Solid solubility and phase transitions in the system LaNb{sub 1-x}Ta{sub x}o{sub 4}

Journal Article · Wed Oct 15 04:00:00 EDT 2008 · Journal of Solid State Chemistry

DOI:https://doi.org/10.1016/j.jssc.2008.06.032· OSTI ID:21212035

Vullum, Fride; Nitsche, Fabian; Selbach, Sverre Magnus ^[1]; Grande, Tor ^[1]

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1. Department of Materials Science and Engineering, Norwegian University of Science and Technology, Trondheim 7491 (Norway)

The solid solubility between LaNbO{sub 4} and LaTaO{sub 4} was investigated by X-ray diffraction, and a two-phase region was observed in the composition region LaNb{sub 1-x}Ta{sub x}O{sub 4} where 0.4{<=}x{<=}0.8. Single-phase LaNb{sub 1-x}Ta{sub x}O{sub 4} (0{<=}x{<=}0.4) with the monoclinic Fergusonite structure at ambient temperature, was observed to transform to a tetragonal Scheelite structure by in-situ high-temperature X-ray diffraction, and the phase transition temperature was shown to increase with increasing Ta-content. This ferroelastic to paraelastic second-order phase transition was described by Landau theory using spontaneous strain as an order parameter. The thermal expansion of LaNb{sub 1-x}Ta{sub x}O{sub 4} (0{<=}x0.4) was shown to be significantly higher below the phase transition than above. Single-phase LaNb{sub 1-x}Ta{sub x}O{sub 4} (0.8{<=}x{<=}1) with another monoclinic crystal structure at ambient temperature was shown to transform to an orthorhombic crystal structure by X-ray diffraction and differential scanning calorimetry. The phase transition temperature was observed to decrease with decreasing Ta-content. Finally, orthorhombic LaTaO{sub 4} could also be transformed to monoclinic LaTaO{sub 4} at ambient temperature by applying a uniaxial pressure of 150-170 MPa, reflecting the lower molar volume of monoclinic LaTaO{sub 4}. - Graphical abstract: LaNb{sub 1-x}Ta{sub x}O{sub 4} (0{<=}x{<=}0.4) with the monoclinic Fergusonite structure, was observed to transform to a tetragonal Scheelite, while monoclinic LaNb{sub 1-x}Ta{sub x}O{sub 4} (0.8{<=}x 1) was shown to transform to an orthorhombic crystal structure.

• OSTI ID: 21212035
• Journal Information: Journal of Solid State Chemistry, Journal Name: Journal of Solid State Chemistry Journal Issue: 10 Vol. 181; ISSN 0022-4596; ISSN JSSCBI
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
AMBIENT TEMPERATURE
CALORIMETRY
LANTHANUM COMPOUNDS
MONOCLINIC LATTICES
NIOBIUM COMPOUNDS
ORDER PARAMETERS
ORTHORHOMBIC LATTICES
PHASE TRANSFORMATIONS
PRESSURE RANGE MEGA PA 100-1000
SOLID SOLUTIONS
SOLIDS
SOLUBILITY
TANTALATES
THERMAL EXPANSION
TRANSITION TEMPERATURE
X-RAY DIFFRACTION