Elastic and anelastic anomalies in (Ca,Sr)TiO 3 perovskites: Analogue behaviour for silicate perovskites

Abstract

Values of bulk modulus ( K), shear modulus ( G) and mechanical quality factor ( Q) have been determined for polycrystalline samples across the CaTiO 3 (CST0)–SrTiO 3 (CST100) solid solution by resonant ultrasound spectroscopy. Because of similarities with low frequency elastic and anelastic anomalies due to twin wall motion reported in previous studies, a working hypothesis is developed in which dissipation processes are interpreted in terms of twin wall displacements. At high temperatures in CST50 the stability field of the I4/ mcm structure is marked by the disappearance of all resonance peaks (superattenuation). This is attributed to anelastic domain wall sliding. At room temperature the I4/ mcm phase of CST70 and CST80 has values of G which are lower than those of cubic or orthorhombic phases, and a concomitant drop in Q is interpreted as implying that the domain wall pinning process reported elsewhere to occur below ∼400–450 K is only partial. A similar drop in G and Q was found in CST95 below the P m 3 ¯ m ↔ I 4 / m c m transition at ∼238 K. The I4/ mcm ↔ Pbcm transition in CST70 at ∼230 K is marked by an abrupt increase in Q, suggesting that mobile twins in crystals with the I4/ mcm structure become effectively immobile in antiferroelectric crystals with the Pbcm structure. The I4/ mcm ↔ Pnma transition in CST50 is marked by a similarly abrupt increase in Q, consistent with twin walls becoming effectively immobile also in crystals with the Pnma structure. A fall in Q below ∼800 K in CST0, however, could imply that a degree of twin wall mobility might develop in Pnma crystals if the tetragonal spontaneous strain departs significantly from zero. The remarkable attenuation behaviour of crystals with the I4/ mcm structure at the relatively low stress conditions which apply during resonances of a parallelepiped with edge dimensions of ∼2–4 mm, is consistent with the view that a characteristic signature for tetragonal CaSiO 3 in the Earth's lower mantle should be a marked attenuation of seismic waves.