PEROVSKITE SOLID SOLUTIONS — A MONTE CARLO STUDY OF THE DEEP EARTH ANALOGUE (K, Na)MgF3

perovskite structural phase transitions Monte Carlo neighborite parascandolaite.

HIGH-FIELD SOLID-STATE 35Cl NMR IN SELENIUM(IV) AND TELLURIUM(IV) HEXACHLORIDES, РЕЛЯТИВИСТСКИЕ ЭФФЕКТЫ В ОКСИДАХ И ГАЛОГЕНИДАХ ПЕРЕХОДНЫХ И ПОСТПЕРЕХОДНЫХ МЕТАЛЛОВ, ...

Here we report a detailed exchange Monte Carlo study using a classical ionic model of the model perovskite parascandolaiteneighborite (K,Na)MgF3 solid solution and its end-members for temperatures in the range 300—1000 K and pressures from 0—8 GPa. <...> Full account is taken of the local environment of the individual cations, clustering and thermal effects. <...> Properties considered include the crystal structure, phase transitions, the thermodynamics of mixing and the non-ideality of the solid solution. <...> Clustering of the potassium ions is examined via a short-range order parameter. <...> Energy differences between different phases can be small and subtle cation ordering effects can be often crucial in determining phase stability and thermodynamic and chemical properties. <...> This solution is not only an excellent test of any theoretical model, but also serves as a useful analogue for the silicate perovskite (Mg, Fe)(Al,Si)O3, which is a dominant phase under the conditions of the lower mantle of the Earth (pressure > 25 GPa, temperature > 2000 K) [ 1—4]. <...> Because of the difficulties of generation of lower mantle conditions experimentally, possible structural phase transitions and substitution mechanisms in silicate perovskites still remain the subject of extensive debates, and both experimental and theoretical studies [ 5—10]. <...> Neighborite NaMgF3 [ 11 ] is isoelectronic and isostructural with MgSiO3; the ratios of the formal cation charges are the same in both compounds (1:2) and the ratio of their ionic radii are about the same. <...> KMgF3 was until very recently known only as a synthetic crystal but has now been identified as the new mineral parascandolaite, found as a volcanic sublimate at Vesuvius [ 12 ]. <...> KMgF3 is a cubic perovskite (Fig. 1, a), while in NaMgF3 there is an orthorhombic distortion such that the Mg—F—Mg bridges linking the MgF6 octahedra are not linear (Fig. 1, b). <...> Cubic perovskite structure AMF3 (a), Representative tilting of MF6 octahedra in an orthorhombic perovskite (b) rules (the so-called tolerance factor) is possible [ 13, 14 ]. <...> The exact nature and order of structural transitions along the Na1–xKxMgF3 series continue to be the subject of a large number of experimental and theoretical studies [15—26 ]. <...> Smith et al. [ 18 ] have complemented <...>

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