Structures and anisotropic thermal expansion of the <i>α</i> , <i>β</i> , <i>γ</i> , and <i>δ</i> polymorphs of Y ₂ Si ₂ O ₇

Abstract

The α , β , γ , and δ polymorphs of Y 2 Si 2 O 7 were synthesized using sol-gel and solid-state methods. The structures of the α and γ polymorphs were determined by identification of isostructural rare-earth disilicates, and the structures were refined using Rietveld analysis of X-ray powder diffraction data. The α polymorph crystallizes in space group P 1 , with a =6.5872(6) Å, b =6.6387(7) Å, c =12.032(1) Å, α =94.501(7)°, β =90.984(8)°, γ =91.771(7)°, and volume=524.16(9) Å 3 . The γ form is described by space group P 2 1 / c , a =4.68824(5) Å, b =10.84072(9) Å, c =5.58219(6) Å, and γ =96.0325(3)°. The anisotropic thermal expansion of each phase was measured using high temperature diffraction up to 1200 or 1400 °C, depending on the stability of the polymorph. The thermal expansion is highly anisotropic for all polymorphs, with the low-expansion direction normal to the long axis of the corner-shared SiO 4 tetrahedra.