Structure determination of a low temperature phase of calcium and strontium amide by means of neutron powder diffraction on Ca(ND2)(2) and Sr(ND2)(2)

Calcium and Strontium amide are ionic compounds crystallising in a tetragon ally distorted anatase structure-type at ambient temperatures. The amide io ns (NH2-/ND2-) resemble water molecules in structure and in charge distribu tion. By means of temperature dependent neutron diffraction investigations weak super-structure reflections were observed at temperatures below 90 K ( Ca(ND2)(2)) and 60 K (Sr(ND2)(2)), respectively, indicating the existence o f a so far unknown low-temperature (LT) phase. Using high resolution neutro n powder diffraction at temperatures below 10 K the structure was determine d for both compounds. The LT-phases are isotypic and crystallise monoclinic in the space group P2(1)/c with four formula units within the unit cell: C a(ND2)(2) at 10 K a = 7.257(2) Angstrom, b = 7.2434(2) Angstrom, c = 6.300( 1) Angstrom, beta = 124.73(1)degrees Sr(ND2)(2) at 5 K a = 7.6950(1) Angstr om, b = 7.68374(9) Angstrom, c = 6.6324(3) Angstrom, beta = 124.917(2)degre es. Their structure is closely related to the tetragonal HT-phase, but an order ing of the amide ions occurs due to freezing of a lattice mode which is dom inated by the librational motion of the amide ions in the (1 0 0) planes of the HT-phase.