Fomos-Materials (Russia)

Growth of solid solutions of intermediate compositions LiNb1-xTaxO3 crystals allows you to adjust the physical parameters of the material and obtain crystals with the required properties, which is why obtaining such crystals and studying their properties taking into account anisotropy is an urgent task. Crystals of solid solutions of lithium niobate tantalate LiNb1-xTaxO3 of LiNb0.88Ta0.12O3 and LiNb0.93Ta0.07O3 compositions were grown in the work. Polished samples oriented in a standard installation were prepared from these crystals. The samples were subjected to monodomenization. The polarity of the piezoactive faces has been determined. By optical microscopy in transmitted light, patterns in the form of a sponge-like structure were observed on all samples of Z-cuts, patterns in the form of vertical regions parallel to the optical axis (Z-axis) were observed on samples of X- and Y-cuts. Taking into account anisotropy and polarity, measurements of the mechanical characteristics of the samples at a load of 25 gs were carried out: the microhardness of the samples according to Vickers HV calculated on the Mohs scale HM and the brittleness score Zx. According to the obtained data, the parameters of the «viscosity» S according to the Palmquist method and the degree of ionic bonds are calculated. The effect of the Nb : Ta ratio on the microhardness of Z-cuts is ambiguous, this may be due to the significant heterogeneity of such samples. In the case of X-cut samples, the results of microhardness measurements are close and within the error limits of the method. In the case of Y-cuts, there is a significant difference in the values of microhardness and brittleness of Y«+» and Y«–» sections, while the microhardness of the sample with a higher Nb content is higher.

The effect of crystal anisotropy and defects of the structure formed upon mechanical cyclic deformation and thermal shock on acoustic parameters such as phase velocity, attenuation coefficient, Q-factor of bulk acoustic wave (BAW) has been studied in lanthanum-gallium tantalate (LGT, La3Ta0,5Ga5,5O14) anisotropic piezoelectric single crystals using inner friction (IF) method with multiple piezoelectric vibrator at a frequency of 105 Hz. The anisotropy of the effective elasticity modulus (E), BAW phase velocity (Vp), attenuation coefficient and Q-factor was observed in anisotropic LGT single crystals. It is shown that cyclic deformation of LGT samples under a load of 2.5 kN with the number of load cycles up to 5 x 105 with a cycling frequency of 100 MHz and thermal shock (100 - 120°C) have no effect on the values of the effective elasticity modulus and phase velocity of the longitudinal BAW, respectively: for X-cut — E =111 GPa, Vp = 4250 m/sec; for Z-cut — E = 181 GPa, Vp = 5430 m/sec. The attenuation coefficient of the longitudinal BAW increased by 1.5 - 2 times after cyclic deformation for both X-and Z-cuts, which resulted in a two-fold decrease of the quality factor. Thermal shock has almost no effect on the attenuation coefficient and Q-factor for X-cut samples. For Z-cut samples thermal shock leads to a three-fold increase of the attenuation coefficient and decrease of the Q-factor. Sensitive elements of piezopressure sensors based on langatate should be protected from thermal shock at a temperature above 150°C, and the total number of the mechanical compression cycles of the material should not exceed 5 x 105 cycles at a frequency of 100 - 150 Hz with the loads not exceeding 2.5 kN.

Molybdate single crystals are perspective for the cryogenic scintillating bolometers that operate in ultra-low temperature conditions and used for the registration of rare events. The article describes the favorable features and imperfections of molybdate cryogenic scintillators, the peculiarities of the crystal growth, the luminescence properties and the influence of charge carriers trapping on the scintillation properties of molybdates.

In this study the important factors having influence on the occurrence of color centers (crystal growth conditions and postgrowth effect on the crystal) in LGT (La3Ga5,5Ta0.5O14, langatate) crystals were investigated. In order to check the model of defect formation in crystals of langatate the effect of ionizing radiation on the spectral dependence of the attenuation coefficients of langatate crystals was studied. The ionizing radiation doses were 1011 – 1015 cm−2. The optical homogeneity was evaluated by interferometric method on a Fizeau interferometer. The refractive indices of the crystals obtained in different atmospheres and after the isothermal annealing were measured by the prism method.

Resonators with different designs and different resonance frequencies were fabricated on LGS. Resonators were studied at a temperature of 650°C. Annealing of LGS sensors was carried out in a furnace during 1000 hours. Dependences of the resonators behavior versus temperature were obtained. It was found that the first 100-300 hours of thermal annealing lead to the largest change in resonator properties, while further changes over the next hours are much smaller. Pt/Ti based resonators and Ir based resonators showed slow ageing trends, while Pt/Ti resonators passivated with an Al2O3 thin film with a thickness of 160 nm demonstrated steeper initial frequency changes. In spite of the obvious damage to electrodes, resonator properties remained reasonably robust to such kinds of degradation and clearly proved the possibility of long-term measurements with LGS based SAW sensor structures. The results showed that initial annealing of high temperature sensors is required to stabilize their ageing behavior. Using the XRD technique, we observed the process of oxidation of Ir and Pt electrodes of SAW-resonators during the first 100 hours of thermal annealing.

The prospects for the combined use of scanning and transmission electron microscopy in studying defects in the matrix elements of the infrared photodetectors are considered. It is shown that combining these techniques allows us to fully realize the potential of high-voltage transmission microscopy for detecting defects that impair matrix parameters.