HR: 11:50h
AN: MR52A-07    [Abstracts]
TI: Polyhedral Control of Tilt Transitions in Perovskites
AU: * Angel, R J
EM: rangel@vt.edu
AF: Virginia Tech, Dept Geosciences, Virginia Tech, Blacksburg, VA 24060, United States
AU: Zhao, J
EM: jzhao@vt.edu
AF: Virginia Tech, Dept Geosciences, Virginia Tech, Blacksburg, VA 24060, United States
AU: Ross, N L
EM: nross@vt.edu
AF: Virginia Tech, Dept Geosciences, Virginia Tech, Blacksburg, VA 24060, United States
AB: Advances in single-crystal structure determinations have shown that the structural evolution of ABO3 perovskites with increasing pressure is determined by the relative compressibility of the octahedral "B" cation site and the extra-framework "A" cation site. With increasing pressure, the octahedral tilts change and the octahedral site compresses in such a way that the increase in bond valence sums at the A and B sites is equal [1]. The extent of tilting of a given perovskite in a given space group is limited. With increasing temperature phase transitions occur to different tilt systems with smaller tilts and higher symmetry. We have previously shown that the bond-valence matching principle leads to the prediction that the Clapeyron slopes of phase transitions between two perovskite tilt systems [2]. The P-T slopes of phase boundaries are negative for perovskites with +3 cations in both sites, and positive for perovskites such as MgSiO3 and CaSiO3 with +2 cations on the A site and +4 cations in the octahedral B site. Examination of structural data for a wide variety of perovskites now shows that phase transitions from one tilt system to another occur when the structural tolerance factor tobs (derived from bond lengths) reaches a critical value. Experimental data show that the critical value corresponds to the point at which the global instability indices of the two phases become equal. Each space-group change has its own critical value of tobs, but otherwise appears to be invariant with chemistry or pressure or temperature. In combination with the bond-valence matching principle, the critical values can therefore be used to calculate phase diagrams for perovskites. [1] Zhao, Ross, Angel (2004) Acta Cryst., 60, 263-271. [2] Angel, Zhao, Ross (2005) Physical Review Letters, 95, 025503
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
DE: 4465 Phase transitions
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting