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DY: Dynamik und Statistische Physik

DY 50: Gitter- und Spindynamik

DY 50.3: Vortrag

Freitag, 26. März 1999, 10:00–10:15, R4

Symmetry-adapted microscopic Hamiltonian for the phase transition sequence in A₂BX₄ compounds — •Boris Neubert¹, Jorge Íñiguez González², Iñigo Etxebarria², Mois Aroyo¹, and Juan Manuel Pérez Mato¹ — ¹Universidad del País Vasco, Facultad de Ciencias, Física de la Materia Condensada, Apartado 644, E-48080 Bilbao — ²Universidad del País Vasco, Facultad de Ciencias, Física Aplicada II, Apartado 644, E-48080 Bilbao

The phase transition sequence (P6₃/mmc→) Pnam → IC → Pna2₁ observed in the class of A₂BX₄ compounds (e.g. Rb₂ZnCl₄) is used to explain the basic concepts for the derivation of an effective symmetry-adapted microscopic Hamiltonian. The latter is a function of pseudo spins and local modes which are centered at the 2c Wyckoff positions of the hexagonal phase and transform according to irreducible representations of the site symmetry group 6m2. The local modes induce the symmetry of the unstable normal modes (negative eigenenergies of the force constant matrix) and thus are expected to expand the subspace of atomic displacements relevant for the transitions. Lowest-order couplings of the model variables to themselves and to elastic strains are rigourously derived by group theoretical methods. The values of all parameters as well as the inner structure of the local modes are determined making use of a semiempiric force model with atom-atom potentials. Possible treatments of the Hamiltonian are sketched. Supported by the DAAD (Deutscher Akademischer Austauschdienst, Grant D/98/14573) and the Government of the Basque Country (Project PI97/71).