Aluminium-doped LiFePO4 single crystals Part I. Growth, characterization and total conductivity

Abstract

Single crystals of Al-doped LiFePO₄ (1% Al) were grown by an optical floating zone technique. After cleaving from the as-grown ingot they exhibited a blackish-green color. The grown crystals have been characterized by the Laue X-ray technique, single-crystal and powder X-ray diffraction. Phase composition has been determined by chemical analysis to be Li_0.985±0.009Fe_0.984±0.12Al_0.0126PO_3.993±0.06. Secondary ion beam spectroscopy (SIMS) indicates a homogeneous distribution of doped Al in the single crystal block. The total conductivities are shown to be electronic conductivities and have been measured along different directions with the help of the cell Ti/LiFe(Al)PO₄/Ti. The samples exhibit effectively two-dimensional electronic conductivities along b- and c-directions similar as in pure LiFePO₄. This decrease of conductivity on Al-doping compared with undoped crystals is in agreement with our previous conclusion of p-type conductivity of LiFePO₄. Unlike nominally pure material not only the association of holes with lithium vacancies plays an important role but also purely ionic association.