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Samples of lanthanum lithium titanate with the general formula La 2/3 -x Li 3x TiO 3 with a different vacancy content, which is varied by varying the ratio of lanthanum to lithium, are investigated. Methods of synthesis, neutron diffraction data for the samples obtained, measurements of the lithium-ion conductivity of equilibrium and quenched samples, the results of nuclear magnetic resonance on 7 Li nuclei, the dynamic characteristics of heat release (heat absorption), and the magnetic susceptibility of some samples are presented.Materials based on lanthanum lithium titanate with perovskite structure have been studied intensively for the last 10 years in virtually all industrially developed countries. Being purely ionic conductors with respect to lithium they are drawing the attention of researchers primarily because these conductors have unique electric properties, making them promising as separators of low-and medium-temperature lithium sources of electric current. The conductivity of samples of materials with the general formula La 2/3 -x Li 3x TiO 3 (LLTO) depends strongly on the ration of lanthanum and lithium, since this ratio determines the concentration of cationic vacancies. In a series of works [1 -3] the formula for the compound is written in the form La 2/3 -x Li 3x 1/3 -2x TiO 3 , where the symbol denotes a cationic vacancy, showing this dependence directly. For x~0.1 the compound at room temperature has a lithium-cationic conductivity of approximately 10 -3 S/cm [1,2]. In practice, this is the highest ionic conductivity known today at these temperatures in crystalline bodies. The electronic conductivity of materials in air is so low that it cannot measured within the limits of sensitivity of modern instruments.Materials based on LLTO possess high ceramic characteristics: they press well and sinter at 1200 -1300°C. This makes it possible to miniaturize devices which are manufactured using LLTO materials. Articles made from raw materials with a low content of cationic impurities with variable valence have a dull white color.The present article presents the results of investigations of the effect of the structural and thermal properties and the cooling regimes of the samples on the ionic conductivity of LLTO. To this end the samples were obtained under equilibrium conditions or quenched in air and liquid nitrogen.The samples were synthesized using the well-known ceramics technology [1,2]. Calcined powders served as the initial materials: chemically pure La 2 O 3 , pure TiO 2 , and chemically pure Li 2 CO 3 . Weighed batches were mixed in an agate mortar, pressed at pressure 200 -220 MPa into 13 mm in diameter and 3 -5 mm thick pellets. The pellets were placed into a corundum crucible and a Silit furnace. Synthesize was conducted in air in four stages. At the first stage the samples were calcined at 900°C for 9 h. After cooling they were ground and pressed once again under the same pressures. Subsequent calcinations were performed at 1200°C for 24 h with intermediate millings of the materials. The fina...