• bond-valence parameters;
  • lone-pair electrons;
  • polyhedral distortion

Bond-valence parameters r0 and b have been re-determined for TeIV—O: r0 = 1.9605 Å, b = 0.41; TeVI—O: r0 = 1.921 Å, b = 0.56; and TeIV—Cl: r0 = 2.3115 Å, b = 0.555. Bond distance data from 208 independent TeIV—O polyhedra, 118 TeVI—O polyhedra and 26 TeIV—Cl polyhedra were used, and all neighbours out to 3.5 Å were included. Root-mean square deviations of bond-valence sums on Te from ideal values were 0.1934, 0.1939 and 0.0865 v.u. The good fit for TeIV—O over a range of Te coordination numbers from 3 to 12 demonstrates that there is no essential difference in character between short `primary' Te—O bonds, oriented away from the Te lone pair, and longer `secondary' Te—O bonds on the same side of the Te atom as the lone pair. Comparison of bond-valence sums for Te—O polyhedra obtained using the new parameters with those calculated using earlier literature values shows that the new parameters give a narrower spread of calculated bond-valence sums, which means much closer to the formal valence for both oxidation states of tellurium.