Structural phase transitions in ferroelastic RbAlF4. II. Linear birefringence investigations

For pt.I see ibid., vol.15, p.183 (1982). The linear birefringence (LB) of RbAlF 4 is measured in the temperature range 5-630K. The discontinuous structural phase transitions (SPT) at T cl =559.4K (P4/mmm to P4/mbm) gives rise to a small jump of the tetragonal LB of delta ( Delta n ac )=6*10 -7 . The continuous SPT at T c2 =282.1K (P4/mbm to Pmmm) is accompanied by orthorhombic LB, Delta n ab , rising from zero at T c2 to 3.4*10 -4 at T=0, whereas n ac changes by about 8*10 -4 . The critical exponent of the order parameter obtained from Delta n ab , beta =0.324, does not change on application of uniaxial stress up to 8 MPa. The deformation of the Delta n ab curve due to internal strains is discussed and the switching of ferroelastic domains by uniaxial stress is demonstrated. The relationship between birefringence, spontaneous strain and the order parameter is discussed.