STRUCTURAL AND OPTICAL-PROPERTIES OF CALCIUM NEODYMIUM HEXAALUMINATES SINGLE-CRYSTALS, POTENTIAL LASER MATERIALS

The structural and optical properties of calcium-neodymium hexaaluminates crystals Ca1-xNdxMgx Al12-xO19 (labeled Ca1-xNdx) with a magnetoplumbite (MP) structure are investigated. The floating zone method is used to grow single crystals in the composition range 0.1 ≤ x ≤ 0.7, although for high calcium content, the melting of the compounds is no longer congruent. The X-ray structural determination, optical absorption at 4 K, and ESR investigation agree in the localization of Nd3+ at the regular large cations site of the MP structure with axial (D3h) symmetry. A set of crystal field and free ion parameters which fits the absorption spectrum of Nd3+ in this site is calculated. When x increases, Nd3+ ions tend to occupy also a second site with lower symmetry. Moreover some anomalous behavior observed in the absorption and ESR spectra at high neodymium concentration may be related to Nd3+-Nd3+ ion pairing. Fluorescence intensity and lifetime measurements as a function of the x value are reported. There is evidence of strong cross-relaxation between neighboring neodymium ions for high x values. The results obtained for the Ca1-xNdx compounds can be extended to other series in which Nd3+ is replaced by another lanthanide ion. Preliminary investigations have been performed with Pr3+ and are also reported. © 1992.