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

Abstract

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.

Hydride-Based Electride Material, LnH2 (Ln = La, Ce, or Y)

“…These phenomena, which are observed in metals including Ln and intermetallics, suggest a unique route for the synthesis of new electride materials possessing vacancies through the utilization of hydrogen. The primary features of the calculated band structure of LaH 2 agree well with previous reports . The electronic structure shows a metallic nature owing to a Fermi surface (FS) originating from La 5d e g .…”

Configurational Thermodynamics of Alloyed Nanoparticles with Adsorbates

“…where Θ αβ (σ,δ) = Φ α (σ)Ψ β (δ) are the products of cluster correlation functions on two coupled lattices and V αβ are the corresponding effective cluster interactions (ECIs) to be determined via structural inversion and cluster selection. 45 Because the orthogonality of primitive basis functions is always preserved in the construction of high-dimensional vector space by outer (tensor) products, a coupled lattice with nonexchanging species that works for ionic systems 20 will also work for adsorbates on alloyed surfaces 46,47 and NPs. The second line is explicitly for a binary alloy substrate with one type of adsorbate, using site-occupation variables, σ i or δ i , equal 1 (0) if site i is occupied (not occupied).…”

Tight-binding modeling of interstitial ordering processes in metals: Application to zirconium hydrides

“…The difficulty is then to get an atomistic energetic model that is accurate enough to treat ordering processes (hundredth of an eV accuracy) while remaining cheap in CPU time. For several transition-metal hydrides, the CEM has already furnished good results (Ti-H [6], Zr-H [7,8], La-H [9]). However, this approach remains questionable and limited.…”