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

Phases of general formula (REA)₂M₃O₇ (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, 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)₂M₃O₇ 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.