Materials Science and Engineering A 442 (2006) 204–207 Mechanical properties and domain wall mobility of LaGaO 3 perovskite over a first-order phase transition C.V. Jakeways ∗ , R.J. Harrison, S.A.T. Redfern Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK Received 13 July 2005; received in revised form 16 December 2005; accepted 16 December 2005 Abstract La x Nd 1-x GaO 3 perovskites have been studied by dynamic mechanical analysis to relate internal friction to the mechanisms controlling first- order phase transitions in both single crystal and polycrystalline samples, in the presence of an external oscillating stress. Reflected light images of the crystal during the transition provide insight into the mechanism of the transition and the corresponding attenuation and storage modulus. La x Nd 1-x GaO 3 has a disordered perovskite structure. At room temperature the structure is orthorhombic, Pbnm, with a first-order phase transition to rhombohedral, R ¯ 3c, occurring on heating. In both polycrystalline and single crystal samples the internal friction can be considered to arise from the motion of the phase interface across the sample. First, internal friction rises as the phase interface moves across the sample, then microstructural reorganisation and relaxation within the sample causes the internal friction to decrease back to a background level. The velocity of the phase interface can be expressed as an exponential of the form V ∞ +V 0 exp(-t/C), where V ∞ , V 0 and C are dependent on the sample composition, and the force and frequency of applied external stress. © 2006 Elsevier B.V. All rights reserved. Keywords: Interface velocity; Internal friction; First-order phase transition; Storage modulus 1. Introduction Internal friction measurements have been fairly widely used to study the processes involved with first-order structural phase transitions [1,2]. Many theories have been proposed to account for the onset of the internal friction peak close to the transition [1,3,4], ranging from changing elastic constants during the tran- sition to motion of dislocations at the boundary between the two phases. A good review of all the past work is provided in these papers, and one of the main results is that the internal friction, or inverse quality factor Q -1 , can be represented as a function of temperature, T, heating rate, dT/dt, and frequency of applied stress, ω, in the form: Q -1 = C(T )ω -2l ((dT/dt )/ω) n , 0 < n, l < 1. (1) Zhang et al. [1,2] consider the main mechanical influence of a first-order phase transition in terms of the motion of a phase interface across the sample, as the daughter phase consumes the parent. They define n as a parameter, which determines ∗ Corresponding author. Tel.: +44 1223 333367; fax: +44 1223 333450. E-mail address: cj249@cam.ac.uk (C.V. Jakeways). the energy dissipation rate during a first-order phase transition, whilst l is a constant for a specific transition. The frequency dependence arises as the forced oscillation frequency will cou- ple with the motion of the phase interface. Therefore, variation in frequency will alter the velocity of the phase boundary, affecting the energy dissipated [1,2]. The following general points were deduced from the early theoretical investigations and comple- mentary experimental mechanical spectroscopy: 1. The height of the internal friction peak at the transition decreases as the frequency of the applied stress increases. 2. The shear modulus minimum is less pronounced at higher frequency above the resonant frequency. 3. An increase in ramp rate through the transition temperature T C increases the amplitude and breadth of the internal friction peak. 4. The minimum in modulus is also more pronounced as the ramp rate is increased. Polycrystalline and single crystal samples of lanthanum- neodymium gallate perovskites within the solid solution of La x Nd 1-x GaO 3 were studied as a structural analogue to mantle perovskite. This group of perovskites falls into the structural 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.12.084