On the deposition of Cr_3-δO₄ spinel particles upon laser-heated pedestal growth of Cr:YAG fiber

The microstructures of Cr⁴⁺:YAG crystal fibers grown along [1 1 1] direction by laser-heated pedestal growth method were studied by optical and analytical electron microscopy. Curved growth ledges were found on the round side surface of the fiber grown at a relatively high speed. In contrast, nonepitaxial Cr_3-δO₄ crystallites with spinel-type structure were formed on the {1 1 2} side surfaces, via an intermediate surface melt, when the fibers were grown at a lower speed. The deposition of Cr_3-δO₄, rather than segregation of Cr and Ca codopants, as of concern to the Cr⁴⁺ fluorescence, can be rationalized by a high crystal field stabilization of octahedrally coordinated Cr³⁺ attained in rather refractory and close packed oxide.