Spectra of tetravalent chromium in calcium fluorophosphate

Publisher Website
Google Scholar
Abstract
The spectroscopic properties of Cr‐doped Ca5(PO4)3F known as Cr:FAP were examined using several different chemical and physical means. Results from analytical titration measurements indicate that chromium is tetravalent in the samples investigated. Based on electron‐spin resonance measurements at temperatures as low as 11 K, no trivalent chromium and only a trace of pentavalent chromium (0.5–10 ppm) was detected in the samples used for optical studies. We postulate that Cr4+ is stabilized in the crystal as a result of the small amount of Y2O3 that was added during the crystal growth process and that charge balance is achieved when Y3+ replaces Ca2+ and Cr4+ replaces P5+. Assuming tetravalent chromium, absorption, and emission spectra were interpreted by analyzing phonon sidebands and zero‐phonon transitions observed at various temperatures. The observed crystal‐field splitting is compared with calculated levels based on a Tanabe–Sugano diagram for quartet and doublet states of Cr4+(3d2) in tetrahedral symmetry. The parametrized Hamiltonian includes the free‐ion parameters for Cr4+(3d2) and an initial set of crystal‐field parameters obtained from a point‐charge lattice‐sum calculation where ionic distances were obtained from x‐ray crystallographic data. The parametrized Hamiltonian with crystal‐field terms in C4 symmetry, when diagonalized within the complete 3d2 configuration, yields calculated Stark levels in reasonable agreement with levels established from the experiment.