Structure and ion exchange properties of tunnel type titanium silicates

A large number of titanium silicates are known to exist in the mineral realm and recently a considerable literature has developed concerned with the synthesis and properties of these and related compounds. This paper describes two such families that have tunnel structures filled with cations that may easily be exchanged. A sodium titanosilicate of ideal formula, Na₂Ti₂O₃(SiO₄)₂·2H₂O, is tetragonal and built up of Ti₄O₄ cubane-like structures linked together by silicate groups in the a and b directions in the form of a square. In the c-axis direction, the cubane groups are linked by oxo-groups to form a framework enclosing tunnels parallel to the c-axis direction. The several ion exchange sites were identified based upon X-ray diffraction studies and the reason for the great affinity of this compound for Cs⁺ elucidated. The second family of compounds have the general composition M₃H(TiO)₄(SiO₄)₃·4H₂O (M=alkali metal cation) and have the pharmacosiderite structure. The sodium or potassium phases are selective for Sr²⁺ and Cs⁺. These compounds are cubic and have similar Ti₄O₄ cubane-like groups. In this case, the connectivity via silicate groups extends along all three crystallographic axes equally. This change in crystal system has a profound effect upon the ion exchange behavior. This effect, as well as the effect of germanate substitutions for silicate, will be described.