Crystal Structure of B-Type Tm₂Si₂O₇ (≡Tm₄[Si₃O₁₀][SiO₄])

Abstract

Single crystals of B-type thulium oxodisilicate Tm 2 Si 2 O 7 were obtained by the reaction of Tm, S and RbCl with the wall of a silica tube in an attempt to synthesize ternary alkalimetal thulium sulfides (e.g. Rb 3 Tm 7 S 12 ). It crystallizes triclinically in space group P1̄(a = 655.91(5), b = 659.04(5), c = 1195.32(9) pm, α = 94.361(8), β = 91.102(8), γ = 92.005(8)◦), with four formula units of Tm 2 Si 2 O 7 per unit cell. Instead of pyro-anionic [Si 2 O 7 ] 6 − groups the title compound contains both ortho-silicate tetrahedra [SiO 4 ] 4 − and catena-trisilicate units [Si 3 O 10 ] 8 −. Therefore the formula Tm 4 [Si 3 O 10 ][SiO 4 ] (Z = 2) appears more adequate. The four crystallographically independent Tm 3+ cations show coordination numbers from six to eight with distorted octahedral, bicapped trigonal prismatic and square antiprismatic oxygen coordination spheres.