Artículos SCI

Phase transitions in LaYSi ₂O ₇ 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 RE ₂Si ₂O ₇-type disilicates predict a stable tetragonal A-LaYSi ₂O ₇ polymorph at temperatures below 1500°C. This study demonstrates that A-LaYSi ₂O ₇ 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 ~250h 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 LaYSi ₂O ₇ polymorphs, but indicate preferential site occupancies in the G-type polymorph.

Er³⁺–TiO₂ synthesized by a surfactant free hydrothermal method exhibits good photoactivities under sun-like excitation for the degradation of phenol. The presence of Er³⁺ does not affect the structural and morphological features of the TiO₂ significantly. The best photocatalytic performance was attained for the samples with 2 wt% of Er. Different photocatalytic runs indicated that the incorporation of the Er³⁺ cation would be responsible for the enhanced photocatalytic activity, which participates in different mechanisms under UV and NIR excitation.