Crystalline phase sequence of Li3ThF7: A study based on the Landau theory and linear birefringence measurements

Linear birefringence measurements in three special geometries were done in Li3ThF7 single crystals, from 100 to 550 K, covering the range where this m aterial presents three structural phase transitions. We demonstrate the pur e ferroelastic nature of the transitions occurring at 281 and 368 K. Beside s, we propose that the higher temperature phase transition at 468 K should be nonferroic. We develop the calculations for the coupling between linear birefringence and ferroelastic deformations. The use of an appropriated Lan dau functional with two interacting order parameters allows us to describe the observed birefringence features of our samples and to propose crystallo graphic point groups for the four pre-established phases; The model predict s the possibility of an interesting phase transition below 100 K. A re-entr ant room-temperature phase could appear below this temperature.