Yttrium Trihydride Research Articles

Reactivity of a Neutral Yttrium(III) Trimethyl Complex toward Brønsted and Lewis Acids.

The present study corroborates that the neutral tridentate N-ligand 1,4,7-trimethyl-triazacyclononane (Me3TACN) qualifies as a versatile platform to study selective ligand exchange with rare-earth-metal alkyl complexes, herein [(Me3TACN)YMe3]. Treatment with Brønsted-acidic bis(dimethylsilyl)amine, HN(SiHMe2)2, gave selectively the mono-exchanged heteroleptic complex [(Me3TACN)YMe2{N(SiHMe2)2}]. Depending on the molecular ratio employed, the reaction of [(Me3TACN)YMe3] with AlMe3 resulted in the isolation/crystallization of [(Me3TACN)YMe2(AlMe4)] [1 : 1] or ion-separated [(Me3TACN)YMe(AlMe4)][AlMe4] [1 : 2] and [(Me3TACN)YMe(AlMe4)][Al2Me7] [1 : 3]. Analogous reactions with the heavier group 13 methyls GaMe3 and InMe3 generated mixed methyl/tetramethylgallato complex [(Me3TACN)YMe2(GaMe4)] and ion-separated [{(Me3TACN)YMe2}2{μ-Me}][InMe4]. Finally, dimethylalane, HAlMe2, converted [(Me3TACN)YMe3] into heteroaluminate [(Me3TACN)Y(HAlMe3)3], representing an AlMe3-supported, molecular yttrium trihydride complex. All compounds were investigated by single crystal X-ray diffraction (SC-XRD), homo- and heteronuclear (13C, 27Al, 89Y, 115In) NMR as well as IR spectroscopies and elemental analyses.

Infrared Gasochromic Devices Based on Metal Thin Films

AbstractVariable infrared emissive materials and devices are effective for the dynamic modulation of infrared radiation. Herein, a new idea of dynamic emissivity modulation based on the hydrogen‐induced metal–insulator phase transition from metallic yttrium (or yttrium dihydride) to dielectric yttrium trihydride is proposed, and infrared gasochromic devices based on yttrium/rhodium metal thin films and infrared‐transparent covers are developed. By alternately injecting 4% hydrogen–argon gas mixture and air into the gasochromic device, the infrared emissivity of the device is dynamically modulated reversibly. The emissivity changes in the 340‐nm‐yttrium/6‐nm‐rhodium device are 0.25 and 0.39 in the 3–5 and 7.5–14 µm atmospheric window regions, respectively, and the response time is <40 s. The emissivity change is affected by the surface morphology and thickness of the yttrium/rhodium film. Moreover, the infrared gasochromic device operates effectively over 100 cycles, and subsequent failure is attributed to oxidation of the yttrium film. The proposed device is easily enlarged and patterned because of its simple structure, thereby indicating that such metal thin films provide a new material platform for dynamic infrared radiation modulation, which has great application prospects in the fields of adaptive thermal camouflage, smart thermal management, and infrared information display.

Formation of vacancies and metallic-like domains in photochromic rare-earth oxyhydride thin films studied by in-situ illumination positron annihilation spectroscopy

Rare-earth (RE) oxyhydride thin films show a color-neutral, reversible photochromic effect at ambient conditions. The origin of the photochromism is the topic of current investigations. Here, we investigated the lattice defects, electronic structure, and crystal structure of photochromic ${\mathrm{YH}}_{x}{\mathrm{O}}_{y}$ and $\mathrm{Gd}{\mathrm{H}}_{x}{\mathrm{O}}_{y}$ thin films deposited by magnetron sputtering using positron annihilation techniques and x-ray diffraction, in comparison with Y, ${\mathrm{YH}}_{\ensuremath{\sim}1.9}$, ${\mathrm{Y}}_{2}{\mathrm{O}}_{3}$, Gd, $\mathrm{Gd}{\mathrm{H}}_{\ensuremath{\sim}1.8}$, and ${\mathrm{Gd}}_{2}{\mathrm{O}}_{3}$ films. Positron annihilation lifetime spectroscopy (PALS) reveals the presence of cation monovacancies in the as-deposited Y and ${\mathrm{YH}}_{\ensuremath{\sim}1.9}$ films at concentrations of $\ensuremath{\sim}{10}^{\ensuremath{-}5}$ per cation. In addition, vacancy clusters and nanopores are found in the as-prepared ${\mathrm{YH}}_{x}{\mathrm{O}}_{y}$ and ${\mathrm{Y}}_{2}{\mathrm{O}}_{3}$ films. Doppler broadening positron annihilation spectroscopy (DB-PAS) of the Y- and Gd-based films reflects the transition from a metallic to an insulating nature of the RE metal, metal hydride, semiconducting oxyhydride and insulating oxide films. In-situ illumination DB-PAS shows the irreversible formation predominantly of di-vacancies, as PALS showed that cation mono-vacancies are already abundantly present in the as-prepared films. The formation of di-vacancies supports conjectures that ${\mathrm{H}}^{\text{--}}$ (and/or ${\mathrm{O}}^{2\text{--}}$) ions become mobile upon illumination, as these will leave anion vacancies behind, some of which may subsequently cluster with cation vacancies present. In addition, in RE oxyhydride films, partially reversible shifts in the Doppler parameters are observed that correlate with the photochromic effect and point to the formation of metallic domains in the semiconducting films. Two processes are discussed that may explain the formation of these metallic domains and the changes in optical properties associated with the photochromic effect. The first process considers the reversible formation of metallic nanodomains with reduced O : H composition by transport of light-induced mobile hydrogen and local oxygen displacements. The second process considers metallic nanodomains resulting from the trapping of photoexcited electrons in an ${e}_{g}$ orbital at the yttrium ions surrounding positively charged hydrogen vacancies that are formed by light-induced removal of hydrogen atoms from octahedral sites. When a sufficiently large concentration, on the order of \ensuremath{\sim}10%, is reached in a certain domain of the film, band formation of the ${e}_{g}$ electrons may occur, leading to an Anderson-Mott insulator-metal transition like the case of yttrium trihydride in these domains.

The origin of the highly crystallized face-centered cubic YH[formula omitted] high-pressure phase when quenched to ambient condition

The origin of the face-centered cubic (FCC) structure in yttrium trihydride stabilized (or quenched) at ambient pressure was investigated using transmission electron microscopy (TEM) and in situ synchrotron x-ray diffraction (XRD) measurements under high pressures in the gigapascal range. Although the FCC-YH3 high-pressure phase is normally unstable at ambient pressure, in this study, we found that the FCC-YH3 phase, with high crystallinity, could be obtained via sintering at 1373 K for 24 h under 6 GPa, and subsequent quenching at a rate greater than 1000 K/min. The XRD profile of the FCC-YH3 phase showed sharp peak patterns, i.e.,it had a high crystallinity; however, the TEM observations revealed that the surfaces of the particles were significantly disordered. It was observed that these surface defects played an important role in keeping the FCC-YH3 high-pressure phase present at the ambient pressure. In situ XRD measurements during high-pressure and temperature processing confirmed that the YH3 phase structure had completely transformed from HCP to FCC at 1173 K and 9 GPa; however, the amount of HCP-YH3 ambient pressure phase increased continuously during cooling and depressurization. The sample cooling rate during in situ measurements was 90 K/min, which was more than ten times slower than that in the ex situ experiments. Since the defects introduced into the surface of the particles during the slow cooling process were insufficient, a reversal of the HCP-FCC reversible structural change was observed during the depressurization after sintering.

Face-centered-cubic yttrium trihydride high-pressure phase stabilized at ambient pressures by mechanical milling

The crystal structure of yttrium trihydride (YH3) is usually characterized by hexagonal close-packed (HCP) metal ordering at ambient conditions and transforms into an FCC structure (FCC-YH3−δ) at pressures greater than 8 GPa. FCC-YH3−δ was previously considered to only be stable under high pressures, but we found that it can be stabilized even at ambient pressures by the mechanical milling of hexagonal YH3(HCP-YH3) in this study. The lattice constant of the FCC-“YH3−δ” sample was evaluated to be a = 0.52801(2) nm, which corresponds to the expected value calculated in other studies. This structural change from HCP to FCC enlarged band gap of the YH3 from 2.2 eV to 2.45 eV measured by UV–vis spectrometry. FCC-YH3 was dehydrogenated to an FCC “YH2” dihydride phase at 520 K, which is known to be conventionally present at ambient pressures. However, the YH2 generated from FCC-YH3−δ maintained its FCC structure upon re-hydrogenation to trihydride, unlike how YH2 usually changes its structure from FCC to HCP. The volume change after the dehydrogenation of FCC-YH3−δ was approximately 5%, which is smaller than that of HCP-YH3 (approximately 10%).

Pressure induced nodal line semimetal in YH3

Abstract The electronic structure of yttrium trihydride (YH3) under pressure has been explored by using the first-principle calculation. The existence of semiconductor phase of YH3 is predicted at low pressure with symmetry group p 3 ¯ c 1 $p\overline{3}c1$ (165). In the range of 10–24 GPa, electron- and hole-like bands near the Fermi level are overlapped and form a snake-like nodal ring around Γ point. Different from previous literature (D. Shao, T. Chen, Q. Gu, et al., “Nonsymmorphic symmetry protected node-line semimetal in the trigonal YH3,” Sci. Rep., vol. 8, 2018.; J. Wang, Y. Liu, K.-H. Jin, et al., Phys. Rev. B, vol. 98, p. 201112, 2018), which assumes the band degeneracy is protected by mirror symmetry, we argue that the nodal line is protected by the space inversion symmetry and the time reversal symmetry. For weak spin-orbital coupling (SOC), the fermion modes at the band crossings are real 3D Majorana fermions. This is a typical double charged nodal-line semimetal, meaning that there are two topological invariants of this nodal line: a 1D Berry’s phase and a Z 2 monopole charge, which are related to the first and the second Stiefel-Whitney classes of the Berry bundle and can be given by the first-principle calculation. It turns out that the 1D Berry’s phase is nontrivial, but the Z 2 monopole charge is trivial. Therefore, this nodal line can shrink to a point and gapped out without breaking the topological constraints.

Role of oxygen in materials properties of yttrium trihydride

Numerous experiments have shown that the oxygen-free films of YH3 possess the face-centered cubic (fcc) structure only at high pressures whereas oxygen-containing YH3 films crystallize into the fcc lattice at ambient conditions. In this report, by means of first-principles simulations, we provide a detail understanding of the role of oxygen in stabilization of the fcc YH3 under normal pressure. We performed the oxygen position preferences screening within the fcc unit cell along with geometrical optimization series and verified the major stability conditions. The main aggregate and electronic characteristics have been calculated in order to gain an insight into how the Y–O bond activation and coordination in the metallic matrix create new materials properties. The possibility of the compositional architecture on the base of fcc crystalline silicon and oxygen-containing YH3 compounds has been suggested.

The electronic state of thin films of yttrium, yttrium hydrides and yttrium oxide

Thin films of yttrium hydride have for almost 20 years been under investigation for optoelectronic and solar energy applications due to the hydrogen-induced switching in electronic state from the metallic elemental yttrium and yttrium dihydride to the transparent semiconductor material yttrium trihydride. In this study, we investigate the electronic structure of yttrium, yttrium hydride and yttrium oxide by using X-ray photoelectron spectroscopy and kelvin probe measurements. The investigated samples have been prepared by reactive sputtering deposition. We show that the electronic work function of transparent yttrium hydride is of 4.76eV and that the recently discovered photochromic reaction lowers the electronic work function of the transparent hydride by 0.2eV.

Yttrium Hydride Nanoantennas for Active Plasmonics

A key challenge for the development of active plasmonic nanodevices is the lack of materials with fully controllable plasmonic properties. In this work, we demonstrate that a plasmonic resonance in top-down nanofabricated yttrium antennas can be completely and reversibly turned on and off using hydrogen exposure. We fabricate arrays of yttrium nanorods and optically observe, in extinction spectra, the hydrogen-induced phase transition between the metallic yttrium dihydride and the insulating trihydride. Whereas the yttrium dihydride nanostructures exhibit a pronounced particle plasmon resonance, the transition to yttrium trihydride leads to a complete vanishing of the resonant behavior. The plasmonic resonance in the dihydride state can be tuned over a wide wavelength range by simply varying the size of the nanostructures. Furthermore, we develop an analytical diffusion model to explain the temporal behavior of the hydrogen loading and unloading trajectories observed in our experiments and gain information about the thermodynamics of our device. Thus, our nanorod system serves as a versatile basic building block for active plasmonic devices ranging from switchable perfect absorbers to active local heating control elements.

On the dynamical stability and metallic behavior of YH3 under pressure

We report on the behavior of structural and electronic properties of yttrium trihydride under pressure using first principles calculations. We show that YH3 undergoes a structural transformation and its high pressure phase is dynamically stable under pressure since the peak at the imaginary frequencies of the phonon density of states, which account for the structural instability disappears at high pressure. Additionally, our GW calculations indicate a metallization of the high pressure cubic phase of YH3.

金属水素化物の高圧研究のフロンティア

The highly compressed state of metal hydride has been investigated to explore structural and electronic transitions characteristic to hydride. The hydrogen atoms play the lead in the transitions, showing displacement in or transfer between the interstitial sites of a metal lattice. One example is the band gap closing observed for a long period lattice of yttrium tri-hydride, YH3, appearing at pressures above 12 GPa. Another example is photochromic event observed again for yttrium hydride, YHx. The former is likely accompanied by the in-site displacement of hydrogen atoms and the latter by the site-to-site transfer.

Long-period stacking structures in yttrium trihydride at high pressure

The pressure-induced structural transition of $\mathrm{Y}{\mathrm{H}}_{3}$ has been investigated by x-ray diffraction experiments under hydrostatic pressure at room temperature. The diffraction patterns of the intermediate state between the hexagonal and cubic phases are interpreted in terms of long-period polytypes with the periodic arrangements of hexagonal-type and fcc-type stacking layers of the Y metals. The hydrogen-hydrogen repulsive interactions and hydrogen-metal bonding likely play a dominant role in the gradual hexagonal-fcc lattice conversion via the long-period polytypic structures.

Pressure induced phase transformation of REH 3

High-pressure X-ray diffraction studies of the number of rare earth trihydrides (REH 3) have been carried out in a diamond anvil cell up to 30 GPa at room temperature. This includes the family of rare earth trihydrides that possess hexagonal structure at ambient conditions. A reversible structural phase transformation from the hexagonal to cubic phase has been observed for the Sm, Gd, Er, Ho and luthetium trihydrides. The same type of phase transition has been found also for yttrium trihydride. The lattice parameters of the new cubic phases and the volume changes at transition points were determined for the all compounds investigated. The parameters of the equation of state for all the hexagonal and cubic phases have been determined in the pressure range up to 30 GPa. The role of H–H repulsive interaction as a probable promoter of the transition and a systematics of transition are discussed.

X-ray diffraction investigation of the hexagonal–fcc structural transition in yttrium trihydride under hydrostatic pressure

Pressure induced structural transition of yttrium hydride has been investigated using synchrotron radiation X-ray diffraction measurement up to 24 GPa at room temperature. A reversible hexagonal–fcc transition with a wide intermediate region from 11 to 20 GPa is confirmed, which is consistent with previously reported X-ray results. The diffraction patterns measured for the intermediate state are not interpreted in terms of a hexagonal–fcc coexisting state or as rare-earth structures with various metal-layer stacking sequences. The equation of state determined for the hexagonal–YH 3 provides a bulk modulus B 0 of 71.9 GPa, which is nearly half of the previously reported value, but is within the range of values reported for other rare-earth metal trihydrides with hexagonal structures.

Infrared spectroscopic study of the band-gap closure inYH3at high pressure

Infrared vibrational absorption spectra are measured for yttrium trihydride at room temperature and pressures up to $30\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The spectral change that begins near $12\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ is interpreted in terms of a hcp-fcc structural transition, which agrees with previous x-ray diffraction measurements. For the hydrogen vibrations in the low-pressure hcp phase, the mode Gr\uneisen parameters are derived from the observed peak frequency shifts with pressure and the reported bulk modulus. The value of 1.91 for the octahedral-site vibration is three times larger than those for the tetrahedral-site vibrations, suggesting hybridization between the hydrogen $1s$ and yttrium $4d$ orbitals at the octahedral site. The infrared transmission spectra collapse when the high-pressure fcc phase is compressed beyond $23\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The band gap abruptly closes without a structural change in the fcc metal lattice. The experimental results are in contrast to the previous theoretical calculations predicting the electronic transition either with hcp-fcc structural change or in the hcp low-pressure phase without structural change. The transition mechanism is still inconclusive.

Crystal structure and lattice dynamics of high-pressure scandium trihydride

Powder samples of hcp $\mathrm{Sc}{\mathrm{H}}_{2.9}$ and $\mathrm{Sc}{\mathrm{D}}_{2.9}$ have been synthesized under a $1\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ pressure of gaseous hydrogen or deuterium and studied at ambient pressure and $95\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ by neutron diffraction. A profile analysis of the diffraction patterns indicated a crystal structure with $P{6}_{3}∕mmc$ symmetry that differs from the structures of hcp trihydrides of yttrium and rare-earth metals by the absence of long-range order in the displacements of hydrogen atoms from high-symmetry positions. The lattice dynamics of scandium trihydride and, for comparison, yttrium trihydride were studied by inelastic neutron scattering at $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the range of energy transfers $4--250\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. The spectra of optical H vibrations in these two trihydrides proved to be similar, suggesting similar arrangements of hydrogen atoms on a short-range scale.

Hexagonal to cubic phase transition in YH 3 under high pressure

X-ray diffraction studies on bulk yttrium trihydride, in a diamond anvil cell, have been carried out up to 25 GPa. Pressure induced hexagonal-to-cubic phase transformation in YH 3 has been found at pressure of about 8 GPa. The lattice parameter of the new cubic phase was determined as equal to 5.28 Å. This finding confirms the theoretical predictions based on first principle calculations of such a transformation. Equations of state have been determined for both the hexagonal hcp and cubic fcc YH 3 phases. As compared to the pure yttrium metal, bulk modulus for YH 3 is about four times bigger. The similarity of this transition to that observed in the other 4-f trivalent hydrides has been discussed.

Unusual doublet structure in proton magnetic-resonance spectra of yttrium and lutetium trihydrides

The proton magnetic resonance spectra of yttrium and lutetium trihydride, $\mathrm{Y}{\mathrm{H}}_{3}$ and $\mathrm{Lu}{\mathrm{H}}_{3}$, respectively, show an unusual doublet structure qualitatively similar to the Pake doublet that results from the mutual dipolar interaction of closely spaced, isolated proton pairs. However, both the magnitude of the splittings, roughly $70\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}$, and the second moment of the spectra, roughly $200\phantom{\rule{0.3em}{0ex}}{(\mathrm{Oe})}^{2}$, greatly exceed classical dipolar theoretical expectations based on known nearest-neighbor spacings. On the other hand, the $\mathrm{Y}{\mathrm{H}}_{3}$ doublet narrows to a single line above $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, consistent with the temperature dependence of the spin-lattice relaxation rate, whereas the $\mathrm{Lu}{\mathrm{H}}_{3}$ doublet shows no narrowing up to $454\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In addition, the peaks themselves are close to Lorentzian in shape, indicative of strong indirect exchange narrowing. These features of the spectra indicate that substantial contributions from electron-mediated pseudodipolar and indirect exchange interactions exceed the classical dipolar interaction.

Optical properties of the γ-phase yttrium trihydride YH3−δ

The transmission spectra of thin films of γ-phase yttrium trihydride YHx, x≅2.87, are measured in the energy range 0.05–6 eV at room temperature. The measured quantities are used to determine the frequency-dependent index of refraction, extinction coefficient, dielectric function, and optical conductivity. In the energy range (E=0.2–6 eV), interband transition and impurity-related transitions dominate the optical properties. In the low-energy range (E=0.05–0.2 eV), hydrogen vibration modes dominate the optical properties. In addition, strong absorption due to free carriers is observed. This work also evaluates several parameters such as plasma frequency, effective carrier density, and donor ionization energy. The possible sources for optical features observed in the spectra are also described.

Structural and electronic properties of hexagonal yttrium trihydride.

The structural and electronic properties of yttrium trihydride with metal atoms in the hexagonal-close-packed (hcp) structure are studied by the pseudopotential method within the local-density-functional approximation (LDA). It is found that the hydrogen positions within the metal lattice have a major role in determining these properties. Calculations confirmed that hexagonal ${\mathrm{YH}}_{3}$ with unusual wavelike hydrogen displacements (space group ${\mathit{D}}_{3\mathit{d}}^{4}$ or P3\ifmmode\bar\else\textasciimacron\fi{}c1) is energetically more stable than the cubic structure. This result is consistent with neutron-diffraction data for ${\mathrm{YD}}_{3}$ and for a heavy rare-earth trihydride ${\mathrm{HoD}}_{3}$. These hydrogen displacements are identified as Peierls-like distortions in this three-dimensional system. The calculated final LDA band structure for ${\mathrm{YH}}_{3}$ gives a semimetal rather than a semiconductor. With electron and hole pockets overlapping around the \ensuremath{\Gamma} point, the system is highly unstable. Other possible symmetry-breaking-distortion and many-body effects such as excitonic condensation are discussed.