Crystal chemistry of ( RE , A ) 2 M 3 O 7 compounds (RE=Y, lanthanide; A=Ba, Sr, Ca; M=Al, Ga)

Phases of general formula ( RE , A ) 2 M 3 O 7 (RE=lanthanide, A=Ca, Sr, Ba; 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, Ca 2 Al 2 SiO 7 , 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 M 3 O 7 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.