Influence of cation species on thermal expansion of Y₂Si₂O₇–Gd₂Si₂O₇ solid solutions

Mixtures of Y₂Si₂O₇ and Gd₂Si₂O₇ were synthesized by solid-state reaction at 1600°C and characterized via in situ x-ray diffraction (XRD) to determine their coefficients of thermal expansion (CTE). All solid solutions within the system exhibited the orthorhombic δ-RE₂Si₂O₇ (Pna2₁) structure. Thermal expansion measurements of Y₂Si₂O₇ and Gd₂Si₂O₇ correlated well with reported values in literature, and all synthesized solid solutions exhibited CTEs between Y₂Si₂O₇ and Gd₂Si₂O₇. Generally, there was a slight decrease in CTE exhibited by the materials with increasing Gd₂Si₂O₇ content, with Gd₂Si₂O₇ having the lowest CTEs and Y₂Si₂O₇ the highest CTEs. The decrease in CTE was attributed to stronger bonds of Gd–O over Y–O, as determined by calculated crystal orbital Hamilton populations using density functional theory. However, such differences were very small and crystal structure was the dominating factor in CTE trends.