Crystal chemistry of (RE,A)(2)M3O7 compounds (RE=Y, lanthanide; A=Ba, Sr, Ca; M=Al, Ga)

Phases of general formula (RE,A)(2)M3O7 (RE=lanthanide, A=Ca, Sr, Pa; M=Ga, Al) are of interest for their nonlinear optical properties and have potential as solid state lasers. Their structures have been refined using X-ray and neutron diffraction data and are related to gehlenite, Ca2Al2SiO7, With M cations occupying Al and Si sites and RE and A cations sharing the Ca site with no evidence for ordering, although they occur in the ratio 1:1 because of stoichiometry; the M cations are tetrahedrally coordinated. (RE,Ba)(2)M3O7 compounds cannot be synthesized for M=Al or for RE cations smaller than Sm; this limitation is believed to be due to size mismatch between cations within the structure. (C) 1999 International Centre for Diffraction Data. [S0885-7156(99)00501-1].