Crystal structure and bond valence of CaH2 from neutron powder diffraction data

The crystal structure of CaH2, has been stitched from neutron powder diffraction (NPD) at 295 K in a non-deuterated sample; a good quality NPD pattern was obtained in spite of the hydrogen incoherent scattering. The structure was refined by the Rietveld method in the Prima space group (No 62), Z = 4, with unit-cell parameters a = 5.9600(1), b = 3.6006(7) and c = 6.8167(1) angstrom. The two kinds of crystallographically independent H atoms, H1 and H2, are located in tetrahedral and square-pyramidal cavities, respectively, while Ca ions are nine-fold coordinated to hydrogen atoms. The average < Ca-H1 > and < Ca-H2 > bond lengths are 2 279 and 2.544 angstrom. respectively. Bond valence calculations show that Ca-H1 bonds are under compressive stress, whereas Ca-H2 bonds undergo tensile stress in a structure with a relatively high global Instability index. It is also remarkable that the displacement factors for H2 are significantly larger than for H1, suggesting an increased lability for the Ca-H2 bonds. We provide with an analysis of the isotope effect, by comparing the present results on CaH2 with literature data on CaD2; we indeed observe a higher distortion of the H1 and H2 coordination polyhedra with respect to the deuteride, as observed in other isostructural dihydrides.