CsTe2O(6-x): novel mixed-valence tellurium oxides with framework-deficient pyrochlore-related structure

Structures of CsTe₂O(6-x) phases were investigated by single-crystal X-ray diffraction and neutron powder diffraction. Stoichiometric CsTe₂O₆ is a mixed-valence Cs₂Te⁴⁺Te₃⁶⁺O₁₂ compound with a rhombohedral pyrochlore-type structure where there is complete order of Te⁴⁺ and Te⁶⁺. On heating, this compound develops significant electrical conductivity. As CsTe₂O₆ becomes oxygen deficient above 600 °C, the rhombohedral pyrochlore-type structure is replaced by a cubic pyrochlore-type structure with disordered Te⁴⁺/Te⁶⁺ and oxygen vacancies. However, for CsTe₂O(6-x) phases prepared at 500 °C, the observed pyrochlore-type structure has symmetry. The Te⁴⁺ and O vacancies are all on chains running along the b axis, and the maximum value of x observed is about 0.3. At still higher values of x a new compound was discovered with a structure related to that reported for Rb₄Te₃⁴⁺Te₅⁶⁺O₂₃.