LiBx (0.82 < x <= 1.0) - an incommensurate composite structure below 150 K

The lithium boride LiBx (0.82 < X <= 1.0) contains linear boron chains isolelectronic to carbyne. The negatively charged boron chains are embedded in and stabilized by a lithium matrix. At room temperature they are disordered with respect to each other and also not commensurate with the Li matrix. Below about 150 K a phase transition takes place, where the boron chains successively order with respect to each other, but remaining still incommensurate with the Li substructure. The crystal structure of LiBx at 2 K was determined from neutron powder diffraction data and refined in the (3+1)-dimensional superspace group P (1) over bar(alpha beta gamma) The unit cell parameters are a = 4.012(1) angstrom, b = 4.012(1) angstrom, c = 2.794(1) angstrom, alpha = 90 degrees, beta = 90 degrees, gamma = 120 degrees for the lithium and a = 4.017(1) b = 4.019(1) angstrom, c = 1.578(1) angstrom, alpha = 93.35(1), beta = 92.83(1)degrees, gamma = 119.71(1)degrees for the boron substructure, respectively. The q-vector is (0.1267 0.1482 1.7707). The boron atoms within the linear chains are equidistant, suggesting a cumulated double bond system, similar to a cumulenic chain in the carbon case. Band structure calculations and the electron localization function give more insight into the unusual physical properties of LiBx.