Synergetic Influence of Alkali-Metal and Lone-Pair Cations on Frameworks of Tellurites.

Four new tellurites were hydrothermally synthesized by the adjustment of different alkali-metal ions, and all of the compounds demonstrate 2D layer structures. Rb2Te4O9·2H2O in centrosymmetric space group Pccn features a new [Te4O9]∞2- anion sheet consisting of rare [Te8O22]12- groups. RbNaTe8O14(OH)6·8H2O in CS space group P1̅ exhibits a [Te4O9]∞2- anion layer by linkage of the TeO3 and TeO4 units. NaPb4Te4O12F located in CS space group P4/ n displays an unusual [PbTeO3]∞ neutral layer made up of PbO3 and TeO3 trigonal pyramids. RbK3Te8O18·5H2O lying in noncentrosymmetric (NCS) space group Cc shows a [Te4O9]∞2- anion layer composed of the TeO3 and TeO4 units; its second-harmonic-generation response is about 0.2 times that of KH2PO4; structure analysis and local dipole moment calculation verify that the weak polarization mostly from the [Te4O9]∞2- layer results from the inverse arrangement of TeO n units, and further theoretical calculation confirms that TeO n groups dominate the band gap of RbK3Te8O18·5H2O and optical properties. Meanwhile, systematic analyses of a series of metal tellurites reveal that the alkali-metal cations exert a considerable impact on polarization of the crystal structures, which puts forward a feasible idea about the design of new NCS materials.