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J. Matt Farmer and Lynn A. Boatner Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received 26 October 2003; accepted 14 April 2004)

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.

I. INTRODUCTION

Double phosphates of the rare earths with the general formula M3RE(PO4)2 (where M denotes an alkali metal and RE represents either a rare-earth element or Y or Sc) are promising materials for ultra-fast gamma-ray scintillators when activated with Ce due to their high light yield and rapid decay times.1,2 For more effective synthesis and processing of these technologically important materials, fundamental structural and thermodynamic data are needed. Data on the chemical stability and structural phase transformations for these compounds are fragmentary. To fill this gap, the thermochemistry of single crystals of double phosphates of potassium and various rare earths from Ce to Lu (plus Y and Sc) and the rubidium and cesium double phosphates with lutetium is investigated in the current work. The structures of these compounds are closely related to the glaserite structure. Glaserite (or Aphthitalite) is an anhydrous sulfate (K,Na)3Na(SO4)2, found in fumaroles of volcanoes.3 The glaserite structure and its derivatives are very common in the A2X2O4 molybdates, tungstates, phosphates, and silicates.3–8 The A3RE(XO4)2 compounds with A ⳱ Rb, K, and RE ⳱ rare earth, Sc, or Y, and X ⳱ P, As, or V were

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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0283 J. Mater. Res., Vol. 19, No. 7, Jul 2004

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first reported by Melnikov et al.9–11 These workers prepared the double phosphates by a reaction of the orthophosphate REPO4 with A2CO3 in the presence of H3PO4. Melnikov et al. divided the synthesized phases into three groups: hexagonal (isostructural to glaserite) and two distinct monoclinic structures. They found that the Rb double salts Rb3RE(XO4)2 (RE ⳱ rare earth, Sc, Y; and X ⳱ P, As, V) are isostructural with glaserite as are the ternary cesium rubidium rare-earth phosphates Rb2CsRE(PO4)2 with RE ⳱ Sm to Lu, Y. Rghioui12 reported that K2CsYb(PO4)2 also crystalli