cations, respectively. So, A 2 FeSbO 6 , where A is Sr, Ca, Conditions for formation of a synthetic analog of the natural are perovskites (2), Mn 2 FeSbO 6 is ilmenite (3), and mineral melanostibite with the composition Mn 2 FeSbO 6 which Cu 2 FeSbO 6 is bixbyite (C-type) (4). The reason for morcrystallizes in the structural type of ilmenite and of some other photropism in this series of the same type compounds is compounds belonging to the series Mn 2 BSbO 6 , B ؍ Cr, V, Ga, a decrease in ionic radii of bivalent cations (1.18 and 1.00 Al, have been found. It has been established that none of these A ˚for Sr 2ϩ and Ca 2ϩ , respectively, 0.83 A ˚for Mn 2ϩ , and compounds is formed from simple oxides when their mixtures 0.73 A ˚for Cu 2ϩ for the coordination number 6 ( )).
are sintered in air at 900-1320؇C. The products of synthesis
The region of existence of the perovskite structure on under these conditions were binary oxides MnSb 2 O 6 , Mn 2 Sb 2 O 7 , the stability fields diagram of the ABO 3 type compounds and spinel phases of the MnB 2 O 4 composition. The compound can be extended by using high pressures and high tempera-Mn 2 FeSbO 6 with the structure of ilmenite has been synthesized tures in the process of synthesis. The compound MnVO 3 under a pressure of 3 GPa and temperature 900؇C. The lattice provides an example of the ilmenite-perovskite transforparameters of this phase (a ؍ 5,238(1), c ؍ 14,344(3) A ˚, hexagonal unit cell) approximate those determined for the natural mation at 45 kbar. Some information concerning polymormineral. A phase transition from the ilmenite to the perovskite phous transitions to the perovskite modification occurring modification was registered for Mn 2 FeSbO 6 at 5 GPa and 900؇C. under high pressure and high temperature conditions in The perovskite phase in the pure form has been obtained at 6 compounds, which in usual conditions form ilmenite, co-GPa and 900؇C. The orthorhombic unit cell with a ؍ 5.394(2), rundum, or pyroxene structures, is generalized in (7). The b ؍ 5,246(1), c ؍ 7,652(3) A ˚and ordered distribution of Fe 3؉ reason the perovskite structure is formed under high presand Sb 5؉ cations in octahedral sites of the perovskite structure sures has to do with a closer packing of atoms resulting was established for this phase. The other compounds from this from an increased coordination of anions around cations.
series also form ilmenite and perovskite structures under high On the other hand, the fact that some compounds, which pressures. In the process of high pressure synthesis it was also are not formed under usual conditions, can be synthesized found that all the investigated systems contain phases with the under high pressures is of considerable significance.
cubic pyrochlore structure, the composition of which was not identified.