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 La ₂Ta₂O₇(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 cationcation 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.