[4+2]-COORDINATED MOLYBDENUM IN THE CHAIN STRUCTURE OF Rb2Cu2(MoO4)3. S.F.Solodovnikov, Z.A.Solodovnikova, V.I.Alekseev, Institute of Inorganic Chemistry, Novosibirsk, Russia
The atomic arrangement of Rb2Cu2(MoO4)3 with a new structure type was determined. The main features of the structure are the following: (i) couples of infinite wolframite-like ribbons of CuO(4+2)-octahedra running along [010] and sharing vertices with two topologically different types of bridging MoO4-tetrahedra (fig.), (ii) the tetradentate MoO4-tetrahedra sandwiched between two such ribbons interact weakly with each other to give CN = 4 + 2 for molybdenum, (iii) the separate couples of the ribbons are connected in the structure by rubidium atoms.
The new structure type represented by A2Cu2(MoO4)3 (A = Rb, Cs) only completes the morphotropic series of double molybdates A2M2(MoO4)3 (A = K, Rb, Cs, Tl; M = Mg, Mn, Co, Ni, Cu, Zn, Cd) which were known to possess the structures of K2Zn2(MoO4)3, K2Mn2(MoO4)3 or K2Mg2(SO4)3 (langbeinite). The substantial difference of the Rb2Cu2(MoO4)3 type with a chain structure from the other structures containing frameworks of M2+O6-octahedra and MoO4-tetrahedra seems to be caused by stereochemical peculiarities of Cu(II).
Synthesis: the light green needle-like crystals were obtained by spontaneous crystallization of the mixture CuMoO4 + Rb2Mo2O7 from 650 to 400oC at cooling rate of 3o/hr.
Crystal data: monoclinic, sp. gr. C2/c, a = 27.698(2), b = 5.102(1), c = 19.292(1) , = 107.26(1)o, Z = 8, MoK, R = 0.016 for 882 I > 3(I).
Interatomic distances: Mo-O (tetrahedra) 1.708-1.814 , Mo-O ([4+2]-coordination) 1.708-1.859 + 2.545 + 2.853 , Cu-O ([4+2]-coordination) 1.914-2.743 , Rb-O 2.816-3.924 (CN = 9-10).