RESUMEN
We report a new phase transition in cobalt from the magnetic varepsilon(hcp) to a beta(fcc) phase, likely nonmagnetic, at 105 GPa. It occurs martensitically in an extended pressure region between 105 and 150 GPa without any apparent volume change. The fcc phase of Co is in systematic accordance with the 4d and 5d counterparts. The pressure-volume isotherm of beta-Co resembles those of alpha(fcc)-Ni and varepsilon(hcp)-Fe within 1%. The phase diagram of cobalt suggests that the fcc stability increases with increasing occupancy of d-band electrons from Fe to Co to Ni.
RESUMEN
We present resonant inelastic x-ray scattering and x-ray emission spectroscopy results on Gd metal to 113 GPa which suggest Kondo-like aspects in the delocalization of 4f electrons. Analysis of the resonant inelastic x-ray scattering data reveals a prolonged and continuous delocalization with volume throughout the entire pressure range, so that the volume-collapse transition at 59 GPa is only part of the phenomenon. Moreover, the Lgamma1 x-ray emission spectroscopy spectra indicate no apparent change in the bare 4f moment across the collapse, suggesting that Kondo screening is responsible for the expected Pauli-like behavior in magnetic susceptibility.
RESUMEN
We present evidence for an isostructural, first-order Mott transition in MnO at 105+/-5 GPa, based on high-resolution x-ray emission spectroscopy and angle-resolved x-ray diffraction data. The pressure-induced structural and spectral changes provide a coherent picture of MnO phase transitions from paramagnetic B1 to antiferromagnetic distorted B1 at 30 GPa, to paramagnetic B8 at 90 GPa, and to diamagnetic B8 at 105+/-5 GPa. The last is the Mott transition, accompanied by a significant loss of magnetic moment, an approximately 6.6% volume collapse and the insulator-metal transition as demonstrated by recent resistance measurements.
RESUMEN
The stability of CO(2) phases has been investigated up to 50 GPa and 750 K by in situ Raman spectroscopy and visual observations using externally heated diamond-anvil cells. A new phase (CO(2)-II) exists above 20 GPa and 500 K, which can be quenched to ambient temperature. The vibrational spectrum of this new CO(2) polymorph suggests the dimeric pairing of molecules. Based on the present in situ data and previous laser-heating results, we present new constraints for the phase diagram of carbon dioxide to 50 GPa and 2000 K. We find that carbon dioxide exhibits dramatic changes, both in the molecular configuration and in the nature of intermolecular interaction at high pressures and temperatures.
RESUMEN
A nonlinear molecular carbon dioxide phase IV was discovered by laser heating CO2-III (Cmca) between 12 and 30 GPa, followed by quenching to 300 K. The Raman spectrum of quenched CO2-IV exhibits a triplet bending mode nu2(O = C = O) near 650 cm (-1), suggesting a broken inversion symmetry because of bending. The 650 cm (-1) bending modes soften with increasing pressure, indicating an enhanced intermolecular interaction among neighboring bent CO2 molecules. At 80 GPa, the low-frequency vibron collapses into high-frequency phonons, and CO2-IV becomes an extended amorphous solid.
RESUMEN
An extended-solid phase, carbon dioxide phase V (CO2-V), was synthesized in a diamond anvil cell by laser heating the molecular orthorhombic phase, carbon dioxide phase III, above 40 gigapascals and 1800 kelvin. This new material can be quenched to ambient temperature above 1 gigapascal. The vibration spectrum of CO2-V is similar to that of the quartz polymorph of silicon dioxide, indicating that it is an extended covalent solid with carbon-oxygen single bonds. This material is also optically nonlinear, generating the second harmonic of a neodymium-yttrium-lithium-fluoride laser at a wavelength of 527 nanometers with a conversion efficiency that is near 0.1 percent.