Viscoelasticity of the titanate perovskites CaTiO3 and SrTiO3 at high temperature

The frequency-dependence of the shear modulus and dissipation in polycrysta lline CaTiO3 and SrTiO3 perovskites has been determined at high temperature using micro-creep (epsilon = 10(-4)), seismic frequency forced oscillation (epsilon = 10(-5)), and ultrasonic interferometric measurements. Both of th ese perovskites display viscoelastic behaviour in the seismic frequency reg ime; with the temperature of onset of this frequency-dependent behaviour be ing grainsize-dependent. The frequency-dependent and time-dependent data ha ve been described in terms of the elastic, anelastic and viscous components of deformation using the Andrade model. The relatively smooth variation wi th temperature of G and Q(-1) for CaTiO3 (sampled at relatively widely spac ed (50 degrees C) temperature steps) suggests that the high temperature pha se transitions occurring in CaTiO3 have little effect on the viscoelastic b ehaviour. In the absence of grain-boundary impurities, twinning and disloca tions, the long-timescale viscoelastic deformation of SrTiO3 is attributed to grain-boundary sliding accommodated by lattice or grain-boundary diffusi on, controlled by the slowest diffusing species, which is expected to be ei ther Sr or Ti. The low viscosities determined for the CaTiO3 samples, toget her with the low values (with respect to the extrapolated ultrasonic data) of shear modulus associated with Andrade model fits to the data suggest tha t the long-timescale viscoelastic deformation of CaTiO3 may reflect relativ ely faster grain-boundary diffusion of the slowest moving species through t he thin (< 2 nm) impurity films and larger impurity segregations present in these samples. The ultrasonic measurements, combined with those of Kung [K ung, J., 1997. Systematics among the elastic properties of perovskite-struc tured compounds and geophysical implications. PhD, Australian National Univ ersity, Canberra, 112 pp.] for ScAlO3 perovskite, indicate relatively tight ly clustered high-frequency (anharmonic) values of /(partial derivative K-S /partial derivative T)(P)/ and /(partial derivative G/partial derivative T) (P)/ within the ranges 0.023-0.033 and 0.015-0.021 GPa K-1, respectively, n either of which includes the most recent determinations for MgSiO3 perovski te. 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 extrapol ation of the rheology of fine-grained SrTiO3 and CaTiO3 perovskites to 5 mm grainsize shows that a similar to 20% dispersion in wavespeed is expected for the period range 1-1000 s, with dissipation(Q(-1)) similar to 10(-2) fo r 1 s period waves at 1300 degrees C. Viscoelastic behaviour related to dif fusional creep in the silicate perovskites in the lower mantle may result i n a similar reduction in seismic shear-wave speed accompanied by an increas e in shear-wave dissipation, and an apparent increase in the temperature de pendence of seismic shear-wave speed. (C) 1999 Elsevier Science B.V. All ri ghts reserved.