Crystal structures, strain analysis, and physical properties of Sr0.7Ce0.3MnO3.

We have studied the crystal structure of mixed-valence Sr0.7Ce0.3MnO3 from 4.2 to 973 K using high-resolution neutron powder diffraction. The crystal structure is tetragonal in space group I4/mcm at 4.2-923 K and cubic in Pm3m at T ≥ 948 K. Lattice parameters and Mn-O bond distances, obtained by Rietveld refinement, have been used to derive the spontaneous strains and MnO6 octahedral distortion, which are interpreted in terms of strain/order parameter coupling using a single Landau free-energy expansion for a Pm3m reference structure with two instabilities (R+4 and &Tao;+3). Two phase transitions were proposed: an octahedral tilting transition at Tc,ø ∼ 938 K (Pm3m ⟷ I4/mcm, R+4), and an isosymmetric, electronically driven (Jahn-Teller-like) transition at Tc,JT ∼ 770 K (I4/mcm, R+4 ⟷ I4/mcm, (R+4 and &Tao;+3)). The nature of the tilting transition appears to be tricritical, while that of the Jahn-Teller-like transition is second order. In addition to the contributions from octahedral tilting and Jahn-Teller-like distortions, there is an excess octahedral distortion at temperatures below 250 K; this is speculated to be associated with an anomaly observed over the temperature range of 275-300 K in the heat-capacity measurements. [ABSTRACT FROM AUTHOR]