Pressure induced phase transitions in LiNaSO4

Raman spectroscopic investigations are carried out on LiNaSO4 up to a press ure of 140 kbar. The behavior of the internal modes as a function of pressu re suggests a structural transition at 22 +/- 2 kbar. Further, the evolutio n of the Raman spectra suggests that the sulphate ions in the high pressure phase are orientationally disordered in the beginning. Upon subsequent inc rease of pressure, the misoriented sulphate ions are found to gradually ord er, and at pressures above 90 kbar, the system gets completely ordered. Com parison of the present results with the high pressure behavior of other bin ary and disulphates reveal that the increase in cationic size ratio beyond a critical value leads to orientational disordering in the high pressure ph ases. (C) 1999 Elsevier Science Ltd. All rights reserved.