Role of NH4 ions in successive phase transitions of perovskite type (NH4)2ZnX4 (X = Cl, Br) by 1H MAS NMR and 14N NMR

The 1H chemical shifts and the spin-lattice relaxation time, T1ρ, in the rotating frame of (NH4)2ZnX4 (X = Cl, Br) are observed in order to investigate local phenomena related to successive phase transitions. The temperature dependence of T1ρ values for 1H showed a minimum, and the T1ρ values for 1H appeared to be governed by tumbling molecular motions at high temperatures. In addition, 14N NMR spectra are studied in each phase of (NH4)2ZnX4 single crystals in the laboratory frame. The phase transition temperatures strongly affect the 14N number of symmetry related nitrogen centers within the unit cell. The 1H MAS NMR and 14N NMR results are discussed to elucidate the roles of NH4 ions during the phase transitions of (NH4)2ZnX4.


Introduction
Perovskite A 2 BX 4 type (A ¼ NH 4 , K, Rb, Cs; B ¼ Zn, Co, Cu, Fe, Zn, Cd; X ¼ Cl, Br) crystals have received a great deal of attention because of their nonlinear optical properties, and also because of the signicant diversity of their structural phase transitions. [1][2][3][4][5][6][7][8][9][10] The prototype of the crystal structures of this family is that of b-K 2 SO 4 , which consists of isolated BX 4 2À tetrahedra and monovalent A + cations placed in two inequivalent cavities. Ammonium tetrachlorozincate, (NH 4 ) 2 ZnCl 4 , and ammonium tetrabromozincate, (NH 4 ) 2 ZnBr 4 , belong to the family of crystals of the perovskite A 2 BX 4 type and are known to undergo several phase transitions. Although the physical properties of (NH 4 ) 2 ZnCl 4 and (NH 4 ) 2 ZnBr 4 have been studied by several research groups, the structural geometry changes during the phase transitions of the two compounds have not been fully understood. Here, the phase transition temperatures and dynamics of the cations in (NH 4 ) 2 ZnCl 4 and (NH 4 ) 2 ZnBr 4 are important. The potential applications of these materials are strongly affected by the phase transitions and dynamics of the cations. 11 (NH 4 ) 2 ZnCl 4 undergoes ve phase transitions: those between phases I and II at 406 K (¼T C1 ) and phases II and III at 364 K (¼T C2 ) are well-known, and the successive phase transitions at 319 K (¼T C3 ), 271 K (¼T C4 ), and 266 K (¼T C5 ) have also been reported; [12][13][14] the phases involved in these transitions are denoted by VI, V, IV, III, II, and I in order of increasing temperature, as shown in Table 1. The structure of (NH 4 ) 2 ZnCl 4 in the normal phase, phase I (above 406 K), is orthorhombic with a o ¼ 9.274Å, b o ¼ 12.620Å, and c o ¼ 7.211Å, and space group Pnma. 12 Upon cooling, there is a phase transition at 406 K to an incommensurate phase that is stable down to 364 K. 15 The structure in phase III between 364 K and 319 K is orthorhombic with a ¼ a o , b ¼ b o , c ¼ 4c o , and the space group Pn2 1 a. 12,16,17 The room temperature phase, phase IV, is antiferroelectric with a pseudo-orthorhombic monoclinic structure and space group Pa. 12 The region between 271 K and 266 K is mixed phase. 14 Below 266 K, the lattice is constant with an orthorhombic structure, where a ¼ a o , b ¼ b o , and c ¼ 3c o . 12,17 On the other hand, the successive phase transitions of (NH 4 ) 2 ZnBr 4 have been reported at 216 K (¼T C3 ), 395 K (¼T C2 ), and 432 K (¼T C1 ), 18-20 as shown in Table 1; the phases involved in these transitions are represented by IV, III, II, and I in order of increasing temperature. The structures of (NH 4 ) 2 ZnBr 4 crystals in phases I, III, and IV are shown in Fig. 1. In phase I (above 432 K), the structure of (NH 4 ) 2 ZnBr 4 is orthorhombic with , and space group Pmcn. 21,22 Upon cooling, there is a phase transition at 432 K to an incommensurate phase II that is stable down to 395 K. The structure in phase III, between 395 K and 216 K, is monoclinic (3) , and space group P2 1 / c11. 22 The low-temperature phase IV is orthorhombic with a ¼ a o , b ¼ b o , c ¼ 3c o , and space group P2 1 cn. 22 The 1 H spin-lattice relaxation times of (NH 4 ) 2 ZnCl 4 and (NH 4 ) 2 ZnBr 4 crystals have been obtained in the laboratory frame by Lim et al. [23][24][25] and Ramesh et al., 26 respectively. The molecular dynamics and phase transitions of (NH 4 ) 2 ZnCl 4 single crystals were reported previously. There were two crystallographically inequivalent NH 4 sites, namely NH 4 (1) and NH 4 (2), in the (NH 4 ) 2 ZnCl 4 . The 1 H spin-lattice relaxation time T 1 in the laboratory frame was observed to vary continuously with temperature without jumps or changes. The 1 H T 1 passes through a minimum value near 220 K; the presence of this minimum was attributed to the reorientation of the NH 4 groups. In addition, the 14 N nuclear magnetic resonance (NMR) results in phase I of (NH 4 ) 2 ZnCl 4 were reported for the two inequivalent sites N(1) and N(2): the quadrupole coupling constant, e 2 qQ/h, and asymmetry parameter, h, were e 2 qQ/h ¼ 105.5 kHz and h ¼ 0.96 for N(1), and e 2 qQ/h ¼ 48.2 kHz and h ¼ 0.087 for N(2). 27 The NMR method enables the study of a lattice's local properties, and is particularly useful in those cases that require information on the behavior of individual structural groups. Measurements of T 1r obtained by magic angle spinning (MAS) NMR in the rotating frame are advantageous in that they allow for probing of molecular motion in the kHz range, whereas T 1 values obtained by state NMR in a laboratory frame reect motion in the MHz range. 28 The aim of this paper is to clarify the structural changes associated with the successive phase transitions in (NH 4 ) 2 ZnX 4 (X ¼ Cl, Br). Detailed studies of the molecular motions are necessary in order to explain the mechanisms of the phase transitions of (NH 4 ) 2 ZnX 4 . The temperature dependences of the MAS NMR spectra and the spin-lattice relaxation times, T 1r , in the rotating frame for the 1 H nuclei in (NH 4 ) 2 ZnX 4 were investigated using a pulsed NMR spectroscopy. In addition, the 14 N NMR spectra in (NH 4 ) 2 ZnX 4 single crystals were obtained by static NMR in the laboratory frame, as a function of temperature. The 1 H MAS NMR and 14 N static NMR results were analyzed to elucidate the roles of NH 4 ions during the phase transitions of (NH 4 ) 2 ZnCl 4 and (NH 4 ) 2 ZnBr 4 . The T 1r values by 1 H MAS NMR obtained here and the previously reported T 1 values by 1 H static NMR are compared. In addition, the information regarding the structural geometry of nitrogen environments in NH 4 + is discussed as a function of temperature.

Experimental method
Single crystals of (NH 4 Reference  22  22  20  21 and 22 In addition, the 14 N NMR spectra of the (NH 4 ) 2 ZnCl 4 and (NH 4 ) 2 ZnBr 4 single crystals in the laboratory frame were measured using a Unity INOVA 600 NMR spectroscopy at the Korea Basic Science Institute, Western Seoul Center. The static magnetic eld was 14.1 T, and the Larmor frequency was set to u 0 /2p ¼ 43.342 MHz. The 14 N NMR experiments were performed using a solid echo sequence of p/2-t-p/2-t. The widths of the p/2 pulse for 14 N in (NH 4 ) 2 ZnCl 4 and (NH 4 ) 2 ZnBr 4 were 4 ms and 3.7 ms, respectively. The measurements of 1 H MAS NMR in the rotating frame and 14 N NMR in the laboratory frame were obtained over the temperature range of 180-430 K. Sample temperatures on MAS NMR and static NMR were held constant within AE0.5 K by controlling the helium gas ow and heater current.

Phase transition temperatures
The phase transition temperatures for (NH 4 ) 2 ZnCl 4 and (NH 4 ) 2 -ZnBr 4 single crystals have not yet been accurately established, as shown in Fig. 2. Here, the small vertical bars were represented the phase transition temperatures reported by several groups. In the case of (NH 4 ) 2 ZnCl 4 , this crystal exhibits three temperature dependence anomalies in the dielectric, thermal, and X-ray diffraction measurements at 270 K, 319 K, and 406 K, respectively, as reported by Matsunaga et al. 13 Furthermore, anomalies characteristic of phase transitions reported by Agarwal et al. 31 have been found in the Raman spectra investigations at 194 K, 266 K, 271 K, 319 K, and 406 K. According to Gillet et al., 32 the occurrence of phase transitions at 253 K, 256 K, 319 K, 364 K, and 406 K was also conrmed on the basis of the Brillouin investigation. 32 In addition, thermal expansion changes at temperatures of 253 K, 255 K, 323 K, 362 K, and 406 K were reported by Tylczynski et al. 29 In the case of (NH 4 ) 2 ZnBr 4 , phase transition temperatures have been reported at 216 K, 395 K, and 432 K by Osaka et al., 18 and an additional phase transition at 365 K was reported by Moskalev et al. 15 in their investigation of a (NH 4 ) 2 ZnBr 4 crystal using 81 Br nuclear quadrupole resonance (NQR), differential thermal analysis (DTA), and dielectric measurements. Fig. 2 shows that the phase transition temperatures obtained by several experiments are inconsistent for (NH 4 ) 2 ZnCl 4 and (NH 4 ) 2 ZnBr 4 , respectively.
In order to determine the phase transition temperatures for the (NH 4 ) 2 ZnCl 4 and (NH 4 ) 2 ZnBr 4 single crystals obtained here, differential scanning calorimetry (DSC) measurements were taken with a DuPont 2010 DSC instrument at a heating rate of 10 C min À1 . The DSC measurements revealed three endothermic peaks at 270 K, 320 K, and 406 K for (NH 4 ) 2 ZnCl 4 , and four endothermic peaks at 216 K, 362 K, 396 K, and 432 K for (NH 4 ) 2 -ZnBr 4 , as shown in Fig. 3. These endothermic peaks were related to the phase transitions, and the temperatures were consistent with those previously reported by Matsunaga et al. 13 and Moskalev et al. 15 The phase transition temperatures of (NH 4 ) 2 ZnX 4 may vary according to the conditions of crystal growth.    which directly measures the rate of motion. The experimental T 1r value can be expressed in terms of s C using molecular motion, as suggested by the Bloembergen-Purcell-Pound (BPP) theory. 33 The T 1r value in the rotating frame can also be expressed in terms of s C using molecular motion. 28,34

Molecular motion near phase transition temperatures from 1 H MAS NMR
Here: In the equation, g H and g N are the gyromagnetic ratios for the 1 H and 14 N nuclei, respectively; n is the number of directly bound protons; r H-N is the H-N internuclear distance; ħ is the reduced Planck constant; u H and u N are the Larmor frequencies of 1 H and 14 N, respectively; and u 1 is the spin-lock eld frequency of 67.56 kHz. Here, the f(u 1 ) is non-zero, i.e., the s C is much less than the Larmor frequencies, therefore all of the other terms is far smaller than the f(u 1 ) term. We analyzed our data by assuming that T 1r would show a minimum when u 1 s C ¼ 1, and that the relation between T 1r and the characteristic frequency of motion, 1/s C , could be applied. The coefficient in eqn (1) can be determined because the T 1r curve displays a minimum and because the value of s C can be obtained from u 1 s C ¼ 1; thus, (n/20)(g H g N ħ/r H-N 3 ) 2 z 4.66 Â 10 6 in the BPP formula. We were then able to calculate the correlation time s C as a function of temperature. The temperature dependence of s C follows a simple Arrhenius expression: 34,35 where s o is a pre-exponential factor, T is the temperature, R is the gas constant, and E a is an activation energy. Thus, the slope of the linear portion of a semi-logarithmic plot should yield E a . On the other hand, the 1 H spin-lattice relaxation time T 1 in the laboratory frame in (NH 4 ) 2 ZnCl 4 previously reported was obtained as a function of temperature, as shown in Fig. 5. [23][24][25] In the high temperature region, T 1 increases monotonically with temperature, and T 1 was continuous at the phase transition temperatures. However, the T 1 undergoes a change in slope near T C4 . 1 H T 1 passes through a minimum value in the vicinity of 220 K, and the presence of this minimum was attributed to the effects of molecular motion. The relaxation process from the 1 H T 1 curve was affected by molecular motion, as described by the BPP theory. 33 The activation energy in the low and high temperatures was reported 29.95 AE 0.85 kJ mol À1 and 10.99 AE 0.37 kJ mol À1 , respectively.

Structural changes near phase transition temperatures from 14 N NMR
In order to investigate local phenomena related to successive phase transitions, the NMR spectra of 14 N (I ¼ 1) was obtained as a function of temperature using static NMR at a Larmor frequency of u 0 /2p ¼ 43.342 MHz. 14 N (I ¼ 1) NMR is a sensitive method for probing local structural properties in each phase. The 14 N NMR spectra consisted of pairs of lines at frequencies corresponding to the transitions Dm ¼ AE1 4 Dm ¼ 0. The crystal was oriented such that the magnetic eld was aligned with the crystallographic c-axis. Temperature-dependent changes in the 14 N resonance frequency are generally attributed to changes in the structural geometry, indicating a change in the quadrupole coupling constant of the 14 N nuclei. The 14 N NMR spectra at phase I, II, III, IV, and VI in (NH 4 ) 2 ZnCl 4 crystals were plotted in Fig. 6. Here, the 14 N peaks positions were denoted by close circles. Two resonance lines were expected because of the quadrupole interaction of the 14 N nucleus. However, many resonance lines were observed, and they were much narrower in line width.
The resonance frequencies of 14 N signals in (NH 4 ) 2 ZnCl 4 and (NH 4 ) 2 ZnBr 4 single crystals are respectively plotted in Fig. 7(a) and (b) as a function of temperature. In the case of (NH 4 ) 2 ZnCl 4 , the phase transitions occurring at T C1 , T C3 , and T C4 were observed from our DSC results, whereas those at T C2 and T C5 were not observed. Therefore, T C1 , T C3 , and T C4 are denoted by solid lines, and T C2 and T C5 are denoted dotted lines in Fig. 7(a). The resonance frequencies near T C1 , T C4 , and T C5 changed, whereas those near T C2 and T C3 did not change. In phase I, each unit cell contains four formula units, and there are also two different kinds of 14 N nuclei, termed N(1) and N (2). Therefore, the 14 N NMR spectra exhibited eight resonance lines in four pairs. Here, the two inequivalent sites N(1) and N(2) are distinguished by the quadrupole coupling constant previously reported: e 2 qQ/h ¼ 105.5 kHz and h ¼ 0.96 for N (1), and e 2 qQ/h ¼ 48.2 kHz and h ¼ 0.087 for N (2). 27 Additional lines in phases II, III, and IV were obtained, although they exhibited very small intensities compared with phase I. In phases III and IV, the unit cell is quadrupoled along the c-direction of phase I. The unit cell of phases III and IV contains 16 formula units, and thus 32 resonance lines of 16 pairs are expected. According to the crystallography results shown in Fig. 1(b), eight atoms N(11), N (21), N (31), and N(41) are surrounded by ve Cl atoms, while the other atoms N (12), N (22), N (32), and N(42), which are located between the layers created by the ZnCl 4 tetrahedra, are surrounded by eight Cl atoms. From the NMR spectra results of 16 pairs of 14 N, the approximately 32 resonance lines in phases III and IV were measured, as shown in Fig. 7(a). The 32 resonance lines from the 16 pairs of 14 N in the NH 4 ion were consistent with the previously reported crystallography structure. 12,16,17 In addition, phase VI, below T C5 , contains Z ¼ 12 formula units, N (11), N (12), N (21), N (22), N (31), and N(32), as shown in Fig. 1(c). Therefore, approximately 24 resonance lines Paper in 12 pairs were obtained. In these results, the splitting of the 14 N resonance lines for seven of the pairs slightly decreased with increasing temperature, whereas those of the 14 N resonance lines for the other ve pairs slightly increased with increasing temperature. On the other hand, the resonance frequencies in phases I, II, III, and IV for the case of (NH 4 ) 2 -ZnBr 4 are shown in Fig. 7(b). The 14 N NMR spectra from (NH 4 ) 2 ZnBr 4 could not be easily observed in detail because of their very low intensity. However, the resonance frequencies near T C2 and T C3 changed discontinuously.
As mentioned above, in the case of (NH 4 ) 2 ZnCl 4 , four pairs of lines ascribed to N(1) and N(2) nuclei appeared in phase I, above 406 K. Because the frequency distributions in phase II discontinuously emerged from these high-temperature lines, the notations N(1) and N(2) were not retained. Based on this temperature dependence, the assignment of the resonance lines in phases III and IV are also not denoted. The resonance frequency from phase V cannot be distinguished because of the much narrower temperature range. The resonance frequencies in phase VI, below T C4 , changed discontinuously from those of phases V and IV. The changes in the resonance lines near T C4 for (NH 4 ) 2 ZnCl 4 and T C3 for (NH 4 ) 2 ZnBr 4 also indicated a phase transition where the new phase exhibited orthorhombic symmetry, which equates to a higher degree of symmetry compared with monoclinic symmetry. Abrupt changes in the resonance frequencies for 14 N near the phase transition temperatures are generally attributed to structural phase transitions.

Conclusion
Data on structural geometries near successive phase transition temperatures of (NH 4 ) 2 ZnX 4 (X ¼ Cl, Br) were obtained by 1 H MAS NMR and 14 N NMR as a function of temperature. We studied the molecular motions in (NH 4 ) 2 ZnX 4 , based on the 1 H chemical shis and spin-lattice relaxation time, T 1r , in the rotating frame. The 1 H chemical shis near the phase transition temperatures for the two materials did not show any drastic change, and this result might be related to proton ordering near the phase transition temperatures. From the 1 H T 1r results, the activation energies for the tumbling motion of 1 H had very similar values, and the tumbling motion of NH 4 + ions occurred within the high-temperature range. We compared the 1 H MAS NMR in the rotating frame measured here and the previously reported 1 H static NMR results in the laboratory frame 23-25 for (NH 4 ) 2 ZnCl 4 . The trends in T 1r values for 1 H in (NH 4 ) 2 ZnCl 4 are different from the trends in the T 1 values. The molecular motion by T 1r in the rotating frame was dominant at high temperature, whereas that by T 1 in the laboratory frame was dominant at low temperature. The activation energy values extracted from T 1r and T 1 measurements are different for the molecular motions in the kHz and MHz ranges. The 14 N NMR spectra exhibited a sudden shi in the 14 N peak positions and number of peaks at the phase transition temperatures. The electric eld gradient (EFG) tensors at the N sites varied, reecting the changing atomic congurations around the 14 N nuclei. This is because the phase transition temperature strongly affect the 14 N number of symmetry related nitrogen centers within the unit cell. Therefore, 14 N NMR provides insight into changes in crystal symmetry and cation reorientation rates induced by heating and phase transitions.
The two crystals have different phase transition temperatures, but seemingly similar phase transition mechanisms. Although (NH 4 ) 2 ZnX 4 has different bond lengths in the Zn-X (X ¼ Cl, Br) structure, and different X atomic radii, the different halide ions (X ¼ Cl, Br) do not appear to signicantly inuence the 1 H relaxation time.

Conflicts of interest
There are no conicts to declare.