Structural Transformations and Phenomenological Description
of the Formation of Phase States in Elpasolites Cs₂RbDyF₆
and Rb₂KB 'F₆ (B ' = Ho, Dy, Tb)

I. N. Safonov^a, *, S. V. Misyul’^a, M. S. Molokeev^b, and M. P. Ivliev^c

^a Siberian Federal University, Svobodnyi pr. 79, Krasnoyarsk, 660041 Russia

* e-mail: isafonov@sfu-kras.ru

^b Kirensky Institute of Physics, Siberian Branch of the Russian Academy of Sciences,
Akademgorodok 50–38, Krasnoyarsk, 660036 Russia

^c Research Institute of Physics, Southern Federal University,
pr. Stachki 194, Rostov-on-Don, 344090 Russia

Abstract—The structures of the cubic and monoclinic phases of an Rb₂KHoF₆ crystal from a series of elpas-
olites Rb₂KB'F₆ (B' = Ho, Dy, Tb), which undergoes a trigger phase transition, have been investigated using X-
ray powder diffraction. The critical and noncritical displacements of atoms in the Rb₂KHoF₆ structure have
been determined from the group-theoretical analysis of the complete condensate of order parameters. It has
been reliably established that a change in the symmetry due to the phase transition in this crystal can be repre-
sented in the form Fmm P12₁/n1. A phenomenological analysis of the conditions respon-
sible for the formation of phase states upon the phase transitions in elpasolites Cs₂RbDyF₆ and Rb₂KB'F₆ (B' =
Ho, Dy, Tb) has been carried out. Taking into account the structural data and using the phenomenological the-
ory, it has been found that the main factor determining the formation of phase states in these and related crystals
is the instability of the elpasolite structure with respect to rotational distortions of two types. It has been shown
that an important role in the formation of a sequence of structural transformations in these crystals is played by
the interaction of rotational order parameters and displacements of the cation from the center of a cuboctahedral
hole.

DOI: 10.1134/S1063783415030282

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