indices | −7 ≤ h ≤ 7 |
−7 ≤ k ≤ 7 | |
−9 ≤ l ≤ 9 | |
Number of reflections | 92 |
Weight parameters, a, b | 0.0156, 4.0817 |
Goodness-of-fit on F2, S | 1.392 |
R 1, wR2 (I > 2σ(I)) | 0.0206, 0.0441 |
R 1, wR2 (all data) | 0.0246, 0.0454 |
Atom | Site | Occ. | x | y | z |
---|---|---|---|---|---|
U ij × 10−2 nm2 (= Å2). | |||||
Ba | 4a | 1 | 0 | 0 | 0.25 |
C | 4d | 1 | 0.5 | 0 | 0 |
N | 8h | 1 | 0.3533(11) | 0.1447(11) | 0 |
Atom | Site | U 11 = U22 | U 33 | U 12 | U 23 = U13 |
---|---|---|---|---|---|
Ba | 4a | 0.0106(3) | 0.0118(4) | 0 | 0 |
C | 4d | 0.008(3) | 0.009(5) | 0.003(9) | 0 |
N | 8h | 0.008(2) | 0.015(4) | −0.001(3) | 0 |
C–N | ×2 | 0.1233(7) |
Ba–N | ×8 | 0.2928(5) |
Ba–Ba | ×2 | 0.35962(3) |
×4 | 0.42602(2) |
The FT-IR spectrum of this compound exhibits an asymmetric stretch peak (νas) at 1960 cm−1 and deformation vibration peaks (δ) at 670 and 680 cm−1 (Fig. 2). These peak positions are in good agreement with those generated by both rhombohedral BaCN2 and α-SrCN2, each of which are metal carbodiimides having two symmetric NCN double bonds.10,14 The tetragonal BaCN2 obtained in this work is therefore believed to contain carbodiimide anions ([NCN]2−). To confirm the tetragonal structure, a powder XRD pattern was calculated using the Rietveld program based on the structural parameters in Tables 1 and 2, and a comparison of the observed and the calculated powder XRD patterns is presented in Fig. 3. The experimental diffraction lines are in good agreement with the calculated pattern, although several additional weak reflections are also present in the former. The impurity phase associated with these reflections has not yet been identified but might consist of a metal cyanamide having both triple and single bonds between C and N, for which there is some evidence in the FT-IR spectrum (Fig. 2).
Fig. 2 FT-IR spectrum of the nitrided product. The symbols νas, νs, and δ correspond to the absorptions for asymmetric stretch, symmetric stretch and deformation vibrations, respectively. |
Fig. 3 Powder XRD pattern of the nitrided product (red line) and the pattern calculated using the structural parameters in Tables 1 and 2 (blue line). Black triangles indicate diffraction lines attributed to an impurity phase. |
Thermogravimetric and differential thermal analyses (TG-DTA) for the tetragonal BaCN2 under a N2 flow were performed in our previous study and indicate an endothermic peak at approximately 910 °C without a weight loss corresponding to the melting of BaCN2.27 A solidified product of BaCN2 after annealing above the melting temperature contains rhombohedral BaCN2 instead of the tetragonal phase. We did not find an additional signal in the DTA curve indicating the phase transition, however we believe the transition temperature was obscured by the large endothermic peak of the melting, and the high temperature phase is the rhombohedral BaCN2. The phase transition from the tetragonal to rhombohedral BaCN2 is irreversible because there is no trace of the tetragonal phase in the XRD pattern of the annealed product. Density of tetragonal BaCN2 is 4.512 g cm−3, which is higher than that of rhombohedral BaCN2 (3.738 g cm−3).14 Interestingly, a low-temperature but high-symmetry phase (I4/mcm) transformed to a high-temperature but low-symmetry (Rc) phase. The mechanism of the phase transition is still unknown, however this counterintuitive phase transition was also reported in SrCN2, in which the phase transition from a hexagonal high-symmetry β-phase to an orthorhombic low-symmetry α-phase occurs around 920 K and the phase transition is irreversible upon cooling, similar to the case of BaCN2.28
The luminescence of Eu2+ ions in a host material is influenced by structural parameters, including covalency, bond-length and coordination number, and the 4f–5d transition energy of phosphors doped with Eu2+ can be estimated by using an empirical equation.29,30 The equation indicates that an increase in the coordination number and the size of the substituted cation will induce a blue-shift in the emission wavelength of a Eu2+-doped phosphor. As noted, red emission at 660 nm was observed in the case of the Eu2+-doped tetragonal BaCN2. By comparing its crystal structure with that of Ba(SCN)2:Eu2+, which emits green light at 511 nm, it is evident that the tetragonal BaCN2 had a shorter Ba–N bond length (0.2928(5) nm) compared to the Ba–N (and S) bonds in Ba(SCN)2, for which the average bond length is 0.3086 nm.18 The Ba2+ ions in BaCN2 were coordinated in a regular square antiprism (D4d point symmetry) environment having identical Ba–N bond lengths, while Ba(SCN)2 is based on a distorted square antiprism coordination (C2 point symmetry) in association with a wide range of Ba–N and Ba–S bond lengths. The greater negative charge of the NCN2− anions forming the symmetric environment in BaCN2 will tend to stabilize the d orbitals of the Eu2+ ions, in contrast to the distorted coordination of the monovalent SCN− anions. As a result, increased crystal field splitting is expected in the BaCN2 host material, leading to the red emission of the Eu2+-doped BaCN2, because of the reduced energy difference between the 4f7 ground state and the 4f65d1 excited state. Conversely, this explanation does not support the shorter emission wavelength of Eu2+-doped SrCN2, in which each cation is coordinated with six NCN2− anions in a distorted octahedral environment.13,17 The empirical equation noted above is, of course, not applicable to all phosphors. As an example, the emission peak is shifted toward longer wavelengths upon changing the M site cation from Ca → Sr → Ba in M2Si5N8:Eu2+ (Ca2Si5N8: 610 nm, Sr2Si5N8: 630 nm, Ba2Si5N8: 640 nm).31 The equation does not take into account the effect of site symmetry on the emission peak shift, so the symmetric square antiprism coordination in BaCN2 might increase the crystal field splitting to a greater extent than distorted octahedral coordination, such that the energy position of the 5d excited state is reduced.
Thermally stable phosphors such as the nitrides mentioned above do not exhibit significant changes in emission wavelength with increasing temperature (typically less than 1 nm on going from room temperature to 423 K and above), which is indicative of the high thermal stability of the chromatic behavior.32–34 This thermal stability is caused by the rigid structure constructed from [SiN4] and/or [AlN4] tetrahedra. However, a large shift in the emission wavelength (from 680 nm at 80 K to 640 nm at 500 K) of BaCN2:Eu2+ is evident in Fig. 5(c). In the case of nitride phosphors, the emission wavelength is usually tailored by substitution between alkaline earth ions that modifies the environment around the activator ions, such as the crystal field strength, symmetry and polyhedron volume. As an example, a blue shift in the emission spectrum is observed for CaAlSiN4:Eu2+ upon substituting Sr for Ca.39 The emission peak shifts from 650 nm for CaAlSiN4:Eu2+ to 610 nm for SrAlSiN4:Eu2+, which has a 4% larger unit cell volume. In addition, a red shift of the emission peak of the Sr2Si5N8:Eu2+ phosphor is achieved by replacing Sr with Ca, accompanied by a reduction in the unit cell volume.40 To assess these effects in the present material, the temperature dependence of the lattice parameters of BaCN2 was examined between 90 K and 290 K. The normalized lattice parameters and the unit cell volume values are plotted in Fig. 6. The lattice shrinkage along the c-axis was estimated to be 0.4% over this temperature range, which was two times that along the a-axis (0.2%) because of the stacking structure of the Ba2+ and NCN2− ions along the c-axis. The unit cell volume was reduced by 0.8% over this same temperature range, which would be associated with a reduction in the Ba–N bond length resulting in significant crystal field splitting of the 5d states. The reduced 5d energy position in turn decreases the energy difference between the ground 4f7 state and the excited 4f65d1 state and induces a large red-shift in the emission wavelength.
No crystalline phase transition of the BaCN2:Eu phosphor was observed in the powder XRD patterns between 138 K and 500 K, as shown in Fig. S3 (ESI†). The tetragonal BaCN2 crystal structure was also confirmed at 90 K by using single crystal XRD (Tables S1 and S2, ESI†). To further investigate the emission property depending on the temperature, fluorescence decay curves upon changing temperature were measured by monitoring 650 nm luminescence, and they are shown in Fig. S4 (ESI†). The decay curves were fitted using a single exponential function to estimate the lifetime. Fig. 7 shows the temperature dependence of the lifetime for the 5d level of Eu2+ doped in the BaCN2 host lattice. From the lifetime, the quenching temperature T1/2,lifetime of 277 K was obtained and found to be different from the T1/2 value estimated from the temperature dependence of emission intensity. It is already known that photoluminescence intensity as a function of temperature can be influenced by many factors such as the changes in absorption strength with temperature and additional intensity due to thermoluminescence.
Both the radiative rate constant (kr) and nonradiative rate constant (knr) were estimated by the combination of the lifetime and estimated QE at different temperatures. Variation of kr upon changing the temperature shown in Fig. S5 (ESI†) implies a change in crystal field splitting of the 5d states of Eu2+ with temperature,42,43 which was induced by the significant change of the lattice parameters.
Interestingly, this wide variation in the emission spectrum upon changing the temperature could have applications in temperature sensing devices.41 Optical temperature sensing has been previously investigated using various molecules and phosphors having a multi emission-center.44–46 The pronounced temperature dependence of the emission wavelength of the BaCN2:Eu2+ phosphor is attributed to changes in the crystal field strength resulting from the varying distance between Eu and N atoms. BaCN2 was also found to have large thermal expansion coefficients, αa = 1.5 × 10−5 K−1 and αc = 2.3 × 10−5 K−1 at 290 K, as estimated from the lattice parameters. These values are almost one order of magnitude larger than that of Si3N4 (3.0 × 10−6 K−1).47 The relatively “soft” host lattice of BaCN2 thus leads to a wide variation in the emission wavelength with temperature, induced by changes in the crystal field splitting of the 5d energy levels of Eu2+ ions. BaCN2:Eu obtained in this work is highly air-sensitive. Recently, we found the product is stable in organic solvents such as hexane and acetone and silicone resin. Moisture resistive coating in organic solution should be further investigated to realize applications as a temperature sensing phosphor.
Footnote |
† Electronic supplementary information (ESI) available: Crystallographic Data (CSD 434273) in CIF format, powder XRD, lattice parameters. See DOI: 10.1039/c8tc01289j |
This journal is © The Royal Society of Chemistry 2018 |