Dihydride Phase Research Articles

Electronic properties and structure of the plutonium-hydrogen system

The resistivity and magnetic susceptibility from 1–300 K have been measured on five compositions of plutonium hydride, PuH x . The resistance vs. temperature data show metallic to semiconductive behavior as x is varied from 1.93 to 2.65 in the cubic dihydride phase field. The samples order ferromagnetically between 44 and 67 K as an increasing function of x. The effective paramagnetic moments are 0.83 to 0.70 μ B, and the ordered moments about half these values. X-ray photoemission measurements place the hybridized 5f-6d band 1.8 eV below the Fermi energy. The core level 4f spectra and the magnetic data are consistent with a trivalent plutonium configuration.

Conduction electron density of states and proton spin-lattice relaxation in the dihydrides of scandium, yttrium, lanthanum and lutetium

We report determinations of the proton Korringa product T 1 e T in the dihydride phases of scandium, yttrium, lanthanum and lutetium based on samples prepared from the highest purity metals available in the Ames Laboratory and in addition in some cases utilizing the technique of partial deuteration to suppress further the paramagnetic impurity contribution to the spin-lattice relaxation rate. We conclude that the quantity (T 1e T) − 1 2 , which is essentially proportional to the electronic density of states N ( E F ), has the value 0.055 ± 0.002 s − 1 2 K − 1 2 at the dihydride composition in all systems. The dependence of ( T 1e T) − 1 2 on hydrogen concentration within the dihydride phase appears to be weak; however, in LaH x ( T 1 e T ) − 1 2 decreases substantially for x ⪢ 2 indicating a decrease in N ( E f ) in this hydrogen concentration range. The implications of these results for relative s and d band contributions and hyperfine fields are discussed in the light of recent low temperature heat capacity measurements.

Mono- and dihydride phases on silicon surfaces — A comparative study by EELS and UPS

Mono- and dihydride phases on silicon surfaces — A comparative study by EELS and UPS

Mono- and dihydride phases on silicon surfaces- a comparative study by eels and ups

Abstract Vibrational and photoemission spectra show that a monohydride phase develops on all low index Si surfaces at elevated temperatures upon exposure to atomic hydrogen. This investigation demonstrates that on these surfaces also a dihydride phase forms at lower temperatures. Both phases are clearly distinguished by respective UPS and EELS spectra.

Mono- and dihydride phases on silicon surfaces — a comparative study by EELS and UPS

Vibrational and photoemission spectra show that a monohydride phase develops on all low index Si surfaces at elevated temperatures upon exposure to atomic hydrogen. This investigation demonstrates that on these surfaces also a dihydride phase forms at lower temperatures. Both phases are clearly distinguished by respective UPS and EELS spectra.

Nuclear magnetic resonance study of the dihydride phase of the Ti-V-H system

We report the results of nuclear magnetic resonance (NMR) measurements of 1. (1) the line shape, the second and fourth moments and the spin-lattice relaxation of the 1H resonance, and 2. (2) the Knight shift and spin-lattice relaxation of the 51V resonance in the dihydride phase Ti 1− x V x H 2 (0.04 ⩽ x ⩽ 0.65). The X-ray and magnetic susceptibility data are also reported. A two-line system was observed in both the 1H and 51V NMR spectra for x = 0.65 indicating that the range of existence of the single dihydride phase in vanadium-rich dihydrides is much narrower than reported previously. Comparison of the experimental and theoretical values of the linewidth and the second and fourth moments of 1H NMR shows that the dipolar interaction is the main source of the line broadening. The temperature dependence of the 1H value T 1 T and the magnetic susceptibility in dilute vanadium hydrides is interpreted in terms of the transition from an f.c.c. phase to a face-centred tetragonal phase induced by the Jahn-Teller effect. Comparison of the values of vanadium Knight shifts in Ti 1− x V x H 2 with existing data for the titanium Knight shift in TiH 2 strongly suggests that the local densities of states at titanium and vanadium sites in the dilute vanadium hydrides Ti 1− x V xH 2 are different. Opposite dependences of the 1H and 51V spinlattice relaxation rates on the vanadium concentration support this suggestion.

Chemisorption of hydrogen on the Si(100) surface: Monohydride and dihydride phases

We present a systematic study of the clean and chemisorbed-hydrogen-covered Si(100) surfaces. Experimentally, we used He i and He ii ultraviolet photoemission spectroscopy and were able to resolve important chemisorption features which had not been observed previously. We carried out electronic energy calculations using both empirical tight-binding and extended H\"uckel methods, from which we deduce a consistent interpretation of the spectra. It is found that the low-binding-energy surface states resulting from the asymmetric dimer-bond model are in good agreement with the experimental data, whereas one high-binding-energy surface state associated with the displacements of the subsurface atoms does not appear in the spectra. Hydrogen chemisorbing on the Si(100)-(2\ifmmode\times\else\texttimes\fi{}1) surface forms a monohydride phase, and removes most of the surface states, except for the dimer bond and some of the backbonding states. Based on the theoretical and experimental results it is suggested that at high coverage the Si---H bonds of the dihydride phase rotate and depart from the tetrahedral directions. The occurrence of this phase on the other surfaces of silicon, and its possible connections with the observed spectral features, are extensively discussed.

THE THERMAL DESORPTION SPECTRA STUDY OF HYDROGEN CHEMISORPTION ON Si(100) CLEAN SURFACE

The thermal desorption spectra of hydrogen chemisorpted on Si(100) clean surface have been measured by means of flash heating the sample under ultra high vacuum. The result shows that with the low hydrogen exposure at room temperature there is only one desorption peak A, while a second desorption peak B with lower peak temperature appears after increasing the exposure time. If the hydrogen exposures are carried out at elevated temperatures, the peak B does not appear in the desorption spectra at the temperature higher than 230℃, and no peak A appeared at the temperature above 530℃. With heat annealing the sample after room temperature exposure, the peak B will not existed if the annea- ling tempersture is higher than 350℃, and the peak A will also disappeared at 530℃ annealing. From all these features of thermal desorption, we believe that the peak A and B are related to the desorptions of monohydride and dihydride phase on Si (100) surface respectively. The measured activation energies of desorption of these two phases are 52.9 kcal/mol and 14.5 kcal/mol. It could be deduced from the peak asymmetry and the invariance of peak temperature with the initial hydrogen coverage that the desorption of peak A is happened as a first order desorption process. This has been futhur confirmed by the order plot. But the mechanism of hydrogen desorption on Si (100) surface is different from both the ordinary first order and second order desorption.

RHEED observations of disorder on Si(001)-(2×1) surfaces

RHEED patterns from Si(001) surfaces show the familiar (2×1) reconstruction. However, often there are also extra features in the diffraction patterns which indicate that the surface is not perfectly ordered (2×1). Some of the RHEED streaks are split and there are also extra curved streaks present. The splitting of the streaks is due to stepping of the surface. The most prominent set of curved streaks passes through the half order diffraction features arising from the reconstruction but not through the integral order diffraction features. It is therefore suggested that they are due to rows of disorder that disrupt the (2×1) lattice but allow all lattice points to lie on a (1×1) lattice. This model is tested by optical diffraction. A structural model of the disorder is proposed, based on the asymmetric dimer model of the (2×1) reconstruction. The rate at which the specimen is cooled to room temperature, after heating to clean it, is found to affect the disordering of the surface. The changes in the diffraction patterns when atomic hydrogen is chemisorbed to produce the monohydride and dihydride phases are described and explained. These changes support the asymmetric dimer model rather than the symmetric dimer model.

The surface structure of a-Sn{100} and the effect of hydrogen adsorption

Preliminary LEED studies, in conjuction with electron energy loss measurements, have been carried out on the surface structure of the semi-conducting a-Sn{100} surface and its interaction with atomic hydrogen. a-Sn was grown epitaxially on InSb{100} surfaces. The results show that the stable a-Sn{100} surface has a (2 × 1) reconstruction in common with Si and Ge{100}. Adsorption of atomic hydrogen appears to retain the (2 × 1) mesh but does induce some structural and electronic changes. It is suggested that this corresponds to the monohydride phase also seen on Si{100}, but the dihydride phase of the Si surface is not produced on Sn.

Room temperature hydrogenation of the Si(001)2 × 1 surface studied by angle-resolved electron energy loss spectroscopy

We observed the hydrogen adsorption on the Si(001)2 × 1 surface achieved at room temperature by angle-resolved electron energy loss spectroscopy (AR-ELS) and elastic low-energy electron diffraction. From measurements of the intensities of elastically diffracted beams, we found a characteristic hydrogen covered surface (called Si(001)2 × 1H(RT) surface in this paper), where all the diffracted beam intensities were enhanced drastically and a sharp 2 × 1 LEED pattern was observed. The angular dependence of the elastically diffracted beams on the 2 × 1H(RT) surface was different from that on the monohydride 2 × 1:H surface. On the 2 × 1H(RT) surface the S 3, transition from the back bond surface state disappeared in contrary to the 2 × 1:H surface and two hydrogen induced transitions were observed at 7.0 and 8.0 eV in AR-ELS spectra. We revealed that the 2 × 1H(RT) surface consisted of the monohydride and the dihydride phases with comparable weights. Additionally, we found the new transition S' 1, ascribed to the newly produced dangling bond surface state due to the rupture of the dimerization bond with hydrogen adsorption.

Electronic energy band structure of UH 3

We have calculated the electronic energy bands of α- and β-UH 3. These systems are unique among hydride formers in that a transformation directly to a trihydride occurs without an intervening dihydride phase and in that two trihydride structures form, both of which are based on the A15 (V 3Si) structure. We have found evidence of a fairly large bonding interaction between U f electrons and H s electrons, which is allowed and favored by these structures. β-UH 3 has eight formula units in the unit cell with the hydrogen atoms tetrahedrally coordinated. As usual there is a low-lying set (24) of hydrogen-derived states which overlaps the U f levels. This leads to the result of two types of f character: localized (magnetic) and itinerant. The itinerant type may be as large as one electron per uranium atom with a greater contribution from the “linear chain” atoms which form three of the four hydrogen neighbors than from the b.c.c.-like uranium atoms. This makes the localized contribution more consistent with f 1 and agrees more with the observed moment, 1.18 β B , than that expected for U 4+f 2.

Hydrogen chemisorption on silicon (100) 2 × 1

Changes in the valence band density of states (DOS) of a (100) silicon surface that accompany he chemisorption of atomic hydrogen onto that surface are deduced from a study of the changes in the L 2,3VV Auger lineshape. Complementary changes in the conduction band DOS are inferred from changes in L 2,3VV-core-level characteristic loss spectra (CLS). The chemisorbed hydrogen layer is identified as the dihydride phase from low energy electron diffraction measurements. Upon hydrogen adsorption the DOS at the top of the valence band decreases and new energy levels associated with the Si-H bonds appear lower in the band. Assuming that the Auger signal from the hydrogen covered sample consists of a superposition of a signal from silicon atoms bonded to hydrogen in the dihydride layer and an elemental-Si signal from the substrate, a N( E) difference spectrum with features due only to the dihydride is obtained by subtracting the background corrected, loss deconvoluted L 2,3VV signal for a clean (100)Si surface rom the corresponding signal for the hydrogen covered surface. Comparisons of the energy position of the major peak in this difference spectrum with that of the main peak in a gas phase silane Si-L 2,3VV spectrum, and of the corresponding Auger energy calculated empirically, indicate a hole—hole interaction energy of ~8 eV for the two-hole final state in the gaseous system and zero for the dihydride surface system. Hydrogen induced changes in the conduction band DOS are less apparent than those of the valence band DOS with only the possibility of a decrease in the DOS at the bottom of the conduction band being inferred from the CLS measurements. Electron stimulated desorption of hydrogen from the dihydride layer is adduced from changes in the Auger lineshape under electron beam irradiation of the surface. Hydrogen induced changes in the near-elastic electron energy loss spectra (ELS) are also reported and compared with previously published ELS results.

Isotope effects in the lanthanum dihydrides

Equilibrium pressures of protium, deuterium, and tritium over lanthanum metal at atomic ratios up to 2.2 have been measured for temperatures ranging from approx. 800 K to slightly over 1000 K. The equilibrium pressures in the plateau regins increased as expected from the protide to the deuteride to the tritide. Sieverts' constants were determined at the higher temperature and decreased in this same order. Within experimental error the terminal solubilities of the three isotopes in the metal, defined by the separation of the dihydride phase from the metal, are the same.

Determination of the relative number of monohydride and dihydride bonding sites in amorphous silicon-hydrogen alloys using pulsed NMR techniques

Proton spin-lattice relaxation time (T 1) measurements (made at 300K and 35 MHz) on reactively sputtered amorphous silicon-hydrogen alloys are correlated with the Fourier Transformed Free Induction Decay (FTFID) spectra and infrared absorption measurements of these alloys. These measurements and their interpretation yield the following assignments of T 1 values on the basis of bonding type: for purely monohydride material, T 1 = 35±5 sec. In a material containing two distinct, co-existing phases, the dihydride phase regions are characterized by T 1 = 1.4±0.5 sec. Relaxation of protons in the monohydride regions of this material proceeds via cross-relaxation to the dihydride regions, the process being spin-diffusion limited and yielding a net T 1 = 8.8±1.0 sec.

Hydrogen locations, diffusion and the electronic density of states in yttrium dihydrides: A nuclear magnetic resonance investigation

We report results of wide-line and pulsed proton magnetic resonance measurements on yttrium dihydrides, YH x (1.63 ⩽ x ⩽ 1.98), covering the temperature range 140–760 K and yielding information on hydrogen locations, diffusion and the density of states in the dihydride phase. Our principal conclusions are as follows. (1) At x = 1.98, wide-line second moment measurements indicate that 15% of the octahedral (O) sites in the f.c.c. CaF 2 structure are occupied by hydrogen. (2) Spin-lattice relaxation time measurements in the rotating frame ( T 1 ρ versus T) yield hydrogen jump frequencies ν j showing three distinct regions in which the activation energy E act for hydrogen diffusion increases with T. (3) For x = 1.98 ν j is consistently higher than for x = 1.92, corroborating the partial occupation of O sites for x values below 2.0. (4) At the lower limiting composition ( x ≈ 1.8) E act becomes anomalously large (approximately 1.1 eV atom −1), compared with the value 0.5 eV atom −1 at x = 1.92, and the diffusion rate is about 10 3 times smaller. (5) At low temperatures the spin-lattice relaxation rate T −1 1 is dominated by the conduction electron contribution T −1 1 e . The quantity (T 1eT) −1 2 which is proportional to the density of states at the Fermi level, is 0.061 ± 0.002 s −1 2 K −1 2 for the dihydride compositions studied, indicating a constant density of states in the dihydride phase.

Studies of the Separation of Hydrogen Isotopes by a Pressure Swing Adsorption Process

Abstract A theoretical and experimental study was made of the mechanism of the separation of a mixture of hydrogen isotopes by a two-column pressure swing adsorption process in which vanadium hydride was used as the solid phase. Theoretical predictions of process performance were compared with data from process experiments using hydrogen containing a trace of HT as feed. For process operation wholly within the monohydride phase, a postulated isotope effect in the rates of hydrogen absorption and desorption appeared to control process performance. When process operation involved transitions back and forth between the monohydride and dihydride phases, process performance was determined mainly by an equilibrium isotope effect.

Low-Temperature Heat Capacity Studies of the Dihydride Phase of Titanium-Niobium-Hydrogen Alloys

Low-Temperature Heat Capacity Studies of the Dihydride Phase of Titanium-Niobium-Hydrogen Alloys

Electronic structure of metal hydrides. I. Optical studies of ScH2, YH2, and LuH2

We have examined the electronic structure of Sc${\mathrm{H}}_{2}$, Y${\mathrm{H}}_{2}$, and Y${\mathrm{H}}_{2}$ using optical absorptivity and thermoreflectance techniques in the photon energy range from 0.2 to 5 eV between 4.2 and 340 K. The measured quantities were used to determine the frequency-dependent dielectric functions and the dependence of the dielectric functions on temperature modulation. The results show that the low-energy properties ($h\ensuremath{\nu}\ensuremath{\lesssim}1.5$ eV) are dominated by intraband absorption and a plasmon falling near 1.5-1.8 eV. Interband absorption is observed to be strong and structured above the interband onsets of \ensuremath{\sim} 1.25, \ensuremath{\sim} 1.6, and \ensuremath{\sim} 1.9 eV for Sc${\mathrm{H}}_{2}$, Y${\mathrm{H}}_{2}$, and Lu${\mathrm{H}}_{2}$, respectively. The observed interband features can be interpreted in terms of the self-consistent band calculations of Sc${\mathrm{H}}_{2}$ and Y${\mathrm{H}}_{2}$ presented in the companion paper, and experimental features can be related to specific bands in particular parts of the Brillouin zone. The systematics observed in these three trivalent metal dihydrides can be correlated well to theory. Extensive studies with samples of varying hydrogen to metal ratio ($x$) within the dihydride phase were carried out to examine the influence of hydrogen sublattice disorder on the optical properties and electronic structure. It was observed that, for $x$ approaching 2, interband features which could be related to $d$-band absorption were broadened by increasing lattice disorder, and new features in the interband absorption spectra were observed which could not be interpreted without postulating the hydrogen occupancy of significant numbers of octahedral sites. The strong $x$ dependence of the optical features emphasizes the importance of studying well-characterized samples.

ChemInform Abstract: THE GADOLINIUM‐HYDROGEN SYSTEM

AbstractMößbauer‐spektroskopische Untersuchungen (Isomerenverschiebung, magnetisches Hyperfeinfeld am Gd‐Kern, Quadrupolaufspal‐ tung) oberhalb und unterhalb der Neel‐Temp.