Effect of hydrostatic pressure on the structural phase transition in K2OsCl6:a35Cl nuclear quadrupole resonance

The 35Cl nuclear quadrupole resonance frequency is reported in K2OsCl6 at temperatures in the vicinity of the structural phase transition for three externally applied hydrostatic pressures. The phase-transition temperature, Tc, decreases linearly with pressure, and the pressure coefficient, (dTc/dP), is measured to be −(2.2 ± 0.3) × 10−2 K∙MPa−1. This value is compared with that measured for K2ReCl6 and is consistent with the prediction of a calculation based on a modified rigid-sphere model. Measurements of (∂v/∂P)T show precursor behaviour as Tc is approached from the high-temperature cubic phase.

Investigation of the Rotary Lattice Mode in R2PtCl6 Compounds. I. From Measurements of the 35Cl Nuclear Quadrupole Resonance Frequency

Measurements of the temperature and pressure variation of the 35Cl nuclear quadrupole resonance (NQR) frequency in Rb2PtCl6 and Cs2PtCl6 are reported. The resonance frequency is measured at atmospheric pressure for temperatures from 4 to 500 K and at four temperatures between 290 and 380 K for pressures to 5000 kg cm−2. Previously published data for K2PtCl6 are also included in the analysis. Static lattice NQR frequencies are deduced. The differences between the static lattice frequencies are compared with the calculations of Smith and Stoessiger. Thermal averaging of the electric field gradient at a chlorine site is assumed to be dominated by the Q3, Q4, Q5, and Q6 internal modes of the PtCl6 octahedra and by the rotary lattice mode. The rotary mode frequencies are deduced; they are of similar magnitude and increase in the same sequence as the frequencies deduced from infrared and Raman data. An analysis of the pressure dependence of the NQR frequencies leads to pressure coefficients for the rotary mode frequencies. The influence of the cage of R atoms surrounding a PtCl6 octahedron is shown to increase through the series K2PtCl6, Rb2PtCl6, Cs2PtCl6. Finally, a thermodynamic analysis of the NQR data is presented which shows the importance of taking specific volume effects into account.