Lithium Subhydrides under Pressure and Their Superatom‐like Building Blocks

Abstract

Evolutionary structure searches are used to predict a new class of compounds in the lithium-rich region of the lithium/hydrogen phase diagram under pressure. First principles computations show that Li m H, 4 < m < 9, are stabilized with respect to LiH and Li between 50-100 GPa. The building block of all of the lithium subhydrides is an Li 8 H cluster, which can be thought of as a superalkali.The geometries and electronic structures of these phases is analogous to that of the well-known alkali metal suboxides. * Electronic address: ezurek@buffalo.edu 1 arXiv:1207.6122v1 [cond-mat.mtrl-sci] 25 Jul 2012 Considerable attention has been given towards the pressure-induced synthesis of hydrogen-rich systems with unusual stoichiometries due to their potential import in energy applications, and because they may provide routes towards hydrogen's metallization and concomitant superconductivity.[1] A number of intriguing systems which exhibit strong intermolecular interactions under pressure, including the van der Waals solids Xe-H 2 ,[2] CH 4 -H 2 ,[3] SiH 4 -H 2 ,[4, 5] and H 2 S-H 2 ,[6] have been prepared. Pressure-induced hydrogen uptake has been observed in metals with low hydrogen solubility yielding RhH 2 ,[7] Re 2 H,[8] PtH,[9-11] or noble metal hydrides.[12] Yet, not much work has been devoted to studies of compounds which are hydrogen poor.Recently, we have shown that under pressure the alkali metal polyhydrides, MH n with n > 1, become stable with respect to decomposition into MH and H 2 at 100, 25, and 2 GPa for lithium,[13] sodium, [14], and rubidium, [15] respectively. LiH n is particularly interesting because the most stable structure above 150 GPa, LiH 6 , was found to be metallic already at 1 atm.[13] Elemental lithium also exhibits unusual behavior: when compressed its melting temperature initially decreases so that between 40-60 GPa it has the lowest melting point among the elemental metals.[16] These reasons prompted us to computationally explore the alkali-metal/hydrogen phase diagram further, specifically its lithium-rich region. Even though the structures we find, some of which are illustrated in Figure 1, show no evidence for significant metallicity or low-temperature melting behavior, they hint that pressure may be used to synthesize a new class of compounds whose atomic and electronic structures are remarkably similar to the well-known rubidium and cesium suboxides. [17,18] Because of the similarities described below, we refer to these phases as lithium subhydrides.The open-source evolutionary algorithm (EA) XtalOpt [20] has been employed to find low enthalpy structures of Li m H with 2 < m < 9 up to P = 100 GPa.[21] The calculated enthalpies of formation, ∆H F , of the most favorable structures for a given stoichiometry are provided in Figure 2(a). The ∆H F values are computed from the enthalpy differences between the right-and left-hand sides of the reaction shown at the top of Figure 2; quantum and temperature effects are not included in this plot. In this pressure range (between 50-100 GPa) a n...

Synthesis of lithium polyhydrides above 130 GPa at 300 K

“…The latter is at higher frequency and exhibits a negative pressure dependence. This rules out the possibility of a dissociation of LiH to form LiH x (x < 1) and solid H 2 that was recently predicted around 90 GPa (20). LiH 2 is thought of as containing interpenetrating Li + H − and H 2 sublattices, with a weak interaction between them.…”

Decomposition Products of Phosphine Under Pressure: PH2 Stable and Superconducting?

“…A priori crystal structure prediction calculations were carried out using the open-source evolutionary algorithm (EA) XTALOPT Release 8 and 9 [23,24] that has previously been used to predict the structures of a variety of binary hydrogenrich phases under pressure [25][26][27][28][29]. EA runs were carried out on the PH 3 stoichiometry at 100, 150, and 200 GPa employing simulation cells with 1-6 formula units (FU) at 100 GPa and 2-3 FU at 150 and 200 GPa.…”

Prediction of Stable Ground-State Lithium Polyhydrides under High Pressures

“…Phase transitions in solid H2; formation enthalpies (ΔH) of solid LiHn (n = 2-10, 13) with zero-point energies (ZPE) at 150 GPa; convex hull of LinH (n =3-9) at 90 GPa and NaHx (x =3, Hooper et al 52 The abscissa x is the fraction of LiH in the structures, and the filled symbols located on the convex hull (solid lines) represent stable species against any type of decomposition.…”