Predicted Novel High-Pressure Phases of Lithium

Abstract

Under high pressure, "simple" lithium (Li) exhibits complex structural behavior, and even experiences an unusual metal-to-semiconductor transition, leading to topics of interest in the structural polymorphs of dense Li. We here report two unexpected orthorhombic high-pressure structures Aba2-40 (40 atoms/cell, stable at 60-80 GPa) and Cmca-56 (56 atoms/cell, stable at 185-269 GPa), by using a newly developed particle swarm optimization technique on crystal structure prediction. The Aba2-40 having complex 4- and 8-atom layers stacked along the b axis is a semiconductor with a pronounced band gap >0.8 eV at 70 GPa originating from the core expulsion and localization of valence electrons in the voids of a crystal. We predict that a local trigonal planar structural motif adopted by Cmca-56 exists in a wide pressure range of 85-434 GPa, favorable for the weak metallicity.

Perspective: Crystal structure prediction at high pressures

“…4 Crystal structure of this semiconducting phase has attracted numerous attentions. 4,25,28,[127][128][129][130][131] The observed insulation seems in accordance with earlier prediction of pairing mechanism, 128 which however is unlikely in view of the proposed energetically much preferred metallic Cmca-24 (24 atoms/cell) structure. 127 Using CALYPSO method on structure prediction, a complex orthorhombic Aba2-40 structure (Fig.…”

“…3(b), 40 atom/cell, Pearson symbol, oC40) was predicted to be energetically more stable than all earlier proposed structures [127][128][129][130] in the pressure range 60-80 GPa. 25 Band structure calculation of Aba2-40 structure reveals indeed a semiconducting behavior with a band gap >0.8 eV at 70 GPa. Similar to that in the insulating Na, the semiconducting state in Aba2-40 structure of Li arises from the strong localization of valence electrons in the interstices of the lattice.…”

“…The use of fingerprint distances in energy landscape as search space metric 113 is less productive since an identical fingerprint distance would correspond to infinite number of structures. 114,115 Currently, CALYPSO has many attractive features including predictions of the energetically stable/metastable structures at given chemical compositions for isolated nanoparticles/clusters or molecules, 115 2D layers (single/ multi layers and buckled layers), 116 25,27 have already received the experimental confirmation. Below, we discuss three examples on the successful applications of CALYPSO into HP structures.…”

Spiral chain O 4 form of dense oxygen

“…However, creation of the insulating θ-O 4 from the compression of a metallic (and superconducting) solid clearly goes against the conventional wisdom that high pressure metallizes materials. The predicted formation of θ-O 4 in dense oxygen provided another case on the metal-insulator transition as previously exemplified in dense lithium (24,35) and sodium (36) and represents a significant step forward in understanding the behavior of solid oxygen and other oxygen-related materials at extreme conditions, such as in the interiors of giant planets.…”

“…Our CALYPSO method has been implemented in the CALYPSO code and successfully benchmarked on a number of known systems (22). Several predictions of high-pressure structures of dense Li, Mg, Bi 2 Te 3 , and water ice (23)(24)(25)(26) were successfully made, and predictions of the high-pressure insulating Aba2-40 (Pearson symbol oC40) structure of Li and the two low-pressure monoclinic structures of Bi 2 Te 3 were confirmed by independent experiments (25,27).…”