The high pressure behavior of optical phonons in wurtzite zinc oxide (w-ZnO) has been studied using room temperature Raman spectroscopy and ab-initio calculations based on a plane wave pseudopotential method within the density functional theory. The pressure dependence of the zonecenter phonons (E2, A1 and E1) was measured for the wurtzite structure up to the hexagonal→cubic transition near 9 GPa. Above this pressure no active mode was observed. The only negative Grüneisen parameter is that of the E low 2 mode. E1(LO) and (TO) frequencies increase with increasing pressure. The corresponding perpendicular tensor component of the Born's transverse dynamic charge e * T is experimentally found to increase under compression like e * T (P) = 2.02 + 6.4 • 10 −3 .P whereas calculations give e * T (P) = 2.09−2.5•10 −3 .P (in units of the elementary charge e, P in GPa). In both cases, the pressure variation is small, indicating a weak dependence of the bond ionicity with pressure. The pressure dependence of the optical mode energies is also compared with the prediction of a model that treats the wurtzite-to-rocksalt transition as an homogeneous shear strain. There is no evidence of anomaly in the E2 and A1 modes behavior before the phase transition.

The high-pressure local structure of zinc oxide has been studied at room temperature using combined energy-dispersive x-ray-diffraction and x-ray-absorption spectroscopy experiments. The structural parameter u and the lattice-parameter ratio c/a of the wurtzite phase is given as a function of pressure and compared with results from ab initio calculations based on a plane-wave pseudopotential method within the density-functional theory. It is shown that an accurate study of ZnO requires the explicit treatment of the d electrons of Zn as valence electrons. In good agreement with present calculations, our experimental data do not show any variation of u(P) in the low-pressure wurtzite phase between 0 and 9 GPa, pressure at which the phase transition to the rocksalt phase occurs. Moreover, no dramatic modification of the r-phase K-edge position up to similar to20 GPa is observed, indicating the absence of metallization. In view of all these results, theoretical models identifying the wurtzite-to-rocksalt transition as an homogeneous path are discussed

The phase transition of zinc oxide from B4 (hexagonal wurtzite structure) to B1 (cubic rocksalt structure) has been studied by energy dispersive powder diffraction technique up to 11 GPa and 1273 K. Analysis of equation-of-state (PVT) data for the rocksalt phase yields precise values of the bulk modulus and its temperature derivative. The previously accepted P-T phase diagram is shown to be incorrect. It is established that the B1 phase is not recoverable. The equilibrium transition pressure of the B4-to-B1 transformation is near 6 GPa (at ambient temperature) and the dP/dT slope close to zero. These new results are confirmed by using simultaneously three other different types of experiment (imaging, ultrasonic and X-ray diffraction studies on single crystal specimens).

Dense powder of nanocrystalline ZnO has been recovered at ambient conditions in the metastable cubic structure after a heat treatment at high pressure (15 GPa and 550 K). Combined x-ray diffraction (XRD) and x-ray absorption spectroscopy (XAS) experiments have been performed to probe both long-range order and local crystallographic structure of the recovered sample. Within uncertainty of these techniques (about 5%), all the crystallites are found to adopt the NaCl structure. From the analysis of XRD and XAS spectra, the cell volume per chemical formula unit is found to be 19.57(1) and 19.60(3) Å3, respectively, in very good agreement with the zero-pressure extrapolation of previously published high-pressure data.

The cerium γ⇄α transition was investigated using high-pressure, high-temperature angle-dispersive x-ray diffraction measurements on both poly- and single-crystalline samples, explicitly addressing symmetry change and transformation paths. The isomorphic hypothesis of the transition is confirmed, with a transition line ending at a solid-solid critical point. The critical exponent is determined, showing a universal behavior that can be pictured as a liquid-gas transition. We further report an isomorphic transition between two single crystals (with more than 14% of volume difference), an unparalleled observation in solid-state matter interpreted in terms of dislocation-induced diffusionless first-order phase transformation.

The elasticity of hcp-Fe-5wt%Si has been investigated by synchrotron X-ray diffraction up to 110 GPa and 2,100 K and by picosecond acoustics measurements at ambient temperature up to 115 GPa. The established Pressure-Volume-Temperature equation of state shows that the density of the Earth's inner core can be matched by an Fe-Si alloy with 5wt% Si for all reasonable core temperatures, but that its compressional and shear velocities remain too high with respect to seismological observations. On the other hand, Fe-Si alloys whose velocities are expected to get close to seismological observations are too dense at relevant temperatures. Thus, based on these combined velocity-density measurements, silicon is not likely to be the sole light element in the inner core.

The mechanism responsible for the wurtzite-to-rocksalt structural phase transition in zinc oxide has been studied through the ab initio calculations of the full phonon properties. We derive a new and general path of the transformation from the parent to the daughter phase which involves an intermediate tetragonal phase. This hypothesis has been tested through analogous calculations on other wide-gap semiconductors, and confirmed in GaN and InN, but not in AlN and SiC. As a consequence of these results, we formulate a general prediction on the effect of d-electrons on the structural and elastic properties of wurtzite semiconductors under pressure.

We report here highly accurate ultrasonic measurements on ultrapure polycrystalline cerium up to 1 GPa. By simultaneously fitting the complete measured data set, bulk and shear moduli have been deduced without any independent input. We observe a maximum in the pressure evolution of the bulk modulus and show that this peculiar behavior can be qualitatively interpreted by taking into account the pressure-induced effects on electron-electron interaction and the anharmonicity of bonding in the ␥ phase. The vibrational contribution to the total entropy change across the ␥ to ␣ transition is estimated to be on the order of 15%, highlighting the need to consider the lattice dynamics for an accurate description of the phase transition.The understanding of how even small changes in temperature, pressure, or doping alter the correlations between electrons, which in turn tune several fundamental physical and chemical properties, provides a rich experimental and theoretical field. In particular, among the rare-earth metals, the unique properties of cerium have generated a long-standing and broad interest. One of the most intriguing phenomena is the instability of the single 4f 1 electron along the isostructural ͑fcc͒ ␥ to ␣ phase transition, and the effects that this has on the behavior of Ce. At ambient conditions, ␥-Ce is magnetic with a localized moment. Upon compression, it transforms at 300 K and P T = 0.75 GPa to the ␣ phase with loss of magnetic moment ͑Pauli paramagnetism͒ and a volume collapse of about 17%. 2 Upon release of pressure, the ␥ phase is totally recovered at ambient pressure. At higher temperature, the ␥ to ␣ transition is suggested to end in a critical point at about T c = 600 K and P c = 2 GPa, 1-3 and the extrapolated line of the ␥ to ␣ boundary seems to terminate at the minimum in the fusion curve. To date, two types of mechanism have been proposed to describe the electronic instability that drives this transition. The Mott transition model ͑within which the 4f electron is considered localized and not binding in the ␥ phase, and itinerant and binding in the ␣ phase͒ was first put forward but, the Kondo-VolumeCollapse ͑KVC͒, where the spd-f hybridization is the dominant effect, has also been suggested. Despite the important number of experimental 4 and theoretical 5-10 studies, the discussion on the validity of these two different pictures remains open.Only recently, a few studies have started to explicitly point out the need of carefully considering the lattice contribution to fully understand the driving mechanism of the ␥ to ␣ transition. 3,[11][12][13] In fact, while the key role played by entropy on the physics of the ␥ to ␣ phase transition is by now quite well-established ͑see for instance Ref. 10 and 14͒, the relative importance of spin and lattice contribution is still under debate, with studies suggesting that vibrational entropy changes across the transition can account for about half of the total entropy change 12 and others suggesting that the lattice contribution is negligible. 3,11,15 S...