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 degrees C, which converts to a previously reported LaTaO4 polymorph at 1200 degrees 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 LnTaO(4) and Ln(2)Ta(2)O(7)(OH)(2) analogues of Ln=Pr, Nd to examine the relationship between cation size and the resulting phases. Published by Elsevier Inc.