Description
Calcium orthovanadate Ca3(VO4)2 belongs to a large family of whitlockite-structure orthovanadates. They crystallize in the R3c space group where the Ca atoms are located in five inequivalent sites. In the unsubstituted crystals a small fraction (typically up to ~10%) of Ca atoms can be replaced by others, of valences from +1 to +4, without a change of structure. In such cases, the corresponding structural modification consists of a partial or full replacement of Ca atoms at one (substitutionally ordered compound) or more of Ca sites (disordered compound) by the substituting atom. The whitlockite-type orthophosphates and orthovanadates jointly substituted with one divalent and one trivalent metal are of interest since they are considered for applications in optoelectronics, being potentially useful for white light generation.
In this work we have focused on the calcium orthovanadate doped with Ni atoms, namely the Ca10Ni0.5(VO4)7 crystal. Our aim was to determine the host site for Ni ions. The perfect technique for such a case is the X-ray absorption fine structure spectroscopy (XAFS) which probes the local neighborhood around specific element. The XAFS measurements were performed at the ASTRA beamline at the SOLARIS synchrotron (Kraków, Poland). The Ni K-edge had been measured in the fluorescence mode at room temperature.
The analysis revealed that the Ni atoms mainly locate at M5 site, the only regular octahedral site for Ca, with six oxygen atoms in the nearest neighborhood.
The authors acknowledge SOLARIS Centre for the access to the ASTRA beamline, under the provision of the Polish Ministry of Science and Higher Education project "Support for research and development with the use of research infra-structure of the National Synchrotron Radiation Centre SOLARIS” under contract no. 1/SOL/2021/2.
