A non-twinned polymorph of CaTe2O5 from a hydrothermally grown crystal.

In contrast with the multiple twinning and/or domain formation found in the mica-like polymorphs of CaTe(2)O(5), calcium pentaoxidoditellurate(IV), that have been prepared by solid-state reactions and for which complete structure determinations have not been successful up to now, the crystal structure of a hydrothermally grown phase was fully determined from a non-twinned crystal. The structure is made up of alternating layers of Ca(2+) cations and of (2)(infinity)Te(2)O(5) anions stacked along [100]. The lone-pair electrons E of the Te(IV) atoms are stereochemically active and protrude into channels within the anionic layer. In comparison with analogous M(II)Te(2)O(5) structures (M = Mg, Mn, Ni or Cu) with ditellurate(IV) anions that are exclusively made up of corner-sharing TeO(x) (x = 3-5) polyhedra resulting in flat (2)(infinity)Te(2)O(5) layers, the anionic layers in CaTe(2)O(5) are undulating and are built of corner- and edge-sharing [TeO(4)] polyhedra.