Synthesis and structure of LiCaPo4 by combined X-ray and neutron powder diffraction

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Abstract
LiCaPO4 has been synthesized and characterized by combined X-ray and neutron powder diffraction. The phase adopts the trigonal space group P31c, a= 7.5247(1)Å, c= 9.9657(2)Å. The structure is composed of a three-dimensional 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.