ESR and Optical Spectroscopy of CrO43− in Chlorospodiosite, Ca2PO4Cl

Abstract

Studies of the spectral properties of CrO₄³⁻ ion in Ca₂PO₄Cl (chlorospodiosite) single crystals confirm the structural results established by single‐crystal x‐ray structure analyses. Detailed electron spin resonance measurements of a single crystal of Ca₂(PO₄, CrO₄)Cl are described and the appropriate spin‐Hamiltonian parameters determined. It is shown that the unpaired electron (3d¹) in its ground state occupies a d_z2 orbital, which is largely concentrated on the chromium atom. The visible and ultraviolet absorption spectrum of a single crystal of Ca₂PO₄Cl containing CrO₄³⁻ is described and shown to be similar to the spectrum of MnO₄²⁻, which had been interpreted on the basis of a Ballhausen and Liehr molecular‐orbital level scheme. The infrared absorption spectra show more splitting of the vibrational bands of the chromate (V) compound (Ca₂CrO₄Cl) than of the phosphate (Ca₂PO₄Cl), in agreement with the structural results which indicated a greater distortion of the CrO₄³⁻ than that of the PO₄³⁻ tetrahedra. The infrared spectra of spodiosites and related compounds with apatite structure are compared.