Crystal structures, strain analysis, and physical properties of Sr _0.7Ce _0.3MnO ₃

We have studied the crystal structure of mixed-valence Sr _0.7Ce _0.3MnO ₃ 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 Pm3̄m at T 948 K. Lattice parameters and Mn-O bond distances, obtained by Rietveld refinement, have been used to derive the spontaneous strains and MnO ₆ octahedral distortion, which are interpreted in terms of strain/order parameter coupling using a single Landau free-energy expansion for a Pm3̄m reference structure with two instabilities (R4+ and Γ3+). Two phase transitions were proposed: an octahedral tilting transition at T _c,φ ∼ 938 K (Pm3̄m I4/mcm, R4+), and an isosymmetric, electronically driven (Jahn-Teller-like) transition at T _c,JT ∼ 770 K (I4/mcm, R4+ I4/mcm, R4+ and Γ3+). 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.