Emergent reduction of electronic state dimensionality in dense ordered Li-Be alloys

Abstract

High pressure is known to influence electronic structure and crystal packing, and can in some cases even induce compound formation between elements that do not bond under ambient conditions [1][2][3] . Here we present a computational study showing that high pressure fundamentally alters the reactivity of the light elements lithium (Li) and beryllium (Be), which are the first of the metals in the condensed state and immiscible under normal conditions 4,5 . We identify four stoichiometric Li x Be 12x compounds that are stable over a range of pressures, and find that the electronic density of states of one of them displays a remarkable steplike feature near the bottom of the valence band and then remains almost constant with increasing energy. These characteristics are typical of a quasi-two-dimensional electronic structure, the emergence of which in a three-dimensional environment is rather unexpected. We attribute this observation to large size differences between the ionic cores of Li and Be: as the density increases, the Li cores start to overlap and thereby expel valence electrons into quasi-two-dimensional layers characterized by delocalized free-particle-like states in the vicinity of Be ions.Our extensive structural search exploring possible Be-Li compound formation under pressure uses two conceptually different approaches. In the first, we propose possible structures on the basis of established chemical/physical heuristics of alloy stability at 1 atmosphere (atm) and/or high pressure. Phenomenological 1-atm binary intermetallic structure maps 6 proved useful in this endeavour. In the second approach, initial structures are formed from randomly generated unit cells and atom positions, and subsequently optimized using density-functional theory (DFT) 7 , with the random search targeting unit cells with 15 or fewer atoms. This combined staticlattice structure search explores the stoichiometries most common in binary intermetallic compounds, including LiBe, LiBe

LiBeB: A predicted phase with structural and electronic peculiarities

“…For the LiBe mixtures, which are stabilized under pressure, we took relative stabilities from a recent computational study [30]. For the Li--B and Be--B mixtures, structural data from the ASM Alloy Phase Diagrams Center [31][32][33] was combined with results from structure searches by us to obtain binary tieline diagrams at the pressures of interest.…”

Application of density functional theory calculations to the statistical mechanics of normal and anomalous melting

“…4 DFT calculations by Feng et al found the electronic DOS of LiBe alloy shows a remarkable quasi-two-dimensional electronic structure that arises from a planar arrangement of valence electrons promoted from the Li cores. 5 From a study of the structural competition as function of volume in Group IIIA elements, Simak et al found that the structure-determining mechanism originates in the degree of s-p mixing of valence electrons. 6,7 This mixing governs the stability of B, Al, Ga, In and Tl at zero pressure, 7 and also qualitatively accounts for high pressure phase transitions in B and Ga and predicts similar behavior for In.…”

Double-diamond NaAl via pressure: Understanding structure through Jones zone activation

“…For instance, lithium and sodium, or their combination with a second element, have been shown to undergo intricate structural transformations under pressure, in both liquid and solid phases, departing in fascinating ways from close packing. [1][2][3][4][5][6][7][8] The dramatic change in reactivity and emergence of new compounds under pressure provide a productive arena for reexamining and deepening our understanding of the underlying structural principles. [7][8][9][10] Obvious questions arise in this context: what new crystalline compounds might form under pressure and in what structures?…”

“…7,8,[11][12][13] However, these methods generally offer little physical insight on why a particular configuration is favored or what reduced parameter space is pertinent to the structure of a class of materials under pressure. To set the problem in context, we note that a synthetic inorganic chemist, well versed in valence rules, is much less frequently bothered by the question "what can be made?," than by "how to make it?"…”

“…In particular, it has been pointed out, notably by Ackland and Macleod 14 and by Degtyareva,15,16 that the Jones 17 and later Mott and Jones 18 stability arguments often play a critical role in determining structures in novel metallic phases under high pressure. 7 The essence of this reasoning resides in an argument that the contact of bands with Brillouin zone ͑BZ͒ planes, associated with crystal ͑pseudo͒po-tentials V K ͑generally local͒, and states near the Fermi level can be a key stabilizing factor. The value of V K varies in general in complex fashion with density, but straightforward computation of relevant V K 's in simple forms may offer an efficient way of searching for stabilizing factors at high density.…”