Phase transitions in tetramethylammonium cadmium tribromide

Nuclear Quadrupole Resonance Investigation of Phase Transition in Tetramethylammonium Cadmium Tribromide

“…1. No hysteresis phenomena have been found in the phase transition region within the experimental accuracy of ±0.5 K. This fact does not contradict the data of dielectric measurements where the hysteresis was equal to 0.2 K [5]. The authors of this article classify the phase transition as ferroelectric of the order-disorder type with the first-order transition features.…”

“…These ions are orientationally disordered at room temperature [1,2]. At low temperatures these compousss are characterized by the structural phase transition of the order-disorder type, accompanied by ordering of the tetramethylammonium ions [3,4].At room temperature the TMCB crystal is isomorphous to the above-mentioned compounds and the first-order structural phase transition occurs at the temperature T = 163 K [5]. However, the NMR investigations did not reveal any anomalies in the temperature dependence of the proton spin-lattice relaxation time at 163 K [6].…”

“…If we assume that the bromine nuclei do not take part directly in the critical dynamics and other atoms groups (for example N(C.H3)4 ) ussergo ordering at the phase transition in this crystal, the critical behaviour of NQR spectum parameters at the phase transition is determined by the influence of macroscopic changes of the crystal on the local electric field gradient tensor of the bromine nuclei. Taking into account that the ferroelectric phase is present below the phase transition temperature [5] it can be shown that the parameters of the tensor of electric field gradient at the bromine nuclei has the following form:…”

“…At room temperature the TMCB crystal is isomorphous to the above-mentioned compounds and the first-order structural phase transition occurs at the temperature T = 163 K [5]. However, the NMR investigations did not reveal any anomalies in the temperature dependence of the proton spin-lattice relaxation time at 163 K [6].…”

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

“…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.…”