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Under ambient condition PdSe2 has the PdS2-type structure. The crystal structure of PdSe2 under pressure (up to 30 GPa) was investigated at room temperature by X-ray diffraction in an energy-dispersive configuration using a diamond anvil cell with a mixture of water/ethanol/methanol as a pressure transmitting medium. A reversible structural transition from the PdS2-type to the pyrite-type structure occurs around 10 GPa, and the applied pressure reduces the spacing between adjacent 2/proportional to [PdSe2] layers of the PdS2-type structure to form the three-dimensional lattice of the pyrite-type structure. First principles and extended Hückel electronic band structure calculations were carried out to confirm the observed pressure-induced structural changes. We also examined why the isoelectronic analogues NiSe2 and PtSe2 adopt structures different from the PdS2-type structure on the basis of qualitative electronic structure considerations.

The temperature- and pressure-driven solid state valence tautomeric transformations of [CoII(3,5DTBSQ)2(phen)]·C6H5CH) and nonsolvated [CoII(3,5DTBSQ)2(phen)] have been studied by EXAFS and XANES. When the toluene solvate sample is cooled it converts in the 270−215 K region (T c = 240 K) from a high-spin CoII complex at higher-temperatues to a low-spin CoIII complex at temperatures below ∼215 K. EXAFS data indicate that the HS-CoII tautomer has bond lengths of Co−O = 2.08 ± 0.02 Å and Co−N = 2.13 ± 0.02 Å, whereas the LS-CoIII tautomeric complex is smaller with Co−O = 1.91 ± 0.02 Å and Co−N = 1.93 ± 0.02 Å. The XANES spectra obtained in the 270−215 K interconversion region were simulated to obtain a plot of the fraction of HS-CoII complexes, n(HS-CoII), vs temperature. This plot of n vs T agrees with that obtained from magnetic susceptibility data for the toluene solvate. In the XANES spectral region transitions characteristic of either a HS-CoII or a LS-CoIII complex could also be used to monitor the valence tautomeric conversion. The nonsolvated phen complex does not convert at low temperatures (2 K) to a LS-CoIII tautomer. However, both complexes could be driven from the larger HS-CoII form to the smaller LS-CoIII form upon application of pressure. EXAFS and XANES data readily detected these changes. For the toluene solvate, this conversion occurred within the 0.075−0.700 GPa range with a midpoint of P c↑ = 0.370 GPa. The nonsolvated complex required higher pressures of 0.10−2.5 GPa with P c↑ = 1.1 GPa. These are the first reported examples of valence tautomeric transformations driven by pressure. Both the thermally driven and pressure-driven valence tautomeric transformations are found to be reversible.