Repulsive carbon–deuterium interactions in zirconium and titanium carbodeuterides

Zirconium and mixed zirconium/titanium carbodeuteride samples of nominal compositions ZrC0.44D0.89, Ti0.66Zr0.33C0.57D0.92 and Ti0.66Zr0.33C0.53D0.29 have been investigated by powder X-ray and neutron diffraction and joint Rietveld structure refinements. They contain two types of phases, one crystallizing with a face-centred cubic (fcc) metal atom arrangement and the other with a hexagonal close-packed (hcp) metal atom arrangement having trigonal symmetry. While the pure Zr sample contains both a cubic and a trigonal phase, the mixed Zr/Ti samples contain either a cubic phase only (C/D-poor sample), or two trigonal phases having different Ti/Zr ratios (C/D-rich sample). The carbon atoms occupy octahedral interstices in nearly ordered (hcp) or disordered (fcc) arrangements, and the deuterium atoms occupy either tetrahedral (hcp) or octahedral (fcc) interstices. In the hcp phase metal octahedra around carbon atoms contract while metal tetrahedra around deuterium atoms expand. The structure results are consistent with the assumption of repulsive interactions between non-metal atoms resulting in their longest possible separation (D–D > 2.1 Å, C–D > 2.6 Å).