Temperature-dependent Raman scattering of KTa1-xNbxO3 thin films

We report a Raman scattering investigation of KTa1-xNbxO3 (x = 0.35, 0.5) thin films deposited on MgO and LaAlO3 as a function of temperature. The observed phase sequence in the range from 90 K to 720 K is similar to the structural phase transitions of the end-member material KNbO3. Although similar in the phase sequence, the actual temperatures observed for phase transition temperatures are significantly different from those observed in the literature for bulk samples. Namely, the tetragonal (ferroelectric) to cubic (paraelectric) phase transition is up to 50 K higher in the films when compared to bulk samples. This enhanced ferroelectricity is attributed to biaxial strain in the investigated thin films

Relationship between the structure and optical properties of lithium tantalate at the zero-birefringence point

The structure of lithium tantalate powders has been investigated using neutron diffraction between room temperature and 445 K, which includes the zero-birefringence point. As the temperature increases, the displacement of the Ta atom from the centre of the O octahedra and the tilt of the octahedra both decrease linearly in the range investigated. The measured structures form the basis of a range of density functional theory calculations utilizing the WIEN2 k code, with a focus on calculating the optical properties. These calculations are sensitive to the small structural changes measured in this temperature range; the calculated birefringence changes are consistent with the changes measured experimentally. Further, by investigating the effect of each atom in isolation, it can be shown that the birefringence of lithium tantalate mainly depends on the Ta displacement and the octahedral tilt, with the linear change in these as a function of temperature producing the change in birefringence with temperature, which results in it becoming zero-birefringent. This work demonstrates the unique material insights that can be obtained by combining density functional calculations with precise structural studies

Influence of composition gradient on domain structure in lithium niobate and lithium tantalate crystals

Исследовалась доменная структура в монокристаллах ниобата лития (LN) и танталата лития (LT) с градиентным составом двух типов: отклонение от стехиометрии, полученное при высокотемпературном VTE отжиге, и восстановление при ионно-плазменном облучении в атмосфере Ar.The domain structure in lithium niobate (LN) and lithium tantalate (LT) single crystals with two types of composition gradients: stoichiometry deviation from VTE process and reduction with plasma-source ion irradiation.В работе использовалось оборудование Уральского центра коллективного пользования «Современные нанотехнологии» ИЕНиМ УрФУ

AS-GROWN DOMAIN STRUCTURE IN LITHIUM TANTALATE WITH SPATIALLY INHOMOGENEOUS COMPOSITION DISTRIBUTION

The dependence of as-grown domain structure in LiTaO3 single crystals on spatially inhomogeneous composition distribution was shown. The inhomogeneous composition, de-fined by Li concentration, was produced by vapor transport equilibration. The formation of wide charged domain boundary was revealed.В работе использовалось оборудование Уральского центра коллективного пользования «Современные нанотехнологии» ИЕНиМ УрФУ

Piezoelectric Response in Hybrid Micropillar Arrays of Poly(Vinylidene Fluoride) and Reduced Graphene Oxide

International audienceThis study was dedicated to the investigation of poly(vinylidene fluoride) (PVDF) micropillar arrays obtained by soft lithography followed by phase inversion at a low temperature. Reduced graphene oxide (rGO) was incorporated into the PVDF as a nucleating filler. The piezoelectric properties of the PVDF-rGO composite micropillars were explored via piezo-response force microscopy (PFM). Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) showed that α, β, and γ phases co-existed in all studied samples, with a predominance of the γ phase. The piezoresponse force microscopy (PFM) data provided the local piezoelectric response of the PVDF micropillars, which exhibited a temperature-induced downward dipole orientation in the pristine PVDF micropillars. The addition of rGO into the PVDF matrix resulted in a change in the preferred polarization direction, and the piezo-response phase angle changed from -120° to 20°-40°. The pristine PVDF and PVDF loaded with 0.1 wt % of rGO after low-temperature quenching were found to possess a piezoelectric response of 86 and 87 pm/V respectively, which are significantly higher than the |d33eff| in the case of imprinted PVDF 64 pm/V. Thus, the addition of rGO significantly affected the domain orientation (polarization) while quenching increased the piezoelectric response

Domain structure and Raman modes in PbTiO3

A comparative analysis of Raman spectra from poly-domain PbTiO3 thin films with poly-domain and single-domain PbTiO3 crystals is presented. Raman spectroscopy reveals that the profiles of A1-symmetry modes can be significantly modified by the existence of the domain structure. The possible origins of these complex profiles of the A1(LO) and A1(TO) modes are discussed. It is emphasized that analysis of stresses, domain structure and phase transitions in the PbTiO3 thin films has to be based on the E(TO) modes presenting profiles that are not affected by the domain structure.Peer Reviewe

GaAs-ZnO acoustic microsensors: Focus on ZnO thin film characterization

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AlN/IDT/AlN/Sapphire as packageless structure for SAW applications in harsh environments

International audiencePackageless heterostructure AlN/IDT/AlN/Sapphire was investigated as a solution protection of the interdigital transducer against agglomeration phenomena at high temperature. For this purpose, the performance of conventional IDT/AlN/Sapphire and protected AlN/IDT/AlN/Sapphire heterostructures were compared. The capability of AlN protective thin film to minimize the deterioration of electrodes was shown

Crystallographic and optical study of LiNb1 − xTaxO3

International audiencePowders of lithium niobate-tantalate across the full compositional range have been made and crystals grown using a lithium vanadate flux growth technique. The Li-content of a lithium tantalate crystal has been determined using the zero-birefringence temperature and Curie measurements, confirming the Li content is between that of congruent and stoichiometric crystals. X-ray diffraction measurements show the Nb/Ta displacement and octahedral tilt both decrease as the Ta content is increased. This also results in a decrease in the lattice parameters from lithium niobate to lithium tantalate. Birefringence measurements on the crystals as a function of temperature have been used to determine the point that the crystals become zero-birefringent, and by comparison with the structural studies have confirmed that it is not related to a phase transition and the structures remain polar through the zero-birefringence points