Ca2Te3O8, a new phase in the CaO-TeO2 system.

Single crystals of dicalcium octa-oxidotritellurate(IV), Ca2Te3O8, were obtained from a CsCl/NaCl melt with CaO and TeO2 as educts in the molar ratio of 1:2. Ca2Te3O8 crystallizes isotypically with Pb3Te2O8 and is comprised of two unique Ca, four Te and eight O sites. One calcium cation has eight and the other nine coordination partners. Both coordination polyhedra are considerably distorted. Two kinds of oxotellurate(IV) anions with the same formula [Te3O8]4- are present. One is an infinite zigzag chain anion consisting of pairs of [TeO4] bis-phenoids linked to a trigonal-pyramidal [TeO3] group with a connectivity of [(TeO1/1O2/2)(TeO2/1O2/2)2] n , while the other is a finite anion made up of one central [TeO4] bis-phenoid linked to two [TeO3] trigonal pyramids and has a connectivity of [(TeO2/1O1/2)2(TeO2/2O2/1)]. In the crystal, the anions are organized in layers extending parallel to (100). Adjacent layers are held together by the calcium cations to define a three-dimensional framework structure.