The crystal structure of trimounsite-(Y), (Y,REE)₂Ti₂SiO₉: an unusual TiO₆-based titanate chain

The intriguing crystal structure of type material trimounsite-(Y), (Y,REE)₂Ti₂SiO₉, a = 12.293(2), b = 11.124(2), c = 4.8610(10) Å, β = 95.69(3)°, V = 661.5(2) ų, Z = 4, was determined using single-crystal diffractometer data (MoKα X-radiation, CCD area detector) and has been refined in space group P2_1/c to R(F) = 3.45 % for 1821 ‘observed’ reflections with F₀ > 4σ(F₀). 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 TiO₆ octahedra. Each octahedron shares four edges with four neighbouring octahedra. The chain can also be thought of as composed of edge-sharing Ti₂O₁₀ dimers in a obliquely staggered arrangement. Ti-O distances in both TiO₆ octahedra are similar and exhibit a range from 1.759 to 2.179 Å. Shortest Ti-Ti distances in the chains range between 3.098 Å (2x Til-Til), and 3.242 and 3.277 Å (Til-Ti2). The chains are aligned parallel to [001], i.e., the morphological elongation. They are decorated, in a staggered arrangement, with isolated, slightly distorted SiO₄ tetrahedra. These are connected to two TiO₆ 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 (∼ 24 %). A titanate chain identical to that in trimounsite-(Y) is found in Ag₂TiO₃, while a closely related vanadate chain is present in Ag_4-xV₄O₁₂. The structure is only remotely similar to those of the known synthetic rare earth titanosilicates and chemically related minerals.