THE STRUCTURE OF HIGH-PRESSURE CA(OD)(2)-II FROM POWDER NEUTRON-DIFFRACTION - RELATIONSHIP TO THE ZRO2 AND EUI2 STRUCTURES

The ''unquenchable'' high pressure form of Ca(OD)(2) [Ca(OH)(2)II] has been synthesized at 9 GPa and 400 degrees C and recovered to ambient pressure at cryogenic temperatures. The structure was determined from powder neutron diffraction data using the Rietveld technique, The symm etry is monoclinic P2(1)/c with a = 5.3979(4) Angstrom, b = 6.0931(4) Angstrom, c = 5.9852(4) Angstrom, beta = 103.581(6)degrees, Z = 4 at 1 atm and 11 K. R-wp = 2.8%, R-p = 1.9%, reduced chi(2) = 6.6. for 117 variables. The calcium and oxygen substructure is intermediate between that in alpha-PbO2 and that in fluorite; it was previously described as isostructural with bad-deleyite (ZrO2), but it is more accurately d escribed as isostructural with EuI2. This structure is distinguished b y the presence of a 3(6) anion net parallel to (100). Only one of the two kinds of D atoms in the structure shows appreciable hydrogen bondi ng to O, with a second neighbor D ... O distance of 1.91 Angstrom, and an O-D ... O angle of 153.2 degrees; the other D atom has 3 second-ne ighbor oxygens near 2.6 Angstrom away. (C) 1997 Academic Press.