The energetics of lanthanum tantalate materials

Lanthanum tantalates are important refractory materials with application in photocatalysis, solid oxide fuel cells, and phosphors. Soft-chemical synthesis utilizing the Lindqvist ion, [Ta6O19]8−, has yielded a new phase, La2Ta2O7(OH)2. Using the hydrated phase as a starting material, a new lanthanum orthotantalate polymorph was formed by heating to 850 °C, which converts to a previously reported LaTaO4 polymorph at 1200 °C. The stabilities of La2Ta2O7(OH)2 (LaTa−OH), the intermediate LaTaO4 polymorph (LaTa-850), and the high temperature phase (LaTa-1200) were investigated using high-temperature oxide melt solution calorimetry. The enthalpy of formation from the oxides were calculated from the enthalpies of drop solution to be −87.1±9.6, −94.9±8.8, and −93.1±8.7 kJ/mol for LaTa−OH, LaTa-850, and LaTa-1200, respectively. These results indicate that the intermediate phase, LaTa-850, is the most stable. This pattern of energetics may be related to cation–cation repulsion of the tantalate cations. We also investigated possible LnTaO4 and Ln2Ta2O7(OH)2 analogues of Ln=Pr, Nd to examine the relationship between cation size and the resulting phases.