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. RbTeO·2HO in centrosymmetric space group Pccn features a new [TeO] anion sheet consisting of rare [TeO] groups. RbNaTeO(OH)·8HO in CS space group P1̅ exhibits a [TeO] anion layer by linkage of the TeO and TeO units. NaPbTeOF located in CS space group P4/ n displays an unusual [PbTeO] neutral layer made up of PbO and TeO trigonal pyramids. RbKTeO·5HO lying in noncentrosymmetric (NCS) space group Cc shows a [TeO] anion layer composed of the TeO and TeO units; its second-harmonic-generation response is about 0.2 times that of KHPO; structure analysis and local dipole moment calculation verify that the weak polarization mostly from the [TeO] layer results from the inverse arrangement of TeO units, and further theoretical calculation confirms that TeO groups dominate the band gap of RbKTeO·5HO 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.