The energetics of lanthanum tantalate materials

Journal Article · Mon Nov 15 00:00:00 EST 2010 · Journal of Solid State Chemistry

DOI:https://doi.org/10.1016/j.jssc.2010.08.024· OSTI ID:21504018

Nyman, May ^[1]; Navrotsky, Alexandra ^[2]

---

1. Sandia National Laboratory, POB 5800, Albuquerque, NM 87185 (United States)
2. Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California at Davis, Davis, CA 95616 (United States)

Lanthanum tantalates are important refractory materials with application in photocatalysis, solid oxide fuel cells, and phosphors. Soft-chemical synthesis utilizing the Lindqvist ion, [Ta{sub 6}O{sub 19}]{sup 8-}, has yielded a new phase, La{sub 2}Ta{sub 2}O{sub 7}(OH){sub 2}. Using the hydrated phase as a starting material, a new lanthanum orthotantalate polymorph was formed by heating to 850 {sup o}C, which converts to a previously reported LaTaO{sub 4} polymorph at 1200 {sup o}C. The stabilities of La{sub 2}Ta{sub 2}O{sub 7}(OH){sub 2} (LaTa-OH), the intermediate LaTaO{sub 4} 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{sub 4} and Ln{sub 2}Ta{sub 2}O{sub 7}(OH){sub 2} analogues of Ln=Pr, Nd to examine the relationship between cation size and the resulting phases. - Graphical abstract: The energetics of three lanthanum tantalates were investigated by the high-temperature oxide melt solution calorimetry. The enthalpies 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 La{sub 2}Ta{sub 2}O{sub 7}(OH){sub 2}, LaTaO{sub 4} (850 {sup o}C), and LaTaO{sub 4} (1200 {sup o}C), respectively. These results indicate that the intermediate phase, LaTaO{sub 4} (850 {sup o}C), is the most stable in energy. Display Omitted

• OSTI ID: 21504018
• Journal Information: Journal of Solid State Chemistry, Vol. 183, Issue 11; Other Information: DOI: 10.1016/j.jssc.2010.08.024; PII: S0022-4596(10)00358-0; Copyright (c) 2010 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved.; ISSN 0022-4596
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
CALORIMETRY
CATIONS
FORMATION HEAT
HEATING
LANTHANUM COMPOUNDS
OXIDES
PHOSPHORS
PHOTOCATALYSIS
SOLID OXIDE FUEL CELLS
SYNTHESIS
TANTALATES
TEMPERATURE RANGE 1000-4000 K
CATALYSIS
CHALCOGENIDES
CHARGED PARTICLES
DIRECT ENERGY CONVERTERS
ELECTROCHEMICAL CELLS
ENTHALPY
FUEL CELLS
HIGH-TEMPERATURE FUEL CELLS
IONS
OXYGEN COMPOUNDS
PHYSICAL PROPERTIES
RARE EARTH COMPOUNDS
REACTION HEAT
REFRACTORY METAL COMPOUNDS
SOLID ELECTROLYTE FUEL CELLS
TANTALUM COMPOUNDS
TEMPERATURE RANGE
THERMODYNAMIC PROPERTIES
TRANSITION ELEMENT COMPOUNDS