Polymorphism, phase transitions, and thermal expansion of K3Lu(PO4)2

Highlights: • Alkali rare-earth double phosphates are candidate scintillators for γ-rays detection. • X-ray and neutron diffraction were used to study the structure of K3Lu(PO4)2 as a function of temperature. • Two low temperature phases transitions occur at 230 K and 130 K, both with monoclinic symmetry. • This is an unusual example of an isosymmetric phase transition with a coordination change, driven by temperature. -- Abstract: Alkali rare-earth double phosphates have been studied for use as long-wavelength scintillators for γ-ray detection using Si photodiodes. Single-crystal and powder X-ray diffraction (XRD) and powder neutron diffraction have been used to study the structure as a function of temperature. K3Lu(PO4)2 crystallizes with a hexagonal unit cell at room temperature, space group P3¯. The Lu ion is six-coordinated to the oxygen atoms of the phosphate groups. Two lower-temperature phases were characterized using single-crystal XRD and powder neutron diffraction. The first transition occurs at 230 K with a transformation to a monoclinic P21/m space group symmetry, and the Lu retains six coordination. The second phase transition occurs at 130 K, with a large change in the cell volume, keeping the same P21/m space group symmetry; however, one of the phosphate groups rotates to increase the coordination of the Lu ion to seven. This is an unusual example of an isosymmetric phase transition with a coordination change, driven by temperature. High-temperature powder neutron diffraction and high-temperature powder XRD have been used to study the thermal expansion of K3Lu(PO4)2 and indicate a large thermal expansion anisotropy. The crystallographic axes with largest changes account for the structural collapse, which rotates the phosphate group to increase the Lu coordination. The lowest temperature form of K3Lu(PO4)2 is the same as the room temperature form for all the lighter RE compounds of the same type, which is not surprising, given the lighter (larger) RE ions would prefer a higher coordination number