Temperature and pressure variation of the 35Cl nuclear quadrupole resonance frequency in K2OsCl6

Measurements of the temperature and pressure dependence of the 35Cl nuclear quadrupole resonance (NQR) frequency in K2OsCl6 are reported. The resonance frequency is measured at atmospheric pressure for temperatures from 4.2 to 430 °K and for five temperatures between 284 and 410 °K for pressures to 5000 kg cm−2. A second-order phase transition occurs at about 45 °K. In the high temperature phase all of the chlorine atoms are crystallographically equivalent. The analysis carried out deals exclusively with the data obtained in this phase. A thermodynamic relation is used to relate the experimental quantities (∂v/∂T)P and (∂v/∂P)T to the theoretical quantity (∂v/∂T)V. The latter quantity is calculated for a particular model to describe the motional averaging of the electric field gradient at the chlorine sites. The model adopted includes two distinct mechanisms—the usual Bayer–Kushida averaging mechanism and a mechanism resulting from the partial destruction of π bonding by the lattice vibrations. The thermodynamic relation is used in conjunction with the combined data for K2PtCl6, K2IrCl6, and K2OsCl6 to evaluate the validity of the model proposed. It is concluded that the model provides a consistent explanation of both the temperature and pressure variation of the NQR data. In addition, the analysis provides information on the nature of the molecular orbitals of the [MCl6]2− complex ion, gives a rough estimate of the ratio of the coefficient of thermal expansion to the isothermal compressibility, and lastly, yields a value for the average frequency of the rotary lattice mode in the three substances.