Structural aspects of pyrochlores stability: influence of the cations in [8] A, [6] B positions and lattice parameters
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REVIEW ARTICLE
Structural aspects of pyrochlores stability: influence of the cations in [8]A, [6]B positions and lattice parameters Alexander F. Redkin1 · Andrey M. Ionov2 · Alexey N. Nekrasov1 · Rais N. Mozhchil2 · Olga V. Reutova3 Received: 11 May 2020 / Accepted: 28 September 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The influence of mole fraction and nature of the cations in positions [8]A and [6]B of the pyrochlores containing N a+, Ca2+, 2+ 4+ 5+ 5+ 4+ 5+ 4+ 3+ 4+ 5+ 6+ Cd , U , Nb , Ta , Ti , Sb , Zr , V , V , W , W synthesized under hydrothermal conditions at 800 °C, 200 MPa and controlled oxygen fugacity in NaF solution of 1.0 mol·kg−1 H2O on lattice parameters has been considered. The unit cell parameters of the fluorpyrochlores where for N b5+ or T a5+ were substituted by other cations showing distinct linear [8] variations from the effective cations radii in positions A and [6]B. Based on the literary data analysis, the two-parameter nonlinear equation describing dependence of pyrochlore lattice parameters on the cation radii for the oxide-pyrochlores has been proposed. Keywords Pyrochlores · Hydrothermal synthesis · Cations substitution · Lattice parameters
Introduction The nomenclature of the pyrochloric supergroup of minerals, relating to the cubic structure and the space group Fd3m , approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA) is discussed in detail in Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00269-020-01114-6) contains supplementary material, which is available to authorized users. * Alexander F. Redkin [email protected] http://www.iem.ac.ru Andrey M. Ionov [email protected] http://www.issp.ac.ru Olga V. Reutova [email protected] 1
Academician D.S. Korzhinskii Institute of Experimental Mineralogy RAS, 4 Academician Ossipyan Str, 142432 Chernogolovka, Moscow District, Russia
2
Institute of Solid State Physics RAS, 2 Academician Ossipyan Str, 142432 Chernogolovka, Moscow District, Russia
3
Geology Department of the M.V. Lomonosov, Moscow State University, GSP‑1, Leninskie Gory, Moscow 119991, Russia
the works (Hogarth 1977; Lumpkin, Ewing 1995; Atencio et al. 2010) and recently (Atencio et al. 2017) has been supplemented with previously known fluoride mineral species. Of particular practical interest as indicators of geochemical processes, catalysts in organic synthesis, research of special magnetic properties (magnetic frustration) and other fields of science and technology is represented by the pyrochloresupergroups formed by oxide compounds of the general formula
A2−m B2 O6 Y1−n , such as pyrochlore, microlite, romeite, betafite, (elsmoreite) and other Ti-Zr-solid solutions with the pyrochlore structure in which A are the cations with charges from + 1 to + 4 and ionic radii 1.1 ÷ 1.5 Å, B are the cations with charges from + 3 to + 5 and ionic radii 0.7 ÷ 0.9 Å, Y are O2-, OH-
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