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.

Introduction

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]. [N(CH3)4]2ZnBr4 and [N(CH3)4]2CuBr4 are members of the [N(CH3)4]2MX4 (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[3-11]. In the case of [N(CH3)4]2ZnBr4, the paraelastic orthorhombic phase at the phase transition temperature TC = 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(CH3)4]2ZnBr4 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(CH3)4]2ZnBr4 has a monoclinic structure with the space group P12/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(CH3)4]2CuBr4 undergoes three phase transitions at 272 K (=TC1), 242 K (=TC2), and 237 K (=TC3) 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 Pbc21, and finally monoclinic with space group P121/c1[17-19]. At room temperature, [N(CH3)4]2CuBr4 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(CH3)4+ ions and an MBr42− (M = Zn, Cu) ion. The MBr4 tetrahedron is almost undistorted, while the N(CH3)4 tetrahedra have large distortions. Figure 1 shows the crystal structures of [N(CH3)4]2ZnBr4, [N(CH3)4]2Zn0.5Cu0.5Br4, and [N(CH3)4]2CuBr4 at room temperature. The two compounds of [N(CH3)4]2ZnBr4 and [N(CH3)4]2CuBr4 have the ferroelastic property at low temperatures.

Figure 1

Structure of [N(CH3)4]2Zn1-CuBr4 on the bc plane. Cu/ZnBr42− anions are represented by gray tetrahedrons. N(CH3)4+ cations are represented by empty tetrahedrons. (a) [N(CH3)4]2ZnBr4, (b) [N(CH3)4]2Zn0.5Cu0.5Br4, and (c) N(CH3)4]2CuBr4.

Until now, various experimental techniques have been used to report the crystal structure, phase transitions, and ferroelectricity of [N(CH3)4]2ZnBr4 and [N(CH3)4]2CuBr4[3-5,14-18]. Perret et al.[19] used 79Br nuclear quadrupole resonance (NQR) to measure the second-order phase transition between the orthorhombic and monoclinic structures in [N(CH3)4]2ZnBr4. Recently, static nuclear magnetic resonance (NMR) and magic angle spinning (MAS) NMR spectrometry have been used to measure the chemical shifts and spin-lattice relaxation times of 1H and 13C nuclei in [N(CH3)4]2ZnBr4 as a function of temperature[20]. Two chemically inequivalent sites, N(1)(CH3)4 and N(2)(CH3)4, have been distinguished by using 13C cross-polarization (CP)/MAS NMR. Based on these results, the behaviors of these two chemically inequivalent N(CH3)4 groups were discussed.

In this work, perovskite-type [N(CH3)4]2Zn1-CuBr4 (x = 0, 0.5, 0.7, and 1) mixed crystals were grown from aqueous solutions by the slow evaporation method. The 1H MAS NMR spectrum and 13C CP/MAS NMR spectrum of [N(CH3)4]2Zn1-CuBr4 were measured as a function of temperature. The spin-lattice relaxation times in the rotating frame T1ρ were determined for 1H and 13C nuclei in [N(CH3)4]2Zn1-CuBr4 for varying amounts of paramagnetic Cu2+ ions. In addition, the 14N NMR spectrum for [N(CH3)4]2Zn1-CuBr4 was observed in order to understand the role of the phase transitions. The results allowed the structural properties of pure [N(CH3)4]2ZnBr4 and [N(CH3)4]2CuBr4 to be compared, and the effect of substituting Zn2+ ions in [N(CH3)4]2ZnBr4 with Cu2+ ions was examined. And, the ferroelastic phase transition of [N(CH3)4]2Zn1-CuBr4 at low temperatures was considered. This study represents the first investigation of the local structures of [N(CH3)4]2Zn1-CuBr4, and the results were used to analyze the role of N(CH3)4 ions.

Experimental Method

[N(CH3)4]2Zn1-CuBr4 (x = 0, 0.5, 0.7, and 1) single crystals were grown at room temperature by slow evaporation of an aqueous solution containing ZnBr2, CuBr2, and N(CH3)4Br in stoichiometric proportions. The [N(CH3)4]2Zn1-CuBr4 single crystals varied in color according to the amount of Cu2+ ions, as shown in Fig. 2.

Figure 2

Colors of mixed crystals [N(CH3)4]2Zn1-CuBr4 (x = 0, 0.5, 0.7, and 1).

At room temperature, the structures of the [N(CH3)4]2Zn1-CuBr4 (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. Table 1 presents the lattice constants of the four crystals at room temperature. All of the [N(CH3)4]2Zn1-CuBr4 crystals containing Cu2+ ions had the same orthorhombic structure as pure [N(CH3)4]2ZnBr4 and [N(CH3)4]2CuBr4.

Table 1

Lattice constants of [N(CH3)4]2Zn1-CuBr4 (x = 0, 0.5, 0.7, and 1) at room temperature.

	a	b	c
[N(CH3)4]2ZnBr4 (x = 0)	12.691	9.244	16.012
[N(CH3)4]2Zn0.5Cu0.5Br4 (x = 0.5)	12.676	9.249	16.039
[N(CH3)4]2Zn0.3Cu0.7Br4 (x = 0.7)	12.675	9.245	16.035
[N(CH3)4]2CuBr4 (x = 1)	12.647	9.341	15.906

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(CH3)4]2Zn1-CuBr4 (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 Cu2+ ions was varied. The three endothermic peaks at 237 K (TC3), 245 K (TC2), and 272 K (TC1) for [N(CH3)4]2CuBr4 are related to phase transitions, and these temperatures are consistent with those previously reported[1]. When the amount of paramagnetic Cu2+ ions was varied, the phase transition temperatures for x = 0.5 and 0.7 were nearly unchanged and were similar to those for pure [N(CH3)4]2ZnBr4, although the colors of the samples were different. Thus, the impurity Cu2+ ions had an insignificant effect on the phase transition temperature.

Figure 3

Differential scanning calorimetry (DSC) thermogram of [N(CH3)4]2Zn1-CuBr4 (x = 0, 0.5, 0.7, and 1) single crystals.

The 1H MAS NMR and 13C CP/MAS NMR spectra of [N(CH3)4]2Zn1-CuBr4 (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. 1H MAS NMR and 13C 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 1H MAS and 13C CP/MAS to minimize the spinning sideband overlap. The chemical shifts of the spectrum for 1H and 13C nuclei were expressed with respect to tetramethylsilane (TMS). The spin–lattice relaxation times in the rotating frame T1ρ for 1H and 13C were measured by using π/2-t-acquisition. The T1ρ values were measured by varying the length of the spin-locking pulses. The π/2 pulse widths used for T1ρ were 3.85 µs for 1H and 13C; this corresponded to the frequency of the spin-locking field of 64.94 kHz.

The 14N NMR spectra of the [N(CH3)4]2Zn1-CuBr4 (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 ω0/2π = 28.90 and 43.34 MHz. The 14N 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.

Experimental Results and Analysis

1H MAS NMR in [N(CH3)4]2Zn1-CuBr4 (x = 0, 0.5, 0.7, and 1)

The 1H chemical shifts in order to the structural analysis of [N(CH3)4]2Zn1-CuBr4 (x = 0, 0.5, 0.7, and 1) were carried out with the MAS NMR method. The 1H chemical shifts in [N(CH3)4]2Zn1-CuBr4 were measured over the temperature range of 180–420 K, as shown in Fig. 4(a and b). At room temperature, the NMR spectrum of [N(CH3)4]2ZnBr4 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 1H NMR signal showed a slight and continuous decrease near TC. At x = 0.5 and 0.7, the chemical shifts at room temperature were 3.58 and 3.54 ppm higher, respectively, than the 1H chemical shift in pure [N(CH3)4]2ZnBr4. The 1H chemical shifts increased continuously as the temperature increased and differed from those of pure [N(CH3)4]2ZnBr4.

Figure 4

(a) Chemical shifts of the 1H MAS NMR spectrum as a function of temperature for [N(CH3)4]2Zn1-CuBr4 (x = 0, 0.5, and 0.7). (b) Chemical shifts of the 1H MAS NMR spectrum as a function of temperature for [N(CH3)4]2Zn1-CuBr4 (x = 1).

On the other hand, the chemical shifts at 300 K for [N(CH3)4]2CuBr4 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(CH3)4 molecule: N(1)(CH3)4 and N(2)(CH3)4. The chemical shift below TC2 has only one resonance line. In contrast, two resonance lines were present above TC2, as shown in Fig. 4(b). The chemical shifts near TC1 and TC3 were the only continuous changes, whereas there was an abrupt change near TC2. 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 Zn2+ and Cu2+ ions.

The recovery traces of the magnetization for the 1H nuclei in [N(CH3)4]2Zn1- CuBr4 (x = 0, 0.5, 0.7, and 1) were obtained at several temperatures. The saturation recovery pulse sequence was utilized to obtain the T1ρ 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) = M0exp(−t/T1ρ), where M(t) is the magnetization at the time t, and M0 is the total nuclear magnetization of 1H at thermal equilibrium[21]. The recovery traces of the 1H nuclei were measured at several delay times. Based on the slope of the plot of log M(t)/M0 versus the delay time t, the spin-lattice relaxation times in the rotating frame T1ρ for the proton in [N(CH3)4]2Zn1-CuBr4 (x = 0, 0.5, 0.7, and 1) were obtained as a function of the temperature, as shown in Fig. 5. When the paramagnetic Cu2+ ions were included, T1ρ for x = 0.5 and 0.7 differed from 1H T1ρ for pure [N(CH3)4]2ZnBr4, whereas the trends of 1H T1ρ were similar with that of 1H T1ρ for pure [N(CH3)4]2ZnBr4. 1H T1ρ was generally continuous near TC. On the other hand, 1H T1ρ of [N(CH3)4]2CuBr4 with x = 1 increased with the temperature. The T1ρ values for N(1)(CH3)4 and N(2)(CH3)4 were nearly identical within the experimental error range. The proton T1ρ data did not show any evidence of an anomalous change near the phase transition temperatures of TC1, TC2, and TC3. However, the 1H T1ρ curves of [N(CH3)4]2Zn0.5Cu0.5Br4 with x = 0.5 and [N(CH3)4]2Zn0.3Cu0.7Br4 with x = 0.7 were markedly different from those observed for pure [N(CH3)4]2ZnBr4 with x = 0 and [N(CH3)4]2CuBr4 with x = 1. The 1H T1ρ values for x = 0.5 and x = 0.7 were larger than those for [N(CH3)4]2ZnBr4 and [N(CH3)4]2CuBr4.

Figure 5

Temperature dependences of the 1H spin-lattice relaxation time in the rotating frame T1ρ for [N(CH3)4]2Zn1-CuBr4 (x = 0, 0.5, 0.7, and 1).

13C CP/MAS NMR in [N(CH3)4]2Zn1-xCuxBr4 (x = 0, 0.5, 0.7, and 1)

Structural analysis of [N(CH3)4]2Zn1-CuBr4 (x = 0, 0.5, 0.7, and 1) was carried out with a 13C CP/MAS NMR method. The chemical shifts for 13C in [N(CH3)4]2Zn1- CuCl4 were measured over the temperature range of 180–420 K, as shown in Fig. 6. In the case of [N(CH3)4]2ZnBr4, the 13C CP/MAS NMR spectrum at room temperature showed two signals at chemical shifts of δ = 57.97 and 57.72 ppm with respect to the reference TMS signal. These signals can be attributed to the methyl carbons in the two chemically inequivalent ions N(1)(CH3)4 and N(2)(CH3)4. At all temperatures, the 13C CP/MAS NMR spectrum of [N(CH3)4]2Zn1-CuBr4 (x = 0, 0.5, 0.7, and 1) consisted of two resonance lines, one for N(1)(CH3)4 and the other for N(2)(CH3)4. This is shown in Fig. 6. This is because the 13C environments at these two chemically inequivalent sites were slightly different. The two different 13C resonances of N(1)(CH3)4 and N(2)(CH3)4 had almost the same chemical shift differences. This difference did not change as the temperature increased because the 13C environments at the two chemically inequivalent N(1)(CH3)4 and N(2)(CH3)4 changed almost equally with the temperature. The chemical shifts of the N(1)(CH3)4 ions were larger than those of the N(2)(CH3)4 ions, which is consistent with the results of previous X-ray and 14N NMR analyses on [N(CH3)4]2ZnCl4, which belongs to this family[22,23]. Hasebe et al.’s[23] X-ray diffraction study indicated that the deformation of the N(2)(CH3)4 ion in [N(CH3)4]2ZnCl4 is larger than that of the N(1)(CH3)4 ion. Based on these results, N(1)(CH3)4 and N(2)(CH3)4 were defined according to the change in the relaxation time as a function of temperature, which was previously reported[24]. For x = 0.5 and 0.7, the 13C CP/MAS NMR spectrum of CH3 in the two inequivalent kinds of N(1)(CH3)4 and N(2)(CH3)4 were measured within this temperature range, and their chemical shifts were similar with that in [N(CH3)4]2ZnBr4 with x = 0. At 286 K, i.e., the transition temperature, the 13C chemical shifts for x = 0, 0.5, and 0.7 slowly and monotonically increased with increasing temperature. For [N(CH3)4]2CuBr4 with x = 1, the 13C CP/MAS NMR spectrum at room temperature had two signals at δ = 76.74 and 165.21 ppm. The signals at δ = 76.74 ppm and δ = 165.21 ppm represent the methyl carbons in the inequivalent N(1)(CH3)4 and N(2)(CH3)4, respectively. The chemical shifts near TC2 changed abruptly, whereas those near TC3 and TC1 showed a continuous change. Near TC2, the change in chemical shift for N(2)(CH3)4 was larger than that for N(1)(CH3)4. These results are consistent with the deformation of the N(2)(CH3)4 ion being greater than that of the N(1)(CH3)4 ion, as shown by Hasebe et al.’s[23] X-ray diffraction study The 13C 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)(CH3)4 and N(2)(CH3)4 can be defined by the change in the relaxation time as a function of the temperature. This is discussed in more detail below.

Figure 6

Chemical shifts of the 13C CP/MAS NMR spectrum as a function of temperature for [N(CH3)4]2Zn1-CuBr4 (x = 0, 0.5, and 0.7). Inset: Chemical shifts of the 13C CP/MAS NMR spectrum as a function of temperature for [N(CH3)4]2Zn1-CuBr4 (x = 1).

The nuclear magnetization recovery curves for carbons in [N(CH3)4]2Zn1-CuBr4 (x = 0, 0.5, 0.7, and 1) were fitted to a single exponential function. The recovery traces of the 13C nuclei were measured at various delay times. Based on these results, the spin-lattice relaxation times in the rotating frame T1ρ in the [N(CH3)4]2Zn1-CuBr4 were obtained for each carbon as a function of temperature. Figure 7 shows the T1ρ values for 13C in the cases of x = 0, 0.5, 0.7, and 1. The 13C T1ρ values of N(1)(CH3)4 and N(2)(CH3)4 in [N(CH3)4]2Zn1-CuBr4 (x = 0, 0.5, and 0.7) were very similar, and those for N(2)(CH3)4 were longer than those of N(1)(CH3)4. The slopes of the T1ρ values near 287 K (=TC) were nearly continuous. In the case of [N(CH3)4]2CuBr4 with x = 1, the 13C T1ρ values for N(1)(CH3)4 and N(2)(CH3)4 showed a similar trend, especially at higher temperatures. However, the change in the 13C T1ρ value for N(2)(CH3)4 near TC2 was discontinuous. This result is consistent with the larger change of the 13C chemical shift for N(2)(CH3)4. The 13C T1ρ values for [N(CH3)4]2CuBr4 were very small, and these T1ρ values of materials containing paramagnetic Cu2+ ions were shorter than those of materials without paramagnetic ions. The T1ρ values for CH3 were not affected when Zn2+ ions were substituted with Cu2+ ions in [N(CH3)4]2ZnBr4.

Figure 7

Temperature dependences of the 13C spin-lattice relaxation time in the rotating frame T1ρ for [N(CH3)4]2Zn1-CuBr4 (x = 0, 0.5, 0.7, and 1).

14N NMR in [N(CH3)4]2Zn1-CuxBr4 (x = 0, 0.5, 0.7, and 1)

The NMR spectra of 14N (I = 1) in the [N(CH3)4]2Zn1-CuBr4 (x = 0, 0.5, 0.7, and 1) single crystal were obtained with static NMR in the laboratory frame at Larmor frequencies of ω0/2π = 28.90 and 43.34 MHz. Figures 8–11 show the in situ 14N NMR spectra and resonance frequencies of the 14N NMR spectra in the [N(CH3)4]2Zn1- CuBr4 (x = 0, 0.5, 0.7, and 1) single crystal as functions of the temperature. Four resonance lines for two groups at the 14N site in the two chemical inequivalent N(1)(CH3)4 and N(2)(CH3)4 were expected because of the quadrupole interaction of the 14N nucleus in [N(CH3)4]2Zn1-CuBr4 single crystals. In the case of [N(CH3)4]2Zn1-CuBr4 with x = 0, 0.5, and 0.7, the 14N NMR spectra above TC exhibited four groups of two signals for both N(1)(CH3)4 and N(2)(CH3)4, as shown in Figs 8–10. Therefore, the eight resonance lines above TC were due to two chemically inequivalent N(1)(CH3)4 and N(2)(CH3)4 ions and two magnetically inequivalent N(1)(CH3)4 and N(2)(CH3)4. This phase I–II transition resulted in an abrupt splitting of the 14N NMR line into several groups of lines corresponding to N(1)(CH3)4 and N(2)(CH3)4. The chemical shifts of the 14N signals below TC varied almost continuously, and those of the 14N signals above this temperature also changed continuously. At low temperatures below TC, the 14N NMR signals split into 16 resonance lines. The number of resonance lines varied near the phase transition temperature, which indicates the ferroelastic twin characteristic.

Figure 8

(a) In-situ 14N NMR spectrum as a function of temperature for [N(CH3)4]2Zn1- CuBr4 (x = 0). (b) Resonance frequency of the 14N NMR spectrum as a function of temperature for [N(CH3)4]2Zn1-CuBr4 (x = 0).

Figure 11

(a) In-situ 14N NMR spectrum as a function of temperature for [N(CH3)4]2Zn1-CuBr4 (x = 1). (b) Resonance frequency of 14N NMR spectrum as a function of temperature for [N(CH3)4]2Zn1-CuBr4 (x = 1).

Figure 10

(a) In-situ 14N NMR spectrum as a function of temperature for [N(CH3)4]2Zn1-CuBr4 (x = 0.7). (b) Resonance frequency of the 14N NMR spectrum as a function of temperature for [N(CH3)4]2Zn1-CuBr4 (x = 0.7).

(a) In-situ 14N NMR spectrum as a function of temperature for [N(CH3)4]2Zn1- CuBr4 (x = 0.5). (b) Resonance frequency of the 14N NMR spectrum as a function of temperature for [N(CH3)4]2Zn1-CuBr4 (x = 0.5).

On the other hand, the patterns of the resonance frequencies for [N(CH3)4]2CuBr4 with x = 1 changed abruptly at the phase transition temperature, as shown in Fig. 11(a and b). Between TC2 and TC1, the two lines were due to N(1) and N(2) in N(1)(CH3)4 and N(2)(CH3)4 ions, respectively. In the low-temperature region below TC3, the 14N NMR signals were split into approximately 16 resonance lines. The 14N NMR spectra were split into several lines for the signals arising from N(1)(CH3)4 and N(2)(CH3)4. Although the unit cell at all temperatures had Z = 4, the 14N resonance lines showed several resonance lines at low temperature.

Consequently, the splitting of several resonance lines near the phase transition temperatures in the [N(CH3)4]2Zn1-CuBr4 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 14N 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 14N nuclei. Near the phase transition temperature, the splitting of several resonance lines of the 14N NMR lines for N(1)(CH3)4 and N(2)(CH3)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(CH3)4]2Zn1- CuBr4 (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(CH3)4]2ZnBr4 and [N(CH3)4]2CuBr4 crystals were investigated for the effect of the random presence of a cation with a similar size. After the partial replacement of Zn2+ ions with Cu2+ ions, the Cu2+ ions occupied the same locations in the lattice as the Zn2+ ions did. The structures and phase transition temperatures of the perovskite-type [N(CH3)4]2Zn1-CuBr4 (x = 0, 0.5, 0.7, and 1) mixed crystals were almost unchanged when [N(CH3)4]2ZnBr4 crystals were doped with Cu2+ ions. The environments for the local structures in [N(CH3)4]2Zn1-CuBr4 (x = 0, 0.5, 0.7, and 1) were understood by considering the differences in chemical shifts of the 1H MAS NMR and 13C CP/MAS NMR spectra. The chemical shifts for 1H nuclei in [N(CH3)4]2Zn1-CuBr4 varied according to the concentration of Cu2+ ions, whereas those for 13C nuclei did not change for mixed crystals with x = 0.5 and 0.7 when Cu2+ ions were added. In addition, the two crystallographically inequivalent kinds of N(1)(CH3)4 and N(2)(CH3)4 in [N(CH3)4]2Zn1-CuBr4 (x = 0, 0.5, 0.7, and 1) were identified by using 13C CP/MAS NMR. The 1H and 13C spin-lattice relaxation times T1ρ were obtained with varying concentrations of Cu2+ ions in [N(CH3)4]2Zn1-CuBr4. The T1ρ values for 1H and 13C nuclei were not governed by the same mechanism for a given amount of paramagnetic impurity Cu2+.

The roles of N(CH3)4 for the mixed systems containing the paramagnetic Cu2+ impurity were explained based on the 1H MAS NMR, 13C CP/MAS NMR, and 14N NMR data for [N(CH3)4]2Zn1-CuBr4. The NMR spectra and T1ρ for 1H and 13C nuclei near the phase transition temperature were not affected when Zn2+ ions were substituted with Cu2+ ions. However, the 14N NMR spectra were affected near the phase transition temperature. Consequently, the main indicators of the phase transition in [N(CH3)4]2Zn1-CuBr4 (x = 0, 0.5, 0.7, and 1) were related to the ferroelastic characteristic with different orientations.