Investigation of the Crystallization Features, Atomic Structure, and Microstructure of Chromium-Doped Monticellite

A series of Cr4+:CaMgSiO4 single crystals is grown using floating zone melting, and their microstructure, composition, and crystal structure are investigated. It is shown that regions with inclusions of second phases, such as forsterite, akermanite, MgO, and Ca4Mg2Si3O12, can form over the length of the sample. The composition of the single-phase regions of the single crystals varies from the stoichiometric monticellite CaMgSiO4 to the solid solution Ca(1-x)Mg(1+x)SiO4 (x = 0.22). The Cr:(Ca0.88Mg0.12)MgSiO4 crystal is studied using X-ray diffraction. It is revealed that, in this case, the olivine-like orthorhombic crystal lattice is distorted to the monoclinic lattice with the parameters a = 6.3574(5) angstrom, b = 4.8164(4) angstrom, c = 11.0387(8) angstrom, beta = 90.30(1)degrees, Z = 4, V = 337.98 angstrom(3), and space group P2(1)/c. In the monoclinic lattice, the M(l) position of the initial olivine structure is split into two nonequivalent positions with the center of symmetry, which are occupied only by Mg2+ cations with the average length of the Mg-O bond R-av = 2.128 angstrom. The overstoichiometric Mg2+ cations partially replace Ca2+ cations (in the M(2) position of the orthorhombic prastructure) with the average bond length of 2.347 angstrom in the [(Ca,Mg)-O-6] octahedron. The average distance in SiO4 distorted tetrahedra is 1.541 angstrom.