Pressure-induced Band Research Articles

Pressure-sensitive Optical Properties of Bis(1,2-dionedioximato)-Palladium (II) Complexes.

The absorption spectra of Pd(niox)2, Pd(dmg)2 and Pd(dpg)2 have been studied at high pressures. The absorption bands based on the 4dz2-5pz transition shifted rapidly to longer wavelengths with increasing pressure. The mean rates of the red shift with pressure were ca. -1170cm-1/GPa for Pd(niox)2, ca. -1300cm-1/GPa for Pd(dmg)2 and ca. -1990cm-1/GPa for Pd(dpg)2. A new pressure-induced absorption band at around 410nm was observed for Pd(niox)2 above 10GPa. The electronic states of the palladium complexes are discussed. The colors of the complexes in the diamond-anvil pressure cell were observed visually at various pressures. The colors of the complexes turn from yellow to orange and then to successive colors in the visible spectrum with increasing pressure. These materials can be utilized as a pressure indicator over the wide pressure range.

Insulator-to-Metal-to-Semiconductor Transitions of One-Dimensional Bis(1,2-dione dioximato)Pt(II) Complexes at High Pressures

Abstract The electrical resistivities and absorption spectra of one-dimensional bis(1,2-dione dioximato)Pt(II) have been studied at high pressures. The resistivities of needle crystals of Pt(dmg)2 and Pt(bqd)2 were measured in detail under a quasi-hydrostatic pressure of up to 7 GPa. Pt(dmg)2 is an insulator with a resistivity of 1015 Ω cm, and Pt(bqd)2 is a semiconductor with 103 Ω cm at atmospheric pressure. A pressure-induced insulator (semiconductor)-to-metal transition was observed for both complexes. This transition arises from a crossing of the 5dz2 valence and 6pz conduction bands. When the pressure was further increasued, a metal-to-semiconductor transition took place at around the pressure that showed a resistivity minimum. A new pressure-induced absorption band was found in the visible region for the Pt complexes. The new band did not shift with pressure; the intensity of the band, however, increased with pressure over a narrow pressure region. The pressure-induced band may be due to intramolecular d–d transitions. The metal-to-semiconductor transition may arise from a change in the electronic states at high pressure, since no anomaly in the strucutre was observed.

ELECTRONIC STRUCTURE OF MATTER AT HIGH COMPRESSION : ISOSTRUCTURAL TRANSITIONS AND APPROACH OF THE FERMI-GAS LIMIT

This paper is concerned with the effect of atomic-shell structure on the cold pressure of highly compressed matter up to 1 Gbar and above. The method is to compare band-structure calculations based on augmented spherical waves with results of the quantum-statistical model. Elements treated explicitly are Li, He, Be, Al, and K. No indication for anomalies due to pressure ionization is found. However, an oscillatory behavior of cold-pressure curves is systematically obtained due to isostructural electronic transitions. They are traced back to pressure-induced band shifts and electron redistribution from bands of low angular momentum (typically s and p waves) to bands of higher angular momentum (typically d and f waves). The shifts are shown to occur generally when squeezing normal matter with extended atoms into highly degenerate matter. The anomalies are particularly pronounced for alkali metals: Strong pressure flattening is found for Li at fivefold compression and 5--10 Mbar, and very sharply for K somewhat below fivefold compression and 500 kbar. The oscillations are much weaker for Be and Al. Metallization of He is obtained at 12.1 g/${\mathrm{cm}}^{3}$ and 110 Mbar. For Be, a structural hcp\ensuremath{\rightarrow}bcc transition at 2 Mbar is predicted.

Electronic structure of matter at high compression: Isostructural transitions and approach of the Fermi-gas limit.

This paper is concerned with the effect of atomic-shell structure on the cold pressure of highly compressed matter up to 1 Gbar and above. The method is to compare band-structure calculations based on augmented spherical waves with results of the quantum-statistical model. Elements treated explicitly are Li, He, Be, Al, and K. No indication for anomalies due to pressure ionization is found. However, an oscillatory behavior of cold-pressure curves is systematically obtained due to isostructural electronic transitions. They are traced back to pressure-induced band shifts and electron redistribution from bands of low angular momentum (typically s and p waves) to bands of higher angular momentum (typically d and f waves). The shifts are shown to occur generally when squeezing normal matter with extended atoms into highly degenerate matter. The anomalies are particularly pronounced for alkali metals: Strong pressure flattening is found for Li at fivefold compression and 5--10 Mbar, and very sharply for K somewhat below fivefold compression and 500 kbar. The oscillations are much weaker for Be and Al. Metallization of He is obtained at 12.1 g/${\mathrm{cm}}^{3}$ and 110 Mbar. For Be, a structural hcp\ensuremath{\rightarrow}bcc transition at 2 Mbar is predicted.

Pressure-induced band cross-over in Pb1−x Sn x Te

Resistivity and thermoelectric power studies have been carried out on two semiconductor alloy systems viz Pb0·8Sn0·2Te and Pb0·6Sn0·4Te up to 35 kbar pressure. Thermoelectric power and resistivity data on Pb0·8Sn0·2Te indicate that the energy gapE g=E L 6− −E L +6 decreases with pressure resulting in a zero gap state near 35 kbar pressure. TEP studies on the alloy system Pb0·6Sn0·4Te provide direct evidence for a pressure induced L −6 →L +6 cross over transition.

Electrical and optical behavior of one-dimensional organic conductors at high pressures

Electrical and optical behavior of 10 one-dimensional organic conductors have been studied at high pressures. The electrical resistivity of the organic conductors decreases with increasing pressure in the low-pressure region and reaches each resistivity minimum. The lowest resistivity of TTF-TCNQ and TTF derivative salts at high pressures is much lower than that of other salts. For conducting TCNQ salts, a pressure-induced absorption band has been observed around 20 × 10 3 cm −1; it may be assigned to the new charge transfer band between TCNQ. A pressure-induced band is not found in TTF-TCNQ, but the change of spectra with pressure is observed around 12 × 10 3 cm −1. Above certain pressures the electrical resistivity of the organic conductors increases rapidly with increasing pressure and drifts upward with time. These phenomena arise from the solid phase reaction. The differences of the physicochemical properties of one-dimensional organic conductors will be discussed.

Pressure Effects of Absorption Bands in NaBr: Cu+

Absorption spectra of NaBr: Cu + have been investigated over the pressure range from 1 bar to about 6 kbar at a low temperature. There occurs a critical pressure-induced local phase transition (off-center to on-center transition), accompanying appearance of new absorption. bands on the lower energy side of the known absorption bands. The above pressure-induced absorption bands are assigned to the A band (spin-forbidden transition) and the B band in order of increasing photon energy, refering to the absorption spectrum of an on-center system, NaCl: Cu + . The origin of the off-center is discussed in connection with the pressure effects.

Spectral properties of hydrogen, helium, methane, and ammonia at thermal infrared wavelengths

We present some results of a theoretical and laboratory program to determine the thermal infrared spectral properties of the principal gaseous constituents of the atmosphere of Jupiter. G. Birnbaum has measured laboratory spectra in the 16 to 1000 um wavelength range for hydrogen and hydrogen-helium mixtures at Jovian temperatures. These are compared to theoretically computed spectra in order to determine the temperature dependence of the line strengths in the pressure-induced rotational band and the overlap parameters from the translational band. Existing spectral data for methane do not agree well with measurements of the ν 4 band at room temperature. A revised allocation of line intensities is proposed. Existing data for the ν 2 (10 um) band of ammonia do agree reasonably well with measurements at room temperature and at −77δC, but there are some important discrepancies which remain to be explained.

Pressure-induced absorption by H 2 in the atmosphere of Jupiter and Saturn

The S(1) line of the pressure-induced fundamental band of H 2 was identified and measured in the spectra of Saturn and Jupiter. This broad line at 4750 cm −1 lies in a region free from telluric and planetary absorptions. It is about 99% absorbing in the core; the high-frequency wing extends to at least 5100 cm −1. We compare the obseved line shape to the predictions of both a reflecting-layer model (RLM) and a homogeneous scattering model (HSM). The RLM provides a good fit to the Saturn line profile for temperatures near 150K; the derived base-level density is 0.52 (+0.26, −0.17) amagat and the H 2 abundance is 25 (+10, −9) km-amagat, assuming a scale height of 48 km. The Jupiter line profile is fit by both the RLM and HSM, but for widely differing temperatures, neither of which seems probable. The precise fitting of the observed S(1) line profile to computed models depends critically on the determination of the true continuum level; difficulties encountered in finding the continuum, especially for Jupiter, are discussed. Derived RLM densities and abundances for both planets are substantially lower than those derived from RLM analyses of the H 2 quadrupole lines, the 3 ν 3 band of CH 4, and from other sources.

ChemInform Abstract: PRESSURE EFFECT ON THE ABSORPTION SPECTRA OF IRON COMPOUNDS

AbstractBei Einwirkung von Druck bis zu 50 kbar auf Fe‐Verbindungen (FePO4 ′ nHlO.

Pressure Effect on the Absorption Spectra of Iron Compounds

Abstract The effect of pressures up to 50 kbar has been measured on the absorption spectra in five ferric compounds, two ferrous compounds, and one mixed valence compound. In six compounds, but not in basic ferric acetate and soluble prussian blue, we observed pressure-induced bands which were assigned to the d-d transition of the converted Fe(II) for the ferric compounds and to the spin-forbidden d-d transition for the ferrous compounds. The absorption spectra for single crystals of potassium ferri- and ferrocyanide were studied at hydrostatic pressures up to 5 kbar. The band-peak intensity decreased with an increase in the pressure in both cases. The charge-transfer band from Fe(II) to Fe(III) of soluble prussian blue showed a blue shift (69 cm−1/kb) at hydrostatic pressures up to 7.2 kbar.

Radiative transfer within the atmospheres of the major planets

A modified Eddington approximation is presented which describes radiative transfer due to the pressure-induced far infrared spectrum of hydrogen. When applied to the atmospheres of the major planets, the radiative transfer model produces atmospheric temperature profiles which are in good agreement with more detailed calculations. Solar absorption and infrared transmission by the fundamental bands of methane are further included for Jupiter and Saturn, resulting in a temperature inversion within both atmospheres above an altitude corresponding to a pressure of roughly 0.01 atm. This relatively high inversion region appears to be qualitatively consistent with infrared observations of methane and ammonia bands for Jupiter. Additional sources of solar absorption, consisting of the pressure-induced vibration-rotation bands of hydrogen and the overtones of the v 3 methane fundamental, are shown to have a minor effect upon the atmospheric thermal structure for Jupiter and Saturn.

Studies in Molecular Dynamics by Collision-Induced Infrared Absorption in H2–Rare Gas Mixtures. II. Diffusional Narrowing of Quadrupole-Induced Transitions

The profiles of the quadrupole-induced transitions (S1(0), S1(1)) of the pressure-induced fundamental infrared absorption band of hydrogen are studied in H2–Ar, H2–Kr, and H2–Xe gas mixtures up to high densities a few degrees above the critical temperature of the rare gas and in the corresponding liquid mixtures along the liquid–vapor line. The half-width δq of the transitions remains practically constant for the gas mixtures up to densities of several hundred amagat and then decreases rapidly. Values of δq for the liquid mixtures, when corrected for the temperature difference, are the same as the gas values at the same density. The narrowing of the transitions in the high-density region is interpreted in terms of the diffusional narrowing theory of Zaidi and Van Kranendonk, and is used to calculate diffusion coefficients D12 of H2 molecules in dense rare gas fluids. The results are in reasonable agreement with values of D12 previously deduced from the intercollisional interference effect in the overlap-induced components (Q1(0), Q1(1)) of the same band.

Intensity of the Pressure-Induced Fundamental Infrared Band of Gaseous Chlorine

The integrated intensity of the pressure-induced fundamental band of gaseous chlorine measured by Winkel, Hunt, and Clouter is about 5 times that calculated assuming that the transition moment arises from the oscillation of quadrupole-induced dipole moments. This provides good evidence that valence-type interaction between gaseous chlorine molecules occurs.

Investigation of the gapless state in bismuth-antimony alloys

The results are presented from an experimental study of the gapless state produced in semiconducting alloysBi1−xSbx by pressure-induced band inversion. The magnetoresistance properties of the alloys have been investigated both in weak magnetic fields (μH « 1) and in strong fields (H≤75 kOe) at liquidhelium temperatures in the Sb concentration interval 0.06≤x≤0.15 and pressure interval 1 bar ≤p<20 kbar. At pressuresp close to the pressurepk at which the gapless state is realized a “semiconductor-semimetal-semiconductor” transition is detected inBi1−xSbx alloys withx=0.070 and 0.071. The rates of change of the gap egL before and after inversion are determined: −(2.5±0.5)×10−6 eV/bar and (1.5±0.5)×10−6 eV/bar, respectively. A reduction in the carrier effective mass as egL → 0 is observed down to values of ∼ 10−4m0. It is shown that as egL → 0 the carrier mobilities in the alloys increase abruptly, the effect being a maximum in the purest alloys, where forT=4.2 K the mobility along the binary axis attains the record-high value of ∼ 3×108 cm2/V · sec.

Anisotropic Intermolecular Force Effects in Spectra of H2– and D2–Rare-Gas Complexes

Spectra of H2–Ar, H2–Kr, and H2–Xe Van der Waals complexes, accompanying the Q1(0), S1(0), Q1(1), and S1(1) transitions of the pressure-induced fundamental absorption band of hydrogen, have been studied in a path length of 165 m at temperatures in the range 85–158°K. At the low total gas densities used, 1–2 amagat, the lifetimes of the complexes were sufficiently long to show a great deal of new detail in the spectra. The R and P branches (δl = ± 1, where l is the angular momentum of the complex) accompanying the overlap-induced Q1(0) transitions, could be analyzed on a nonrigid rotation model to give Lennard-Jones ε and Σ parameters for the molecular pairs. The spectra accompanying the quadrupole-induced S1(0), Q1(1), and S1(1) transitions show T and N (δl = ± 3) as well as P and R branches, and many of the lines are split by the anisotropy of the intermolecular forces. The splitting is qualitatively in agreement with a model with loose coupling between J, the angular momentum of the H2 molecule, and 1. Well-resolved spectra of D2–Ar, D2–Kr, and D2–Xe complexes accompanying the S1(0) transition of deuterium were also obtained; these show more bound states than the H2–rare-gas spectra and anisotropic interaction effects are evident. The H2–N2 complex shows a spectrum with a diffuse structure not previously observed.

Pressure Narrowing of the Rotational Lines of the Fundamental Infrared Band of H2 in Collision-Induced Absorption

The enhancement by argon of the pressure-induced fundamental infrared absorption band of hydrogen is studied for Ar densities in the range 8–820 amagat at 152 K. The half-width, Δv1/2, of the quadrupole-induced S1(1) transition remains constant at 55 cm−1 up to ~300 amagat and then decreases to 25 cm−1 at the highest density. In the higher density range Δv1/2 varies inversely as the density. The S1(1) line of H2 in liquid H2–Ar solutions shows a similar pressure narrowing for Ar densities in the range 640–833 amagat at 115 K.

Estimate of the H2 abundance in the atmosphere of uranus from the pressure induced spectrum

Theoretical values of matrix elements of the quadrupole moment (Birnbaum and Poll, 1969; Dalgarno and Allison, 1969) and of the polarizability of H2 (Birnbaum and Poll, to be published) are used to calculate the integrated intensities of a number of lines in the pressure-induced overtone spectra of molecular hydrogen. These calculated intensities are used to obtain an estimate of the H2 abundance in the atmosphere of Uranus from the equivalent width of a feature in the 4-0 pressure-induced band observed by Giver and Spinrad (1966). An estimate of the equivalent width of the corresponding bands on Jupiter is also given.

Anomalous density dependence of the v1 band of carbon dioxide in pressure-induced absorption

The anomaly in the density variation of the integrated absorption of the pressure-induced v1 band of carbon dioxide is shown experimentally to be represented by a linear term in the density expansion of the intensity, which is important only in the lower density region (0–20 amagat). A calculation indicates that the linear term arises from the infrared-active absorption of the v1 band of C16O18O.

The Effect of Pressure on the Electronic Absorpion Spectra of TCNQ Ion Radical Salts

Abstract The effect of pressure up to 7 kbar on the electronic absorption spectra of the crystalline TCNQ anion radical salts and Li(TCNQ) in PVA has been studied. For simple TCNQ salts such as Li(TCNQ), a blue shift with an increase in the pressure has been observed in the absorption band associated with the intramolecular transition. The magnitude of the blue shift is between 45 and 86 cm−1/kbar. The observed shift has been discussed in terms of the crystal-field effect of the cations. For N-MePhe(TCNQ) and M(TCNQ)2, composed of the formally-neutral TCNQ and its anion radical such as TEA(TCNQ)2, a pressure-induced absorption band has been observed at around 20 kK; it may be assigned to the charge-transfer band between the anion radicals.