Structural phase transitions in ferroelastic TlAlF4: DSC investigations and structures determinations by neutron powder profile refinement
- 10 July 1987
- journal article
- Published by IOP Publishing in Journal of Physics C: Solid State Physics
- Vol. 20 (19) , 2885-2900
- https://doi.org/10.1088/0022-3719/20/19/014
Abstract
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.Keywords
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