Effect on Local Structure and Phase Transition of Perovskite-Type [N(CH3)4]2Zn1-xCuxBr4 (x = 0, 0.5, 0.7, and 1) Crystals with the Various Doping of Cu2+ Ions

This study focused on how the local structures in pure [N(CH3)4]2ZnBr4 crystal are affected by the partial replacement of Zn2+ ions with Cu2+ ions. The structures and phase transition temperatures TC of perovskite-type [N(CH3)4]2Zn1-xCuxBr4 (x = 0, 0.5, 0.7, and 1) mixed crystals were almost unchanged by the partial doping of Cu2+ ions. The environments for the local structures of [N(CH3)4]2Zn1-xCuxBr4 mixed systems were studied according to differences in the chemical shifts of the 1H magic angle spinning (MAS) NMR, 13C cross-polarization (CP)/MAS NMR, and 14N NMR spectra. The 1H and 13C NMR results showed that the local environments of 1H and 13C nuclei near TC are not affected by substituting Zn2+ ions with Cu2+ ions, whereas the 14N NMR results showed that the local environment is affected near TC. Consequently, the main indicators of the phase transition in [N(CH3)4]2Zn1-xCuxBr4 are related to the ferroelastic characteristics with different orientations.

Metal-organic hybrids, which consist of organic and inorganic components, have recently attracted much attention because these materials have many possibilities for the tailoring of their functionalities and physical properties including optical, electrical and magnetic properties. Hybrid organic-inorganic compounds based on perovskite structures are an interesting class of materials 1,2 4 ] 2 MX 4 (M = transition metal ion; Co, Cu, Zn, Cd, and X = halide; Br, Cl) family. These structures undergo successive structural phase transitions, including an incommensurate-commensurate phase transition [3][4][5][6][7][8][9][10][11] . In the case of [N(CH 3 ) 4 ] 2 ZnBr 4 , the paraelastic orthorhombic phase at the phase transition temperature T C = 287.6 K undergoes a second-order transition to the ferroelastic monoclinic phase 3,5,6 . The paraelastic and ferroelastic phases are denoted as I and II in order of decreasing temperature. In phase I, [N(CH 3 ) 4 ] 2 ZnBr 4 has an orthorhombic structure with the space group Pmcn in the paraelastic phase. Its orthorhombic lattice constants are a = 12.681 Å, b = 9.239 Å, c = 16.025 Å, and Z = 4 12 . In phase II, [N(CH 3 ) 4 ] 2 ZnBr 4 has a monoclinic structure with the space group P12 1 /c1, and the lattice constants are a = 12.534 Å, b = 9.142 Å, c = 15.772 Å, γ = 89.69°, and Z = 4 13 . On the other hand, [N(CH 3 ) 4 ] 2 CuBr 4 undergoes three phase transitions at 272 K (=T C1 ), 242 K (=T C2 ), and 237 K (=T C3 ) as it gradually cools 14 . The four phases are denoted as I, II, III, and IV in order of decreasing temperature. At room temperature, the crystal is in the orthorhombic phase I. As the temperature decreases, the crystal transforms to the intermediate phase II at about 272 K and then to the ferroelectric orthorhombic phase III at about 242 K. The ferroelectric phase III transforms to the lowest-temperature ferroelastic monoclinic phase IV at about 237 K 15,16 . With decreasing temperature, the crystal structure of each phase becomes orthorhombic with space group Pnma, incommensurate, orthorhombic with space group Pbc2 1 , and finally monoclinic with space group P12 1 /c1 [17][18][19] . At room temperature, [N(CH 3 ) 4 ] 2 CuBr 4 has an orthorhombic structure with the lattice constants a = 12.600 Å, b = 9.326 Å, c = 15.825 Å, and Z = 4 20 . For two crystals, the unit cell at room temperature contains four formula units consisting of two crystallographically independent N(CH 3 20 . Two chemically inequivalent sites, N(1) (CH 3 ) 4 and N(2)(CH 3 ) 4 , have been distinguished by using 13 C cross-polarization (CP)/MAS NMR. Based on these results, the behaviors of these two chemically inequivalent N(CH 3 ) 4 groups were discussed.
In this work, perovskite-type [N(CH 3 ) 4 ] 2 Zn 1-x Cu x Br 4 (x = 0, 0.5, 0.7, and 1) mixed crystals were grown from aqueous solutions by the slow evaporation method. The 1 H MAS NMR spectrum and 13 C CP/MAS NMR spectrum of [N(CH 3 ) 4 ] 2 Zn 1-x Cu x Br 4 were measured as a function of temperature. The spin-lattice relaxation times in the rotating frame T 1ρ were determined for 1
At room temperature, the structures of the [N(CH 3 ) 4 ] 2 Zn 1-x Cu x Br 4 (x = 0, 0.5, 0.7, 1) crystals were determined with an X-ray diffraction system (PANalytical, X'pert pro MPD) with a Cu-Kα (λ = 1.5418 Å) radiation source at the Korea Basic Science Institute, Western Seoul Center. Measurements were taken with θ-2θ geometry from 10° to 60° at 45 kV and with a tube power of 40 mA. In order to determine the phase transition temperatures, differential scanning calorimetry (DSC) was carried out on the crystals with a Dupont 2010 DSC instrument. The measurements were performed at a heating rate of 10 °C/min in the temperature range of 190-550 K. Figure 3 shows the endothermic peaks for x = 0, 0.5, 0.7, and 1. For [N(CH 3 ) 4 ] 2 Zn 1-x Cu x Br 4 (x = 0, 0.5, and 0.7), the DSC measurements showed only one endothermic peak at 287 K, and the phase transition temperature hardly changed when the amount of impurity Cu 2+ ions was varied. The three endothermic peaks at 237 K (T C3 ), 245 K (T C2 ), and 272 K (T C1 ) for [N(CH 3 ) 4 ] 2 CuBr 4 are related to phase transitions, and these temperatures are consistent with those previously reported 1 . When the amount of The 1 H MAS NMR and 13 C CP/MAS NMR spectra of [N(CH 3 ) 4 ] 2 Zn 1-x Cu x Br 4 (x = 0, 0.5, 0.7, and 1) in the rotating frame were measured by using a Bruker DSX 400 FT NMR spectrometer at the Korea Basic Science Institute, Western Seoul Center. 1 H MAS NMR and 13 C CP/MAS NMR experiments were performed at the Larmor frequencies of 400.12 and 100.61 MHz, respectively. The samples were placed in a 4 mm CP/MAS probe as powders. The MAS rate was set to 5 kHz for 1 H MAS and 13 C CP/MAS to minimize the spinning sideband overlap. The chemical shifts of the spectrum for 1 H and 13 C nuclei were expressed with respect to tetramethylsilane (TMS). The spin-lattice relaxation times in the rotating frame T 1ρ for 1 H and 13 C were measured by using π/2-t-acquisition. The T 1ρ values were measured by varying the length of the spin-locking pulses. The π/2 pulse widths used for T 1ρ were 3.85 µs for 1 H and 13 C; this corresponded to the frequency of the spin-locking field of 64.94 kHz.
The 14 N NMR spectra of the [N(CH 3 ) 4 ] 2 Zn 1-x Cu x Br 4 (x = 0, 0.5, 0.7, and 1) single crystals in the laboratory frame were measured by using the Bruker DSX 400 FT NMR spectrometer and Unity INOVA 600 NMR spectrometer at the Korea Basic Science Institute, Western Seoul Center. The static magnetic fields were 9.4 and 14.1 T, and the Larmor frequency was set to ω 0 /2π = 28.90 and 43.34 MHz. The 14 N NMR experiments were performed by using a solid-state echo sequence: 4 µs-t-4 µs-t. The samples were maintained at a constant temperature with an accuracy of ±0.5 K by controlling the nitrogen gas flow and heater current. The temperature-dependent NMR measurements were carried out in the temperature range of 180-420 K.  Fig. 4(a and b). At room temperature, the NMR spectrum of [N(CH 3 ) 4 ] 2 ZnBr 4 consisted of one peak at a chemical shift of δ = 3.32 ppm, which was assigned to the methyl proton. The chemical shifts of the 1 H NMR signal showed a slight and continuous decrease near T C . At x = 0.5 and 0.7, the chemical shifts at room temperature were 3.58 and 3.54 ppm higher, respectively, than the 1   On the other hand, the chemical shifts at 300 K for [N(CH 3 ) 4 ] 2 CuBr 4 with x = 1 consisted of two peaks at chemical shifts of δ = 3.60 ppm and δ = 6.65 ppm. Two chemical shifts were assigned to the methyl protons, and they may be due to two inequivalent sites of the N(CH 3 ) 4 molecule: N(1)(CH 3 ) 4 and N(2)(CH 3 ) 4 . The chemical shift below T C2 has only one resonance line. In contrast, two resonance lines were present above T C2 , as shown in Fig. 4(b). The chemical shifts near T C1 and T C3 were the only continuous changes, whereas there was an abrupt change near T C2 . The change in the chemical shift indicates that a structural phase transition occurred at this temperature. The chemical shift for x = 1 was completely different from those for x = 0, 0.5, and 0.7. This difference was due to variations in the electronic structure of the Zn 2+ and Cu 2+ ions.
The recovery traces of the magnetization for the 1 H nuclei in [N(CH 3 ) 4 ] 2 Zn 1-x Cu x Br 4 (x = 0, 0.5, 0.7, and 1) were obtained at several temperatures. The saturation recovery pulse sequence was utilized to obtain the T 1ρ values over the whole temperature range. The nuclear magnetization recovery curves obtained for protons can be described by the following single exponential function: M(t) = M 0 exp(−t/T 1ρ ), where M(t) is the magnetization at the time t, and M 0 is the total nuclear magnetization of 1 H at thermal equilibrium 21 = 0, 0.5, 0.7, and 1). Structural 4 . This is shown in Fig. 6. This is because the 13 C environments at these two chemically inequivalent sites were slightly different. The two different 13 C resonances of N(1)(CH 3 ) 4 and N(2)(CH 3 ) 4 had almost the same chemical shift differences. This difference did not change as the temperature increased because the 13 C environments at the two chemically inequivalent N(1)(CH 3 ) 4 and N(2)(CH 3 ) 4 changed almost equally with the temperature. The chemical shifts of the N(1)(CH 3 ) 4 ions were larger than those of the N(2)(CH 3 ) 4 ions, which is consistent with the results of previous X-ray and 14 4 and N(2)(CH 3 ) 4 , respectively. The chemical shifts near T C2 changed abruptly, whereas those near T C3 and T C1 showed a continuous change. Near T C2 , the change in chemical shift for N(2)(CH 3 ) 4 was larger than that for N(1)(CH 3 ) 4 . These results are consistent with the deformation of the N(2)(CH 3 ) 4 ion being greater than that of the N(1)(CH 3 ) 4 ion, as shown by Hasebe et al. 's 23 X-ray diffraction study The 13 C chemical shifts for x = 0, 0.5, and 0.7 increased with the temperature, whereas those for x = 1 decreased with increasing temperature. Based on these results, N(1)(CH 3 ) 4 and N(2)(CH 3 ) 4 can be defined by the change in the relaxation time as a function of the temperature. This is discussed in more detail below.

C CP/MAS NMR in [N(CH 3 ) 4 ] 2 Zn 1-x Cu x Br 4 (x
The nuclear magnetization recovery curves for carbons in [N(CH 3 ) 4 ] 2 Zn 1-x Cu x Br 4 (x = 0, 0.5, 0.7, and 1) were fitted to a single exponential function. The recovery traces of the 13 C nuclei were measured at various delay times. Based on these results, the spin-lattice relaxation times in the rotating frame T 1ρ in the [N(CH 3 ) 4 ] 2 Zn 1-x Cu x Br 4 were obtained for each carbon as a function of temperature. Figure 7 shows the T 1ρ values for 13 C in the cases of x = 0, 0.5, 0.7, and 1. The 13 C T 1ρ values of N(1)(CH 3 ) 4 = 0, 0.5, 0.7, and 1). The NMR spectra of 14 N (I = 1) in the [N(CH 3 ) 4 ] 2 Zn 1-x Cu x Br 4 (x = 0, 0.5, 0.7, and 1) single crystal were obtained with static NMR in the laboratory frame at Larmor frequencies of ω 0 /2π = 28.90 and 43.34 MHz. Figures 8-11 show the in situ 14 N NMR spectra and resonance frequencies of the 14    On the other hand, the patterns of the resonance frequencies for [N(CH 3 ) 4 ] 2 CuBr 4 with x = 1 changed abruptly at the phase transition temperature, as shown in Fig. 11(a and b). Between T C2 and T C1 , the two lines were due to N(1) and N (2) in N(1)(CH 3 ) 4 and N(2)(CH 3 ) 4 ions, respectively. In the low-temperature region below T C3 , the 14 N NMR signals were split into approximately 16 resonance lines. The 14 N NMR spectra were split into several lines for the signals arising from N(1)(CH 3 ) 4 and N(2)(CH 3 ) 4 . Although the unit cell at all temperatures had Z = 4, the 14 N resonance lines showed several resonance lines at low temperature.

N NMR in [N(CH 3 ) 4 ] 2 Zn 1-x CuxBr 4 (x
Consequently, the splitting of several resonance lines near the phase transition temperatures in the [N(CH 3 ) 4 ] 2 Zn 1-x Cu x Br 4 indicated that a phase transition into a new phase with monoclinic symmetry occurred at this temperature, which corresponded to symmetry reduction from orthorhombic symmetry. Temperature-dependent changes in the 14 N resonance frequency are generally due to a change in structural geometry. The electric field gradient (EFG) tensor at the N sites varied, which reflects configuration changes of atoms neighboring the 14 N nuclei. Near the phase transition temperature, the splitting of several resonance lines of the 14 N NMR lines for N(1)(CH 3 ) 4 and N(2)(CH 3 ) 4 were due to a ferroelastic twin domain with different orientations.
In order to confirm the ferroelastic property, the domain wall orientations were evaluated according to the spontaneous strain tensors given by Aizu 25 and Sapriel 26 . In the transition from an orthorhombic structure with the point symmetry group mmm to monoclinic with the point symmetry group 2 /m, the domain wall orientations are expressed by the following equations: x = 0, z = 0. These equations of the twin boundaries indicate the mmmF2/m ferroelastic species. During the phase transition, the point group symmetry in the crystal changed from mmm (phase I in case of x = 0, 0.5, and 0.7: phase III on case of x = 1) to 2/m (phase II in case of x = 0, 0.5, and 0.7: phase IV in case of x = 1). Consequently, the NMR spectra of [N(CH 3 ) 4 ] 2 Zn 1-x Cu x Br 4 (x = 0, 0.5, 0.7, and 1) at low temperature were attributed to the ferroelastic property, respectively.

Discussion and Conclusion
The variation in the structural geometry as a function of the impurity concentration in the mixed system was considered according to differences in the size and electron structure between the host and impurity ions. The local structures in pure [N(CH 3 ) 4 ] 2 ZnBr 4 and [N(CH 3 ) 4 ] 2 CuBr 4 crystals were investigated for the effect of the random presence of a cation with a similar size. After the partial replacement of Zn 2+ ions with Cu 2+ ions, the Cu 2+ ions occupied the same locations in the lattice as the Zn 2+ ions did. The structures and phase transition temperatures of the perovskite-type [N(CH 3 ) 4 ] 2 Zn 1-x Cu x Br 4 (x = 0, 0.5, 0.7, and 1) mixed crystals were almost unchanged when [N(CH 3 ) 4 ] 2 ZnBr 4 crystals were doped with Cu 2+ ions. The environments for the local structures in [N(CH 3 ) 4 ] 2 Zn 1-x Cu x Br 4 (x = 0, 0.5, 0.7, and 1) were understood by considering the differences in chemical shifts of the 1 H MAS NMR and 13 C CP/MAS NMR spectra. The chemical shifts for 1 H nuclei in [N(CH 3 ) 4 ] 2 Zn 1-x Cu x Br 4 varied according to the concentration of Cu 2+ ions, whereas those for 13 C nuclei did not change for mixed crystals with x = 0.5 and 0.7 when Cu 2+ ions were added. In addition, the two crystallographically inequivalent kinds of N(1)(CH 3 ) 4 and N(2)(CH 3 ) 4 in [N(CH 3 ) 4 ] 2 Zn 1-x Cu x Br 4 (x = 0, 0.5, 0.7, and 1) were identified by using 13 C CP/ MAS NMR. The 1 H and 13 C spin-lattice relaxation times T 1ρ were obtained with varying concentrations of Cu 2+ ions in [N(CH 3 ) 4 ] 2 Zn 1-x Cu x Br 4 . The T 1ρ values for 1 H and 13 C nuclei were not governed by the same mechanism for a given amount of paramagnetic impurity Cu 2+ .
The roles of N(CH 3 ) 4 for the mixed systems containing the paramagnetic Cu 2+ impurity were explained based on the 1 H MAS NMR, 13 C CP/MAS NMR, and 14 N NMR data for [N(CH 3 ) 4 ] 2 Zn 1-x Cu x Br 4 . The NMR spectra and T 1ρ for 1 H and 13 C nuclei near the phase transition temperature were not affected when Zn 2+ ions were substituted with Cu 2+ ions. However, the 14 N NMR spectra were affected near the phase transition temperature. Consequently, the main indicators of the phase transition in [N(CH 3 ) 4 ] 2 Zn 1-x Cu x Br 4 (x = 0, 0.5, 0.7, and 1) were related to the ferroelastic characteristic with different orientations.