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Thermochemistry of the alkali rare-earth double phosphates, A3RE(PO4)2

Published online by Cambridge University Press:  03 March 2011

Sergey V. Ushakov
Affiliation:
Thermochemistry Facility and NEAT ORU University of California at Davis, Davis, California 95616
Alexandra Navrotsky*
Affiliation:
Thermochemistry Facility and NEAT ORU University of California at Davis, Davis, California 95616
J. Matt Farmer
Affiliation:
Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
Lynn A. Boatner
Affiliation:
Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
*
a) Address all correspondence to this author. e-mail: [email protected]
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Abstract

The formation enthalpies for alkali rare-earth compounds of the type K3RE(PO4)2 where RE = Sc, Y, Lu, Er, Ho, Dy, Gd, Nd, or Ce and for A3Lu(PO4)2 compounds with A = K, Rb, or Cs were determined using high-temperature oxide-melt solution calorimetry. Structural phase transitions were observed and characterized using differential scanning calorimetry and high-temperature x-ray diffraction. The formation enthalpy of the K3RE(PO4)2 phases from oxides becomes more exothermic with increasing rare-earth radius for the K3RE(PO4)2 series and with increasing alkali radius for the A3Lu(PO4)2 compounds. The K3RE(PO4)2 phases are stable with respect to anhydrous K3PO4 and REPO4. The monoclinic K3RE(PO4)2 compounds undergo a reversible phase transition to a hexagonal (glaserite-type) structure with a phase transition temperature that increases from −99 to 1197 °C with increasing RE ionic radius going from Lu to Ce.

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Articles
Copyright
Copyright © Materials Research Society 2004

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