Viscoelasticity of the titanate perovskites CaTiO₃ and SrTiO₃ at high temperature

The frequency-dependence of the shear modulus and dissipation in polycrystalline CaTiO₃ and SrTiO₃ perovskites has been determined at high temperature using micro-creep (ε = 10^-4), seismic frequency forced oscillation (ε = 10^-5), and ultrasonic interferometric measurements. Both of these perovskites display viscoelastic behaviour in the seismic frequency regime; with the temperature of onset of this frequency-dependent behaviour being grainsize-dependent. The frequency-dependent and time-dependent data have been described in terms of the elastic, anelastic and viscous components of deformation using the Andrade model. The relatively smooth variation with temperature of G and Q^-1 for CaTiO₃ (sampled at relatively widely spaced (50°C) temperature steps) suggests that the high temperature phase transitions occurring in CaTiO₃ have little effect on the viscoelastic behaviour. In the absence of grain-boundary impurities, twinning and dislocations, the long-timescale viscoelastic deformation of SrTiO₃ is attributed to grain-boundary sliding accomodated by lattice or grain-boundary diffusion, controlled by the slowest diffusing species, which is expected to be either Sr or Ti. The low viscosities determined for the CaTiO₃ samples, together with the low values (with respect to the extrapolated ultrasonic data) of shear modulus associated with Andrade model fits to the data suggest that the long-timescale viscoelastic deformation of CaTiO₃ may reflect relatively faster grain-boundary diffusion of the slowest moving species through the thin ( < 2 nm) impurity films and larger impurity segregations present in these samples. The ultrasonic measurements, combined with those of Kung [Kung, J., 1997. Systematics among the elastic properties of perovskite-structured compounds and geophysical implications. PhD, Australian National University, Canberra, 112 pp.] for ScAlO₃ perovskite, indicate relatively tightly clustered high-frequency (anharmonic) values of ζ(partial differentialK(S)/partial differentialT)(P)ζ and ζ(partial differentialG/partial differentialT)(P)ζ within the ranges 0.023-0.033 and 0.015-0.021 GPa K^-1, respectively, neither of which includes the most recent determinations for MgSiO₃ perovskite. The diffusion controlled viscoelastic rheology observed here results in the magnitude of the highest temperature shear moduli determined for fine-grained CaTiO₃ and SrTiO₃ perovskite at seismic frequencies being less than one-quarter that of the ultrasonic moduli, and the temperature dependence of the seismic-frequency moduli being larger by a factor of 10. An extrapolation of the rheology of fine-grained SrTiO₃ and CaTiO₃ perovskites to 5 mm grainsize shows that a ~20% dispersion in wavespeed is expected for the period range 1-1000 s, with dissipation (Q^-1) ~ 10^-2 for 1 s period waves at 1300°C. Viscoelastic behaviour related to diffusional creep in the silicate perovskites in the lower mantle may result in a similar reduction in seismic shear-wave speed accompanied by an increase in shear-wave dissipation, and an apparent increase in the temperature dependence of seismic shear-wave speed.