Theory of an Fm3m I4/m Structural Phase Transition
in an Rb₂KScF₆ Crystal

V. I. Zinenko* and N. G. Zamkova

Kirenskii Institute of Physics, Siberian Division, Russian Academy of Sciences, Krasnoyarsk, 660036 Russia
*e-mail: zvi@post.krascience.rssi.ru

Received December 30, 1999

Abstract—An Rb₂KScF₆ crystal having an elpasolite structure undergoes a sequence of Fm3m I4/m
P12₁/n1 structural phase transitions where the transition to the tetragonal phase is associated with “rotation” of
the ScF₆ octahedron. An effective Hamiltonian is constructed to describe the Fm3m I4/m transition using
the approximation of a local mode for which we selected a “soft mode” whose eigenvector corresponds to the
rotation of the octahedron. The effective Hamiltonian also includes the relationship between the local mode and
the homogeneous elastic strains. The parameters of the effective Hamiltonian were determined using the gen-
eralized Gordon–Kim model of an ionic crystal which allows for the deformability and polarizability of the
ions. The thermodynamic properties of a system with this model Hamiltonian were investigated using the
Monte Carlo method. The calculated phase transition temperature of 250 K is almost the same as the experi-
mental value (252 K). The tetragonal phase remains stable as far as T = 0 K and a second transition (to the mon-
oclinic phase) cannot be obtained using this effective Hamiltonian. This suggests that if the transition to the tet-
ragonal phase is mainly associated with “rotations” of the octahedrons, in order to describe the phase transition
to the monoclinic phase the effective Hamiltonian must allow for additional degrees of freedom mainly associ-
ated with the motion of rubidium ions. © 2000 MAIK “Nauka/Interperiodica”.

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