Phase transition in CaGeO3 perovskite: Evidence from X-ray powder diffraction, thermal expansion and heat capacity

Abstract

Abstract. High-temperature x-ray powder diffraction study by the full pattern Rietveld method of orthorhombic CaGeO3 (Pbnm at ambient condition) perovskite confirms the previously observed phase transition at Tc = 520 K. The measured volumetric thermal expansion coefficients are 3.1 x 10 -s (K -1) below Tc and 3.5x 10 .5 (K -1) above Tc. The space group at T>Tc has been tentatively identified as Cmcm. Such a transition involves the disappearance of one of the two octahedral rotations in the (001) plane, and the doubling of the unit cell volume, with c axis unchanged. Although this transition should be of first order from symmetry considerations, the distortion of the Pbnm phase decreases continuously as the temperate approaches Tc and there is no observable volume discontinuity at Tc. The measured heat capacity places an upper limit on the enthalpy of transition of 50 J/mol, which is quite reasonable in terms of the crystallographic nature of this phase transition.

Raman scattering study of the high-temperature vibrational properties and stability of CaGeO3 perovskite

“…As discussed above, Liu et al (1991) have recently suggested that CaGeO3 perovskite undergoes a firstorder transformation from its Pbnm structure to a pseudotetragonal Cmcm phase near 250 ~ C. As shown in Table 1, the distribution of Raman-active modes is similar for the Pbnm and Cmcm perovskite structures. In particular, both structures have 7 formally allowed Ag modes.…”

“…As discussed in the introduction, Liu et al (1991) have proposed that CaGeO3 perovskite transforms to a Cmcm (D17h) modification above 250 ~ C. In this space group, one of the octahedral tilt angles in the ab plane vanishes, and the structure is represented by a (0 qS~b) tilting scheme. The relationship of the Cmcm structure and crystallographic directions to those for the Pbnm and cubic symmetries is displayed in Fig.…”

“…This transition was inferred from a convergence of the 404 and 440 Pbnm reflections observed in powder x-ray diffraction measurements and was discussed in terms of critical soft-mode behavior involving octahedral rotational modes, similar to that displayed by other GdFeO3-type perovskites. In a subsequent analysis, Liu et al (1991) have reinterpreted their diffraction data to indicate that the high temperature phase is actually a pseudotetragonal modification of an orthorhombic perovskite structure. On the basis of additional weak reflections not allowed by Pbnm symmetry, these authors suggest a Cmcm (DI~) space group for this high temperature phase, similar to that displayed by CsSrC13 between 90 and 102~ (Prokert 1981), and CsPbC13 perovskite from 37-42~ C (Fujii et al 1974;Hirotsu and Suzuki 1978).…”

“…Although symmetry constraints require that the Pbnm-to-Cmcm transition must necessarily be first order, no resolvable discontinuities were observed in the lattice parameters at the transition temperature, although a small increase (13%) in the thermal expansion coefficient was noted. Moreover, no latent heat anomaly was detected in calorimetric measurements through the proposed transition region (Liu et al 1991).…”

Displacive Phase Transitions in and Strain Analysis of Fe-Doped CaTiO3 Perovskites at High Temperatures by Neutron Diffraction

“…However, this behavior could be due to the tilt angles (or displacements of the oxygen atoms) being too small to produce appreciable changes in the unit cell volume. This is the case of CaGeO 3 where a Pnma to Cmcm transition seems to occur with no discontinuity in the unit cell volume (17). In fact, the Pnma-I4=mcm transition in our Fe-doped CaTiO 3 perovskites is also required by symmetry to be of first order.…”