The present status of high-pressure research with the diamond anvil cell (DAC) is reviewed in this article, mainly from an experimental aspect. After a brief description of the different types of DAC's that are currently in vogue, the techniques used in conjunction with the DAC in modern high-pressure research are presented. These include techniques for low- and high-temperature studies, x-ray diffractometry, spectroscopy with the DAC, and other measurements. Results on selected materials, with a view to illustrating the physics behind high-pressure phenomena, are presented and discussed. These include metal-semiconductor transitions, electronic transitions, phonons and high-pressure lattice dynamics, and phase transitions. A whole section is devoted to the behavior of condensed gases, principally ${\mathrm{H}}_{2}$, ${\mathrm{D}}_{2}$, ${\mathrm{O}}_{2}$, ${\mathrm{N}}_{2}$, and rare-gas solids. The concluding section briefly deals with speculations on ultra-high-pressure research with the DAC in the future.

Diamond anvil cell and high-pressure physical investigations

Local-density-functional total-energy calculations are reported for both the structural and metallization transitions of BaSe and BaTe. Good agreement with recent experimental results for BaTe confirms the band-overlap mechanism in this and related closed-shell systems. A metallization pressure of about 400 kbar is predicted for BaSe. These calculations represent the first convincing demonstration of the ability of the local-density-functional approximation to predict band-overlap metallization accurately.

Local-density-functional calculation of the pressure-induced metallization of BaSe and BaTe.

The elastic constants c11, c12 and c44 of U, Np, Pu, and Am telluride single crystals have been obtained by Brillouin scattering from the measured sound velocities in prominent crystallographic directions. In case of the U chalcogenides also by ultrasound techniques. The Cauchy pressure, the Poisson ratio, the anisotropy ratio, the bulk modulus and the Debye temperature have been derived from these data. U and Pu telluride have a negative c12, implying intermediate valence. AmTe has a very low bulk modulus, but a positive c12. It is extremely soft, with a very low sound velocity. For AmTe also the LO and TO phonon frequencies could be determined. The elastic data point to a divalent Am state. The optical reflectivity of the light actinide tellurides (and sometimes of all chalcogenides) has been measured between UV and infrared wavelengths, in the case of AmTe down to the far infrared. The plasma edge of the free carriers has been determined, which yields the ratio of n/m*. Together with the γ value of the specific heat and magnetic data a consistent proposal for the electronic structure of the light actinide chalcogenides can be given. Thus, the Pu chalcogenides are intermediate valent and represent the high-pressure phase of the corresponding Sm chalcogenides. AmTe, as judged by the electronic, optic and magnetic properties seems to be in the 5f7 configuration, i.e. divalent Am, but with a narrow, half-filled (24 meV) wide 5f band, about 0.1 eV below the bottom of the 6d conduction band. We thus propose a new kind of unhybridized heavy fermion. Also AmTe seems to represent a high-pressure phase of EuTe.

Electronic and elastic properties of the light actinide tellurides

New shock-wave (Hugoniot) and release-adiabatic data for Fe_(0.94)O and CaO, to 230 and 175 GPa (2.3 and 1.75 Mbar) respectively, show that both oxides transform from their initial B1 (NaCl-type) structures at about 70 (±10) GPa. CaO transforms to the B2 (CsCl-type) structure and FeO is inferred to do the same. Alternatively, FeO may undergo an electronic transition, but it probably does not disproportionate under shock to Fe and Fe_2O_3 or Fe_3O_4. The Hugoniot data for the B1 phases of FeO and CaO agree with the ultrasonically-determined bulk moduli (K_0= 185, 112 GPa, respectively) and with the ultrasonically-determined pressure derivative for CaO (K′_0= 4.8); K′_0∼ 3.2 for FeO is determined from the present data. The Hugoniot data for both FeO and CaO are consistent with low- and high-pressure phases having identical K_0 and K′_0. Volume changes for B1/B2 transitions in oxides agree with theoretical expectations and with trends among the halides: -ΔV/V_1 ~ 4 per cent and 11 per cent for FeO and CaO respectively. Also, the transition pressures increase with decreasing cation/anion radius ratio for the oxides. The Hugoniot data show that the density of the outer core is equal to that of a 50–50 mix (by weight) of Fe and FeO (∼10 wt per cent oxygen), consistent with geochemical arguments for the presence of oxygen in the core. In terms of a mixture of simple oxides, the density of the lower mantle is satisfied by Fe/(Mg + Fe) ∼ 0.12, however, arbitrarily large amounts of CaO can be present; an enrichment of refractory components in the lower mantle is allowed by the shock-wave data. Because of the relatively low transition pressure in FeO, a B1/B2 transition in (Mg, Fe)O is likely to occur in the lower mantle even if MgO transforms at 150–170 GPa. Such a transition may contribute to the scattering of seismic waves and change in velocity gradient found near the base of the mantle.

/pdf/equations-of-state-of-feo-and-cao-438w9n3ihi.pdf

Equations of state of FeO and CaO

The pressure-volume relationships for EuSe, EuS, YbSe, and YbS have been obtained from high-pressure x-ray-diffraction studies to nearly 300 kbar. Like EuTe, EuSe and EuS exhibit normal compression and undergo the NaCl-to-CsCl-type transition at about 145 and 215 kbar. The compression curves of YbSe and YbS are anomalous in the 150-200-kbar region, which we believe is because of a change in the valence state of Yb from ${2}^{+}$ towards the ${3}^{+}$ state. Among the Eu monochalcogenides EuO is the only substance that shows a valence transformation (\ensuremath{\sim}300 kbar) followed by a NaCl-to-CsCl-type transition (\ensuremath{\sim}400 kbar). However, all the Yb monochalcogenides undergo the valence transformation continuously with pressure. The bulk moduli evaluated from the experimental $P\ensuremath{-}V$ data and the measured pressure coefficients of the energy gap between the $4f$ level and the conduction-band edge from optical-absorption studies of divalent rare-earth monochalcogenides are compared. These data are used to rationalize the occurrence or nonoccurrence of the valence transitions in this series of compounds.

Pressure-volume relationship and pressure-induced electronic and structural transformations in Eu and Yb monochalcogenides

The compressibilities of the monotellurides of Pr, Sm, Eu, Tm, and Yb and the monoselenide and sulfide of Sm have been investigated to \ensuremath{\sim} 300 kbar using high-pressure $x$-ray-diffraction techniques. SmTe, SmSe, TmTe, and YbTe show abnormal volume changes in the 20-50-, 15-40-, 15-30-, and 150-200-kbar regions of pressure, respectively. SmS shows an abrupt decrease in volume at 6.5 kbar. Since there is no change in structure, the anomalously large volume changes have been explained on the basis of a pressure-induced $4f\ensuremath{-}5d$ electronic collapse which involves a change in the valence state of the rare-earth ion from ${2}^{+}$ towards the ${3}^{+}$ state. The results of high-pressure x-ray studies on Sm chalcogenides are consistent with the conclusions drawn in the earlier work from high-pressure resistivity measurements. PrTe, SmTe, and EuTe exhibit a phase transition from NaCl-type to CsCl-type structure at pressures of about 90 \ifmmode\pm\else\textpm\fi{} 10, 110 \ifmmode\pm\else\textpm\fi{} 10, and 110 \ifmmode\pm\else\textpm\fi{} 10 kbar, respectively. It appears that a pressure-induced NaCl-to-CsCl transition may be commonly encountered in rare-earth monochalcogenides.

Pressure-Induced Electronic Collapse and Structural Changes in Rare-Earth Monochalcogenides

PRESSURE INDUCED ELECTRONIC AND NaCl TO CsCl TYPE TRANSITIONS IN SOME RARE EARTH MONOTELLURIDES

The pressure-volume relationship in YbTe has been studied by high-pressure x-ray diffraction up to 300 kbar. This relationship is anomalous in the region 150-190 kbar indicating a continuous electronic collapse with pressure, in which the ${\mathrm{Yb}}^{2+}$ changes to ${\mathrm{Yb}}^{3+}$ state. This must involve $4f\ensuremath{-}5d$ electron promotion and must result in a semiconductor-to-metal transition. The behavior of YbTe suggests that in rare-earth chalcogenides with large band gap a continuous transition would be favored.

/pdf/pressure-induced-4f-5d-electron-collapse-in-ytterbium-jxa22qkxnv.pdf

Pressure-induced 4f-5d electron collapse in ytterbium monotelluride

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Pressure-volume relationship and pressure-induced electronic and structural transformations in Eu and Yb monochalcogenides

Pressure-Induced Electronic Collapse and Structural Changes in Rare-Earth Monochalcogenides

PRESSURE INDUCED ELECTRONIC AND NaCl TO CsCl TYPE TRANSITIONS IN SOME RARE EARTH MONOTELLURIDES

Pressure-induced 4f-5d electron collapse in ytterbium monotelluride