Structure data for K2MgH4 and Rb2CaH4 and comparison with hydride and fluoride analogues

Neutron powder diffraction data on the deuterides of the title compounds confirm their tetragonal K2NiF4 type structures (space group I4/mmm; K2MgD4: a=4.0361(1) Angstrom, c=13.5921(5) Angstrom, c/a=3.37; Rb2CaD4: a=4.4976(2) Angstrom, c=14.8294(7) Angstrom, c/a=3.30; T=293 K). The alkaline earth centred deuterium octahedra in the magnesium compound are compressed along the tetragonal axis (Mg-D=2.00 Angstrom axial, 2.02 Angstrom basal), whereas those in the calcium compound are elongated (Ca-D=2.30 Angstrom axial, 2.25 Angstrom basal). A survey of isostructuraI hydride and fluoride analogues shows that big alkali cations tend to flatten the anion octahedra and to increase the cell parameter ratios c/a, whereas big alkaline earth cations tend to elongate the anion octahedra and to decrease c/a. (C) 2001 Elsevier Science B.V. All rights reserved.