Sol-gel synthesis, structure, and dielectric properties of La0.67LixTi1-xAlxO3 solid solutions.

La0.67LixTi1-xAlxO3 were synthesized using the sol-gel Pechini route. Ceramic samples were sintered in the temperature range of 1240-1300 °C in the air atmosphere. It was found that in the concentration range of 0.05 ≤ x < 0.15 there is a morphotropic phase boundary region with rhombohedral and tetragonal syngonies. In the concentration range of 0.15 ≤ x ≤ 0.3, a single-phase solid solution with rhombohedral R 3 ‾ c syngony is formed. As the value of x increases, the average grain size of La0.67LixTi1-xAlxO3 ceramics increases from 5.23 µm (x = 0.05) to 8.76 µm (x = 0.3). All materials of the La0.67LixTi1-xAlxO3 system at 0.05 ≤ x ≤ 0.3 have colossal values of dielectric constant ε' > 104 at frequencies up to 1 kHz. La0.67LixTi1-xAlxO3 (x = 0.2) solid solution with rhombohedral syngony has the highest value of dielectric constant and the lowest value of the dielectric losses.

Effect of non-stoichiometry of initial reagents on morphological and structural properties of perovskites CH3NH3PbI3.

The properties of films of organic-inorganic perovskites CH3NH3PbI2.98Cl0.02 depending on the ratio of starting reagents in solutions (PbI2:{CH3NH3I + CH3NH3Cl}) has been investigated. It was found that the formation of the perovskite structure with the ratio of the initial reagents PbI2: CH3NH3I = 1:1 occurs at 70-80 °C, and with the increase of the temperature of thermal treatment to 120 °C, the thermal destruction of the perovskite begins. When the ratio of the starting reagents PbI2: CH3NH3I = 1:2, the formation of the perovskite structure occurs through the intermediate compound (CH3NH3)2PbI4, and when the ratio is 1:3-(CH3NH3)3PbI5 and (CH3NH3)2PbI4. Independent on the ratio of the initial components (CH3NH3I:PbI2), the ratio between the content of lead and iodine in the films remains unchanged, that is why a significant difference in the film properties could be explained by the anisotropy of the particle shape, which is consistent with the data of electron microscopy and X-ray diffractometry.

Impedance Analysis of Thin Films of Organic-Inorganic Perovskites CH3NH3PbI3 with Control of Microstructure.

The effect of starting reagents (PbI2:{CH3NH3I + CH3NH3Cl}) with different ratios in raw solutions on the microstructure of films of organic-inorganic perovskites CH3NH3PbI3-xClx, as well as on the electrical properties, has been investigated. It was found that the crystallinity is increased sharply when the ratio of the starting reagents increases from 1:1 to 1:2 and is changed slightly with a further increase of ratio to 1:3. It is shown that when the ratio of starting reagents varies, the morphology of the films changes; at a ratio of 1:1, the films consist of needle-like particles, and when the ratio is increased, particles become roundish and then faceted. Additionally, the average grain size is decreased. Complex impedance curves and I-V curves have been investigated for samples with different ratios of the starting reagents. With increasing this ratio, the concentration of charge carriers remains unchanged, the mobility of charge carriers decreases, and conductivity passes through a maximum at a ratio of 1:2. The electrical properties of film are the highest at the ratio of starting reagents 1:2 due to the effect of two competing factors: the growth of crystallinity and the decrease of grain size.