Abstract
Elastic properties of leucite have been investigated using resonant ultrasound spectroscopy over a temperature range from 300 to 1400 K. According to these measurements, elastic moduli soften by ~50% at the Ia 3 d-I 41/ acd ferroelastic transition temperature T c1 = 940 K relative to the value at 1400 K. A second softening is observed at T c2 = 920 K, corresponding to the structural change from the space group I 41/ acd to I 41/ a . These elastic anomalies are analyzed in a simple model under the assumption that the transitions observed at T c1 and T c2 can be approximated by a single pseudoproper ferroelastic transition. The two phase transitions are accompanied by a single peak in mechanical damping attributed to the high mobility of twin walls in the intermediate phase followed by pinning in the low-temperature phase. To determine whether twin walls in tetragonal leucite are polar, resonant piezoelectric spectroscopy and second harmonic generation measurements were performed, but no evidence of polarity was found.
Highlights
The mineral leucite (KAlSi2O6) undergoes two structural transitions at Tc1 = 938 K and Tc2 = 918 K (Grögel et al 1984; Lange et al 1986; Palmer et al 1988, 1990)
A ferroelastic transition occurs at Tc1 from a cubic phase with the space group Ia3d to a tetragonal phase with the space group I41/acd
Polar character for twin walls has only recently been identified in perovskites CaTiO3 and SrTiO3 (Van Aert et al 2012; Scott et al 2012, 2013)
Summary
Diffuse tetragonal diffraction signals persist up to >1060 K (Palmer and Salje 1990; Palmer et al 1989) The microstructure in this case is likely to correspond to tweed, which is comprised of orthogonal modulations that appear as cross-hatched patterns in electron diffraction (Putnis and Salje 1994; Bratkovsky et al 1994). Microstructures, especially ferroelastic twins, can give valuable information about the order parameters and spontaneous strains associated with a phase transition, as has been elucidated by Palmer et al (1989, 1990) and Hatch et al (1990). We will show in this paper that microstructures and the phase transitions in leucite do cause elastic softening but none of the domain walls show any sign of polarity
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