Ho2Te4O11 and Ho2Te5O13: Two telluriumdioxide-rich oxotellurates(IV) of trivalent holmium

Ho2Te4O11 (monoclinic, C2/c-, a = 1240.73(8) b = 511.21(3), c = 1605.84(9) pm, beta = 106.142(7)degrees Z = 4) and Ho2Te5O13 (triclinic, PT; a = 695.6 7(5), b = 862.64(6), c = 1057.52(7)pm, alpha = 89.057(6), beta = 86.825(6), gamma = 75.056(6)degrees. Z = 2) are obtained by the reaction of holmium s esquioxide with tellurium dioxide in appropriate molar ratios (Ho2O3: TeO2 = 1 :4 and 1 :5, respectively) in evacuated silica tubes within eight days at 800 degreesC. The application of cesium chloride (CsCl) as flux in about five times molar excess secures fast and complete reactions to the single- crystalline products aimed at. In the crystal structure of Ho2Te4O11 [HoO8] polyhedra are connected via oxygen edges thereby building up a network (2) (infinity){[Ho2O10](14-)} (001). On the other hand. the crystal structure o f Ho2Te5O13 exhibits oxygen-linked [(Ho1)O-8] and [(Ho2)O-7] polyhedra, whi ch form ribbons (1)(infinity){[(Ho1)(2-)(Ho2)(2)O-20](28-)} running along [ 100]. Common to both structures. however, is the stereochemical activity of the nonbonding electron pairs ("lone pairs") of all the of the Te4+ cation s (Tel and Te2 in Ho2Te4O11, Te1-Te5 in Ho2Te5O13) causing psi (1)-polyhedr al figures of coordination with 3 + 1, 4 and 3 + 2 oxygen atoms, respective ly, around the central atoms.