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Nanoparticles of Bi2Ti2O7 pyrochlore were synthesized by a coprecipitation method. They exhibit a high visible-light photoactivity for decolorization of a methyl-orange dye solution in the presence of a sacrificial agent, H2O2. The same pyrochlore system with an Ag co-catalyst showed even faster kinetics under only visible light and an unprecedentedly high photoactivity in the UV range.

Chromium niobate and tantalate (CrNbO4 and CrTaO4) were synthesized by pyrolysis of the oxalate‐based heterometallic complexes [Cr2(bpy)4(μ-O)4Nb2(C2O4)4]·3H 2O (Cr‐Nb) and [Cr(bpy)2(H2O)(μ‐O)Ta(C2O4)3]2·3.5H2O (Cr‐Ta) (bpy = 2, 2'‐bipyridine). Compared to conventional solid‐state synthesis, herein studied oxides are prepared at lower temperatures, in one step without repeating grinding procedures. The structure, morphology, and optical properties of the as‐synthesized oxides were characterized using powder X‐ray diffraction (PXRD), field emission scanning electron microscopy (SEM), the thermogravimetric and differential thermal analysis (TG/DTA) and UV‐vis diffuse reflectance spectroscopy (UV‐vis DRS). The determined band gap energies of CrNbO4 and CrTaO4 are 2.39 and 2.82 eV, respectively, which prompted us to investigate photocatalytic activity of these oxides in degradation of dyes. Microscopy studies show that the particles of both oxides began to aggregate into bigger particles, leading to an increase in grain size. Additionally, magnetization measurements on both oxides revealed spin‐glass behavior at low temperatures.

We investigated the effect of annealing temperature on magnetic properties of 2% and 10% Fe-doped $BaTiO_{3}$. To understand the possible structural differences between samples treated at different annealing temperatures, and to correlate them with the magnetic properties, several characterization techniques, such as X-ray diffraction and X-ray absorption spectroscopic methods (XANES and EXAFS) were employed. We found that the 2% Fe-doped $BaTiO_{3}$ pseudocubic perovskite is paramagnetic regardless of the heat-treatment conditions. Initially paramagnetic 10% Fe-doped $6H–BaTiO_{3}$, treated at 1250°C, became ferromagnetic after additional annealing at higher temperature. We have crystalographically characterized the cation ordering processes in the $6H–BaTiO_{3}$ that occurred during the high-temperature annealing. The ferromagnetism that is induced in this stage is most probably associated with the observed diffusion processes but it extrinsic character still cannot be fully disregarded.

An analysis of Mn substitution in SrTiO 3 is performed in order to understand the origin of reported spin coupling in lightly Mn-doped SrTiO 3 . The spin glass state magnetoelectrically coupled to the dipolar glass state has previously been reported for SrTiO 3 substituted with only 2% of Mn on the B-site. An analysis of the substitution mechanism for A- and B-site doping shows a strong infl uence of processing conditions, such as processing temperature, oxygen partial pressure, and off-stoichiometry. The required conditions for a site-selective substitution are defi ned, which yield a single-phase and almost defect-free perovskite. Magnetic measurements show no magnetic anomalies resulting from spin coupling and only a simple paramagnetic behavior. Magnetic anomalies are observed only for the samples in which Mn is misplaced within the cation sublattice of the SrTiO 3 perovskite. This occurs due to improper material processing, which causes initially unpredicted changes in the valence state of the Mn and results in the formation of structural defects and irregularities associated with segregation and nucleation of the magnetic species. Previously reported spin coupling in Mn-doped SrTiO 3 is not an intrinsic phenomenon and cannot be treated as a spin glass.

A new pyrochlore homogeneity field in Bi2O3–Fe2O3–TeO3 system is characterized with respect to its compositional range and phase relations to adjacent phases. The sintering temperature of these pyrochlores is 880°C and they do not react with metallic silver, which makes them suitable for low-temperature cofired ceramics (LTCC) technology. Magnetic measurements indicate a spin-glass formation. The spin freezing temperature that occurs at around 20 K shows a slight, yet systematic, variation with the sample composition. The dielectric constant increases with an increase in Bi content from 37(1) to 55(1). At cryogenic temperatures, they show the dielectric relaxation typical for displacive pyrochlores. The UV–Vis diffuse reflectance data were used for calculation of the electronic band gap. The lowest band gap was determined to be 1.965 eV. Next to the pyrochlore range, another new phase was discovered. Structural analysis showed that Bi3(Fe0.56Te0.44)3O11 crystallizes as KSbO3-type in a displacive disordered cubic Pn 3 symmetry with a lattice parameter of 9.47045(15) A ˚ .

Analysis of kinetic characteristics of the incommensurate—commensurate transformation in the Bi3Nb1−xTaxO7 system with a fluorite-like crystal structure enabled us to define the compositional and thermal stability ranges of the commensurate tetragonal polymorph. We showed that the tetragonal polymorph across the entire homogeneity range is isostructural to the Bi3NbO7 end member. The kinetics of the transformation from the cubic incommensurate to the tetragonal commensurate phase decreases with an increase in Ta concentration within the Bi3Nb1−xTaxO7 system. The formation studies indicate that the tetragonal Bi3TaO7 is thermodynamically stable; however, its formation via the metastable incommensurate-cubic phase is suppressed.

Mn-doped KTaO3 samples were prepared by the solid-state method. From the analysis of A-and B-site substitution characteristics, we determined the solid solubility limits for both doping mechanisms, the influence on unit cell parameters, the phase relations, and the sintering characteristics. The sintered samples were used for the analysis of microwave dielectric properties. From the temperature and frequency dependence of permittivity and dielectric loss as a function of Mn content in KTaO3 the behavior of polar regions, induced by the doping, was estimated.

In functional oxide materials so-called molecular auxetic behavior is extremely rare. Here, it is reported in the CoFe 2 0 4 spinel structure. A CoFe 2 0 4 epitaxial thin film under compressive axial strain also reduces its cell dimensions in the transverse direction with a Poisson's ratio of -0.85. A hinge-like honeycomb network in the spinel structure is identified as being responsible for the negative Poisson's ratio. This phenomenon has a substantial effect on the functional properties of CoFe 2 O 4 and enables the construction of a new class of nano-devices.

A quantitative assessment of the Raman spectrum emitted from a coarse-grained polycrystal of multiferroic BiFeO3 has been carried out by means of a polarized Raman microprobe. The dependence of the intensity of Raman phonon modes has been first theoretically modeled as a function of crystal rotation. Then, the Raman tensor elements have been experimentally determined from the analysis of the Ag and Eg vibrational modes. Copyright © 2009 John Wiley & Sons, Ltd.

Studies of a synthesis of KTaO3 displayed a two-step reaction path with the intermediate formation of K2Ta4O11. Thermal and X-ray diffraction analysis showed that the synthesis is completed at 900°C. Sintering studies showed that KTaO3 powder can produce single phase ceramics with 85% of relative density with the addition of 5% of potassium excess when sintered at 1330°C for 1 h. Longer sintering times or higher temperatures result in enhanced potassium loss and consequent formation of K6Ta10.8O30 secondary phase with tetragonal tungsten bronze structure. Room- and low-temperature microwave dielectric properties of KTaO3 ceramics compare well with measurements performed on single crystals. We evaluated the influence of microstructural features (porosity, structural defects, secondary phases) on the dielectric properties of the ceramic samples.

Dielectric properties of pyrochlores compositions from Bi2O3–Nb2O5–NiO system were analyzed. The dielectric properties are dominated with a low-temperature relaxation that is typical for Bi-pyrochlores. A vast pyrochlore homogeneity range that exists in this system allowed to correlate characteristics of the observed relaxations with a compositional variations within the A2O′- and B2O6 pyrochlore sublattice. It was possible to make a distinction between different influences of the two sublattices, which can be satisfactorily described by the existing relaxation model for Bi3/2ZnNb3/2O7. A new relaxor-like room temperature relaxation was found for Bi1.6Ni0.57Nb1.43O6.55.

Using X-ray diffraction analysis and scanning electron microscopy it was revealed that in an atmosphere of flowing oxygen in the temperature range 700°-800°C, three new compounds are formed in the Bi 2 O 3 -TiO 2 -TeO 2 pseudoternary system. These compounds are Bi 2 Ti 3 TeO 12 , Bi 2 TiTeO 8 , and Bi 6 Ti 5 TeO 22 , and all the compounds include Te 6+ . All three crystal structures were solved and refined using X-ray powder diffraction data. Based on the results of the phase formation, a solid-state compatibility diagram is proposed.

Composite ceramics based on the spinel Mg2TiO4 were prepared by a conventional mixed-oxide route. To achieve the temperature stabilization of the dielectric constant, each of the composites was added with 7 mol% CaTiO3. The effect of the substitution of isovalent Co for Mg on the microstructure and the microwave dielectric properties of the composite ceramics was also investigated. A maximum Q×f value of around 150–160 THz was obtained for the undoped Mg2TiO4, whereas a reduced Q×f value was observed for an increase in the Co concentration in the system (1−x)Mg2TiO4−xCo2TiO4. Upon doping with 7 mol% CaTiO3, the Q×f value passed through a maximum with increasing Co concentration. Adding ZnO–B2O3 to the composite system based on Co-doped Mg2TiO4 resulted in a reduction of the sintering temperature by 150°–200°C without any significant degradation in the Q×f value.

NaxCa1� xAl2� xSi21xO8 plagioclase solid solutions (0rxr1) were synthesized under sub-solidus conditions using a solid-state reaction technique. The plagioclase formation and the sintering temperature decreased with an increase in x from the anorthite (CaAl2Si2O8; x 5 0) to the albite (NaAlSi3O8; x 5 1). Microwave (MW) dielectric measurements revealed that

The Bi2O3–Nb2O5–NiO phase diagram at 1100°C was determined by means of solid-state synthesis, X-ray diffraction, and scanning electron microscopy. A ternary eutectic with a melting point below 1100°C was found to exist in the field between NiO, Bi2O3, and the end-member of the δBi2O3–Nb2O5 solid solution. The existence of the previously reported Bi3Ni2NbO9 phase was disproved. A pyrochlore homogeneity range around Bi1.5Ni0.67Nb1.33O6.25 was determined together with all the phase relations in this phase diagram.

The crystal structure of monophasic AgNb 1/2 Ta 1/2 O 3 was investigated via Rietveld refinement using powder X-ray diffractometry (XRD) data. The study revealed a monoclinic unit cell of the P2/m space group with the following unit-cell parameters: a = 3.9286(3) A, b = 3.9259(2) A, c = 3.9302(3) A, and β = 90.49(1)°. High-temperature XRD studies of the decomposition of AgNb 1/2 Ta 1/2 O 3 showed that the kinetics of the decomposition are influenced by the volatilization of silver. Suppression of silver volatilization by enclosing the sample in a corundum tube and applying an oxygen atmosphere yielded sintered, almost-monophasic AgNb 1/2 Ta 1/2 O 3 ceramics with >97% of the theoretical density. The Ag 8 (Nb,Ta) 26 O 69 phase, which regularly appeared as a result of partial decomposition of Ag(Nb 1-x Ta x )O 3 , was studied by energy-dispersive spectroscopy analysis and XRD. The Ag 8 (Nb,Ta) 26 O 69 phase exhibited a bronze-like orthorhombic structure with the following unit-cell parameters: a = 37.116(5) A, b = 12.432(3) A, and c = 7.826(2) A. Indexed powder diffraction data for Ag 8 (Nb,Ta) 26 O 69 have been reported.

Ceramic samples with the AgNb1−xTaxO3 (0.46 410, whereas the Q×f value, measured at 2 GHz, reaches values of 640–900 GHz. Dependence of the resonant frequency on temperature was not linear but did display a minimum at a particular temperature, which was dependent on the composition of the ceramic. Samples that were measured at radio frequencies displayed low dissipation factors (tgδ= 7 ×10−4−18 × 10−4), a low aging rate (0.2% per time decade), and good dielectric strength (8 V/μm at a thickness of 0.6 mm). Microstructural investigations revealed the presence of precipitates and transformation twins that should be eliminated during additional processing optimization.

A new sillenite compound, Bi 12 (B 0.5 P 0.5 )O 20 , was synthesized using a solid-state reaction method. The stoichiometry was confirmed by XRD analyses, microstructural investigations, and quantitative elemental analysis. An investigation of the dielectric properties at frequencies from 100 Hz to 1 MHz revealed a broad, highly frequency-dispersive, relaxor-like dielectric anomaly, which appeared in the temperature range of -80°-100°C. The permittivity, Q x f value, and temperature coefficient of the resonant frequency, measured at ∼5.5 GHz, were determined to be 37.4, 850 GHz, and -19 ppm/K, respectively.

An addition of just 0.4 wt% Li 2 O to (Ba 0.6 Sr 0.4 )TiO 3 powder was able to reduce the sintering temperature to ≤900°C and produce ceramics with a relative density of 97%. Small amounts of two secondary phases were formed during this process: Li 2 TiO 3 and Ba 2 TiO 4 . The addition of Li 2 O depresses the ferroelectric character of the (Ba 0.6 Sr 0.4 )TiO 3 and, as a result, reduces the permittivity, improves the temperature coefficient of permittivity, and reduces the dielectric losses. The tunability shows no significant variation with Li 2 O concentration and remains between 16.5% and 13.5%. A low-temperature sintering mechanism was proposed. The mechanism involves the intermediate formation of BaCO 3 , its melting and the incorporation of Li + into the BST. The sintering mechanism can be characterized as reactive liquid-phase sintering.

A series of Bi2O3-rich solid solutions with a fluorite-like crystal structure have been synthesized and analyzed with respect to their dielectric properties as functions of temperature and frequency. The existence of several common dielectric characteristics indicates a strong influence of the fluorite prototype cell on the frequency and temperature dependence of the dielectric properties. These characteristics, common to Bi2O3 solid solutions with the face-centered cubic δ structure, the tetragonal β structure, and the rhombohedral anti-α-AgI crystal structure, are (i) a frequency-dispersive increase in permittivity and dielectric losses because of a space-charge polarization, (ii) a frequency-independent permittivity and a linear temperature dependence of permittivity with a positive trend in the low-temperature range where the space-charge polarization is insignificant, and (iii) two frequency-dispersive relaxations above room temperature.

Solid-state synthesis of compositions from the Bi2O3–TeO2 system show that, under an oxygen atmosphere, Te4+ oxidizes to Te6+ and yields four room-temperature stable compounds: Bi2Te2O8, Bi2TeO6, Bi6Te2O15, and new a compound with the nominal composition 7Bi2O3·2TeO2. Dense ceramics can be prepared from all these compounds by sintering between 650° and 800°C under an oxygen atmosphere. The permittivity of these compounds varies from ∼30 to ∼54, the Q×f value from 1.100 to 41.000 GHz (∼5 GHz), and the temperature coefficient of resonant frequency from −43 to −144 ppm/K. Bi6Te2O15 and 7Bi2O3·2TeO2 do not react with silver, and, therefore, they have the potential to be used for applications in low-temperature cofired ceramic (LTCC) technology.

Our analysis of the microwave dielectric properties of the δ-Bi2O3–Nb2O5 solid solution (δ-BNss) showed a continuous increase in permittivity and dielectric losses with an increasing concentration of Nb2O5. The only discontinuity was found for the temperature coefficient of resonant frequency, which is negative throughout the entire homogeneity range but reaches a minimum value for the sample with 20 mol% Nb2O5. At the same composition there is a discontinuity in the grain size of the δ-BNss ceramics. For the sample containing 25 mol% Nb2O5 two structural modifications were observed. A single-phase tetragonal Bi3NbO7, in the literature referred to as a Type-III phase, is formed in a very narrow temperature range from 850° to 880°C. A synthesis performed below or above this temperature range resulted in the formation of the end member of the δ-BNss homogeneity range. Compared with the δ-BNss the Bi3NbO7 ceramics exhibit lower microwave dielectric losses, an increased conductivity, and a positive temperature coefficient of resonant frequency.

Extended thermal annealing treatments were used to modify the B-site cation order in the (1 −x)PMN–xPSN perovskite system (where PMN is lead magnesium niobate, Pb(Mg1/3Nb2/3)O3, and PSN is lead scandium niobate, Pb(Sc1/2Nb1/2)O3). Extensive 1:1 ordering could be induced in compositions with x≥ 0.1. The substitution of PSN into PMN produced a large increase in the thermal stability of the 1:1 ordered phase, with the maximum disordering temperature of ∼1360°C being observed for the x= 0.5 composition. By monitoring the change in the degree of order with temperature, the order–disorder boundary for the PMN–PSN system was established and the transition temperature for pure PMN was estimated to be ∼950°C. The changes in stability across the system were consistent with the “random-site” description of the cation order. The change in enthalpy associated with the ordering was affected by the size difference of the two ordered cation sites and by the size and charge mismatch of the metal cations that occupy the random- site position.

Six sillenite compounds Bi12MO20-δ (M = Si, Ge, Ti, Pb, Mn, B1/2P1/2) were synthesized, and the resulting single-phase powders were then sintered to obtain ∼97% dense ceramics. An analysis of their microwave dielectric properties, performed at ∼5.5 GHz, revealed a permittivity of ∼40 for all six compounds. The temperature coefficient of resonant frequency was the lowest for the Pb analogue (−84 ppm/K) and was found to increase with increasing ionic radius of the B-site ion to a value of −20 ppm/K for the Bi12SiO20 and Bi12(B1/2P1/2)O20 compounds. The Q×f value is a maximum for Bi12SiO20 and Bi12GeO20 with 8100 and 7800 GHz, respectively. The dielectric properties of the sillenites have been correlated with the structure of the oxygen network of the sillenite crystal lattice. As a result of its low sintering temperature (850°C), chemical compatibility with silver, low dielectric losses, and temperature-stable permittivity, the Bi12SiO20 compound is a suitable material for applications in low-temperature cofiring ceramic (LTCC) technology.

Compositions in the Zn2TiO4+xTiO2 system (x= 0–0.43) were synthesized via the solid-state reaction route, using high-purity (≥99.99%) metal-oxide powders. The incorporation of titanium, in the form of TiO2, in Zn2TiO4 spinel ceramics was investigated by analyzing the crystal structure and measuring the dielectric properties. The results of the crystal structure analyses suggested that TiO2 levels of x≤ 0.33 could be incorporated into the Zn2TiO4 spinel at temperatures of T > 945°C, whereas the solid solubility of titanium in Zn2TiO4 decreased for T < 945°C. When x≥ 0.28, the Zn2Ti3O8 phase formed in the Zn2TiO4 grain interior while cooling after heat treatment. Measurement of the microwave dielectric properties also supported the conclusion that the solubility limit of titanium in Zn2TiO4 was close to x= 0.33, as determined through analysis of the crystal structure.

The effect of the chemical substitution of Ti, Zr, and Gd cations on the structure and properties of bismuth-based pyrochlores was investigated for (Bi1.5Zn1.0Nb1.5)O7 and Bi2(Zn2/3Nb4/3)O7. Broad ranges of solid solutions based on cubic (Bi1.5Zn1.0Nb1.5)O7 were observed for Ti and Zr cations: (Bi1.5Zn1−(x/3)Nb1.5−(2 x/3)Zrx)O7 (0.0 ≤x≤ 1.5) and (Bi1.5Zn1−(x/3)Nb1.5−(2 x/3)Tix)O7 (0.0 ≤x≤ 2.25). For Gd cations, the range of solid solution was quite limited in (Bi1.5−xGdxZn1.0Nb1.5)O7 (0.0 ≤x≤ 0.2); however, the range of solid solution is more extensive in the system that contains Ti and Gd cations ((Bi1.5−xGdxZn0.5Ti1.5Nb0.5)O7 (0.0 ≤x≤ 0.6)). The stability fields of the solid solutions could be interpreted by using the ratio of the ionic sizes of the A- and B-site ions in the pyrochlore structure (RA/RB). The substituted phases exhibited permittivities (κ′) in the range of ∼100–200 and relatively low dielectric losses (tan δ < 7 × 10−4) at 1 MHz. The temperature coefficients of permittivity (τκ) varied in accordance with the RA/RB value and could be tuned from −88 ppm/K to −1300 ppm/K. For the orthorhombic Bi2(Zn2/3Nb4/3)O7 pyrochlore, the ranges of solid solution were very limited and the dielectric properties remained similar to those of the undoped phase (κ′= 90, τκ= 150 ppm/K).

The title compound is synthesized by solid state reaction of a stoichiometric mixture of Bi2O3, TeO2, and Fe2O3 (O2 atmosphere, 530 and 840 °C for 5 h each).

Six sillenite compounds Bi12MO20-δ (M = Si, Ge, Ti, Pb, Mn, B1/2P1/2) were synthesized, and the resulting single-phase powders were then sintered to obtain ∼97% dense ceramics. An analysis of their microwave dielectric properties, performed at ∼5.5 GHz, revealed a permittivity of ∼40 for all six compounds. The temperature coefficient of resonant frequency was the lowest for the Pb analogue (−84 ppm/K) and was found to increase with increasing ionic radius of the B-site ion to a value of −20 ppm/K for the Bi12SiO20 and Bi12(B1/2P1/2)O20 compounds. The Q×f value is a maximum for Bi12SiO20 and Bi12GeO20 with 8100 and 7800 GHz, respectively. The dielectric properties of the sillenites have been correlated with the structure of the oxygen network of the sillenite crystal lattice. As a result of its low sintering temperature (850°C), chemical compatibility with silver, low dielectric losses, and temperature-stable permittivity, the Bi12SiO20 compound is a suitable material for applications in low-temperature cofiring ceramic (LTCC) technology.

A new sillenite compound, Bi 12 (B 0.5 P 0.5 )O 20 , was synthesized using a solid-state reaction method. The stoichiometry was confirmed by XRD analyses, microstructural investigations, and quantitative elemental analysis. An investigation of the dielectric properties at frequencies from 100 Hz to 1 MHz revealed a broad, highly frequency-dispersive, relaxor-like dielectric anomaly, which appeared in the temperature range of -80°-100°C. The permittivity, Q x f value, and temperature coefficient of the resonant frequency, measured at ∼5.5 GHz, were determined to be 37.4, 850 GHz, and -19 ppm/K, respectively.

We fi rst summarize the main points of our disagreement with Tkach et al. [ 1 ] They claim that the magnetic peak observed at around 38 K in their essentially paramagnetic Sr 0.98 Mn 0.02 TiO 3 ceramic sample is caused by a spin glass ground state, which is an intrinsic property of the Sr 0.98 Mn 0.02 TiO 3 phase. They support their claim by providing ac magnetic data that shows weak frequency-dependence, and memory effects in the dc magnetic susceptibility, which are indeed consistent with the spin-glass model. In addition, they provide zero-fi eld cooled magnetic data for the second sample in the Sr 1-x Mn x TiO 3 series, namely, Sr 0.975 Mn 0.025 TiO 3 that shows a magnetic anomaly peak at around 43 K. In this way, they demonstrate that the temperature of the magnetic anomaly is dependent on the Mn concentration which serves as evidence that the magnetic anomaly is an intrinsic characteristic of the Sr 1-x Mn x TiO 3 . In contrast, we have demonstrated in our previous studies [ 2 , 3 ]

Dr. F. Le Goupil, Dr. A.-K. Axelsson, Prof. L. J. Dunne, Dr. A. Berenov, Prof. N. McN. Alford Department of Materials Imperial College London London SW7 2AZ , UK E-mail: f.le-goupil09@imperial.ac.uk Dr. A.-K. Axelsson, Prof. L. J. Dunne Department of Engineering Systems London South Bank University London SE1 0AA , UK Prof. M. Valant Materials Research Laboratory University of Nova Gorica Nova Gorica 5000 , Slovenia Dr. G. Manos Department of Chemical Engineering University College London London WC1E 7JE , UK Prof. T. Lukasiewicz Institute of Electronic Materials Technology 133 Wolczynska Str., 01–919 Warsaw , Poland Prof. J. Dec Institute of Materials Science University of Silesia 12 Bankowa Str., PL 40–007 Katowice , Poland