In the fluorite CaF₂ structure (Fm-3m, Z = 4), the Ca atoms are surrounded by 8 fluorines at the vertices of a cube while the F atoms are surrounded by 4 Ca atoms to form a tetrahedron. It is the archetypical structure of many difluorides, dioxides, intermetallics, etc. [1-4]. The structural relation is the closest when the cation:anion ratio is equal to 1:2 with a random arrangement of cations or anions. Ordering induces low-symmetry polymorphs. Another type of superstructures is formed due to deficiency in cations or anions.

In this contribution, studies on structures, stabilities, and phase transitions of fluorides with fluorite superstructures using x-ray powder diffraction will be presented. The experimental methods include in situ measurements in diamond anvil cells and a Paris-Edinburgh press as well as synthesis in a multi-anvil.

Most of the flurite-related materials are hosts for solid state lasers. Since the optical properties mainly depend on ordering of the cations and their crystalline surrounding, a detailed study of crystal structures as a function of pressure and temperature can provide useful information on their solid state chemistry and additional data for crystal field considerations.

1. D.J.M. Bevan, Acta Cryst. A 36, 889 (1980).
2. A.F. Wells, Structural Inorganic Chemistry, Clarendon Press, Oxford (5th Edition, 1984).
3. B.G. Hyde & S. Andersson, Inorganic Crystal Structures, John Wiley & Sons, New York (1989).
4. V.P. Korsun et al., Russ. J. Inorg. Chem. 52, 613 (2007).

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1. Thermal behaviour of the BaMF₄ ferroics
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