Enthalpy of Formation of Rare-earth Silicates Y₂SiO₅ and Yb₂SiO₅ and N-containing Silicate Y₁₀(SiO₄)₆N₂

The enthalpies of formation of two rare-earth silicates (Y₂SiO₅ and Yb₂SiO₅) and a N-containing rare-earth silicate Y₁₀(SiO₄)₆N₂ have been determined using high-temperature drop solution calorimetry. Alkali borate (52 wt% LiBO₂·48 wt% NaBO₂) solvent was used at 800 °C, and oxygen gas was bubbled through the melt. The nitrogen-containing silicate was oxidized during dissolution. The standard enthalpies of formation are for Y₂SiO₅, Yb₂SiO₅, and Y₁₀(SiO₄)₆N₂, respectively, –22868.54 ± 5.34, –22774.75 ± 8.21, and –14145.20 ± 16.48 kJ/mol from elements, and –52.53 ± 4.83, –49.45 6 ± 8.35, and –94.53 ± 11.66 kJ/mol from oxides (Y₂O₃ or Yb₂O₃, SiO₂) and nitride (Si₃N₄). The silicates and N-containing silicate are energetically stable with respect to binary oxides and Si₃N₄, but the N-containing silicate may be metastable with respect to assemblages containing Y₂SiO₅, Si₃N₄, and SiO₂. A linear relationship was found between the enthalpy of formation of a series of M₂SiO₅ silicates from binary oxides and the ionic potential (z/r) of the metal cation.