X-ray diffraction and nuclear magnetic resonance studies of the relationship between electronic structure and the tetragonal distortion in zirconium hydride (ZrHx)

Abstract

Low-temperature (LT) X-ray diffraction (XRD) studies of 13 ZrH, samples in the range 1.50 5 x 5 2.00 were made by using a Guinier-Simon (GS) focusing camera and other methods. The ZrH, phase diagram was made more complete for the fcc (&phase) and fct (€-phase) regions from ambient to low temperature (i.e,, to about 110 K). No new low-temperature phases were obbserved by XRD. Careful determinations of the composition and temperature dependences of the following parameters were made: c/a ratio, V (unit-cell volume), and Al/(ZAT). When the XRD results are combined with the solid-state proton NMR parameters that reflect the density of electron states at the Fermi level, mutual support is obtained for the Jahn-Teller splitting of the Zr d bands to produce the tetragonal distortion of the ideal fcc lattice when the stoichiometry exceeds x = 1.66. The lattice constants from XRD confirm that the maximum in the values of the proton NMR parameters that are observed when 1 .80 5 x I 1.85 is due primarily to the Fermi level falling at the maximum of the lower split d band and is not merely a lattice-temperature effect.A systematic study of the influence of chemical structure on the performance of merocyanine dyes as solar energy converters has been made. Key correlations between current-generating capability and chemical properties such as chain length, electronegativity, and acid strength have been established. While these correlations are helpful in leading to systematic improvements in performance, certain chemical group substitutions give rise to disproportionate increases signaling the simultaneous advancement of several underlying mechanisms. While the highest sunlight efficiencies achieved to date have been with binuclear structures, certain trinuclear structures have been developed which exhibit broader spectral width and higher quantum efficiency than their binuclear counterparts. The inherently higher solar efficiency potential of devices made from these structures has not yet been realized, however, because of instability problems. Squarylium dyes also exhibit performance indicative of potential high efficiency. Broad spectral widths occur for certain structures due to aggregate formation, High quantum efficiency and open circuit voltage, however, seem to correlate with narrow solid-state absorption bands rather than with broadened aggregate-induced absorption.

Effect of electronegativity on the mechanical properties of metal hydrides with a fluorite structure

“…3. The lattice parameter in the present experiment is good accordance with the reported values [21][22][23][24][25][26]. The lattice parameter of zirconium hydride slightly increases with an increase in the hydrogen content as follows:…”

Influence of hydrogen on electron-phonon coupling and intrinsic electrical resistivity in zirconium: A first-principles study

“…The calculated lattice parameters (a and c) of pure Zr and the ZrH x systems are shown in Table I and have very good agreement with the previous calculation results and experimental data 1,13,15,[25][26][27]31,50,51 . It is observed that the ratio c/a decreases with increasing H concentration in Zr, as in experiments 26,27 . As the intrinsic electrical resistivity depends on the crystal volume [Eq.…”

“…There are three indications of the structural instability of the δ phase: the high peak of the EDOS at the Fermi level, imaginary phonon frequencies and the extremely high value of the Fermi surface nesting factor, and they correspond to the electron-, phonon-, and electron-phonon-driven mechanisms of the tetragonal distortion in the Zr-H system. There are two main electron-driven mechanisms, the first is the splitting of the bands at the Fermi level in the Γ − L direction due to Jahn-Teller effect 26,30,31,33 , and the second is the reduction of N (ε F ) by a shift in energy of the band along the Γ − K direction 29,31 . The key point is that the two electron-driven mechanisms lead to the reduction in N (ε F ) from the δ phase to the ε phase ( Fig.…”

“…It was shown in experimental and theoretical works that at low H concentrations, the Zr-H system has a hexagonal close packed (HCP) structure and H atoms are located at tetragonal interstitial sites [20][21][22][23] . At high H concentrations (1 H/Zr 2), the Zr-H system has a face-centered cubic (FCC) or facecentered tetragonal (FCT) structure 15,[24][25][26][27] . The transition from the δ phase (FCC structure) to the ε phase (FCT structure, c < a) was investigated in Refs.…”

Isotope effects in thermal neutron transmission and backscattering processes for ε-phase zirconium hydrides and deuterides

“…During reactor irradiation, a temperature gradient develops radially in the target, and the hydrogen atoms migrate to the lower-temperature target surface by thermal diffusion [2]. A small change of hydrogen concentration has a large influence on thermal, electrical and mechanical properties, due to changes in charge density between Zr and H atoms [3,4]. For example, the hydrogen concentration dependence of the thermal conductivity r, electrical resistivity q and Vickers microhardness H v for the e-phase zirconium hydrides at 300 K were r = 24.4-82.1 W/m K (H/Zr = D/Zr = 1.76-1.99) [5], q = 54.7 Â 10 À2 -24.7 Â 10 À2 X m (H/Zr = D/Zr = 1.81-1.96) [6] and H v = 155 ± 12-164 ± 10 (H/Zr = 1.7-2.0) [7], respectively.…”