Crystal structures of lazulite-type oxidephosphates TiIIITiIV3O3(PO4)3 and MIII4TiIV27O24(PO4)24 (MIII=Ti, Cr, Fe)

Single crystals of the oxidephosphates TiIIITiIV3O3(PO4)3 (black), CrIII4TiIV27O24(PO4)24 (red-brown, transparent), and FeIII4TiIV27O24(PO4)24 (brown) with edge-lengths up to 0.3 mm were grown by chemical vapour transport. The crystal structures of these orthorhombic members (space group F2dd ) of the lazulite/lipscombite structure family were refined from single-crystal data [TiIIITiIV3O3(PO4)3: Z=24, a=7.3261(9) A, b=22.166(5) A, c=39.239(8) A, R1=0.029, wR2=0.084, 6055 independent reflections, 301 variables; CrIII4TiIV27O24(PO4)24: Z=1, a=7.419(3) A, b=21.640(5) A, c=13.057(4) A, R1=0.037, wR2=0.097, 1524 independent reflections, 111 variables; FeIII4TiIV27O24(PO4)24: Z=1, a=7.4001(9) A, b=21.7503(2) A, c=12.775(3) A, R1=0.049, wR2=0.140, 1240 independent reflections, 112 variables). For TiIIITiIVO3(PO4)3 a well-ordered structure built from dimers [TiIII,IV2O9] and [TiIV,IV2O9] and phosphate tetrahedra is found. The metal sites in the crystal structures of Cr4Ti27O24(PO4)24 and Fe4Ti27O24(PO4)24, consisting of dimers [MIIITiIVO9] and [TiIV,IV2O9], monomeric [TiIVO6] octahedra, and phosphate tetrahedra, are heavily disordered. Site disorder, leading to partial occupancy of all octahedral voids of the parent lipscombite/lazulite structure, as well as splitting of the metal positions is observed. According to Guinier photographs TiIII4TiIV27O24(PO4)24 (a=7.418(2) A, b=21.933(6) A, c=12.948(7) A) is isotypic to the oxidephosphates MIII4TiIV27O24(PO4)24 (MIII: Cr, Fe). The UV/vis spectrum of Cr4Ti27O24(PO4)24 reveals a rather small ligand-field splitting Δo=14,370 cm-1 and a very low nephelauxetic ratio β=0.72 for the chromophores [CrIIIO6] within the dimers [CrIIITiIVO9]. - Graphical abstract: Single crystals of the oxidephosphates TiIIITiIV3O3(PO4)3 (black), CrIII4TiIV27O24(PO4)24 (red-brown, transparent), and FeIII4TiIV27O24(PO4)24 (brown) with edge-lengths up to 0.3 mm were grown by chemical vapour transport. The crystal structures of these orthorhombic members of the lazulite/lipscombite structure family were refined from single-crystal data