Calcium and Strontium amide are ionic compounds crystallising in a tetragonally distorted anatase structure-type at ambient temperatures. The amide ions (NH 2 ± / ND 2 ± ) resemble water molecules in structure and in charge distribution. By means of temperature dependent neutron diffraction investigations weak super-structure reflections were observed at temperatures below 90 K (Ca(ND 2 ) 2 ) and 60 K (Sr(ND 2 ) 2 ), respectively, indicating the existence of a so far unknown low-temperature (LT) phase. Using high resolution neutron powder diffraction at temperatures below 10 K the structure was determined 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: Ca(ND 2 ) 2 at 10 K a = 7.257(2) A Ê , b = 7.2434(2) A Ê , c = 6.300(1) A Ê , b = 124.73(1)°Sr(ND 2 ) 2 at 5 K a = 7.6950(1) A Ê , b = 7.68374(9) A Ê , c = 6.6324(3) A Ê , b = 124.917(2)°.
Their structure is closely related to the tetragonal HTphase, but an ordering of the amide ions occurs due to freezing of a lattice mode which is dominated by the librational motion of the amide ions in the {1 0 0} planes of the HTphase.