The crystal structure of trimounsite-(Y), (Y,REE) (sub 2) Ti (sub 2) SiO (sub 9) ; an unusual TiO (sub 6) -based titanate chain

The intriguing crystal structure of type material trimounsite-(Y), (Y,REE) (sub 2) Ti (sub 2) SiO (sub 9) , a = 12.293(2), b = 11.124(2), c = 4.8610(10) Aa, beta = 95.69(3) degrees , V = 661.5(2) Aa (super 3) , Z = 4, was determined using single-crystal diffractometer data (MoKalpha X-radiation, CCD area detector) and has been refined in space group P2 (sub 1) /c to R(F) = 3.45% for 1821 "observed" reflections with F (sub 0) >4sigma (F (sub 0) ). The unique structure contains two non-equivalent Ti and (Y,REE) sites (REE = rare earth element), one Si and nine O atoms. The dominant element of the structure is a spiral-like chain composed of edge-sharing, strongly distorted TiO (sub 6) octahedra. Each octahedron shares four edges with four neighbouring octahedra. The chain can also be thought of as composed of edge-sharing Ti (sub 2) O (sub 10) dimers in a obliquely staggered arrangement. Ti-O distances in both TiO (sub 6) octahedra are similar and exhibit a range from 1.759 to 2.179 Aa. Shortest Ti-Ti distances in the chains range between 3.098 Aa (2x Ti1-Ti1), and 3.242 and 3.277 Aa (Ti1-Ti2). The chains are aligned parallel to [001], i.e., the morphological elongation. They are decorated, in a staggered arrangement, with isolated, slightly distorted SiO (sub 4) tetrahedra. These are connected to two TiO (sub 6) octahedra via a common O ligand. The two non-equivalent (Y,REE) sites separate the chains from each other along both the a- and b-axis. The two sites are seven-fold coordinated, and occupied by Y atoms which are each substituted by REE (predominantly Dy, Er, Yb) to a very similar degree ( approximately 24%). A titanate chain identical to that in trimounsite-(Y) is found in Ag (sub 2) TiO (sub 3) , while a closely related vanadate chain is present in Ag (sub 4-x) V (sub 4) O (sub 12) . The structure is only remotely similar to those of the known synthetic rare earth titanosilicates and chemically related minerals.