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. K 3 Lu(PO 4 ) 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 230K with a transformation to a monoclinic P2 1 /m space group symmetry, and the Lu retains six coordination. The second phase transition occurs at 130K, with a large change in the cell volume, keeping the same P2 1 /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 K 3 Lu(PO 4 ) 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 K 3 Lu(PO 4 ) 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.