V.I. Voronkova

Structure and properties of crystals in the potassium titanyl phosphate family: A review

This paper presents an overview of the data available in the literature and the results obtained by the authors in the study of the influence of the chemical composition on the structure and properties of crystals in the potassium titanyl phosphate family. The structure and properties of phosphates in which univalent cations are substituted for potassium cations, as well as of crystals with isovalent or heterovalent substitutions for tetravalent titanium cations in the structure of potassium titanyl phosphate crystals, are considered.

Superstructure formation and X-ray photoemission properties of the TlTiOPO4 surface

X-ray photoemission spectroscopy (XPS) measurements have been executed for TlTiOPO4 to elucidate the general features in the electronic structure of the KTiOPO4 family compounds. The peculiarities of the valence band structure have been discussed for the crystals. The persistence of core level binding energy differences O1s–P2p and O1s–Ti2p3/2 has been detected in TlTiOPO4 and KTiOPO4, which relates well with the constancy of averaged P–O and Ti–O chemical bond lengths in this crystal family. The superstructure ordering of the TlTiOPO4 surface subjected to polishing and annealing has been detected by reflectance high energy electron diffraction (RHEED). From comparison of surface crystallographic properties of TlTiOPO4 and KTiOPO4, the most typical superstructure indices have been revealed.

Crystal structure of the oxygen conducting compound Nd5Mo3O16

Abstract Structure of the Nd5Mo3O16 single crystal grown in the Nd2O3–MoO3 system was studied using the X-rays diffraction technique at 293 K and 110 K temperatures. The unit-cell values were always cubic relating to that of CaF2 fluorite as a ≈ 2af (af = 5.5 Å). The structure was solved within the Pn-3n symmetry group. It was found that the Nd5Mo3O16 compound has a fluorite-like structure with all atoms disordered. An indirect confirmation for the violation of translational periodicity in the distribution of Mo and Nd atoms was obtained. The possible oxygen diffusion paths were analyzed using the one-particle potentials of the oxygen atoms. The ionic conductivity of Nd5Mo3O16 compound is associated with the disordering of the oxygen atoms in several positions, and their deficiency in comparison with the initial fluorite.

Thermomechanical detwinning of YBa2Cu3O7−x single crystals under reduced oxygen partial pressure

Abstract The kinetics of the thermomechanical detwinning of YBa 2 Cu 3 O 7− x crystals is studied as the influence of the starting domain structure, the oxygen content and the temperature on the detwinning process. We observed that after the release of uniaxial pressure at the detwinning temperatures or when detwinned crystals are heated again up to 120–150°C, nearly the same twin patterns as before the detwinning can reappear spontaneously (memory effect). If appropriate times and temperatures for the detwinning process are used the memory can be cleared. This memory effect is attributed to the presence of impurity and atomic defect clusters near former twin boundaries or to remaining minute twin domains which cannot be detected by optical methods. The time during which the uniaxial pressure has to be applied for clearing the memory follows an Arrhenius relation with an activation energy of 1.09 ± 0.10 eV. This value is close to the activation energy diffusion in YBa 2 Cu 3 O 7− x . The detwinning was found to be easy and complete when the crystal initially was reduced into the tetragonal phase at 560–600°C followed by the application of a uniaxial pressure along 〉100〈. After changing the atmosphere to pure oxygen the crystal transformed again into the orthorhombic phase without dividing into domains.

Specific features of phase transitions and the conduction of La2Mo2O9 oxide-ion conducting compound doped with vanadium

The X-ray powder analysis, calorimetric studies, and conductivity measurements of a series of ceramic La2Mo2−xVxOy specimens with different vanadium content are performed with the aim of following the dynamics of phase formation of the low-temperature α, high-temperature β, and metastable βms phases. At x ≥ 0.06, the cubic phase becomes stable and the monoclinic phase vanishes; therefore, the main α → β transition is suppressed. According to the data of differential thermal analyses, a weak thermal anomaly is observed in the range 450–470°C at x ≥ 0.06. This anomaly is indicative of the βms → β transition due to the conversion of the cubic phase with statically disordered oxygen atoms into the cubic phase with dynamic disorder. The conductivity of the high-temperature β phase obeys the Vogel-Tammann-Fulcher law.

Electrical and nonlinear optical properties of KTiOPO4 single crystals doped by Nb or Sb

Single crystals of K1−xTi1−xNbxOPO4 and K1−xTi1−xSbxOPO4 solid solutions are grown and their dielectric properties, electrical conductivity, and nonlinear optical properties are studied. Maximal content of these dopants x is found to be 0.11 for Nb and 0.23 for Sb. This doping leads to the creation of the additional potassium vacancies and the high ionic conductivity σ33. Ferroelectric phase transitions become more diffuse and their temperature decreases. All the crystals show intensive broad maxima of the dielectric permeability attributed with the relaxation phenomena.

Synthesis and electrical properties of mixed-layer Aurivillius phases

Polycrystalline samples of the mixed-layer Aurivillius phases Bi10Ti3W3O30, Bi7Ti4NbO21, Bi7Ti4TaO21, SrBi6Ti3Nb2O21, BaBi6Ti3Nb2O21, SrBi8Ti7O27, and BaBi8Ti7O27 (ferroelectrics with high oxygen-ion conductivity) have been prepared by solid-state reactions, and their electrical properties have been studied. All of the compounds undergo a second-order ferroelectric phase transition at temperatures from 500 to 800°C and have high electrical conductivity. Of particular interest is the compound Bi10Ti3W3O30, whose conductivity exceeds that of the other mixed-layer Aurivillius phases by more than one order of magnitude and reaches 5 × 10−2 S/cm at 800°C.

Flux growth and characteristics of some ferroelectric and related crystals

Single crystals of 38 ferroelectrics and related compounds: Bi2WO6, Ln2WO6 and Ln2MoO6 (Ln = La-Lu, Y), RNbW2O9 (R = K, Rb), RNbWO6 (R = K, Rb, Tl), Al2(WO4)3, Dy2(MoO4)3, LiNbO3 and KNbO3 have been flux grown. The crystallization was carried out from high temperature solutions of oxide systems including the required components. High-temperature microscopy was used for choosing the starting solution compositions and temperature conditions. Crystal growth was initiated by spontaneous nucleation during slow cooling of the solution. Only KNbO3 crystals were grown on (100) KNbO3 seeds. The composition, morphology, domain structure, optical, dielectric and other characteristics of the crystals have been studied. Bi2WO6 and LiNbO3 crystals grown at temperatures below the ferroelectric transition point were single domain. The morphology of Bi2WO6 crystal reflected their polar structure.

Temperature dependences of the first critical field and critical current in the untwinned TmBa2Cu3Ox superconducting single crystals

Abstract The critical field Hc1 of the monodomain TmBa2Cu3Ox superconducting single crystals are measured along all three crystallographic axes. These data determine, in the logarithmic approximation, the ratio of the penetration depths λa:λb:λc = 1:1.7: 4.4 for T > 10K the dependence of Hc1 on the temperature is a linear function with a slope of about 22 Oe/K for H∥c and 3.1 Oe/K for H∥a. Angular dependences of the Meissner magnetic susceptibility and Hc1 in the (bc)-plane were studied. The expression derived in the ellipsoid approximation shows good agreement with these data. Temperature dependences of the critical current jc (T) are also measured. Different types of single crystals are used, including the monodomain crystal. For all samples the jc(T) curves follow the exponential dependence j c (T) = j c (0) exp (− T T 0 ) with a jc(0) value of about (2–3) × 106 A/cm2. The jc(0) independence on the density of domain walls suggests a weak pinning by a twinning plane.

Crystal growth and physical properties of Cs2Nb4O11 and Rb2Nb4O11 single crystals

The Cs 2 Nb 4 O 11 (CN) and Rb 2 Nb 4 O 11 (RN) single crystals are grown by means of flux method and their physical properties are studied. Despite the similarity of their chemical formulae the crystals are not isostructural. CN crystals undergo ferroelectric phase transition at 164 C. Both the compounds show electrical conductivity of about 10 -4 Ω -1 cm -1 at room temperature.