Raman spectra and phase transitions in single crystal (NH4)2ZnCl4

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

“…Here, the two inequivalent sites N (1) 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.…”

“…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).…”

Deuteron NMR spectra and relaxation in fully and partly deuterated

“…The structure at 233 K was found to be orthorombic with 24 ammonium ions in the unit cell [15]. Below 194 K Raman spectra suggest a triclinic structure [16]. According to the X-ray diffraction experiments there are two kinds of ammonium ions at 233 K; one kind, called I, is surrounded by five practically equidistant chlorine atoms while kind II ions are surrounded by eight roughly equidistant chlorine atoms [15].…”

“…Here we extend such studies on differently deuterated ðNH 4 Þ 2 ZnCl 4 samples, where limited jumps do not play any role. The normal compound, with the negligible deuteron concentration, shows many structural transitions at temperatures 406, 319, 271, 266 and 194 K [13][14][15][16][17]. The transition at 266 K is from an apparently incommensurate antiferroelectric phase (above 266 K) to a commensurate ferroelectric phase [13,15,16].…”

“…According to the X-ray diffraction experiments there are two kinds of ammonium ions at 233 K; one kind, called I, is surrounded by five practically equidistant chlorine atoms while kind II ions are surrounded by eight roughly equidistant chlorine atoms [15]. The Raman experiments hint that the number of nonequivalent ammonium ions could be three below 194 K [16]. The proton relaxation time, measured at the resonance frequency 10 MHz, shows a minimum at 93 K, increases then monotonously with temperature, reaches a maximum near 270 K and starts to decrease at still higher temperatures [18].…”