Fluorite M_1xR_xF_2+x Phases (M = Ca, Sr, Ba; R = Rare Earth
Elements) as Nanostructured Materials

B. P. Sobolev*, A. M. Golubev**, and P. Herrero***

* Shubnikov Institute of Crystallography, Russian Academy of Sciences,
Leninski pr. 59, Moscow, 119333 Russia

e-mail: sobolevb@crossovers.com

** Moscow State Technical University, Moscow, Russia

*** Instituto de Ciencia de Materiales, Madrid, Spain

Received March 20, 2002

Abstract—The structural defects (M²⁺ and R³⁺ in the noncubic environment of F, interstitial F, and anion
vacancies) in nonstoichiometric M_1xR_xF_2+x crystals with the CaF₂ structure form {M₈[R₆F₆₈₆₉]} superclus-
ters of nanometer linear dimensions. This fact allows one to classify the M_1xR_xF_2+x phases as nanostructured
materials (NSM). The superclusters concentrate rare-earth ions (R³⁺ = RE). In a M_1xR_xF_2+x crystal with the
fluorite cation motif, two chemically different parts can be separated: the R³⁺-depleted matrix and the
R³⁺enriched clusters. The M_1xR_xF_2+x phases are the first NSM among fluorides; they constitute a new type
of these materials in which different chemical compositions of the matrix and nanoinclusions are combined
with their isostructurality and coherent conjugation of the crystal lattices. Superclusters can also form associ-
ates with linear dimensions of tens or hundreds of angstroms. A model is suggested which describes the main
characteristic of such NSMs. These materials behave as single crystals in X-ray, neutron, and electron diffrac-
tion experiments. The influence of microheterogeneity on some physical properties of the M_1xR_xF_2+x phases
is also considered. © 2003 MAIK “Nauka/Interperiodica”.

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