Growth studies of YVO4 crystals (II). Changes in Y?V?O-stoichiometry

Incongruent vanadium oxide vaporization of yttrium orthovanadate (YVO₄) melt generates changes in both oxygen and yttrium-vanadium (Y V) stoichiometry. Slightly modified YVO₄ or YVO₄ + Y₈V₂O₁₇ phases are formed from continuously changing melt systems depending on their starting compositions. The occurrence of Y₈V₂O₁₇ in yttrium orthovanadate crystal fibers grown by laser heated pedestal growth (LHPC) technique can be simply eliminated by utilization of suitable V₂O₅-excess starting compositions. In contrast, the oxygen-deficient YVO_4–x black phase is an inherently oxygen deficient phase with limited solid solution in the YVO₄ YVO₃ subsystem of Y₂O₃ V₂O₅ V₂O₃ complex ternary phase diagram close to the congruent YVO₄ composition (49.3 mol% V₂O₅–50.7 mol% Y₂O₃). The greater oxygen deficiency YVO_4–x specimens have smaller lattice parameters as determined from detailed XRD data. Ceramic feed rod and fiber crystal grown from the feed rod with slight yttrium oxide excess starting composition were characterized by electron microprobe analysis. Changes in both Y V and oxygen stoichiometry were observed along the fiber, due to double limited solid solution system of three dimensional ternary phase diagram. Consequently, both the presence of Y₈V₂O₁₇ phase and change of Y V stoichiometry over oxygen deficiency cause difficulties for YVO₄ crystal growth from yttrium oxide excess melt.     

Effect of annealing temperature on the structural and optical properties of amorphous Sb2Te2Se thin films

Thin films of Sb₂Te₂Se were prepared by conventional thermal evaporation of the presynthesized material on Corning glass substrates. The chemical composition of the samples was determined by means of energy‐dispersive X‐ray spectrometry. X‐ray diffraction studies on the as‐deposited and annealed films revealed an amorphous‐to‐crystalline phase transition. The as‐deposited and annealed films at T _a = 323 and 373 K are amorphous, while those annealed at T _a= 423 and 473 K are crystalline with a single‐phase of a rhombohedral crystalline structure as that of the source material. The unit‐cell lattice parameters were determined and compared with the reported data. The optical constants (n , k ) of the investigated films were determined from the transmittance and reflectance data at normal incidence in the spectral range 400–2500 nm. The analysis of the absorption spectra revealed non‐direct energy gaps, characterizing the amorphous films, while the crystalline films exhibited direct energy gaps. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth behavior of CuPc films by physical vapor deposition

Various Cu‐phthalocyanine (CuPc) films were grown from physical vapor deposition on top of indium‐tin‐oxide glass substrates by controlling substrate temperature (T_sub), source temperature (T_sou), and growth time. From side‐view SEM pictures, the growth rates for these CuPc films are estimated and can be categorized into three regions. From the Arrhenius plot of growth rate versus 1/T_sub, the activation energy E_A can be obtained. As T_sou = 390 °C, for region (A) with T_sub < 140 °C, the growth of CuPc films is dominated by the adhesion process with E_A = 810 meV. For region (B) with 140 °C < T_sub < 320 °C, the growth is then limited by the steric character associated with the organic molecular solids with E_A = 740 meV. For region (C) with T_sub > 320 °C, the re‐evaporation of the CuPc adhered molecules from the interface becomes dominant. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Molybdenum and Tungsten Combined Oxidation by Dissociative Evaporation of Products during Refractory Oxides Crystal Growth from the Melt

The main chemical reactions and composition of gas and solid phases have been determined for the equimolar ratio Mo: W: O₂ = 1: 1: 2 at T = 2400 K in the pressure range of 1-1 x 10^-5 bar. It is established that the character of the main processes of combined oxidation depends significantly on the pressure and state of the oxidant (oxygen): at P > 7.52 x 10^-5 bar, oxidation reactions involve mainly molecular oxygen, whereas atomic oxygen dominates at lower pressures. At P ≥ 0.424 bar, the solid phase contains not only Mo but also MoO₂. At P = 1 x 10^-5 bar, the concentration of lower Mo and W oxides and elementary Mo and W in the gas phase sharply increases, which can negatively affect the main crystallization units.     

On symmetry of sillenites

A precise X-ray diffraction study of a sillenite crystal with the initial composition Bi₂₄(Si _x Mn_1−x )₂O₄₀ grown by the hydrothermal method showed the presence of diffraction reflections that make it possible to assign this crystal to the sp. gr. P23, in contrast to the Bi₂₄Si₂O₄₀ crystal with the sp. gr. I23. The distribution of cations over the two positions of the crystal structure of the general composition Bi₂₄(Bi_0.130(3)Si_0.277(3)Mn_1.593(3))O₄₀ with three atoms (Bi, Si, Mn) in one position of the sp. gr. P 23 is established. The disymmetrization of the Bi₂₄(Bi, Si, Mn)₂O₄₀ crystal is explained by the kinetic order-disorder phase transition, which is caused by the presence of several atoms with different crystallochemical properties in one crystallographic position of the structure, in contrast to the Bi₂₄Si₂O₄₀ crystal, where this phenomenon is absent.     

Neutron Diffraction Investigation of Langanite Crystals

The upper and lower parts of a single crystal with a nominal composition La₃Ga_5.5Nb_0.5O₁₄grown by the Czochralski method (Ir crucible, 99%Ar + 1%O₂, growth direction) have been studied by neutron diffraction for the first time. The compositions of the upper and lower parts are found to differ ((La_2.935(2)□_0.065)(Ga_0.450Nb_0.550(3))Ga₃(Ga_1.965(4) □_0.035)(O_13.90(1) □_0.10) and (La_2.940(1) □_0.060)(Ga_0.590Nb_0.410(2))Ga₅(O_13.82(1) □_0.18), respectively), and microtwin formation was observed in the upper part, with two unit cells related by the translation 1/2Z. Based on a comparison of the refined crystal composition and optical transmission spectra, the absorption bands at 30 000, 24 500, and 20 500 cm_-1 were assigned to divacancies (2 V _La^', 3 V _O^..), oxygen vacancies, and color centers, respectively. A relationship between the crystal color and oxygen content is established.     

Role of oxygen vacancies in the origin of ferromagnetism in Mn‐doped ZnO

The relationship between the oxygen vacancy and ferromagnetism in Mn‐doped ZnO has been studied based on the first‐principles calculations. Three possible charge states of oxygen vacancies, i.e., neutral (V_O⁰), 1+ (V_O¹⁺) and 2+ (V_O²⁺) are considered. Results show that the lattice relaxations around oxygen vacancies are large difference under different charge states. It is found that V_O¹⁺ and V_O²⁺ oxygen vacancies induce ferromagnetism. However, Mn‐doped ZnO system shows ferromagnetism with V_O⁰ oxygen vacancies in hydrogenated environment, the ferromagnetism is attributed to the interstitial H, which forms a bridge bond and mediates d ‐d coupling and stabilizes the ferromagnetic state. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and Structure of Ba<Subscript>5</Subscript>(V<Subscript>2</Subscript>O<Subscript>7</Subscript>)<Subscript>2</Subscript>Cl<Subscript>2</Subscript>

Abstract  A new barium chlorovanadate, Ba₅(V₂O₇)₂Cl₂, was isolated by a high-temperature (850 °C) reaction employing a CsCl/RbCl flux. The structure was determined by single crystal X-ray diffraction methods. This compound crystallizes in an orthorhombic crystal system, Pmmn (No. 59), with a = 11.558(2) ?, b = 15.164(3) ?, c = 10.023(2) ?, Z = 4 and V = 1756.7(6) ?³. The structure of Ba₅(V₂O₇)₂Cl₂ was determined by full-matrix, least-squares methods with R ₁ = 0.0398, wR ₂ = 0.1069 and GOF = 1.048 for all data. This new structure can be described as a composite lattice made up of mixed covalent and ionic moities. The extended framework is orchestrated by stacked [Ba(V₂O₇)Cl]³⁻ slabs that are interconnected by Ba²⁺ cations through Ba–O bonds to the [V₂O₇] units. The Ba²⁺ and Cl^- ions form BN-type “[BaCl]” sheets with pseudo-hexagonal windows that are centered by [V₂O₇]⁴⁻ pyrovanadate units. Graphical Abstract  The structure of a new chlorovanadate, Ba₅(V₂O₇)₂Cl₂, exhibits an interesting BN-type salt lattice that consists of an extended [BaCl] sheet containing pseudo-hexagonal windows that are centered by [V₂O₇] pyrovanadate units.      

Far-infrared spectra of amorphous titanium dioxide films

The amorphous TiO₂ film structure on an aluminum substrate was studied by far infrared reflection-absorption spectroscopic (IRAS) analysis. The transverse (TO) and longitudinal (LO) optical modes of the amorphous oxide films were studied experimentally and through simulation. These optical modes were compared to those of the corresponding crystal. While TO modes of the films correspond to those of the crystal, the LO modes do not. The best agreement between the experimental and the simulated IRAS spectra is obtained when an appropriate dielectric function ε_film(ν) is chosen. ε_film(ν) must be related to the amorphous character of the films e.g. by averaging the anisotropic crystal properties. As a consequence of our findings, the Berreman effect does apply in amorphous films to absorption bands at frequencies of the peaks of the loss function Im[−1/ε_film(ν)], which does not correspond to frequencies of the LO modes of the crystal.     

Growth and structure properties of La1−xSrxMnO3−σ (x=0.2, 0.3, 0.45) thin film grown on SrTiO3 (0 0 1) single-crystal substrate by laser molecular beam epitaxy

La_1−xSr_xMnO_3−σ (LSMO) thin films have been grown on SrTiO₃ (0 0 1) single-crystal substrates using the laser molecular beam epitaxy (MBE) technique. The two-dimensional layer-by-layer growth was in-situ monitored by reflection high-energy electron diffraction (RHEED). Kinetic growth with surface relaxation was also observed, and crystallinity of the thin films was investigated by high-resolution X-ray diffraction. Results of 2θ−ω scans revealed a strong correlation between out-of-plane lattice constant and oxygen content as well as strontium doping concentration. However, further analysis of rocking curve measurements around (0 0 2) plane of thin films grown under different oxygen pressure (P_O2) shown the effects of oxygen content on the crystal structure. An exceptionally low full-width at half-maximum (FWHM) of 0.02° was measured from the sample grown at P_O2 of 5.0 Pa, indicating the almost perfect epitaxial growth of LSMO thin films.     

Stability and phase changes in thin layers of rare-earth metals/iron and other binary compounds

The structure, thermal stability and phase transformations in thin Gd–Fe films have been studied. The films were obtained by means of thermal evaporation in vacuum. At room temperature of substrates (T_s) amorphous films were formed, at T_s = 300–500 K – amorphous–crystal condensates, and at T_s > 500K – polycrystalline films. The crystal structure of condensates was determined at various temperatures and crystallization of amorphous films was found to have a heterogeneous character. During crystallization of Gd₂Fe₁₇ amorphous films, formation of two phases, viz. Gd₆Fe₂₃ and a-Fe was observed. In the films obtained at substrates temperature > 500K, the presence of three phases was established: a hexagonal phase Gd₂Fe₁₇ of the Th₂Ni₁₇-type structure, a rhombohedral phase Gd₂Fe₁₇ of the Th₂Zn₁₇-type structure and a hexagonal phase GdFe₅ of the CaCu₅-type structure. GdFe₂ films had a face-centered cubic structure, expected in bulk GdFe₂.     

Pressure Effects of Nematic Liquid Crystals

There exists a variety of important new phenomena and plenty of experimental data on the pressure effects of liquid crystals. Yet, no systematic phenomenological or microscopic theory is available. In this paper, the ordinary Landau-deGennes theory is generalized so that the free energy becomes G = G _o(P, T) + a/2 [T - T*(P)]S ² - B(P)/3 S ³ + C(P)/4 S ⁴, T*(P) = T _o + bP - eP ². All the known pressure experiments of nematic PAA (where B and C are independent of P) are explained in one stroke and all parameters are determined. New results including reentrant I phase (I represents isotropic phase), and the independence of T _c—T* and T*—T _c on P, etc., are predicted. Simple methods of experimental confirmation are proposed. Similar discussions on the cases of MBBA, EBBA, etc. are also given. Our theory differs from that of Lin-Keyes-Daniels but agrees better with experiments. Results related to G _O(P, T) will be reported in a separate paper.     

Growth condition related orientation transition for YBa2Cu3O7−δ films on NdGaO3 substrate

For comprehensive understanding, the crystallographic out-of-plane axis transition for YBa₂Cu₃O_7?δ (YBCO or Y123) films grown on (110) NdGaO₃ (NGO) substrate, using liquid phase epitaxy (LPE), was systematically investigated via changing flux composition, processing temperature and oxygen partial pressure. It is found that LPE films could grow, remarkably, in a wide temperature range between 24 K above and 25 K below the peritectic temperature (T_p). The unpredicted c-oriented films formed at the temperatures above T_p, is deduced to be attributed to the etch-back behavior, i.e., Nd partially dissolved from the NGO substrate into solution, which leads to a locally high supersaturation for facilitating film growth. Even more distinctively, decreasing from the high temperature in this wide range, the YBCO films characteristically experienced the orientation transitions, the double transition of the c-axis oriented→a/c-axis mixed→c-axis oriented in air, and a single evolution of the c-axis→a-axis in the pure oxygen atmosphere. By combining supersaturation with the NGO etch-back, and solute diffusion, the transition origin of the film orientation in various temperature regions was clarified.     

Ionic conductivity of Bi12(V,Bi)O20 + δ single crystal (δ = 0.27) with a sillenite-type structure

The ionic conductivity of nonstoichiometric Bi₁₂(V_0.89Bi_0.03)O_20.27 single crystal with a sillenite-type structure has been investigated by impedance spectroscopy; its conductivity at 673 K is 2 × 10^?3 S/cm, which is about two orders of magnitude higher than the conductivity of oxide superionic conductor single crystal Zr_0.88Y_0.12O_1.94. As follows from crystallochemical analysis, ion transport in Bi₁₂(V_0.89Bi_0.03)O_20.27 is due to additional O^2? ions, which arise due to oxygen nonstoichiometry.     

Characteristic of spontaneous polarization in Pb<Subscript>5</Subscript>Ge<Subscript>3</Subscript>O<Subscript>11</Subscript> crystals

The temperature behavior of the spontaneous polarization of lead tetragermanate, a uniaxial ferroelectric, is studied in the range from 4.2 to 300 K. The results obtained along with the data from the literature make it possible to reconstruct a complete pattern of the behavior of P _s (T) both in the vicinity of the phase transition and at lower temperatures. In the range from 290 K to T_C, the crystal behavior is found to change from the dipole type (β = 1/2) to the pseudoquadrupole type (β = 1/4). This specific crossover manifests itself in the change in the behavior of P _s ^1/β as a function of (T_C-T). In the low-temperature range, weak anomalies in the dependenceP _s >(T) are found, which point to the occurrence of contributions from the dipole moments of separate structural fragments of Ge₂O₇ and GeO₄, which have internal degrees of freedom and are weakly bound to the dynamics of the crystal lattice.     

Photoluminescence of annealed LLGG:Cr3+ crystals under high pressure

High pressure photoluminescence has been used for analysis of lattice disorder in La_2.7Lu_2.29Cr_0.01Ga₃O₁₂ and La_2.32Lu_2.59Cr_0.02Ga_3.07O₁₂ crystals. Photoluminescence of samples before annealing and those annealed for 5 h in oxygen and hydrogen atmosphere at 923 K has been measured. The pressure dependence of Cr³⁺ luminescence has been used to obtain the crystal field distribution (lattice disorder) and its dependence on the thermal treatment. The distribution energy of the ⁴T₂ state has been considered as a measure of disorder. Annealing in oxygen has been found to reduce the energies of ⁴T₂ states for all sites, and annealing in hydrogen – to increase these energies.     

The evolution of structure induced by intensive milling in the system 2Bi2O3 · 3TiO2

Concurrent mechanochemical treatment of either a stoichiometric 2Bi₂O₃ · 3TiO₂ powder mixture or (pulverized) Bi₄Ti₃O₁₂ compound was performed in a planetary ball mill. Relevant structural parameters: crystallite size, amount of amorphous phase and the transformed fraction (as a result of chemical reaction between Bi₂O₃ and TiO₂) of the powders milled for various milling times and intensities were derived from X-ray powder diffraction data. The obtained structural parameters were used to follow the kinetics of the reduction of crystallite size, amorphous phase formation and chemical reaction. In the powder mixture, during the early stage of mechanochemical treatment the Bi₂(CO₃)O₂ phase was found as an intermediate product which transformed into the highly amorphized Bi₄Ti₃O₁₂ phase as the milling progressed. On the other hand, mechanochemical treatment of Bi₄Ti₃O₁₂ powder induced a gradual deformation of the crystal lattice and destruction of the perovskite-type structure. However, in both cases, after a certain milling time, a very disordered, amorphous/nanocrystalline structure was obtained. It was demonstrated that in the steady state, the amorphous/nanocrystalline phase ratio depends on the milling conditions. Higher milling intensities induce ‘nanocrystallization’ of the amorphous phase, i.e., precipitation of crystallites with an average size below 20 nm. A kinetic model involving a nanocrystalline ↔ amorphous reaction, in which the forward and reverse reaction were first-order was postulated and used to analyze the formation of an amorphous phase.     

Hydrothermal crystallization and X-ray structure determination of a new decavanadate with mixed cations [Mn(H2O)6]2[N(CH3)4]2[V10O28]·2H2O

A new decavanadate with mixed cations, [Mn(H₂O)₆]₂[N(CH₃)₄]₂[V₁₀O₂₈]·2H₂O (1), was crystallized from a hydrothermal reaction between MnCO₃ and V₂O₅ in the presence of N(CH₃)₄Br at 100°C. The structure of 1, as determined by x-ray single crystal analysis, consists of cations and anions of hexa-aqua manganese [Mn(H₂O)₆]²⁺, tetramethyl ammonium [N(CH₃)₄]⁺ and decavanadate [V₁₀O₂₈]^6–. The extended H-bonding between the [Mn(H₂O)₆]²⁺ and [V₁₀O₂₈]^6– ions gives rise to a pseudo- two-dimensional network in the crystal lattice x-ray crystallographic data for 1: monoclinic P2₁/n, a = 9.1499(5), b = 12.8725(7), c = 18.625(1) Å, = 92.252(1)°, V = 2192.0(2) ų, MZ = 2, and D _calcd = 2.22 g cm^–3.     

Structure and crystallization of potassium titanium phosphate glasses containing B2O3 and SiO2

Transparent glasses composition of which can be expressed by the formula: (100−x) · (K₂O · 2TiO₂ · P₂O₅) · x(K₂O · 2B₂O₃ · 7SiO₂), where x=5, 10, 15 and 20 mol% (KTP-xKBS), were obtained by melt quenching technique. The structure and crystallization behavior of these glasses have been examined by Fourier transform infrared spectroscopy, differential thermal analysis and X-ray diffraction. In spite of their nominal composition, the studied glasses exhibit a similar oxygen polyhedra distribution. However, significant differences were found in the trigonal BO₃ units amount. During DTA runs all the examined glasses devitrify in two steps. In the former, very small crystals of an unknown crystalline phase are produced. In KTP-5KBS and KTP-10KBS glasses anatase phase was also detected. Attempts were made in order to identify the unknown phase (UTP) for which a AB₃(XO₄)₂(OH)₆ Crandallite-type structure was proposed where the A, B and X sites were occupied by K, Ti and/or Al, and P, respectively. In the second devitrification step the crystallization of the KTiOPO₄ phase occurs while the UTP phase previously formed disappears. Isothermal heat treatments performed at temperature just above T_g have allowed one to obtain transparent crystal-glass nanocomposites, formed by crystalline nanostructure of the UTP phase uniformly dispersed in the amorphous matrix.     

Phase transitions in nonstoichiometric strontium tantalates with the cryolite structure

The structural transformations of Sr₆Ta₂O₁₁ and Sr_5.92Ta_2.08O_11.12 oxygen-deficient cryolites with variations in the partial pressures of oxygen p(O₂) and water vapor p(H2O) are investigated using X-ray diffraction, thermogravimetry, and electrical conductivity measurements. It is found that a change in the oxygen partial pressure leads to a phase transition accompanied by the transformation of the cubic cell into the tetragonal cell, most probably, due to ordering of oxygen vacancies. The intercalation of water into the matrix of the complex oxides under investigation results in a structural-chemical transformation during which the solid solution undergoes a transition to an oxyhydrate phase of variable composition and the cubic cell transforms into the orthorhombic cell.