English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Plus monthly

Global: Zendy Tools annual

Global: Zendy Tools monthly

Global: Zendy Free Plan

Recent discoveries of supposedly pure alpha-tetragonal boron require torevisit its structure. The system is also interesting with respect to a newtype of geometrical frustration in elemental crystals, which was found inbeta-rhombohedral boron. Based on density functional theory calculations, thepresent study has resolved the structural and thermodynamic characteristics ofpure alpha-tetragonal boron. Different from beta-rhombohedral boron, theconditions for stable covalent bonding (a band gap and completely filledvalence bands) are almost fulfilled at a composition B_52 with two 4cinterstitial sites occupied. This indicates that the ground state of purealpha-tetragonal boron is stoichiometric. However, the covalent condition isnot perfectly fulfilled because non-bonding in-gap states exist that cannot beeliminated. The half occupation of the 4c sites yields a macroscopic amount ofresidual entropy, which is as large as that of beta-rhombohedral boron.Therefore, alpha-tetragonal boron can be classified as an elemental crystalwith geometrical frustration. Deviations from stoichiometry can occur only atfinite temperatures. Thermodynamic considerations show that deviations deltafrom the stoichiometric composition B_(52+delta) are small and positive. Forreported high-pressure syntheses conditions delta is predicted to be about 0.1to 0.2. An important difference between pure and C- or N-containingalpha-tetragonal boron is found in the occupation of interstitial sites: thepure form prefers to occupy the 4c sites, whereas in C- or N-containing forms amixture of 2a, 8h, and 8i sites are occupied. The present article providesrelations of site occupation, delta values, and lattice parameters, whichenable us to identify pure alpha-tetragonal and distinguish the pure form fromother ones.

Structure, non-stoichiometry, and geometrical frustration of alpha-tetragonal boron