Structural and crystal chemical properties of alkali rare-earth double phosphates

Abstract

When appropriately activated, alkali rare-earth double phosphates of the form: M3RE(PO4)2 (where M denotes an alkali metal and RE represents either a rare-earth element or Y or Sc) are of interest for use as inorganic scintillators for radiation detection at relatively long optical emission wavelengths. These compounds exhibit layered crystal structures whose symmetry properties depend on the relative sizes of the rare earth and alkali-metal cations. Single-crystal X-ray and powder neutron diffraction methods were used here to refine the structures of the series of rare-earth double phosphate compounds: K3RE(PO4)2 with RE = Lu, Yb, Er, Ho, Dy, Gd, Nd, Ce, plus Y and Sc – as well as the compounds: A3Lu(PO4)2, with A = Rb, and Cs. The double phosphate K3Lu(PO4)2 was reported and structurally refined previously, and it exhibited two lower-temperature phases. The compound K3Yb(PO4)2 reported here also exhibits a new second phase that occurs at T = 120 °C with a transformation to hexagonal P-3 space group symmetry and a Yb-ion coordination number reduction from seven to six. This latter result was confirmed using EXAFS. Comprehensive structural data and structural systematics are reported here for a number of alkali rare-earth double phosphates. Additionally, single-crystal growth methods for the preparation of large single crystals of these compounds are described, and the thermal expansion properties of the present series of alkali rare-earth double phosphates, as determined by both X-ray and neutron diffraction methods, are presented. These data represent a structural and thermal characteristics basis for continuing research on the use of alkali rare-earth double phosphates as scintillators for radiation detection – as well as for future studies of the fundamental phase properties of these compounds at elevated pressures.