A first-principles study of the La–H system

Schöllhammer, Gunther; Wolf, W.; Herzig, Peter; Yvon, K.; Vajda, P.

doi:10.1016/j.jallcom.2008.10.009

Cited by 9 publications

(8 citation statements)

References 17 publications

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“…In the dilute-vacancy limit near the trihydride, close nearestneighbor pairs of octahedral vacancies are favored over two separated vacancies, a fact already known from previous DFT studies 6 and also reflected in the ground-state structures Fig. 4 shows the energetics of configurations with particular properties, namely, configurations with exactly two hydrogen vacancies per unit cell.…”

Section: B Percolating Vacancies and The Metal-insulator Transitionmentioning

confidence: 64%

“…They confirmed that the ground states at LaH 2.50 and LaH 2.71 are metallic, which is the expected result because experiments find the metal-insulator transitions not until around LaH 2.8 . 3,13 For the LaH 3 structure, a band gap appears and the solid is insulating 6 The question about the critical H concentration is answered by experiments: The transition occurs approximately at x = 2.8. 3,13 Without answer, however, remain the questions about the configurational process prompting the transition and the main difference between metallic and insulating hydrides.…”

Section: B Percolating Vacancies and The Metal-insulator Transitionmentioning

confidence: 99%

“…62) with La atoms occupying Wyckoff position 4c (x = 0.12974, z = 0.71477), tetrahedrally coordinated H atoms at Wyckoff position 8d (x = 0.11804, y = 0.00209, z = 0.20169), and octahedrally coordinated H atoms at Wyckoff position 4c (x = 0.36438, z = 0.15679). 5,6 The orthorhombic distortion goes along with a lowering of the total energy by 21 meV per formula unit. We deploy the P nma structure as reference for the H-rich side of the phase diagram.…”

Section: Introductionmentioning

confidence: 99%

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First-principles study of hydrogen ordering in lanthanum hydride and its effect on the metal-insulator transition

et al. 2012

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We discuss the structural details and the ordering of hydrogen in LaH x for 2 x 3. To this end, we combine first-principles calculations with the cluster-expansion method. This approach allows us to follow the H occupation of the interstitial sites within the face-centered cubic matrix of La atoms. We find that LaH x clearly favors the fluorite structure at x = 2 and adds excess H atoms at the octahedral interstitial sites. The ground-state behavior of the system is discussed and is found in agreement with experimentally observed structures at compositions LaH 2.25 and LaH 2.50 ; an additional ground state at composition LaH 2.71 is predicted. The cluster expansion also permits an extensive scan of LaH x structures with two octahedral vacancies per unit cell. For energetically favorable configurations, this scan yields a vacancy percolation threshold at LaH 2.75 that possibly drives the concentration-dependent metal-insulator transition: The band gap calculated for isolated vacancy pairs disappears for percolating vacancy chains. This transition from metallic to insulating state is also experimentally observed near to the composition LaH 2.8 and gives rise to the "switchable mirror" phenomenon.

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Section: B Percolating Vacancies and The Metal-insulator Transitionmentioning

confidence: 64%

Section: B Percolating Vacancies and The Metal-insulator Transitionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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First-principles study of hydrogen ordering in lanthanum hydride and its effect on the metal-insulator transition

et al. 2012

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“…The O-site hydrogen atoms form covalent-like bonds through the 1s(H)-5d(La) hybridization to open the band gap. [8][9][10][11] The dihydride is considered as an intermediate state in hydrogenation process toward trihydride and hence expected to exhibit significant response against external force probably in association with hydrogen transfer between the tetrahedral and octahedral interstitial sites.…”

Section: Introductionmentioning

confidence: 99%

Phase separation of lanthanum hydride under high pressure

et al. 2011

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Structural change of lanthanum dihydride LaH 2.3 , which has a face-centered-cubic (fcc) metal lattice with tetrahedral interstitial sites fully occupied with hydrogen atoms and partially occupied octahedral sites, has been investigated at high pressures up to 20 GPa at ambient temperature by synchrotron radiation x-ray diffraction. Additional Bragg reflections appear just on higher angle sides of the original ones at ∼11 GPa and their peak intensities increase gradually on further compression. The coexistence state of two fcc metal lattices thus observed above 11 GPa is interpreted in terms of phase separation or disproportionation reaction from the dihydride toward a solid solution and trihydride states, in both of which the octahedral interstitial sites are partially occupied with hydrogen atoms. A gradual distortion from the cubic to a tetragonal lattice is observed prior to the phase separation. The coexistence phase goes back to the dihydride fcc phase via the lattice distorted phase with decreasing pressure.

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“…[17][18][19][20] Stabilities of many R fcc H 2+x ͑0 Ͼ x Ͼ 1͒ phases have also been investigated by first-principles calculations. [21][22][23][24][25] Typically, the R hydride/deuteride phases have wide homogeneity ranges around their ideal stoichiometries, and these stoichiometric deviations are associated with various interesting phenomena such as magnetic transitions, metalinsulator transitions, and H ordering, particularly in the R fcc H 2+x phases. [1][2][3] Ideally, stoichiometric R fcc H 2 has the CaF 2 fluorite structure with all tetrahedral sites ͑T sites͒ occupied by H. In the composition range RH 2 -RH 3 , H atoms in R fcc H 2+x solid solutions must occupy both T sites and octahedral interstitial sites ͑O sites͒, and if all T and O sites are filled the stoichiometry is RH 3 .…”

Section: Introductionmentioning

confidence: 99%

Transmission electron microscopy and first-principles calculations of hydrogen ordering inβ-YH2+x

2010

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In YH 2+x hydrides stoichiometric deviations are associated with various interesting phenomena such as magnetic transitions and metal-insulator transitions; ordering of hydrogen on the octahedral sites ͑O sites͒ was suggested to be responsible. Long-range order was discovered in hydrogenated Y films by electron diffraction and high-resolution transmission electron microscopy. Ordering on interstitial O sites in the fcc-based Y fcc H 2+x solid solution ͑0 Ͻ x Ͻ 1͒ was investigated with computational tools from the Alloy Theoretic Automated Toolkit. The resulting set of 94 structure energies was fit to a cluster-expansion Hamiltonian that was used to perform a ground-state analysis for the mostly metastable fcc-based solid solution and for the more stable Y fcc H 2+x +Y hcp H 2+x two-phase mixture. The calculated structures were tested by the observed diffraction conditions and two possible ordered structures were suggested, a ground-state triclinic x = 0.0625 and an I-centered cubic x = 0.375.

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A first-principles study of the La–H system

Cited by 9 publications

References 17 publications

First-principles study of hydrogen ordering in lanthanum hydride and its effect on the metal-insulator transition

First-principles study of hydrogen ordering in lanthanum hydride and its effect on the metal-insulator transition

Phase separation of lanthanum hydride under high pressure

Transmission electron microscopy and first-principles calculations of hydrogen ordering inβ-YH2+x

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