The ''unquenchable'' high pressure form of Ca(OD) 2 [Ca(OH) 2 II] has been synthesized at 9 GPa and 400 o C and recovered to ambient pressure at cryogenic temperatures. The structure was determined from powder neutron diffraction data using the Rietveld technique. The symmetry is monoclinicP2 1 /cwitha=5.3979(4) A,b=6.0931(4) A,c=5.9852(4) A,β=103.581(6) o ,Z=4 at 1 atm and 11 K.R w p =2.8%,R p =1.9%, reducedχ 2 =6.6. for 117 variables. The calcium and oxygen substructure is intermediate between that inα-PbO 2 and that in fluorite; it was previously described as isostructural with baddeleyite (ZrO 2 ), but it is more accurately described as isostructural with EuI 2 . This structure is distinguished by 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 bonding to O, with a second neighbor D ... O distance of 1.91 A, and an O-D ... O angle of 153.2 o ; the other D atom has 3 second-neighbor oxygens near 2.6 A away.