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Under standard conditions, carbon dioxide (CO2) is a simple molecular gas and an important atmospheric constituent, whereas silicon dioxide (SiO2) is a covalent solid, and one of the fundamental minerals of the planet. The remarkable dissimilarity between these two group IV oxides is diminished at higher pressures and temperatures as CO2 transforms to a series of solid phases, from simple molecular to a fully covalent extended-solid V, structurally analogous to SiO2 tridymite. Here, we present the discovery of an extended-solid phase of CO2: a six-fold coordinated stishovite-like phase VI, obtained by isothermal compression of associated CO2-II (refs 1,2) above 50 GPa at 530–650 K. Together with the previously reported CO2-V (refs 3–5) and a-carbonia6, this extended phase indicates a fundamental similarity between CO2 (a prototypical molecular solid) and SiO2 (one of Earth’s fundamental building blocks). We present a phase diagram with a limited stability domain for molecular CO2-I, and suggest that the conversion to extended-network solids above 40–50 GPa occurs via intermediate phases II (refs 1,2), III (refs 7,8) and IV (refs 9,10). The crystal structure of phase VI suggests strong disorder along the c axis in stishovite-like P42/m n m, with carbon atoms manifesting an average six-fold coordination within the framework of s p3 hybridization.

When β-nitrous oxide (Cmca N2O) is laser heated to between 2000 and 3400 K at high pressures (10−55 GPa), it disproportionates into an ionic form of dimeric nitrogen dioxide (NO+NO3-) and nitrogen. Raman spectra of the quenched products suggest that the ionic phase is stable to 55 GPa at ambient temperature but, upon subsequent heating at pressures below 30 GPa, further dissociates into nitrogen, oxygen, and other nitrogen−oxygen compounds. X-ray diffraction patterns of the ionic phase indicate that it has an orthorhombic structure (Z = 4, ρo = 2.69 g/cm3), similar to the aragonite phase of calcium carbonate. The results suggest that the primary driver for ionization is densification at high pressures, whereas dissociation between 10 and 30 GPa results from the combined effects of densification and entropy increases.

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.

We present a review of recent studies of carbon dioxide (CO 2 ) at high pressures and temperatures and describe the experimental evidence for a dipole interacting bent CO 2 phase IV and for a non-molecular extended phase V. Phase IV is stabilized above 12 GPa and 1000 K; the Raman activation of the internal bending mode (v 2 ) indicates a loss of molecular inversion symmetry in this new phase. Above 35 GPa, and 1800 K. the CO 2 molecules associate to form a covalent solid (V) with a structure based on singly bonded CO 4 tetrahedra, similar to that of SiO 2 tridymite, We discuss the stability and properties of this polymeric form of CO 2 .

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.

The crystal structures of two molecular phases (I and III) and a polymeric phase (V) of CO2 have been investigated to 60 GPa. CO2-1 (Pa3) transforms to CO2-III (Cmca) at 12 GPa with almost no change of density. Although CO2-III persists in Cmca to at least 60 GPa at ambient temperature, it transforms when heated to 1800 K above 40 GPa to tridymite (P212121) CO2-V with 15.3% volume change. Each carbon atom of CO2-V is tetrahedrally bonded to four oxygen atoms. CO2-V is likely superhard with low compressibility B0 = 365 GPa, similar to cubic BN.

Deep defects are often assumed to be insensitive to applied pressure because of their localized character. However, in recent photoluminescence (PL) experiments, several deep acceptor bands in ZnSe were found to shift with pressure substantially faster than the ZnSe bandgap. This shows that the optical (viz., PL) levels of these acceptors become more shallow under compression, a result that, if also true for the thermal defect levels, is important for p-type doping problems in II±VI semiconductors. We report investigations of the C3v-relaxed isolated Zn-vacancy (VZn) in ZnSe that help to resolve these issues. High-pressure PL and PL-excitation (PLE) experiments and calculations are performed on this system. We find that the VZn-related PL and PLE bands have pressure coefficients that are, respectively, larger and smaller than that of the ZnSe bandgap. Hence, the Stokes-shift decreases with pressure. These results can not be understood without taking explicit account of lattice relaxation. We employ a defect-molecule model with atomic wavefunctions to calculate semi-empirical configuration-coordinate diagrams for the VZn defect as a function of pressure. We find that compression increases the Jahn-Teller coupling, but not sufficiently to overcome lattice stiffening. Overall, the VZn thermal level deepens, inhibiting p-type doping.

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.

Mineral properties in Earth's lower mantle are affected by iron electronic states, but representative pressures and temperatures have not yet been probed. Spin states of iron in lower-mantle ferropericlase have been measured up to 95 gigapascals and 2000 kelvin with x-ray emission in a laser-heated diamond cell. A gradual spin transition of iron occurs over a pressure-temperature range extending from about 1000 kilometers in depth and 1900 kelvin to 2200 kilometers and 2300 kelvin in the lower mantle. Because low-spin ferropericlase exhibits higher density and faster sound velocities relative to the high-spin ferropericlase, the observed increase in low-spin (Mg,Fe)O at mid-lower mantle conditions would manifest seismically as a lower-mantle spin transition zone characterized by a steeper-than-normal density gradient.

Direct laser heating of molecular ${\mathrm{N}}_{2}$ to above $1400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ at $120--130\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ results in the formation of a reddish amorphous phase and a transparent crystalline solid above $2000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Raman and x-ray data confirm that the transparent phase is cubic-gauche nitrogen (cg-N), while the amorphous phase could be an extension of the already known $\ensuremath{\eta}$ phase. The reddish color of the amorphous phase, however, might also indicate the presence of $\mathrm{N}\mathrm{N}$ bonds. The quenched amorphous phase is stable down to at least $70\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, analogous to cg-N. A chemophysical phase diagram is presented which emphasizes the difference between pressure- and temperature-induced transitions from molecular to nonmolecular solids, as found in other low $Z$ systems.

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.

This research project examined the changes in electronic and magnetic properties of transition metals and oxides under applied pressures, focusing on complex relationship between magnetism and phase stability in these correlated electron systems. As part of this LDRD project, we developed new measurement techniques and adapted synchrotron-based electronic and magnetic measurements for use in the diamond anvil cell. We have performed state-of-the-art X-ray spectroscopy experiments at the dedicated high-pressure beamline HP-CAT (Sector 16 Advanced Photon Source, Argonne National Laboratory), maintained in collaboration with of University of Nevada, Las Vegas and Geophysical Laboratory of The Carnegie Institution of Washington. Using these advanced measurements, we determined the evolution of the magnetic order in the ferromagnetic 3d transition metals (Fe, Co and Ni) under pressure, and found that at high densities, 3d band broadening results in diminished long range magnetic coupling. Our experiments have allowed us to paint a unified picture of the effects of pressure on the evolution of magnetic spin in 3d electron systems. The technical and scientific advances made during this LDRD project have been reported at a number of scientific meetings and conferences, and have been submitted for publication in technical journals. Both the technical advances and the physicalmore » understanding of correlated systems derived from this LDRD are being applied to research on the 4f and 5f electron systems under pressure.« less

We present evidence for an isostructural, first-order Mott transition in MnO at $105\ifmmode\pm\else\textpm\fi{}5\text{ }\text{ }\mathrm{GPa}$, 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 $105\ifmmode\pm\else\textpm\fi{}5\text{ }\text{ }\mathrm{GPa}$. The last is the Mott transition, accompanied by a significant loss of magnetic moment, a $\ensuremath{\sim}6.6%$ volume collapse and the insulator-metal transition as demonstrated by recent resistance measurements.

The crystal structure of carbon dioxide phase IV $({\mathrm{CO}}_{2}\ensuremath{-}\mathrm{IV})$ has been characterized based on powder angle-resolved x-ray diffraction data obtained in situ at 300--750 K and 11--50 GPa. The results confirm that in this high-pressure polymorph $\mathrm{O}=\mathrm{C}=\mathrm{O}$ molecules are nonlinear. Rietveld analyses of the x-ray data yield two plausible structures: ${P4}_{1}{2}_{1}2$ $(\ensuremath{\alpha}\ensuremath{-}{\mathrm{SiO}}_{2}$ cristobalite) and Pbcn $(\ensuremath{\alpha}\ensuremath{-}{\mathrm{PbO}}_{2},$ post-stishovite). Carbon dioxide molecules are bent slightly more in the Pbcn phase $(l\mathrm{C}\ensuremath{-}\mathrm{O}\ensuremath{-}\mathrm{C}=160\ifmmode^\circ\else\textdegree\fi{})$ than in the ${P4}_{1}{2}_{1}2$ (171\ifmmode^\circ\else\textdegree\fi{}). For both structures, the intramolecular C-O bond is increased to 1.50(\ifmmode\pm\else\textpm\fi{}0.1) \AA{} at the distance of a C-O single bond, whereas the intermolecular C\ensuremath{\cdots}O distance is reduced to around 2.1(\ifmmode\pm\else\textpm\fi{}0.2) \AA{}. The bending and elongation of the molecular units suggest that phase IV is an intermediate state between the molecular and nonmolecular extended phases and that the molecular-to-nonmolecular transformation in carbon dioxide occurs gradually via intermediate phases IV and II. Transition pathways among all five ${\mathrm{CO}}_{2}$ phases (I--V) are also discussed in the context of symmetry arguments and transition mechanisms. Considerations of plausible mechanisms of the transitions among phases I, II, IV, and V favor the Pbcn interpretation of the crystal structure of phase IV.

The phase diagram for praseodymium (Pr) has been determined for pressures between 5 and 60 GPa and temperatures between 295 and 830 K using both in situ energy- and angle-dispersive x-ray diffraction with externally heated diamond-anvil cells. Mineral oil and argon were alternatively used as pressure media in order to compare conflicting results in the literature and to ensure the validity of mineral oil as an inert medium. Evidence for the presence of an, as yet, unidentified phase (denoted Pr-VI) above 675 K has been observed, whereas no compelling evidence has been observed for the existence of the recently reported monoclinic phase (Pr-V). The new constraints of the phase diagram, therefore, suggest that the phase transitions occur as Pr-I(dhcp)\ensuremath{\rightarrow}Pr-II(fcc)\ensuremath{\rightarrow}Pr-V$\stackrel{\ensuremath{\rightarrow}}{\mathrm{I}}$Pr-IV(\ensuremath{\alpha}-U) above approximately 700 K. Additionally, there is a Pr-III(distorted fcc), Pr-VI, and Pr-IV triple point at approximately 675 K and 23.8 GPa. Temperature-dependent equations of state have been determined, allowing the temperature-dependent volume collapse at the transition between Pr-III and Pr-IV to be calculated. We report a linear decrease of the volume collapse along the Pr-III to Pr-IV boundary with temperature, $\ensuremath{\Delta}V/V$ $(%)=16.235\ensuremath{-}0.0156[T(\mathrm{K})];$ the extrapolation indicates that the volume collapse should vanish well below the melting point. With the temperature-dependent equation of state data and new phase diagram we demonstrate that the volume collapse can be accounted for by a change in the multiplicity of Pr atoms as the f electrons go from localized to itinerant.

The crystal structure of ${\mathrm{CO}}_{2}\ensuremath{-}\mathrm{II}$ is determined to be tetragonal ${P4}_{2}/mnm$ with $z=2,$ with evidence of some tetragonal-to-orthorhombic (Pnnm) disorder in the ab plane. In this structure, carbon atoms are pseudo-sixfold-coordinated by oxygens; two oxygens are at an elongated intramolecular $\mathrm{C}=\mathrm{O}$ bond distance, 1.331(3) \AA{}, and four are at a collapsed intermolecular distance, 2.377(2) \AA{}, at the apices of highly distorted octahedral. Strong intermolecular association results in a large splitting of the symmetric stretching ${\ensuremath{\nu}}_{1}$ mode and in a high bulk modulus 131.5 GPa. At 11 GPa, ${\mathrm{CO}}_{2}\ensuremath{-}\mathrm{II}$ is about 6.8% denser than ${\mathrm{CO}}_{2}\ensuremath{-}\mathrm{III}$ (Cmca).

A nonlinear molecular carbon dioxide phase IV was discovered by laser heating $\mathrm{CO}{}_{2}\ensuremath{-}\mathrm{III}\phantom{\rule{0ex}{0ex}}(\mathrm{Cmca})$ between 12 and 30 GPa, followed by quenching to 300 K. The Raman spectrum of quenched $\mathrm{CO}{}_{2}\ensuremath{-}\mathrm{IV}$ exhibits a triplet bending mode ${\ensuremath{\nu}}_{2}(\mathrm{O}=\mathrm{C}=\mathrm{O})$ near $650\mathrm{cm}{}^{\ensuremath{-}1}$, suggesting a broken inversion symmetry because of bending. The $650\mathrm{cm}{}^{\ensuremath{-}1}$ bending modes soften with increasing pressure, indicating an enhanced intermolecular interaction among neighboring bent $\mathrm{CO}{}_{2}$ molecules. At 80 GPa, the low-frequency vibron collapses into high-frequency phonons, and $\mathrm{CO}{}_{2}\ensuremath{-}\mathrm{IV}$ becomes an extended amorphous solid.

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.

An extended-solid phase, carbon dioxide phase V (CO 2 -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 CO 2 -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.