Crystal structure refinements of stoichiometric NiSe and NiSe.

Single crystals of NiSe (trinickel diselenide) and NiSe (nickel selenide) with stoichiometric chemical compositions were grown in evacuated silica-glass tubes. The chemical compositions of the single crystals of NiSe and NiSe were determined by scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDS). The crystal structures of NiSe [rhombohedral, space group R32, a = 6.02813 (13), c = 7.24883 (16) Å, Z = 3] and NiSe [hexagonal, space group P6/mmc, a = 3.66147 (10), c = 5.35766 (16) Å, Z = 2] were analyzed by single-crystal X-ray diffraction and refined to yield R values of 0.020 and 0.018 for 117 and 85 unique reflections, respectively, with F > 4σ(F). R32 is a Sohncke type of space group where enantiomeric structures can exist; the single-domain structure obtained by the refinement was confirmed to be correct by a Flack parameter of -0.05 (2). The existence of Ni-Ni bonds was confirmed in both compounds, in addition to the Ni-Se bonds. The value of the atomic displacement parameter (mean-square displacement) of each atom in NiSe was larger than that in NiSe. The larger amplitude of the atoms in NiSe corresponds to longer Ni-Se and Ni-Ni bond lengths in NiSe than in NiSe. The Debye temperatures, θ, estimated from observed mean-square displacements for Ni and Se in NiSe, were 322 and 298 K, respectively, while those for Ni and Se in NiSe were 246 and 241 K, respectively. The existence of large cavities in the structure and the weak bonding force are likely responsible for the brittle and soft nature of the NiSe crystal.