Quadrupole Relaxation of⁷⁹Br Nuclei in Ferroelectric (CH₃)₄NCdBr₃(TMCB)

Abstract

Investigations of temperature dependence of a nuclear quadrupole spinlattice relaxation time of 79 Βr nuclei in ferroelectric (CH3) 4 NCdΒr3 are ρre-sented. It is shown that in paraelectric phase iii the vicinity of phase transition temperature relaxation process is determined h ey critical fluctuations of electric field gradient. In ferroelectric plase relaxation is related to three dicherent mechanisms, namely: lattice vibrations, CH3--group reorientations and TMA cation motions. According to the results of measurements it is concluded that the phase transition is connected with ordering of TMA cations. PACS numbers: 64.70.KbTetramethylammonium tribromocadmate (CH 3)4NCdBr3 (TMCB) belongs to the family of crystals with a general chemical formula (CH3)4NMΧ3 , where Μ = Mn, Ni, Cd; Χ = Cl, Br, I. Among them (CH 3)4NMnCl3 (TMMC) crystal has been widely studied as quasi-one-dimensional magnetic system with infinite chains of -Mn-C13-Mn-Cl3-separated by tetramethylammonium cations (TMA) [1,2]. Similarly as in the case of TMMC isomorphous crystals of (CH 3 )4 NCdC13 (TMCC) and (CH3) 4 ΝΝiCl3 (TMNC) exhibit structural phase transitions from hexagonal high-temperature phase to monoclinic low-temperature phase [3,4]. It was shown that in higl-temperature phase of these crystals TMA cations are disordered [2, 5] and order-disorder phase transitions are related to the ordering of TMA cations and small shifts of the metal-halogen chains [4]. TMCB crystal is another member of this family and undergoes a stuctural phase transition of the first order from hexagonal room temperature phase (space group: P6 3 /m) to another hexagonal phase (space group P61) and disordered states of TMA cations in paraelectric and ordered in ferroelectric phase are found in it [6][7][8]. The low-temperature phase exhibits ferroelectric properties along the c-axis. Spontaneous polarization is equal to 1.2 x 10 -3 C/m2 at 125 K [9]. NMR investigations of molecular dynamics in (357)

A nuclear magnetic resonance study of tetramethylammonium cadmium chloride (TMCC)

“…All compounds of this family undergo structural phase transitions leading to different ordered low-temperature phases, which are expected to be ferroelastic in the case of TMMC and TMCC. A ferroelectric phase was found in the case of (CH 3 ) 4 NCdBr 3 (TMCB) [4,6,[11][12][13]. The crystals of TMCC possess a hexagonal structure at room temperature with the space group P 6 3 /m and with Z = 2 formula units per unit cell.…”

NMR analysis of structural geometry and molecular dynamics in perovskite-type N(CH3)4CdBr3 crystal near high-temperature phase transition

“…[8][9][10][11][12][13][14][15][16][17] Tetramethylammonium tribromocadmate (N(CH 3 ) 4 CdBr 3 ) belongs to the N(CH 3 ) 4 BX 3 (B = 55 Mn, 59 Co, 64 Cu, 65 Zn, 113 Cd; X = 35 Cl or 79 Br) family of ABX 3 -type perovskites. [18][19][20][21][22][23][24][25][26][27][28][29] The crystal structure at room temperature consists of isolated linear chains of face-sharing CdBr 6 octahedra separated by N(CH 3 ) 4 ions. N(CH 3 ) 4 CdBr 3 has two phase transition temperatures, 160 and 390 K, 19,22,23 with three phases denoted as III, II, and I, respectively, in the order of increasing temperature.…”

“…According to a previous report by Baisa et al , 25 a singlet 79 Br nuclear quadrupole resonance (NQR) spectrum was obtained for phases II and I, and a triplet spectrum was recorded for phase III. In addition, the NMR T 1 for 79 Br near 160 K exhibits strong temperature dependence and changes by three orders of magnitude in the temperature range of 77–310 K. 27 Overall, the relaxation processes of 1 H and 79 Br in N(CH 3 ) 4 CdBr 3 near the phase-transition temperature of 160 K were studied in the laboratory frame using static NMR. The structural phase transition near the high phase-transition temperature of 390 K presented by our group in the past was briefly studied using magic angle spinning (MAS) NMR in the rotating frame and static NMR in the laboratory frame.…”