Atomic displacements at and order of all phase transitions in multiferroic YMnO3 and BaTiO3.
نویسنده
چکیده
Coordinate analysis of the multiple phase transitions in hexagonal YMnO(3) leads to the prediction of a previously unknown aristotype phase, with the resulting phase-transition sequence: P6(3)'cm'(e.g.) <--> P6(3)cm <--> P6(3)/mcm <--> P6(3)/mmc <--> P6/mmm. Below the Néel temperature T(N) approximately = 75 K, the structure is antiferromagnetic with the magnetic symmetry not yet determined. Above T(N) the P6(3)cm phase is ferroelectric with Curie temperature T(C) approximately = 1105 K. The nonpolar paramagnetic phase stable between T(C) and approximately 1360 K transforms to a second nonpolar paramagnetic phase stable to approximately 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 BaTiO(3), with space-group sequence R3m <--> Amm2 <--> P4mm <--> Pm3m, provides a basis for deriving the aristotype phase in YMnO(3). Landau theory allows the I <--> II, III <--> IV and IV <--> V phase transitions in YMnO(3), and also the I <--> II phase transition in BaTiO(3), 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.
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ورودعنوان ژورنال:
- Acta crystallographica. Section B, Structural science
دوره 65 Pt 4 شماره
صفحات -
تاریخ انتشار 2009