Defect structure rearrangement of lithium tetraborate (Li$_{2}$B$_{4}$O$_{7}$) in an external electric field

The process of the defect structure rearrangement in a lithium tetraborate single crystal under the influence of high voltage external electric field applied along the polar direction [001] is studied with use of X-ray diffractometry. The results are supplemented by measurements of the conductivity kinetics. Under conditions of electric field of 300–500 V/mm strength, a sharp broadening of the 004 reflection diffraction peak and its integral intensity increasing by several times are observed, however its position and shape practically do not change. Under the influence of DC field with a strength in range of 500 to 1500 V/mm, the broadening process slows down, but the rocking curve asymmetry appears as well as its sharp shift to the smaller angles associated with an increase in the lattice parameter along the $c$-axis. This process is quasi-reversible, since the distorted structure is partially restored at a very slow rate (for several months). Two types of the diffraction peak parameters variation dependencies on the external field are interpreted as the manifestation of two ionic conductivity mechanisms: mobile lithium ions (Li$^+$) at low-intensity electric field and oxygen vacancies (VO$^{2+}$) at stronger fields. The process of charge carriers' migration causes the increase of defects concentration and structure changes in the near-surface region of the crystal.