Potential high-Tc superconducting lanthanum and yttrium hydrides at high pressure.

A systematic structure search in the La-H and Y-H systems under pressure reveals some hydrogen-rich structures with intriguing electronic properties. For example, LaH10 is found to adopt a sodalite-like face-centered cubic (fcc) structure, stable above 200 GPa, and LaH8 a C2/m space group structure. Phonon calculations indicate both are dynamically stable; electron phonon calculations coupled to Bardeen-Cooper-Schrieffer (BCS) arguments indicate they might be high-Tc superconductors. In particular, the superconducting transition temperature Tc calculated for LaH10 is 274-286 K at 210 GPa. Similar calculations for the Y-H system predict stability of the sodalite-like fcc YH10 and a Tc above room temperature, reaching 305-326 K at 250 GPa. The study suggests that dense hydrides consisting of these and related hydrogen polyhedral networks may represent new classes of potential very high-temperature superconductors.

Quasimolecules in Compressed Lithium.

Under high pressure, some materials form electrides, with valence electrons separated from all atoms and occupying interstitial regions. This is often accompanied by semiconducting or insulating behavior. The interstitial quasiatoms (ISQ) that characterize some high pressure electrides have been postulated to show some of the chemical features of atoms, including the potential of forming covalent bonds. It is argued that in the observed high-pressure semiconducting Li phase (oC40, Aba2), an example of such quasimolecules is realized. The theoretical evaluation of electron density, electron localization function, Wannier orbitals, and bond indices forms the evidence for covalently bonded ISQ pairs in this material. The quasimolecule concept thus provides a simple chemical perspective on the unusual insulating behavior of such materials, complementing the physical picture previously presented where the global crystal symmetry of the system plays the major role.

Topological Surface States in Dense Solid Hydrogen.

Metallization of dense hydrogen and associated possible high-temperature superconductivity represents one of the key problems of physics. Recent theoretical studies indicate that before becoming a good metal, compressed solid hydrogen passes through a semimetallic stage. We show that such semimetallic phases predicted to be the most stable at multimegabar (∼300 GPa) pressures are not conventional semimetals: they exhibit topological metallic surface states inside the bulk "direct" gap in the two-dimensional surface Brillouin zone; that is, metallic surfaces may appear even when the bulk of the material remains insulating. Examples include hydrogen in the Cmca-12 and Cmca-4 structures; Pbcn hydrogen also has metallic surface states but they are of a nontopological nature. The results provide predictions for future measurements, including probes of possible surface superconductivity in dense hydrogen.

Dense Hydrocarbon Structures at Megabar Pressures.

The structure, bonding, and other properties of phases in the carbon-hydrogen system over a range of conditions are of considerable importance to a broad range of scientific problems. However, the phase diagram of the C-H system at high pressures and temperatures is still not known. To search for new low-energy hydrocarbon structures, we carried out systematic structure prediction calculations for the C-H system from 100 to 300 GPa. We confirmed several previously predicted structures but found additional compositions that adopt more stable structures. In particular, a C2H4 structure is found that has an indirect band gap, and phonon calculations confirm that it is dynamically stable over a broad pressure range. We also identify more carbon-rich structures that are energetically favorable. The results are important for understanding carbon-hydrogen interactions in high-pressure experiments, dense astrophysical environments and the deep carbon cycle in planetary interiors.

Chemical bonding in hydrogen and lithium under pressure.

Though hydrogen and lithium have been assigned a common column of the periodic table, their crystalline states under common conditions are drastically different: the former at temperatures where it is crystalline is a molecular insulator, whereas the latter is a metal that takes on simple structures. On compression, however, the two come to share some structural and other similarities associated with the insulator-to-metal and metal-to-insulator transitions, respectively. To gain a deeper understanding of differences and parallels in the behaviors of compressed hydrogen and lithium, we performed an ab initio comparative study of these systems in selected identical structures. Both elements undergo a continuous pressure-induced s-p electronic transition, though this is at a much earlier stage of development for H. The valence charge density accumulates in interstitial regions in Li but not in H in structures examined over the same range of compression. Moreover, the valence charge density distributions or electron localization functions for the same arrangement of atoms mirror each other as one proceeds from one element to the other. Application of the virial theorem shows that the kinetic and potential energies jump across the first-order phase transitions in H and Li are opposite in sign because of non-local effects in the Li pseudopotential. Finally, the common tendency of compressed H and Li to adopt three-fold coordinated structures as found is explained by the fact that such structures are capable of yielding a profound pseudogap in the electronic densities of states at the Fermi level, thereby reducing the kinetic energy. These results have implications for the phase diagrams of these elements and also for the search for new structures with novel properties.

Origin of Transitions between Metallic and Insulating States in Simple Metals.

Unifying principles that underlie recently discovered transitions between metallic and insulating states in elemental solids under pressure are developed. Using group theory arguments and first-principles calculations, we show that the electronic properties of the phases involved in these transitions are controlled by symmetry principles. The valence bands in these systems are described by simple and composite band representations constructed from localized Wannier functions centered on points unoccupied by atoms, and which are not necessarily all symmetrical. The character of the Wannier functions is closely related to the degree of s-p(-d) hybridization and reflects multicenter chemical bonding in these insulating states. The conditions under which an insulating state is allowed for structures having an integer number of atoms per primitive unit cell as well as reentrant (i.e., metal-insulator-metal) transition sequences are detailed, resulting in predictions of behavior such as phases having band-contact lines. The general principles developed are tested and applied to the alkali and alkaline earth metals, including elements where high-pressure insulating phases have been reported (e.g., Li, Na, and Ca).

Aromaticity, closed-shell effects, and metallization of hydrogen.

CONSPECTUS: Recent theoretical and experimental studies reveal that compressed molecular hydrogen at 200-350 GPa transforms to layered structures consisting of distorted graphene sheets. The discovery of chemical bonding motifs in these phases that are far from close-packed contrasts with the long-held view that hydrogen should form simple, symmetric, ambient alkali-metal-like structures at these pressures. Chemical bonding considerations indicate that the realization of such unexpected structures can be explained by consideration of simple low-dimensional model systems based on H6 rings and graphene-like monolayers. Both molecular quantum chemistry and solid-state physics approaches show that these model systems exhibit a special stability, associated with the completely filled set of bonding orbitals or valence bands. This closed-shell effect persists in the experimentally observed layered structures where it prevents the energy gap from closing, thus delaying the pressure-induced metallization. Metallization occurs upon further compression by destroying the closed shell electronic structure, which is mainly determined by the 1s electrons via lowering of the bonding bands stemming from the unoccupied atomic 2s and 2p orbitals. Because enhanced diamagnetic susceptibility is a fingerprint of aromaticity, magnetic measurements provide a potentially important tool for further characterization of compressed hydrogen. The results indicate that the properties of dense hydrogen are controlled by chemical bonding forces over a much broader range of conditions than previously considered.

Electronic excitations and metallization of dense solid hydrogen.

Theoretical calculations and an assessment of recent experimental results for dense solid hydrogen lead to a unique scenario for the metallization of hydrogen under pressure. The existence of layered structures based on graphene sheets gives rise to an electronic structure related to unique features found in graphene that are well studied in the carbon phase. The honeycombed layered structure for hydrogen at high density, first predicted in molecular calculations, produces a complex optical response. The metallization of hydrogen is very different from that originally proposed via a phase transition to a close-packed monoatomic structure, and different from simple metallization recently used to interpret recent experimental data. These different mechanisms for metallization have very different experimental signatures. We show that the shift of the main visible absorption edge does not constrain the point of band gap closure, in contrast with recent claims. This conclusion is confirmed by measured optical spectra, including spectra obtained to low photon energies in the infrared region for phases III and IV of hydrogen.

Collective dipole behavior and unusual morphotropic phase boundary in ferroelectric Pb(Zr(0.5)Ti(0.5))O3 nanowires.

Dipole collective behavior and phase transition in ferroelectric (FE) Pb(Zr(0.5)Ti(0.5))O(3) nanowires, caused by modulated electric fields, are reported. Our result also leads to the finding of a rather outstanding electromechanical d(31) response in a 8.4 nm diameter PZT wire, which may potentially outperform bulk PMN-PT and PZN-PT. Moreover, we further demonstrate the existence of a new type of morphotropic phase boundary (MPB) that bridges two dissimilar structure phases of different order parameters. Microscopic insights for understanding the collective behavior and the structural phase within the new MPB are provided.

Cooperative response of Pb(ZrTi)O3 nanoparticles to curled electric fields.

We investigate cooperative responses, as well as a microscopic mechanism for vortex switching, in Pb(Zr0.5Ti0.5)O3 nanoparticles under curled electric fields. We find that the domain coexistence mechanism is not valid for toroid switching. Instead dipoles display unusual collective behavior by forming a new vortex with a perpendicular (not opposite) toroid moment. The correlation between the new and original vortices is revealed to be critical for reversing the toroid moment. We further describe a technological approach that is able to drastically reduce the curled electric field needed for vortex switching.

Spontaneous polarization in one-dimensional Pb(ZrTi)O3 nanowires.

Formation of spontaneous polarization in one-dimensional (1D) structures is a key phenomenon that reveals collective behaviors in systems of reduced dimensions, but has remained unsolved for decades. Here we report ab initio studies on finite-temperature structural properties of infinite-length nanowires of Pb(Zr0.5Ti0.5)O3 solid solution. Whereas existing studies have ruled out the possibility of phase transition in 1D chains, our atomistic simulations demonstrate a different conclusion, characterized by the occurrence of a ferroelectric polarization and critical behaviors of dielectric and piezoelectric responses. The difference is accounted for by the use of depolarizing effects associated with finite thickness of wires. Our results suggest no fundamental constraint that limits the use of ferroelectric nanowires and nanotubes arising from the absence of spontaneous ordering.

Unusual phase transitions in ferroelectric nanodisks and nanorods.

Bulk ferroelectrics undergo structural phase transformations at low temperatures, giving multi-stable (that is, multiple-minimum) degenerate states with spontaneous polarization. Accessing these states by applying, and varying the direction of, an external electric field is a key principle for the operation of devices such as non-volatile ferroelectric random access memories (NFERAMs). Compared with bulk ferroelectrics, low-dimensional finite ferroelectric structures promise to increase the storage density of NFERAMs 10,000-fold. But this anticipated benefit hinges on whether phase transitions and multi-stable states still exist in low-dimensional structures. Previous studies have suggested that phase transitions are impossible in one-dimensional systems, and become increasingly less likely as dimensionality further decreases. Here we perform ab initio studies of ferroelectric nanoscale disks and rods of technologically important Pb(Zr,Ti)O3 solid solutions, and demonstrate the existence of previously unknown phase transitions in zero-dimensional ferroelectric nanoparticles. The minimum diameter of the disks that display low-temperature structural bistability is determined to be 3.2 nm, enabling an ultimate NFERAM density of 60 x 10(12) bits per square inch-that is, five orders of magnitude larger than those currently available. Our results suggest an innovative use of ferroelectric nanostructures for data storage, and are of fundamental value for the theory of phase transition in systems of low dimensionality.