Charge fluctuations and the valence transition in Yb under pressure.

We present a dynamical mean-field theory study of the valence transition (f;{14} --> f;{13}) in elemental, metallic Yb under pressure. Our calculations reproduce the observed valence transition as reflected in the volume dependence of the 4f occupation. The transition is advanced by heating, and suggests quasiparticle or Kondo-like structure in the spectra of the trivalent end state, consistent with the early lanthanides. Results for the local charge fluctuations and susceptibility, however, show novel signatures uniquely associated with the valence transition itself, indicating that Yb is a fluctuating valence material in contrast with the intermediate valence behavior seen in the early trivalent lanthanides Ce, Pr, and Nd.

Thermal signatures of the Kondo volume collapse in cerium.

X-ray diffraction measurements of cerium in the vicinity of the isostructural gamma-alpha transition have been performed with high precision and accuracy from room temperature to almost 800 K. The disputed location of the critical point has been found to occur at 1.5+/-0.1 GPa and 480+/-10 K. The data are well fit by the Kondo volume collapse model plus a quasiharmonic representation of the phonons. The resultant free energy is validated against data for the thermodynamic Grüneisen parameter and, beyond the dominant spin-fluctuation contribution, indicates a dramatic change in the lattice Grüneisen parameter across the transition.

4f delocalization in Gd: inelastic x-ray scattering at ultrahigh pressure.

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.

New cubic phase of Li3N: stability of the N3- ion to 200 GPa.

Diamond-anvil cell experiments augmented by first-principles calculations have found a remarkable stability of the N(3-) ion in Li3N to a sixfold volume reduction. A new (gamma) phase is discovered above 40(+/-5) GPa, with an 8% volume collapse and a band gap quadrupling at the transition determined by synchrotron x-ray diffraction and inelastic x-ray scattering. gamma-Li(3)N (Fm3m, Li(3)Bi-like structure) remains stable up to 200 GPa, and calculations do not predict metallization until approximately 8 TPa. The high structural stability, wide band gap, and simple electronic structure make this N(3-) based system analogous to lower valency compounds (MgO, NaCl, Ne), meriting its use as an internal pressure standard.

Martensitic fcc-to-hcp transformation observed in xenon at high pressure.

Angle-resolved x-ray diffraction patterns of Xe to 127 GPa indicate that the fcc-to-hcp transition occurs martensitically between 3 and 70 GPa in diamond-anvil cells without an intermediate phase. These data also reveal that the transition occurs by the introduction of stacking disorder in the fcc lattice at low pressure, which grows into hcp domains with increasing pressure. The small energy difference between the hcp and the fcc structures may allow the two phases to coexist over a wide pressure range. Evidence of similar stacking disorder and incipient growth of an hcp phase are also observed in solid Kr.

Cerium volume collapse: results from the merger of dynamical mean-field theory and local density approximation.

The merger of density-functional theory in the local density approximation and many-body dynamical mean-field theory allows for an ab initio calculation of Ce including the inherent 4f electronic correlations. We solve the equations by the quantum Monte Carlo technique and calculate the Ce energy, spectrum, and double occupancy as a function of volume. At low temperatures, the correlation energy exhibits an anomalous region of negative curvature which drives the system towards a thermodynamic instability, i.e., the gamma-to- alpha volume collapse, consistent with experiment. The connection of the energetic with the spectral evolution shows that the physical origin of the energy anomaly and, thus, the volume collapse is the appearance of a quasiparticle resonance in the 4f-spectrum which is accompanied by a rapid growth in the double occupancy.