Atomic displacements at and order of all phase transitions in multiferroic YMnO 3 and BaTiO3

摘要

Coordinate analysis of the multiple phase transitions in hexagonal YMnO3 leads to the prediction of a previously unknown aristotype phase, with the resulting phase-transition sequence: P63'cm'(e.g.) → P63cm → P63/mcm → P63/mmc P6Immm. Below the Neel temperature TN 75 K, the structure is antiferromagnetic with the magnetic symmetry not yet determined. Above TN the P63cm phase is ferroelectric with Curie temperature Tc=105 K. The nonpolar paramagnetic phase stable between Tc and ～ 1360 K transforms to a second nonpolar paramagnetic phase stable to ～ 1600 K, with unit-cell volume one-third that below 1360 K. The predicted aristotype phase at the highest temperature is nonpolar and paramagnetic, with unit-cell volume reduced by a further factor of 2. Coordinate analysis of the three well known phase transitions undergone by tetragonal BaTiO3, with space-group sequence R3m → Amm2 → P4mm → Pm3m, provides a basis for deriving the aristotype phase in YMnO3. Landau theory allows the I→ II, III → IV and IV →phase transitions in YMnO3, and also the I→ II phase transition in BaTiO3, to be continuous; all four, however, unambiguously exhibit first-order characteristics. The origin of phase transitions, permitted by theory to be second order, that are first order instead have not yet been thoroughly investigated; several possibilities are briefly considered.