STRUCTURAL BEHAVIOR OF PALLADIUM(II) OXIDE AND A PALLADIUM SUBOXIDE AT HIGH-PRESSURE - AN ENERGY-DISPERSIVE X-RAY-DIFFRACTION STUDY

The behavior of PdO at high pressure was studied by energy-dispersive x-ray diffraction of samples contained in diamond anvil cells. PdO und ergoes a first-order transition at about 12 GPa. The new phase is tetr agonal and of similar cell dimensions to the low-pressure phase. Howev er, it is more compressible along c and much harder along a. The volum e change is 1.7%. It is likely that the new phase has the rocksalt str ucture, tetragonally elongated due to the low-spin d(8) electron confi guration of palladium (II). The zero-pressure cell parameters and bulk moduli are (low pressure phase) a = 3.042(1) Angstrom, c = 5.351(3) A ngstrom, K = 280+/-52 GPa; (high pressure phase) a = 2.982 Angstrom, c = 5.383 Angstrom, K = 545+/-20 GPa. One sample prepared was found to be a mixture of PdO with a cubic material [Fm 3m, a = 4.043(5) Angstro m at ambient], deduced to be a suboxide PdOx with x approximately 0.21 -0.27. Under pressure, the two phases reacted reversibly: PdO was cons umed and x increased with increasing pressure. This led to expansion o f the suboxide unit cell with pressure up to 1.73 GPa, above which pre ssure the calculated value of x remained approximately constant and th e lattice constant decreased in the usual fashion. It is evidently pos sible to construct mixtures of these two phases which show zero total change in the lattice constant of the cubic phase over a selected rang e of pressure.