Crystal structures of the triple perovskites Ba<sub>2</sub>K<sub>2</sub>Te<sub>2</sub>O<sub>9</sub>and Ba<sub>2</sub>KNaTe<sub>2</sub>O<sub>9</sub>, and redetermination of the double perovskite Ba<sub>2</sub>CaTeO<sub>6</sub>
Acta crystallographica. Section E, Crystallographic communications