Compressibility of the nitridosilicate SrYb[Si₄N₇] and the oxonitridoaluminosilicates MYb[Si_4-xAl_xO_xN_7-x] (x = 2; M = Sr, Ba)

The compressibilities of the nitridosilicate SrYb[Si₄N₇] and the oxonitridoaluminosilicates MYb[Si_4-xAl_xO_xN_7-x] (x = 2; M = Sr, Ba) were investigated by in situ high-pressure X-ray powder diffraction. Pressures up to 42 GPa were generated using the diamond-anvil cell technique. The title compounds are structurally stable to the highest pressure obtained. A fit of a third-order Birch-Murnaghan equation-of-state to the p-V data results in V₀ = 302.91 (6) Å³, B₀ = 176 (2) GPa and B' = 4.4 (2) for SrYb[Si₄N₇]; V₀ = 310.4 (1) Å³, B₀ = 161 (2) GPa and B' = 4.6 (2) for SrYb[Si_4-xAl_xO_xN_7-x]; and V₀ = 317.3 (5) Å³, B₀ = 168 (2) GPa and B' = 4.7 (2) for BaYb[Si_4-xAl_xO_xN_7-x]. While the linear compressibilities of the a and c axes of BaYb[Si_4-xAl_xO_xN_7-x] are very similar up to 30 GPa, distinct differences were observed for SrYb[Si₄N₇] and SrYb[Si_4-xAl_xO_xN_7-x], with the c axis being the most compressible axis. In all of the investigated compounds the bulk compressibility is dominated by the compression behaviour of the tetrahedral network, while the size of the substituted cation plays a minor role.