A new series of rare earth aluminum borates has been synthesized from a molten solution with the general formula RX3(BO3)4. Single crystals of these materials have been grown where R has been yttrium or a rare earth and X has been aluminum or chromium. All the members of this series made to date have been found to be piezoelectric as indicated by a positive test when placed in a Giebe-Scheibe circuit. The yttrium-aluminum borate, when doped with rare earth ions, and the rare earth-aluminum borates show a bright, sharp line fluorescence when excited by U.V. with the emitted fluorescence concentrated in only a very few lines.

The crystals are trigonal with a rhombohedral lattice and contain one formula weight in the rhombohedral unit cell. The yttrium-aluminum borate and the rare earth-aluminum borates crystallize as hexagonal rods elongated in the c direction and are colorless except where the rare earth ion imparts its characteristic color. The rare earth-chromium borates occur as distinct rhombohedra and are dark green in color. Optical examination of the yttrium-aluminum borate shows it to be uniaxial (–) with ι= 1.775 ± 0.005 and ϵ= 1.775 α 0.005. H (Mohs’ scale) = ca. 7.5, and the pycnometric density is 3.701 g/cc as compared to a theoretical density of 3.72 g/cc based on x-ray measurements.

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