Structural phase transitions in ferroelastic TlAlF4: DSC investigations and structures determinations by neutron powder profile refinement

The temperature behaviour of TlAlF4 is investigated by DSC and by neutron powder diffraction. Evidence for two structural phase transitions (SPT) is found at Tc1=514 K (a first-order SPT) and at Tc2=435 K. The structures of the three phases are determined by the neutron powder profile-refinement technique at 723, 523, 473, 423, 200 and 4.2 K. They can be described by the space groups P4/mmm above Tc1 (phase I, 'ideal TlAlF4 structure'), I4/mcm between Tc1 and Tc2 (phase II) and I2/a below Tc2 (phase III). It is shown that the transitions are closely related to tilts of AlF6 octahedra with the ion sequences a0a0c0, a0a0c-, aa-aa-c- and can be associated with the freezing of phonons at the A point of the Brillouin zone of the ideal TLAlF4 structure cell. The Tc1SPT is non-ferroic while the Tc2 SPT is of the improper ferroelastic type. The ferroelastic properties of phase III are discussed.