Yanming Ma

Ionic high-pressure form of elemental boron

Boron is an element of fascinating chemical complexity. Controversies have shrouded this element since its discovery was announced in 1808: the new ‘element’ turned out to be a compound containing less than 60–70% of boron, and it was not until 1909 that 99% pure boron was obtained. And although we now know of at least 16 polymorphs, the stable phase of boron is not yet experimentally established even at ambient conditions. Boron’s complexities arise from frustration: situated between metals and insulators in the periodic table, boron has only three valence electrons, which would favour metallicity, but they are sufficiently localized that insulating states emerge. However, this subtle balance between metallic and insulating states is easily shifted by pressure, temperature and impurities. Here we report the results of high-pressure experiments and ab initio evolutionary crystal structure predictions that explore the structural stability of boron under pressure and, strikingly, reveal a partially ionic high-pressure boron phase. This new phase is stable between 19 and 89u2009GPa, can be quenched to ambient conditions, and has a hitherto unknown structure (space group Pnnm, 28 atoms in the unit cell) consisting of icosahedral B12 clusters and B2 pairs in a NaCl-type arrangement. We find that the ionicity of the phase affects its electronic bandgap, infrared adsorption and dielectric constants, and that it arises from the different electronic properties of the B2 pairs and B12 clusters and the resultant charge transfer between them.

High-pressure phase transformations in CaH2

The structural stabilities, lattice dynamics and electronic properties of CaH2 under high pressures were studied using the density functional linear response theory. The calculations showed that CaH2 transforms from the PbCl2-type (Pnma) to the InNi2-type (P63/mmc) structure at ~16xa0GPa in agreement with experiment. The theoretical and experimental Raman investigations demonstrated the occurrence of a high frequency E2g mode of the P63/mmc phase and explained why this mode was not observed in previous Raman experiments, in conflict with the proposed P63/mmc symmetry of the phase structure. The calculations of the phonon dispersion curves revealed the dynamical instability of the P63/mmc phase at pressures below 10xa0GPa caused by a soft transverse acoustic phonon mode at the zone boundary M point. The P63/mmc phase was predicted to be an insulator and to transform to a metallic phase with an AlB2-type structure (P6/mmm) at ~138xa0GPa. According to the electron–phonon coupling calculation, the superconducting temperature of the P6/mmmxa0CaH2 phase cannot be higher than 1xa0mK. The charge transfers in the Pnma, P63/mmc, and P6/mmm phases were also calculated and discussed.

Origin of bcc to fcc phase transition under pressure in alkali metals

The electronic, dynamical and elastic properties of body-centered cubic (bcc) alkali metals (Li, Na, K, Rb and Cs) at high pressure are extensively studied to reveal the origin of the phase transition from bcc to face-centered cubic (fcc) by using ab initio calculations. The calculated 3D Fermi surface (FS) shows an anisotropic deformation by touching the Brillion zone boundary at the N point with pressure for Li, K, Rb and Cs due to the s→p and s→d charge transfers, respectively. However, no clear charge transfer is found in Na, in favor of an isotropic FS even at very high pressure. The traditional charge transfer picture and the newly proposed Home–Rothery model in understanding the bcc→fcc transition are thus questionable in view of their difficulties in Na. In this paper, a universal feature of pressure-induced instability of the tetragonal shear elastic constant Cu2009 and the softening of the transverse acoustic phonons along the [0ξξ]-direction near the zone center for all the alkali metals is identified. Analysis of the total energy results suggests that Cu2009 instability associated with the soft mode is the driving force for the bcc→fcc transition, which could be well characterized by the tetragonal Bains path.

Structural phase transition at high temperatures in solid molecular hydrogen and deuterium

We study the effect of temperature up to 1000K on the structure of dense molecular para-hydrogen and ortho-deuterium, using the path-integral Monte Carlo method. We find a structural phase transition from orientationally disordered hexagonal close packed (hcp) to an orthorhombic structure of Cmca symmetry before melting. The transition is basically induced by thermal fluctuations, but quantum fluctuations of protons (deuterons) are important in determining the transition temperature through effectively hardening the intermolecular interaction. We estimate the phase line between hcp and Cmca phases as well as the melting line of the Cmca solid.

First-principles studies of phonon instabilities in AgI under high pressure

The dynamical instabilities of AgI are extensively studied using the density-functional perturbation theory to probe the mechanism of the pressure-induced phase transitions of wurtzite . Analysis of the phonon calculations suggested that the pressure-induced instabilities of the transverse acoustic (TA) modes at the M/X points of zone boundaries for WZ/ZB AgI are responsible for the phase transitions of . While the transition is not induced by phonon instability, but probably by energetic instability. Moreover, it is found that the transformation of the RS to monoclinic phase is driven by the softening of the TA mode at point X. The elastic constant calculations suggest that although the predicted elastic instabilities in WZ and ZB cannot independently induce the phase transition, they might couple with the softening phonon to lower the transition pressure. Pressure-induced metallization has been predicted for CsCl-type AgI from evidence of a significant band gap overlap. However, the CsCl phase of AgI is predicted to be dynamically unstable at the pressures under study.

Lattice Strains in Gold and Rhenium Under Non-Hydrostatic Compression

Lattice strains have been measured as a function of the angle, ψ , from the diamond cell stress axis in a sample of gold and rhenium at pressures of 15–37 GPa. Experiments were conducted using X-ray transparent gaskets made from beryllium. The differential stresses supported by gold and rhenium have been characterized to 37 GPa. It is also shown that proper choice of the diffraction geometry allows recovery of a quasi-hydrostatic compression curve under these highly non-hydrostatic conditions. X-ray elastic moduli have also been determined, and while good agreement with previous data is achieved for gold, there is a large discrepancy between the present results and extrapolated ultrasonic data for rhenium.