Ionic conductivity and pressure dependence of trigonal-to-cubic phase transition in lithium sodium sulphate

The pressure and temperature dependencies of electrical impedance as a function of frequency (from 0.1 Hz, to 100 kHz) have been measured in crystalline LiNaSO4 over temperature range 400-800 degreesC and pressures 5, 10 and 20 kbar in a piston-cylinder apparatus. The cell for electrical impedance measurements represents a co-axial cylindrical capacitor with a geometric factor 5.5-7 cm. LiNaSO4 undergoes a displacive first order phase transition from trigonal structure with a space group P31c to a body-centred cubic (bbc) structure with a high ionic conductivity > 0.1 S/cm (at 10 kHz). The bulk resistance has been estimated at each temperature from Argand plots. The temperature of the phase transition has been estimated from plots of log[sigmaT] vs. 1/T. A drop of the electrical impedance indicates that the phase transition occurred at 5 kbar at 578 degreesC (+/-1), at 10 kbar at 641 degreesC and at 20 kbar at 764 degreesC. There is small hysteresis of phase transition temperature observed by cooling, the hysteresis disappears with the pressure increase. The slope dT/dP estimated from the electrical conductivity measurements of this phase transition is 12.5 degreesC/kbar and corresponds to Clausius-Clayperon slope with DeltaS similar to 26.4 J/mol/K and DeltaV similar to 3.3 cm(3)/mol. In the extrinsic region, the electrical conductivity has an activation energy which varies from 1.48 eV at 5 kbar to 1.35 eV at 20 kbar, in the intrinsic region, the activation energy (E-a) is ca. 2.4-2.30 eV. In the high temperature conductive phase, E-a is ca. 0.33-0.4 eV, and does not depend on pressure. With the pressure increase, temperature intervals of extrinsic and intrinsic conductivities start to overlap, which results in a non-Arrhenian temperature dependence. The frequency dependence of the electrical conductivity at low frequencies (< 500 Hz) may be approximated as sigma approximate to sigma (0)(1 + (j omega tau (0))(n)), where omega is frequency, tau (0) is a temperature-dependent constant, and n is an experimental exponent characterising a low frequency dispersion (LFD). With the temperature increase from extrinsic region to fast ionic phase, n increases from 0 to similar to 0.5. (C) 2001 Elsevier Science BN. All rights reserved.