Stoichiometric Solid Solutions Research Articles

Design of a metal-oxide solid solution for selective detection of ethanol with marginal influence by humidity

Emerging nanotechnologies improved solid-state sensors design and capabilities and enabled the use of semiconducting metal oxides-based gas sensors for new and increasingly demanding applications. Among them, ethanol monitoring is of great interest because it is an important chemical raw material widely used in chemical industry, medicine, paint and cosmetics. Water vapour is the most common interferent during operating conditions, affecting the baseline stability of sensors, and reducing their sensitivity and accuracy. In this work, a strategy based on the formation of stoichiometric solid solutions of Sn, Ti and Nb oxides have been sought to address humidity dependence of sensor signals and to enhance the sensing response vs. ethanol. Powders were synthesised through sol-gel technique, by keeping the Sn:Ti ratio constant at the optimal value for ethanol detection with different Nb concentrations (1.5, 3.5 and 5 at%), and thermally treated at 650 °C and 850 °C. Such solid solutions exploited the good sensitivity of SnO2 towards most analytes and the low cross-sensitivity of TiO2 to humidity. Nb was added to lower the film resistance and to prevent grains coalescence at temperatures required for the formation of the solid solutions and the anatase-to-rutile phase transition. The results show that the niobium concentration and the heating treatment of powders are fundamental parameters to optimise the sensing features of the film towards ethanol monitoring applications and to lower the influence of humidity on its performance.

Low-temperature heat capacity VO2±δ and solid solutions V1-XFeXO2

The article reports the study results of the heat capacity of vanadium dioxide compounds (VO2) (within the homogeneity range) and stoichiometric solid solutions V1-xFexO2 at temperature range from 5 κ to 100 κ. Separation of various constituents of heat capacity with its additivity assumption has been conducted: crystal lattice heat capacity and defects heat capacity. It is shown that a considerable contribution to the heat capacity is produced by the crystal lattice defects (till ~ 25 κ) at low temperatures, and at higher temperatures the heat capacity is determined mainly with the crystal lattice. Using the Debye function table for the heat capacity for all compounds, the characteristic Debye temperatures θD were determined.

Incorporation of vanadium into the framework of hydroxyapatites: importance of the vanadium content and pH conditions during the precipitation step.

Even though vanadium-modified hydroxyapatite (V-HAp) samples are very promising systems for oxidative dehydrogenation of propane, the incorporation of vanadium into the hydroxyapatite framework was reported to be limited and to lead to over-stoichiometric compounds. Here, the synthesis of a Ca10(PO4)6-x(VO4)x(OH)2 stoichiometric solid solution using a co-precipitation method is monitored in the whole composition range (0 ≤ x ≤ 6) by controlling the pH of the precipitation medium, with continuous (the first series of samples) or periodic (the second series of samples) addition of NH4OH during the precipitation step or during the maturation step, respectively. It is demonstrated that the changes in pH conditions result in materials of a substantial difference in terms of the final composition. From XRD patterns and Rietveld refinements, a solid solution V-HAp phase was found to be exclusively obtained for the first series of samples for x varying from 0 to 6. This also occurred in the second series of samples but only for x lower than 4. For 4 ≤ x ≤ 5.22, the materials were composed of a mixture of V-HAp and Ca2V2O7, whereas for a x value of 6 only Ca2V2O7 was formed. The predominance of polymeric V species in solution at a high vanadium concentration deduced from the diagram of speciation of vanadium accounts for the preferential formation of Ca2V2O7 under these particular conditions. However, provided that a higher pH value was maintained, isolated VO3(OH)2- species are predominant, which accounts for the incorporation of isolated vanadates into the hydroxyapatite framework and for the well-controlled stoichiometry with Ca/(P + V) ratios found to be close to 1.67. Such a very good accommodation of vanadium in the hydroxyapatite framework is illustrated by the characterization of the local surrounding of phosphorus and vanadium species using 31P and 51V NMR, Raman and UV-vis spectroscopies.

Hydrothermal synthesis of yttria stabilized ZrO 2 nanoparticles in subcritical and supercritical water using a flow reaction system

Yttria stabilized zirconia nanoparticles have been prepared by hydrothermal flow reaction system under subcritical and supercritical conditions. ZrO(NO 3) 2/Y(NO 3) 3 mixed solutions were used as starting materials. Reaction temperature was 300–400 °C. Reaction time was adjusted to 0.17–0.35 s. Based on the residual Zr and Y concentrations, the complete conversion of zirconium was achieved irrespective of pH and hydrothermal temperature, whereas the conversion of yttrium increased with an increase in pH and hydrothermal temperature. Stoichiometric solid solution was achieved at pH>8. XRD results revealed that tetragonal zirconia can be formed regardless of yttrium content, where the tetragonality was confirmed by Raman spectroscopy. The average particle size estimated from BET surface area was around 4–6 nm. Dynamic light scattering particle size increased with the solution pH owing to the aggregation of primary particles. TG-DTA analyses revealed that weight losses for adsorbed water and hydroxyl groups decreased with hydrothermal temperature.

Solid solutions Ru x Ir1−x synthesized by thermolysis of coordination compounds

The double complex salts [Ir(NH3)5Cl][IrCl6], [Ru(NH3)5Cl][IrCl6], and [Ru(NH3)5Cl]2[IrCl6]Cl2 have been synthesized and characterized. An X-ray diffraction study demonstrated that these salts are isostructural with [Rh(NH3)5Cl][OsCl6] and [Ir(NH3)5Cl]2[PtCl6]Cl2 synthesized earlier. Thermolysis of these salts in hydrogen and helium has been studied. X-ray powder diffraction analysis show that thermolysis yields stoichiometric solid solutions of metals as the final products. The unit cell parameters of these products correspond to equilibrium phases.

A novel route to synthesize cubic ZrW 2−xMo xO 8 ( x=0–1.3) solid solutions and their negative thermal expansion properties

Cubic ZrW 2− x Mo x O 8 (c-ZrW 2− x Mo x O 8) ( x=0–1.3) solid solutions were prepared by a novel polymorphous precursor transition route. X-ray diffraction (XRD) analysis reveals that the solid solutions are single phase with α- and β-ZrW 2O 8 structure for 0⩽ x⩽0.8 and 0.9⩽ x⩽1.3, respectively. The optimum synthesis conditions of ZrWMoO 8 are obtained from differential scanning calorimetry–thermal gravimetric analysis (DSC–TGA), XRD and mass loss-temperature/time curves. Following the above experience, the stoichiometric solid solutions of c-ZrW 2− x Mo x O 8 ( x=0–1) are obtained within 1 wt% of mass loss. The relationships of lattice parameters ( a), phase transition temperatures ( T c) and instantaneous coefficients of thermal expansion ( α i) against the content x of Mo are discussed based on the variation of order degree parameters of ZrW 2− x Mo x O 8.

Peculiarities of the magnetic state in the system La0.70Sr0.30MnO3−γ (0 ≤ γ ≤ 0.25)

The results of experimental investigation of the chemical phase composition, crystal structure, and magnetic properties of the manganite La0.70Sr0.30MnO3−γ (0 ≤ γ ≤ 0.25) with perovskite structure depending on the concentration of oxygen vacancies are presented. It is found that the mean grain size of the stoichiometric solid solution of La0.70Sr0.30MnO3 amounts approximately to 10 μm, while the grain size for anion-deficient solid solutions of La0.70Sr0.30MnO3−γ is approximately 5 μm. It is found that samples with 0 ≤ γ ≤ 0.13 have a rhombohedral unit cell (with space group $$R\bar 3c$$ , Z = 2), while samples with γ ≥ 0.20 have a tetragonal unit cell (space group I4/mcm, Z = 2). It is proved experimentally that the magnetic phase state of the manganite La0.70Sr0.30MnO3−γ changes upon a decrease in the oxygen content. It is shown that anion-deficient solid solutions of La0.70Sr0.30MnO3−γ experience a number of successive magnetic phase transformations in the ground state from a ferromagnet (0 ≤ γ ≤ 0.05) to a charge-disordered antiferromagnet (γ = 0.25) via an inhomogeneous magnetic state similar to a cluster spin glass (0.13 ≤ γ ≤ 0.20). The mean size of ferromagnetic clusters (r ≈ 50 nm) in the spin glass state is estimated. It is shown that oxygen vacancies make a substantial contribution to the formation of magnetic properties of manganites. The generalized magnetic characteristics are presented in the form of concentration dependences of the spontaneous magnetic moment, coercive force, and the critical temperature of the magnetic transition. The most probable mechanism of formation of the magnetic phase state in Sr-substituted anion-deficient manganites is considered. It is assumed that in the absence of orbital ordering, a decrease in the magnetic ion coordination number leads to sign reversal in indirect superexchange interactions Mn3+-O-Mn3+.

Effect of Composition on Ferroelectric Response of Bi4-xSmxTi3O12 Thin Films

AbstractBismuth titanate (Bi4Ti3O12) is an electroceramic within the Aurivillius phase material with week ferroelectric memory. However, the partial substitution of Bi ion by a trivalent rare earth resulted in improved ferroelectric polarization. We synthesized Bi4-xSmxTi3O12 by a solution chemistry route and thin films were deposited by spin coating on Pt (Pt/TiO2/SiO2/Si) substrate. X-ray diffraction studies of the films showed a stoichiometric solid solution for the composition x ≤0.85. The films showed fatigue free ferroelectric polarization (Pr = 19.8 µC/cm2) for the composition x = 0.70. The films showed low leakage current at room temperature, showing the possibility of lead free ferroelectric devices.

The effects of dopants on the properties of metal oxides

The effect of aliovalent dopants on the defect chemistry of metal oxides is reviewed. The discussion focuses on the stoichiometric solid solution obtained by combination of the stoichiometric dopant oxide and the stoichiometric host oxide without gain or loss of oxygen. The charge of the dopant centers is then compensated solely by ionic defects. As the oxygen activity is raised above that in equilibrium with the stoichiometric composition the lattice defects are gradually replaced by holes. Conversely, when the oxygen activity is reduced below that in equilibrium with the stoichiometric composition the lattice defects are gradually replaced by electrons. The near-stoichiometric region of a pure metal oxide is replaced by two regions with charge compensation by a product of reduction, e.g. oxygen vacancies or electrons, in the region of lower oxygen activity, and by a product of oxidation, e.g. cation vacancies or holes, in the region of higher oxygen activity.

Distortion and cation ordering in LaSr(Ni1−xCux)TaO6

The perovskite-type solid solution LaSr(Ni 1− x Cu x )TaO 6 was synthesized and characterized by powder X-ray diffraction and iodometric titration. The stoichiometric solid solutions were synthesized at 1050–1100°C in air. The compounds for 0≤ x ≤0.6 have the (NH 4 ) 3 FeF 6 -type structure with ordering between (Ni 2+ , Cu 2+ ) and Ta 5+ . Tetragonal distorted (NH 4 ) 3 FeF 6 -type phases due to the Jahn-Teller effect formed in the compositional range, x ≥0.7 and the monoclinic distortion was observed for x =0.9 and 1.0. This result suggests that the tetragonal distortion occurs when the amount of Cu 2+ becomes more than 35% of the octahedral site in this system.

Composition dependence of the lattice parameter in solid solutions

We present an accurate x-ray powder diffraction study of stoichiometric solid solutions performed using an internal-standard method. The results obtained allowed us to establish for the first time that the composition dependence of the lattice parameter a(c) is not linear, as had been asserted, and interpreted within Vegard's model, in previous works. A new model, which is able to explain both the a(c) behaviour and the variation of the local structure around nickel ions, obtained previously by x-ray absorption spectroscopy (Kuzmin A, Mironova N, Purans J and Rodionov A 1995 J. Phys.: Condens. Matter 7 9357), is suggested and discussed.

A Composite Modulated Structure Versus Superstructure Approach to the Refinement of >Zr108N98F138

Zr108N98F138 is an x = 2.185 member of a wide range non- stoichiometric solid solution Zr (N,O,F)x, 2.12 ≤ x ≤ 2.25, in the zirconium nitride oxide fluoride system. The structure is rerefined as a composite modulated structure composed of two mutually incommensurable Q and H substructures with overall superspace group symmetry Acmm (0,0,1.185..)0s0. The Q substructure has lattice parameters a = 5.186, b = 5.368 and cQ = 5.374 Ǻ, and substructure superspace group symmetry Acmm (0,0,1.185..)0s0. The H substructure has lattice parameters a = 5.186, bH = 1/2b = 2.684 and CH = 4.534 Ǻ, and substructure superspace group symmetry Pmcm (0,±,0.844...)s00. While employing almost an order of magnitude fewer positional parameters than the previous study, refinement on 956 unique reflections converged to R = 0.057 instead of R = 0.113.

Magnetic susceptibilities of Y yU 1− yO 2+ x solid solutions

Magnetic susceptibilities of Y y U 1− y O 2+ x solid solutions with fluorite structure were measured from 4.2 K to room temperature. An antiferromagnetic transition was observed even for the solid solutions in which the mean uranium valency is over +5. From the comparison of the susceptibility data of oxygen hyper- and hypostoichiometric solid solutions ( x ≷ 0) with those of stoichiometric solid solutions ( x = 0), it was found that both oxygen interstitials and oxygen vacancies weaken the magnetic exchange interactions between uranium ions. With increasing yttrium and oxygen concentrations, the effective magnetic moment of uranium decreased, and the uranium ions were found to be oxidized from +4 to +6 through the +5 state.

Synthesis of ?stabilized Mn1?xMgxO? system (0.001 ? x ? 0.15) and its structural, thermal, optical and magnetic studies

A novel method of preparation, involving careful formation of a stoichiometric solid solution of stabilized Mn1-x Mg x O system for eight different concentrations ofx (0.001 ⩽x ⩽ 0.15) is described. A variety of physicochemical techniques such as X-ray diffraction, DTA-TGA, reflectance spectra, magnetic susceptibility and XPS data are used to determine structure-property correlations in the solid solutions, which are (i) X-ray diffraction studies reveal the linear decrease of cubic lattice parameter with increasing x, (ii) DTA-TG-DTG studies which confirm the formation of solid solution and the stability of these solid solutions in air which increases from 515 to 565 K withx, (iii) the diffused reflectance spectra shows nearly perfect octahedral symmetry around Mn2+ ions, (iv) magnetic susceptibility data indicates the usual dilution effects of diamagnetic ions in antiferromagnetic host lattice, (v) XPS studies reveal the presence of mixed phase of Mn2O3-MgO on the surface which protects the bulk MnO from its further oxidation to higher oxides of manganese. This shows the important role of Mg2+ in the chemical passivation of manganous oxide.

Alkali metal-doped transition metal oxides active for oxidative coupling of methane

Addition of alkali metals to the oxides of transition elements endowed the oxides with the ability for converting methane into C 2 compounds (C 2H 6 + C 2H 4). Among the combinations of alkali metals and transition metal oxides tested, the lithium added nickel oxide was the most active catalyst for the reaction. X-ray diffraction (XRD) analysis showed that the catalyst was a solid solution of lithium in NiO. The reaction of methane with the lattice oxygen atoms of the stoichiometric solid solution, LiNiO 2, produced C 2 compounds very selectively. The XRD analysis of the sample showed that the reaction proceeded as 2LiNiO 2 + 2CH 4 → Li 2O + 2NiO + C 2H 6 + H 2O. Oxidation of the reduced catalyst by oxygen regenerated LiNiO 2 as Li 2O + 2NiO + 1/2O 2 → 2LiNiO 2. Thus, it is concluded that the oxidative coupling of methane proceeds via the redox mechanism of LiNiO 2. The stability of the solid solution of alkali metals with transition metal oxides under steady state reaction conditions is essential for the alkali-doped oxides to be effective in the oxidative coupling of methane.

Equilibrium oxygen fugacity for the synthesis of zinc ferrite solid solutions

AbstractThermodynamic information is utilized to study the width of the zinc ferritemagnetite phase field, and to establish the oxygen fugacity in equilibrium with this solid solution. Information is provided for the specification of the phase boundaries and for the particular oxygen fugacity that corresponds to the 4/3 oxygen/ cation ratio in the strictly stoichiometric solid solution.

Phase Relations in the System UO<sub>2</sub>-ThO<sub>2</sub>-O

The phase study was carried out, in relation to the effective fabrication of the ceramic nuclear fuel of UO2-ThO2 system, for obtaining informations of phases present on its sintering or heat treatment in various atmospheres. In addition it is also very interesting that non-stoichionetric UO2+x and ThO2 make a non-stoichiometric solid solution UyTh1-yO2+x in various composition.Ammonia solution was added to the mixed soluton of uranyl nitrate and thorium nitrate to co-precipitate the ammonium diuranate plus thorium hydroxide. The intimate mixture of U3O8 and ThO2 containing a solid solution as starting meterial was obtained by calcination of the co-precipitate in air at 500°-800°C. This powder was pressed to small round pellets which were suspended in the furnace tube to hold at certain constant temperature (700°-1500°C) in an atmosphere (oxygen, air or nitrogen), and then quenched. The specimens obtained were examined with their O/(U+Th) atomic ratio by means of the thermal balance hydrogen reduction, and the phases present by X-ray diffraction analysis.Fig. 1 shows the variation of O(U+Th) ratio against various temperatures in several atmosheres. Each curve has no break that was seen in the case of UO2 only. The lattice constants of cubic solutions containing excess oxygen were given in Fig. 2. On these curves the minimum points were obtained. This shows that the lattice parameters of stoichiometric UO2-ThO2 solid solution decreased with the O/(U+Th) ratio till the minimum points and then increased again. This fact explains that the non-stoichiometric dissolution of oxygen into lattice made decrease the lattice parameter, reached a saturation, and then the lattice constant of cubic solution increased owing to the increase in thorium content in it by a separation of U5O13(UO2.60) phase. In Fig. 3a of X-ray diffraction line intensities of the cubic solution versus O/(U+Th) ratio, an appearance of U5O13 phase begins above 2.25 of O/(U+Th) and a disappearance of cubic solution at about 2.6 (Fig. 3b).In order to confirm the limit of disappearance of the cubic solution, the UO2-ThO2 compositions rich in UO2 were examined and the results are shown in Fig. 4. There is no break at 2.60 of O/(U+Th) on the isobaric curve in air for the specimens of 95-5 and 90-10wt.% UO2-ThO2, and the X-ray diffraction line intensities of the cubic solid solution almost disappear at 2.64-2.65 of O/(U+Th) passing 2.60. This shows that the terminal composition coexisting with pure thoria without solid solution is found to be UO2.65.In the compositions rich in ThO2(60-90%), the isobaric curves are shown in Fig. 5 for oxygen, air, or nitrogen atmosphere, and the changes of the lattice parameters of cubic solution phase with O/(U+Th) are given in Fig. 6. From the minimum point of each curve, oxygen content of saturation in non stoichiometric solid solution decreases above the 59±1% ThO2 with increasing in thoria content, and then tends to zero in pure ThO2. This shows that the terminal composition of U5O13 phase in equilibrium with ThO2 or the saturated solid solution in UO2.65 (Table 1).The lattice parameters of the non-stoichiometric solution do not satisfy the Vegard rule as shown in Fig. 7. Thus the phase diagram of the UO2-ThO2-O system was obtained in the temperature range less than 1500°C, as shown in Fig. 9, and the lattice parameters obtained by using this Figure were compared with the actual data and the agreement was found in the composition range rich in UO2 (Table 3). The formation of