Structural and kinetic study of phase transitions in LaYSi2O7

Phase transitions in LaYSi2O7 have been investigated as a function of temperature using XRD, NMR and TEM. Previously described empirical crystal structure guidelines based on average cation radius in rare-earth RE2Si2O7-type disilicates predict a stable tetragonal A-LaYSi2O7 polymorph at temperatures below 1500 °C. This study demonstrates that A-LaYSi2O7 is not thermodynamically stable at these temperatures and suggests that guidelines based on average cation size do not accurately describe the equilibrium behaviour of this silicate system. The A to G-type polymorph transition is extremely sluggish; complete transformation requires ∼250 h at 1200 °C, and more than 3 weeks of calcination at 1100 °C. This observation is important when this type of material is used as environmental barrier coating (EBC) of advanced ceramics. Analysis of XRD and TEM data reveal complete substitution of Y and La on the rare-earth cation sites in both LaYSi2O7 polymorphs, but indicate preferential site occupancies in the G-type polymorph.