Brillouin scattering and ultrasonic studies in K1-xRbxAlF4: evidence of a new phase

Abstract

The layered tetrafluoroaluminates K1-xRbxAlF4 undergo several structural phase transitions, the existence and features of which evolve when varying x. A study of the temperature variations of the elastic constants derived from Brillouin scattering and ultrasonic experiments is presented for x=0.06, 0.09, 0.12 and 0.15. At room temperature, like in the pure KAlF4 and RbAlF4 compounds, all the mixed crystals studied show P4/mbm symmetry (denoted phase II), differing from the aristotypic phase (P4/mmm, phase I) by a tilting of the AlF6 octahedra around the c axis. Whereas pure KAlF4 presents a unique martensitic-like transition on lowering the temperature, a small amount of Rb+ ions (x>0.02) induces an intermediate phase (P21/m, phase IIIa), the existence temperature range of which increases with x. For x>0.09, a new phase (P21/m, phase IIIb) intercalated between phase II and phase IIIa is evidenced. All the phase transitions are monitored by the softening of normal phonon modes related to the rotation of the AlF6 octahedra. The measured elastic constants suffer pretransitional and transitional characteristic behaviours that are explained according to the various mechanisms involved during the transformations.