Studies on¹³³Cs NMR Spectra and Spin-Lattice Relaxation Times in Incommensurate Cs₂HgCl₄

Abstract

133 Cs NMR spectra and the spin-lattice relaxation times, T 1 , in Cs 2 HgCl 4 crystals were measured in the ranges of 160-361 K and 160-371 K, respectively. The central lines of the spectra show small splittings characteristic of the incommensurate (IC) phase. The splitting increases with decreasing temperature. The critical exponent ζ of T 1 at the phase transition from normal (N) to IC phase was determined to be 0.615±0.025 in the high-temperature side of the transition, which is in good agreement with the three-dimensional XY model predicting the one-dimensional modulation wave in the IC phase. The frequency dependence of T 1 in the IC phase can be explained by the fluctuations of phason with a small gap, 0-5 MHz, and amplitudon. ment disorders in HgCl 2− 4 complex anions and one of the crystallographically inequivalent two Cs cations. It has been presumed that the ferroelectric commensurate (C) phase below T C without centrosymmetry have a point symmetry mm2, 4) but detailed studies on structures below T C have not been reported. 35 Cl NQR, 12) dielectric and Raman scattering 8) measurements revealed a different result that the IC phase appears between T IC and T C1 . 12) From the optical polarization measurement, the low-temperature phase immediately below T C1 is anticipated to be a ferroelastic C phase analogous to that in Cs 2 MBr 4 (M=Cd, Hg). §2. Experimental Cs 2 HgCl 4 crystals were grown by cooling a molten mixture containing stoichiometric amounts of CsCl (purity 99.9%) and HgCl 2 (purity 99.9%) purchased from Wako Pure Chemical Industries, Ltd. The crystalline powder obtained was dried in vacuo and then sealed in glass tubes with nitrogen gas for differential thermal analysis (DTA) and NMR measurements.DTA was measured between 120 and 300 K to confirm the phase transitions reported previously. The sample temperature was determined within ±0.2 K using a chromel-constantan thermocouple. The measurement of 133 Cs NMR T 1 was performed with a Bruker MSL-300 spectrometer at frequency of 39.4 MHz in the range 160-371 K and also with a Bruker MSL-400 spectrometer at 52.5 MHz in 196-215 K. 133 Cs NMR spectra were recorded by a Bruker MSL-300 using a saturated CsCl aqueous solution as a standard of frequency shift. The sample temperature was controlled within ±0.5 K by Bruker VT-1000 temperature controller and determined by a copper-constantan thermocouple with the same accuracy. The uncertainty in the T 1 measurement is estimated to be within 5%. §3. Results DTA thermograms on heating displayed endother- §1. IntroductionMany compounds belonging to the A 2 BX 4 family with the pseudo-hexagonal β-K 2 SO 4 type structure (P nma) form incommensurate (IC) phases at low temperatures. We reported lattice dynamics in IC phases of Cs 2 MBr 4 (M=Cd and Hg) studied by 133 Cs NMR. 1) The critical exponents obtained at the phase transitions from normal to incommensurate (N-IC) phase correspond to the three-dimensional XY and the mean field models for Cd and Hg compounds, respectively. This difference seems to be expl...

Soft-mode-driven lattice instabilities in Cs2HgCl4crystal: phenomenological treatment and far-infrared spectroscopy of the structurally modulated phases

“…Cs 2 HgCl 4 exhibits a very rich sequence of structural PTs into both C and IC phases (see table 1) which have been intensively studied in the past by various experimental techniques, among which are the structural analysis [21], dielectric [22], optics and acoustics [23,24], resonance [25,26] and heat capacity [27] methods. A ferroelastic instability induced by the hydrostatic pressure and lattice dynamics associated with it in the high temperature phase of Cs 2 HgCl 4 were explored in [28][29][30].…”