STRUCTURE AND PHASE TRANSITION OF CA(BD_4)_2 FROM COMBINED NEUTRON/X-RAY POWDER DIFFRACTION AND DENSITY FUNCTIONAL CALCULATIONS

摘要

The crystal structure of Ca(BD_4)_2 has been investigated by combined synchrotron radiation X-ray powder diffraction, neutron powder diffraction and ab-initio calculations. A variety of structures depending on the synthesis and temperature of the samples are observed in Ca(BD_4)_2 or Ca(BH_4)_2 samples. An hitherto unsolved tetragonal P-phase crystal structure of Ca(BD_4)_2, has been solved from combined X-ray and neutron powder diffraction data. Combined experimental and ab-initio calculations have shown that out of seven tetragonal structure candidates giving similar fit of the experimental data, the best description of the structure is with the space group P4_2/m. The previously reported orthorhombic α-phase of Ca(BD_4)_2 with space group Fddd has been confirmed and investigated. A preliminary structural model of a hitherto unsolved structure of an orthorhombic γ-phase of Ca(BD_4)_2 was determined with space group Pbca. Ab-initio calculations provide formation energies of the α-phase, β-phase, and γ-phase (based on preliminary model) of the same order of magnitude (ΔH≈0.15 eV). This indicates the possibility of coexistence of these phases at the same thermodynamical conditions.