The ''unquenchable'' high pressure form of Ca(OD) 2 [Ca(OH) 2 II] has been synthesized at 9 GPa and 400Β°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 monoclinic P2 1 /c with a β«Ψβ¬ 5.3979(4) A s , b β«Ψβ¬ 6.0931(4) A s , c β«Ψβ¬ 5.9852(4) A s , β«Ψβ¬ 103.581(6)Β°, Z β«Ψβ¬ 4 at 1 atm and 11 K. R wp β«Ψβ¬ 2.8%, R p β«Ψβ¬ 1.9%, reduced 2 β«Ψβ¬ 6.6. for 117 variables. The calcium and oxygen substructure is intermediate between that in -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 2 O distance of 1.91 A s , and an O-D 2 O angle of 153.2Β°; the other D atom has 3 second-neighbor oxygens near 2.6 A s away.