Electronic Structure of Lithium Tetraborate

Abstract : Due to interest as neutron detection material, an investigation of Li2B4O7(110) and Li2B4O7(100) was undertaken, utilizing photoemission and inverse photoemission spectroscopic techniques. The measured band gap depended on crystallographic direction with the band gaps ranging from 8.9 plus or minus 0.5 eV to 10.1 plus or minus 0.5 eV. The measurement yielded a density of states that qualitatively agreed with the theoretical results from model bulk band structure calculations for Li2B4O7; albeit with a larger band gap than predicted, but consistent with the known deficiencies of LDA and DFT calculations. The occupied states of both surfaces were extremely flat; to the degree that resolving periodic dispersion of the occupied states was inconclusive, within the resolution of the system. However, both surfaces demonstrated clear periodic dispersion within the empty states very close to theoretical Brillouin zone values. These attributes also translated to a lighter charge carrier effective mass in the unoccupied states. Li2B4O7(110) yielded the more consistent values in orthogonal directions for energy states. The presence of a bulk band gap surface state and image potential state in Li2B4O7(110) was indicative of a defect-free surface. The absence of both in the more polar, more dielectric Li2B4O7(100) was attributed to the presence of defects determined to be O vacancies. The results from Li2B4O7(110) were indicative of a more stable surface than Li2B4O7(100). In addition, Li 1s bulk and surface core level components were determined at the binding energies of -56.5+0.4 and -53.7+0.5 eV. Resonance features were observed along the [001] direction and were attributed to a Coster-Kronig process. The pyroelectric and piezoelectric character of Li2B4O7 was explored more deeply and a non-zero, off-axis pyroelectric coefficient for the Li2B4O7(110) direction was discovered.