CRYSTAL STRUCTURE OF Mn-LOW SOBOLEVITE, ITS PHYSICAL PROPERTIES AND THE PROBLEM OF POLYTYPISM
S.V. Krivovichev1,2,@, A.S. Osipov1, M.S. Avdontseva2, J.H. Chen3, G.O. Samburov1, O.F. Goychuk1, I.V. Pekov4, T.L. Panikorovskii1,2, Y.Q. Li3
1 Nanomaterials Research Centre, Kola Science Center, Russian Academy of Sciences, Apatity, Russia
2 Department of Crystallography, Institute of Earth Sciences, St. Petersburg State University, St. Petersburg, Russia;
3 School of Resources, Environment and Materials, Guangxi University, Nanning China
4 Faculty of Geology, Moscow State University, Moscow, Russia)
Keywords: sobolevite, crystal structure, antiperovskite, density functional theory, Kola peninsula, Arctic
Abstract
Sobolevite with low Mn content from the Karnasurt deposit (Lovozero, Kola peninsula) has been characterized by electron microprobe analysis, single-crystal structure refinement, high-temperature X-ray diffraction and density functional theory (DFT) calculations. The chemical formula can be written as Na7.04(Ca0.87Mg0.16)S=1.03(Ti1.48Zr0.20Mn2+0.18 Nb0.10Fe2+0.06)S=2.02Si2.05P2.02O17.12F0.88. The crystal structure refined to R1 = 0.037 (P21/c, a = 7.0908(3), b = 5.4108(2), c = 40.6179(19) Å, b = 93.095(4)o, V = 1556.11(11) Å3) is based upon the [Ti2O2[Si2O7](PO4)]5-
titanosilicate-phosphate HOH layers (TS blocks). The interlayer space is occupied by the AC complex formed by Na+, Ca2+
and Mg2+ cations along with (P1O4)3- groups and F- anions. The F- anions are coordinated octahedrally by Na and Ca to form antiperovskite [FA3] chains (A = Ca, Na) running parallel to the b axis. The crystal chemical formula of sobolevite is (Na6.92Ca0.92Mg0.16)S=2.00(Ti1.46Zr0.18Mn2+0.15Nb0.15Fe2+0.06)S=2.00(Si2O7)(PO4)(F0.81O0.19)S=1.00, which corresponds to the idealized formula Na7CaTi2O2[Si2O7](PO4)2F and allows to consider it as a polymorph of quadruphite. The relations between sobolevite and quadruphite are pseudo-polytypic. The thermal behavior of sobolevite is typical for layered structures corrected by shear deformations. According to DFT calculations, sobolevite exhibits p-type semiconductor behavior with a band gap of approximately 2.75 eV. Altering the Ti/Nb ratio influences the band structure, inducing magnetism in the crystal and transforming it from a semiconductor to a semimetal or even a metal. The material exhibits strong absorption in the ultraviolet region, with an absorption coefficient reaching up to 2×105 cm-1. The overall structural architecture of sobolevite combines features of both titanosilicates and antiperovskite-type structures, which makes it an interesting example of hybrid structures with potentially interesting functional properties.
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