Synthesis and structure of LiCaPO₄ by combined X-ray and neutron powder diffraction

LiCaPO₄ has been synthesized and characterized by combined X-ray and neutron powder diffraction. The phase adopts the trigonal space group P3,c, a=7.5247(1) Å, c=9.9657(2) Å. The structure is composed of a threedimensional framework of vertex-sharing LIO₄ and PO₄ tetrahedra enclosing five-sided channels running parallel to the c axis, which are occupied by the Ca²⁺ ions. This structure is isotypic with LiNaSO₄ but is believed to be novel amongst phosphates. The combined use of X-ray and neutron scattering is found to improve significantly the determination of both the light (Li) and heavy (Ca) atom positional and thermal parameters, and it is proposed that this strategy may be useful in the structural characterization of more complex lithium-containing solids, containing heavier elements of similar neutron scattering length such as La and Sr, where neutron diffraction alone is insufficient to determine the site ordering uniquely.