Synthesis and characterization of metal substituted AlxCr1−x(acetylacetonate)3 single-source precursors for their application to MOCVD of thin films

A detailed study, involving the synthesis of a single-source precursor containing two metal ions sharing the same crystallographic site, has been undertaken to elucidate the use of such a single-source precursor in a CVD process for growing thin films of oxides comprising these two metals, ensuring a uniform composition and distribution of metal ions. The substituted complexes Cr_1−xAl_x(acac)₃, where acac = acetylacetonate, have been prepared by a co-synthesis method, and characterized using UV–Vis spectroscopy, TGA/DTA measurements, and single crystal X-ray diffraction at low temperature. All the studied compositions crystallize in the monoclinic space group P2₁/c with Z = 4 in the unit cell. It was observed that the ratio (Al:Cr) of the site occupancy for the metal ions, obtained from single crystal refinement, is in agreement with the results obtained from complexometric titrations. All the solid state structures have the metal in an octahedral environment forming six-membered chelate rings. M–O acac bond lengths and disorder in the terminal carbon have been studied in detail for these substituted metal–organic complexes. One composition among these was chosen to evaluate their suitability as a single-source precursor in a LPMOCVD process (low-pressure metal–organic chemical vapour deposition) for the deposition of a substituted binary metal oxide thin film. The resulting thin films were characterized by X-ray diffraction, scanning electron microscopy, and infrared spectroscopy.     

Synthesis, Characterization and Humidity Sensitive Properties of Nanocrystalline LaCoxFe1-xO3

The nanocrystalline LaCo_xFe_1-xO₃ with different concentrations of Co was prepared by polyethylene glycol (PEG) sol-gel method and characterized by differential thermal analysis and thermal gravimetric analysis (DTA-TGA), X-ray diffraction (XRD), and scanning electron microscope (SEM). It was found that the crystal structure of perovskite-type could be obtained at 600 °C, and the concentration of Co had significant effects on the solid-state reaction and the average particle size of the obtained nanocrystals. Furthermore, the humidity-sensitive properties of nanocrystalline LaCo_xFe_1-xO₃ were investigated, and it was found that LaCo_0.3Fe_0.7O₃ exhibited higher sensitivity to humidity compared with other samples. The addition of Na₂CO₃ improved the humidity-sensitive properties of this sample, and made its response to humidity good in the whole humidity range of 11%-95% relative humidity (RH).     

Crystal structures of RPt3−xSi1−y(R=Y, Tb, Dy, Ho, Er, Tm, Yb) studied by single crystal X-ray diffraction

The crystal structures of ternary compounds RPt_3−xSi_1−y(R=Y, Tb, Dy, Ho, Er, Tm, Yb) have been elucidated from X-ray single crystal CCD data. All compounds are isotypic and crystallize in the tetragonal space group P4/mbm. The general formula RPt_3−xSi_1−y arises from defects: x≈0.20, y≈0.14. The crystal structure of RPt_3−xSi_1−y can be considered as a packing of four types of building blocks which derive from the CePt₃B-type unit cell by various degrees of distortion and Pt, Si-defects.     

Crystal Structure of Pseudorhombohedral InFe1−xTixO3+x/2 (x=2/3)

The structure of pseudorhombohedral-type InFe_1−xTi_xO_3−x/2 (x=2/3) was refined by Rietveld profile fitting. The crystal is a commensurate member of a series in a solution range on InFeO₃-In₂Ti₂O₇ including incommensurate structures. The structure with the unit cell of a=5.9188(1), b=10.1112(2), and c=6.3896(1) Å, β=108.018(2)°, and a space group P2₁/a is the alternate stacking of an edge-shared InO₆ octahedral layer and an Fe/Ti-O plane along c^*. Metal sites on the Fe/Ti-O plane are surrounded by four oxygen atoms on the Fe/Ti-O plane and two axial ones. Electric conductivities of the order 10⁻⁴ S/cm were observed for the samples at 1000 K, while the oxide ion transport number is almost zero as no electromotive force was detected by an oxygen concentration cell.     

New synthesis of excellent visible-light TiO2−xNx photocatalyst using a very simple method

An excellent visible-light-responsive (from 400 to 550 nm) TiO_2−xN_x photocatalyst was prepared by a simple wet method. Hydrazine was used as a new nitrogen resource in this paper. Self-made amorphous titanium dioxide precursor powders were dipped into hydrazine hydrate, and calcined at low temperature (110 °C) in the air. The TiO_2−xN_x was successfully synthesized, following by spontaneous combustion. The photocatalyst was characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), transmission electron microscope (TEM), UV-Vis diffuse reflectance spectrometer (DRS), and X-ray photoelectron spectroscopy (XPS). Analysis of XPS indicated that N atoms were incorporated into the lattice of the titania crystal during the combustion of hydrazine on the surface of TiO₂. Ethylene was selected as a target pollutant under visible-light excitation to evaluate the activity of this photocatalyst. The newly prepared TiO_2−xN_x photocatalyst with strong photocatalytic activity and high photochemical stability under visible-light irradiation was firstly demonstrated in the experiment.     

Synthesis and electrical conductivity of perovskite-type PrCo1−xMgxO3

Oxides in the system PrCo_1−xMg_xO₃ (x=0.0, 0.05, 0.10, 0.15, 0.20, 0.25) were synthesized by citrate technique and characterized by powder X-ray diffraction and scanning electron microscope. All compounds have a cubic perovskite structure (space group ). The maximum ratio of doped Mg in the system PrCo_1−xMg_xO₃ is x=0.2. Further doping leads to the segregation of Pr₆O₁₁ in PrCo_1−xMg_xO₃. The substitution of Mg for Co improves the performance of PrCoO₃ as compared to the electrical conductivity measured by a four-probe electrical conductivity analyzer in the temperature range from 298 to 1073 K. The substitution of Mg for Co on the B site may be compensated by the formations of Co⁴⁺ and oxygen vacancies. The electrical conductivity of PrCo_1−xMg_xO₃ oxides increases with increasing x in the range of 0.0-0.2. The increase in conductivity becomes considerable at the temperatures ?673 K especially for x?0.1; it reaches a maximum at x=0.2 and 1073 K. From x>0.2 the conductivity of PrCo_1−xMg_xO₃ starts getting lower. This is probably a result of the segregation of Pr₆O₁₁ in PrCo_1−xMg_xO₃ , which blocks oxygen transport, and association of oxygen vacancies. A change in activation energy for all PrCo_1−xMg_xO₃ compounds (x=0-0.25) was observed, with a higher activation energy above 573 K and a lower activation energy below 573 K. The reasons for such a change are probably due to the change of dominant charge carriers from Co⁴⁺ to Vö in PrCo_1−xMg_xO₃ oxides and a phase transition mainly starting at 573 K.     

Analysis of magnetic and structural phase transition behaviors of La1−xSrxCrO3 for preparation of phase diagram

The phase transition behavior of perovskite-type compounds, La_1−xSr_xCrO₃, was investigated by differential scanning calorimetry (DSC), dilatometry, dc magnetic susceptibility measurement and X-ray diffraction analysis. Both second-order magnetic phase transition from antiferromagnetic to paramagnetic and first-order structural phase transition from orthorhombic to rhombohedral were observed in the DSC or dilatometric curve of every specimen. The temperatures of both these magnetic and structural phase transitions decreased linearly with an increase in Sr content. The structural phase transition temperature of La_1−xSr_xCrO₃ with x less than 0.11 is higher than the magnetic phase transition temperature; however, a larger decrease in structural phase transition temperature than in magnetic phase transition temperature was observed with an increase in Sr content, resulting in a structural phase transition temperature lower than the magnetic phase transition temperature for La_1−xSr_xCrO₃ with x of more than 0.12. It was also observed that the heat of absorption of the structural phase transition decreased with an increase in x. In the dependence of dc magnetic susceptibility on temperature, variations by not only magnetic but also structural phase transitions were observed. It was also revealed that thermal expansion coefficient is affected not only by structural phase transition but also magnetic phase transition. Magnetic and structural phase diagram of La_1−xSr_xCrO₃, suggesting the existence of two Sr contents and temperatures at which triple phases coexist, was proposed.     

Structural, spectroscopic and photoluminescence studies of LiEu(WO4)2−x(MoO4)x as a near-UV convertible phosphor

A series of lithium europium double tungsto-molybdate phosphors LiEu(WO₄)_2−x(MoO₄)_x (x=0, 0.4, 0.8, 1.2, 1.6, 2.0) have been synthesized by solid-state reactions and their crystal structure, optical and luminescent properties were studied. As the molybdate content increases, the intensity of the ⁵D₀→⁷F₂ emission of Eu³⁺ activated at wavelength of 396 nm was found to increase and reach a maximum when the relative ratio of Mo/W is 2:0. These changes were found to be accompanied with the changes in the spectral feature, which can be attributed to the crystal field splitting of the ⁵D₀→⁷F₂ transition. As the molybdate content increases the emission intensity of the 615 nm peak also increases. The intense red-emission of the tungstomolybdate phosphors under near-UV excitation suggests them to be potential candidate for white light generation by using near-UV LEDs. In this study the effect of chemical compositions and crystal structure on the photoluminescent properties of LiEu(WO₄)_2−x(MoO₄)_x is investigated and discussed.     

Novel rare-earth borosilicide RE1−xB12Si3.3−δ (RE=Y, Gd-Lu) (0?x?0.5, δ≈0.3): synthesis, crystal growth, structure analysis and properties

New rare-earth boron-rich compounds with the formula of RE_1−xB₁₂Si_3.3−δ (RE=Y, Gd-Lu) (0?x?0.5,δ≈0.3) have been synthesized. They belong to a new type of rhombohedral structure with the space group of R-3m (No. 166) and z=9. The lattice constants were measured from powder XRD data. Crystal structure solved from powder XRD data for Tb_0.68B₁₂Si₃ as a representative has been compared with that of YB_17.6Si_4.6 (or Y_0.68B₁₂Si_3.01), whose structure was solved from single-crystal reflection data. The structure model is confirmed by high-resolution transmission microscope analysis. The vibrational modes of the new crystals were measured by Raman spectroscopy. Temperature dependence of magnetic susceptibility which was measured for RE_1−xB₁₂Si_3.3−δ single crystals by SQUID revealed that they are paramagnetic materials down to 2.0 K.     

Structure and conductivity of strontium-doped cerium orthovanadates Ce1−xSrxVO4 (0≤x≤0.175)

A-site substituted cerium orthovanadates, Ce_1−xSr_xVO₄, were synthesised by solid-state reactions. It was found that the solid solution limit in Ce_1−xSr_xVO₄ is at x=0.175. The crystal structure was analysed by X-ray diffraction and it exhibits a tetragonal zircon structure of space group I4₁/amd (1 4 1) with a=7.3670 (3) and c=6.4894 (1) Å for Ce_0.825Sr_0.175VO₄. The UV-vis absorption spectra indicated that the compounds have band gaps at room temperature in the range 4.5-4.6 eV. Conductivity measurements were performed for the first time up to the strontium solid solution limit in air and in dry 5% H₂/Ar with conductivity values at 600 °C ranging from 0.3 to 30 mS cm⁻¹ in air to 30-45 mS cm⁻¹ in reduced atmosphere. Sample Ce_0.825Sr_0.175VO₄ is redox stable at a temperature below 600 °C although the conductivity is not high enough to be used as an electrode for solid oxide fuel cells.     

Thermochemistry of Li1+xMn2−xO4 (0?x?1/3) spinel

Lithium substituted Li_1+xMn_2−xO₄ spinel samples in the entire solid solution range (0?x?1/3) were synthesized by solid-state reaction. The samples with x<0.25 are stoichiometric and those with x?0.25 are oxygen deficient. High-temperature oxide melt solution calorimetry in molten 3Na₂O·4MoO₃ at 974 K was performed to determine their enthalpies of formation from constituent binary oxides at 298 K. The cubic lattice parameter was determined from least-squares fitting of powder XRD data. The variations of the enthalpy of formation from oxides and the lattice parameter with x follow similar trends. The enthalpy of formation from oxides becomes more exothermic with x for stoichiometric compounds (x<0.25) and deviates endothermically from this trend for oxygen-deficient samples (x?0.25). This energetic trend is related to two competing substitution mechanisms of lithium for manganese (oxidation of Mn³⁺ to Mn⁴⁺ versus formation of oxygen vacancies). For stoichiometric spinels, the oxidation of Mn³⁺ to Mn⁴⁺ is dominant, whereas for oxygen-deficient compounds both mechanisms are operative. The endothermic deviation is ascribed to the large endothermic enthalpy of reduction.     

Synthesis and properties of the Co7Se8−xSx and Ni7Se8−xSx solid solutions

We report the synthesis and elementary properties of the Co₇Se_8−xS_x (x=0-8) and Ni₇Se_8−xS_x (x=0-7) solid solutions. Both systems form a NiAs-type structure with metal vacancies. In general, the lattice parameters decrease with increasing x, but in the Ni₇Se_8−xS_x system c increases on going from x=5 to 7. Magnetic susceptibility measurements show that all samples exhibit temperature-independent paramagnetism from 25-250 K. Samples within the Co₇Se_8−xS_x system, as well as Ni₇Se₈ and Ni₇SeS₇, were found to be poor metals with resistivities of ∼0.20 and ∼0.06 mΩ cm at 300 K, respectively. The Sommerfeld constant (γ) was determined from specific heat measurements to be ∼13 mJ/mol_CoK² and ∼7 mJ/mol_NiK² for Co₇Se_8−xS_x and Ni₇Se_8−xS_x, respectively.     

Effects of gold catalysts and thermal evaporation method modifications on the growth process of Zn1−xMgxO nanowires

In this paper, we investigate the roles of gold catalysts and thermal evaporation method modifications in the growth process of Zn_1−xMg_xO nanowires. Zn_1−xMg_xO nanowires are fabricated on silicon substrates with and without using a gold catalyst. Characterizations reveal that Mg acts in a self-catalyst role during the growth process of Zn_1−xMg_xO nanowires grown on catalyst-free substrate. The optical properties and crystalline quality of the Zn_1−xMg_xO nanowires are characterized by room temperature photoluminescence (PL) measurements and Raman spectroscopy, respectively. The Raman and PL studies demonstrate that the Zn_1−xMg_xO nanowires grown using the catalyst-free method have good crystallinity with excellent optical properties and have a larger band-gap in comparison to those grown with the assistance of gold.     

Modulated structure of the composite crystal InCr1−xTixO3+x/2

Incommensurately modulated structure of the composite crystal InCr_1−xTi_xO_3+x/2 was refined by the profile fitting of powder X-ray diffraction based on the four-dimensional superspace group. The crystal consists of two monoclinic subsystems mutually incommensurate in b. The first subsystem is the alternate stacking of an edge-shared InO₆ octahedral layer and a Cr/Ti triangle-lattice plane along c^*. A sheet of oxygen atoms constructing the second subsystem is also extending on the Cr/Ti plane. The whole structure is the alternate stacking of an edge-shared InO₆ octahedral layer and a Cr/Ti-O plane, where displacive modulation of O ions is prominent. Metal ions on the Cr/Ti-O plane are surrounded by three or four oxygen ions on the plane and, in addition, two axial ones.     

Electrical properties and sulfur tolerance of La0.75Sr0.25Cr1−xMnxO3 under anodic conditions

Complex metal oxides with composition of La_0.75Sr_0.25Cr_1−xMn_xO₃(x=0.4,0.5,0.6) (LSCM) have been synthesized and examined as anode materials for solid oxide fuel cells (SOFCs). LSCM compositions show excellent tolerance to both reduction and oxidation but the crystal structure transforms from hexagonal in air to orthorhombic in H₂. The volume change associated with this phase transformation is only about 1%, thus having little effect on other properties. The total electrical conductivity increases with the content of Mn, whereas the resistance to sulfur poisoning increases with the content of Cr. Fuel cells using LSCM as the anode show very good performance when pure hydrogen is used as the fuel. However, they do not appear to be stable in fuels containing 10% of H₂S.     

Sol-gel synthesis of K3InF6 and structural characterization of K2InC10O10H6F9, K3InC12O14H4F18 and K3InC12O12F18

K₃InF₆ is synthesized by a sol-gel route starting from indium and potassium acetates dissolved in isopropanol in the stoichiometry 1:3, with trifluoroacetic acid as fluorinating agent. The crystal structures of the organic precursors were solved by X-ray diffraction methods on single crystals. Three organic compounds were isolated and identified: K₂InC₁₀O₁₀H₆F₉, K₃InC₁₂O₁₄H₄F₁₈ and K₃InC₁₂O₁₂F₁₈. The first one, deficient in potassium in comparison with the initial stoichiometry, is unstable. In its crystal structure, acetate as well as trifluoroacetate anions are coordinated to the indium atom. The two other precursors are obtained, respectively, by quick and slow evaporation of the solution. They correspond to the final organic compounds, which give K₃InF₆ by decomposition at high temperature. The crystal structure of K₃InC₁₂O₁₄H₄F₁₈ is characterized by complex anions [In(CF₃COO)₄(OH_x)₂]^(5−2x)− and isolated [CF₃COOH_2−x]^(x−1)− molecules with x=2 or 1, surrounded by K⁺ cations. The crystal structure of K₃InC₁₂O₁₂F₁₈ is only constituted by complex anions [In(CF₃COO)₆]³⁻ and K⁺ cations. For all these compounds, potassium cations ensure only the electroneutrality of the structure. IR spectra of K₂InC₁₀O₁₀H₆F₉ and K₃InC₁₂O₁₂F₁₈ were also performed at room temperature on pulverized crystals.     

Transport coefficients of titanium-doped Sb2Te3 single crystals

Titanium-doped single crystals (c_Ti=0-2×10²⁰ atoms cm⁻³) were prepared from the elements Sb, Ti, and Te of 5 N purity by a modified Bridgman method. The obtained crystals were characterized by measurements of the temperature dependence of the electrical resistivity, Hall coefficient, Seebeck coefficient and thermal conductivity in the temperature range of 3-300 K. It was observed that with an increasing Ti content in the samples the electrical resistivity, the Hall coefficient and the Seebeck coefficient increase. This means that the incorporation of Ti atoms into the Sb₂Te₃ crystal structure results in a decrease in the concentration of holes in the doped crystals. For the explanation of the observed effect a model of defects in the crystals is proposed. The data of the lattice thermal conductivity were fitted well assuming that phonons scatter on boundaries, point defects, charge carriers, and other phonons.     

Effects of Pressure and Temperature on the Formation of Tungsten-Bronze-Related Phases, RExWO3+y, with RE=La and Nd

The influence of pressure (P) and temperature (T) on the formation of tungsten-bronze-related phases containing lanthanum and neodymium was investigated. A large number of samples with bulk compositions RE_xWO₃, prepared by solid-state reaction in the pressure and temperature regions P= 10-80 kbar and T= 1170-1620 K were examined by X-ray powder diffraction and electron microscopy, and a (P-T) diagram showing the phase relations was drawn. Three tungsten-bronze-related phases with perovskite (PTB)-, hexagonal (HTB)- and intergrowth (ITB)-type structures were identified. The PTB bronze RE_xWO₃ with x≈ 0.10 was formed at p≤50 kbar. The HTB-related phase with x≈ 0.10 was observed in samples prepared at P≥20 kbar, whereas phases of (n)-ITB-type were observed only in the 25-50 kbar region. In the latter pressure region, the PTB and ITB phases were only seen in samples prepared at T > 1520 K, while the HTB-related phase was found in almost all samples. The HTB- and ITB-related compounds are metastable, probably fully oxidized, high-pressure phases of composition RE_xWO_3+3x/2 with x≤0.13. They transform to a cubic PTB bronze during annealing in inert atmosphere under ambient pressure conditions. According to microanalysis studies of individual crystals, less than 40% of the hexagonal tunnel sites in the HTB and ITB structures are occupied by RE³⁺ ions. A superstructure of HTB-type with ≈60% occupancy of the hexagonal tunnel sites (x≈0.20) was observed in a few crystals from the samples prepared at P= 80 kbar. Ordered, defect and intergrowth structures are presented.     

The Crystal Chemistry of Lithium in the Alluaudite Structure: A Study of the (Na1−xLix)CdIn2(PO4)3 Solid Solution (x=0 to 1)

Several compounds of the (Na_1−xLi_x)CdIn₂(PO₄)₃ solid solution were synthesized by a solid-state reaction in air, and pure alluaudite-like compounds were obtained for x=0.00, 0.25, and 0.50. X-ray Rietveld refinements indicate the occurrence of Cd²⁺ in the M(1) site, and of In³⁺ in the M(2) site of the alluaudite structure. This non-disordered cationic distribution is confirmed by the sharpness of the infrared absorption bands. The distribution of Na⁺ and Li⁺ on the A(1) and A(2)′ crystallographic sites cannot be accurately assessed by the Rietvled method, probably because the electronic densities involved in the Na⁺→Li⁺ substitution are very small. A comparison with the synthetic alluaudite-like compounds, (Na_1−xLi_x)MnFe₂(PO₄)₃, indicates the influence of the cations occupying the M(1) and M(2) sites on the coordination polyhedra morphologies of the A(1) and A(2)′ crystallographic sites.     

Floating Zone Crystal Growth and Structure Analysis of a Novel ScB19 Family Compound, ScB19+xSiy

Single crystals of a novel ScB₁₉ family compound ScB_19+xSi_y were grown by the floating zone method using a four-lamp mirror-type image furnace. A small amount of silicon addition to ScB₁₉ which decomposes at elevated temperatures without melting allowed it to coexist with the liquid phase and as a resultant made the floating zone crystal growth possible. Powder X-ray diffraction analysis confirmed the grown crystals of ScB_19+xSi_y to be isostructural to ScB₁₉. It was found that the crystal structure of ScB_19+xSi_y solved based on single-crystal X-ray data is tetragonal with lattice constants of a, b=1.03081(2) nm, c=1.42589(3) nm, space group P4₁2₁2 or P4₃2₁2 and is basically isotypic with α-AlB₁₂ structure type. In the crystal structure boron atoms form a three-dimensional framework based on interconnected B₁₂ icosahedra and B₂₂ polyhedra. The Sc atoms reside in three of five Al sites in the α-AlB₁₂ structure and Si resides in a bridge site bonding two B₂₂ units.