Phosphate Tungsten Bronzes Research Articles

Synthesis, characterization and electrochemical properties of iron doped phosphate tungsten heteropoly acid (Fe-PWA) and it’s bronze (Fe-PWB): Comparative study

In this work, synthesized 12-tungstenphosphoric acid (H3PW12O40 ? nH2O; PWA) was further ionically exchanged with Fe3+ ions, which led to the formation of the 12- tungstophosporic acid iron salt, (FePW12O40 ? nH2O; Fe-PWA). Fe-PWA was then subjected to thermal analysis (TGA/DTA), determining the phase transition temperature of 576?C from Fe-PWA to its corresponding phosphate tungsten bronze doped with iron, Fe-PWB. Using the X-ray powder diffraction (XRPD), Fourier-transform infrared spectroscopy (FTIR), and Scanning electron microscopy with an energy dispersive X-ray spectroscopy (SEM-EDS) method, the obtained Fe-PWA and Fe-PWB were additionally characterized, and compared. Due to channels and cavities in their structures, Fe-PWA and Fe-PWB were next examined as electrode materials for aqueous rechargeable batteries. Electrochemical measurements were done in aqueous solutions of 6 M LiNO3 by cyclic voltammetry. Fe-PWA and Fe-PWB exhibit different redox processes, which are discussed thoroughly in this work. Electrochemical results are showing that within the Fe-PWA structure, more Li+ ions can be intercalated in the first discharge cycle, but consecutive cycling leads to a fast capacity fade. While the Fe- PWB redox process was stable during cycling, its specific capacity is limited by the material's poor electrical conductivity. Improvements in Fe-PWB conductivity must be addressed in future studies in order to boost material?s electrochemical performance.

Novel cerium and praseodymium doped phosphate tungsten bronzes: Synthesis, characterization, the behavior in the Briggs-Rauscher reaction and photoluminescence properties

Due to the interesting and potentially useful properties, phosphate tungsten bronzes are constantly being studied and attract a lot of attention. In the present work, two different metallic elements, belonging to the group of rare-earth metals, cerium and praseodymium, were used as a dopants for phosphate tungsten bronzes. Novel cerium and praseodymium doped phosphate tungsten bronzes were successfully synthesized and further characterized by thermal analyses, Fourier-transform infrared spectroscopy, X-ray powder diffraction, scanning electron microscopy with energy-dispersive X-ray spectrometer and photoluminescence spectroscopy. The behavior of cerium and praseodymium doped phosphate tungsten bronzes were examined in the Briggs-Rauscher oscillatory reaction. The changes in the oscillatory dynamics, caused by the addition of cerium doped and praseodymium doped phosphate tungsten bronzes, are used for distinction of these bronzes and also as a method for testing the catalytic features of the bronzes. Beside the nonlinear behavior, the bronzes have shown photoluminescence in the visible region (blue region).

Layered Monophosphate Tungsten Bronzes [Ba(PO4 )2 ]Wm O3m-3 : 2D Metals with Locked Charge-Density-Wave Instabilities.

Phosphate tungsten and molybenum bronzes represent an outstanding class of materials displaying textbook examples of charge-density-wave (CDW) physics among other fundamental properties. Here we report on the existence of a novel structural branch with the general formula [Ba(PO4 )2 ][Wm O3m-3 ] (m=3, 4 and 5) denominated 'layered monophosphate tungsten bronzes' (L-MPTB). It results from thick [Ba(PO4 )2 ]4- spacer layers disrupting the cationic metal-oxide 2D units and enforcing an overall trigonal structure. Their symmetries are preserved down to 1.8 K and the compounds show metallic behaviour with no clear anomaly as a function of temperature. However, their electronic structure displays the characteristic Fermi surface of previous bronzes derived from 5d W states with hidden nesting properties. By analogy with previous bronzes, such a Fermi surface should result into CDW order. Evidence of CDW order was only indirectly observed in the low-temperature specific heat, giving an exotic context at the crossover between stable 2D metals and CDW order.

The Influence of Electrode Constituents on Hydrogen Evolution Reaction on Phosphate W- and Mo-Bronze-Based Electrodes

Phosphate tungsten bronze (WPB) and phosphate molybdenum bronze (MoPB) were synthesized and modified with rhenium. The existing phases were established by X-ray powder diffraction (XRPD), electron paramagnetic spectroscopy (EPR) and Field emission scanning electron microscopy (FESEM). The electroactivity of bronze samples, with and without rhenium for hydrogen evolution reaction (HER) was tested. The influence of carbon black presence in the catalytic ink on the electrochemical activity was investigated. Collected results provide insight into the effects of the constituents of an electrode on its electrochemical activity.

A possible connection between phosphate tungsten bronzes properties and Briggs-Rauscher oscillatory reaction response

The calcium phosphate tungsten bronze (Ca-PWB) has been synthesized and characterized (TGA, DSC, XRPD, FTIR, SEM). The influence of solid insoluble materials Ca- PWB, as well as lithium doped (Li-PWB) and cation free phosphate tungsten (PWB) bronzes on the oscillatory Briggs-Rauscher (BR) reaction dynamics, is compared. The results show that doping with Li and Ca reduces sensitivity of the BR reaction towards bronzes addition. These findings suggest the usage of the BR reaction as an innovative method for testing of different properties of bronze material. The behavior of PWB in the BR reaction is significantly changed with divalent cation (Ca2+) doping. The reasons for the different bronzes behavior were found in their calculated unit cell volumes. Namely, the compressed Ca-PWB unit cell volume indicates the difficult availability of the active site for heterogeneous catalysis. Hence, the linear correlation (slope) of the BR oscillogram?s length (?osc) vs. mass of bronze in BR reaction might be considered as a new parameter for the evaluation of the bronzes catalytic activity.

Briggs-Rauscher reaction as a novel electrochemical detector for phosphate tungsten and phosphate molybdenum bronzes

Briggs-Rauscher (BR) oscillatory reaction is a chemical system very sensitive to different analyte addition. Changes in oscillatory dynamics, by analyte addition, could be successfully used for the determination of analyte concentration, as well as for its antioxidative/antiradical activity assessment. In this work, the BR reaction is used as a system detector for distinguishing solid unsolvable materials, such as phosphate tungsten bronze (PWB) and phosphate molybdenum bronze (PMoB), obtained by thermal treatment. The addition of different masses of PWB in BR reaction decreases its oscillation time, while the addition of different masses of PMoB had no effects on the BR oscillation time. Additionally, the BR oscillation time is the linear function of added PWB mass. The mechanism of bronzes action is investigated by using pH and electric conductivity measurements, as well as inductively coupled plasma and the cyclic voltammetry measurements, and it can be ascribed to PWB better catalytic activity then PMoB, under BR reaction condition. This work extends the practical aspect of the BR reaction for testing solid (insoluble) materials, and opening the possibility of using oscillatory reactions in material science and catalysis, in general.

THE MINOR INFLUENCE OF CALCIUM DOPED PHOSPHATE TUNGSTEN BRONZE ON THE BRIGGS-RAUSCHER REACTION DYNAMICS

The Briggs-Rauscher (BR) oscillatory reaction is the oxidation of malonic acid in the presence of hydrogen peroxide and iodate in the acidic environment, which is catalyzed by ions of manganese. This reaction is very sensitive to the presence of additives. In this paper, the BR reaction has been used to investigate the phosphate tungsten bronze as well as calcium doped tungsten bronze, obtained by thermal treatment. The addition (0.01-0.08 g) of phosphate tungsten bronze and calcium doped phosphate tungsten bronze has a different effect on the dynamics of the Briggs-Rauscher reaction. In the case of the addition of phosphate tungsten bronze in the Briggs-Rauscher reaction, the linear dependence of the length of the oscillatory period on the mass of the added bronze was obtained, while in the case of addition of calcium doped phosphate tungsten bronze, the oscillatory period does not significantly change with an increase of added mass. The mechanism of calcium doped and undoped phosphate tungsten bronze action in BR reaction is probably adsorptive, and it will be the subject of future work. Keywords: oscillatory reactions, Briggs-Rauscher reaction, phosphate tungsten bronze, calcium doped phosphate tungsten bronze, thermal treatment.

Oscillatory reaction as a system detector for doped and undoped phosphate tungsten bronzes

Phosphate tungsten bronzes, obtained by thermal treatment, are insufficiently investigated bronzes and there is scarce literature data on their chemical behavior and structure. Due to high-sensitivity of the Briggs-Rauscher (BR) reaction to addition of different analytes, this oscillatory reaction presents a potentially important chemical system for investigation and characterization of phosphate-tungsten bronzes, doped and undoped. The reaction mixture for the oscillatory BR reaction typically consists of H2O2, HClO4, KIO3, Mn(II) (catalyst), and CH2(COOH)2 (malonic acid, as an organic substrate). This paper deals with phosphate tungsten bronzes (PWB) and lithium doped phosphate tungsten bronzes (LiPWB) and their effects on the Briggs-Rauscher reaction dynamics. It is shown that the addition of phosphate tungsten bronzes decreases the oscillatory period length in this reaction. Furthermore, the obtained results show that PWB has a stronger influence on the BR reaction dynamics then LiPWB. In both cases, the oscillatory period is a linear function of the added bronze mass. The obtained linear functions can be successfully used for determination of the unknown bronze mass. Furthermore, due to different slopes of these functions, the Briggs-Rauscher reaction can be used as a system-detector for lithium doped and undoped phosphate tungsten bronzes. In order to elucidate the mechanism of bronze action, the inductively coupled plasma optical emission spectrometry (ICP-OES) was used to measure the total contents of K, Mn, W, Li. The aliquots of the above solution (i.e. CH2(COOH)2, MnSO4, HClO4, KIO3 but without H2O2) with the identical masses of PWB and LiPWB were examined. For the sake of comparison, contents of the metals in the solution without the bronze addition were measured, as well. Results obtained by the ICP-OES analysis show that the bronze structure is disturbed in the strong oxidizing environment (iodate in acidic solution) so that both, tungsten and lithium, leach into the BR solution. Accordingly, the proposed mechanism of the bronze action is probably by the reaction of tungsten ion with hydrogen-peroxide resulting in formation of a tungsten-peroxo complex. This complex is a stronger oxidizing agent then hydrogen peroxide itself. Thus, formation of the tungsten-peroxo complex potentially affects the kinetics of the Briggs-Rauscher reaction.

Electrochemical lithiation/delithiation kinetics and capacity of phosphate tungsten bronze and its chemically pre-lithiated derivatives in aqueous solutions

Monophosphate bronze (PW8O26, WPB) was synthesized by low-temperature (650 °C) heating of 12-tungstophosphoric acid (WPA). Its lithiated derivative with a low lithium content (Li3-WPB) was synthesized by heating lithium salt of WPA at 650 °C. Its lithiated derivative with a high lithium content was synthesized by mechanochemical lithiation of Li3WPB (Lin-WPB) (n > 3), followed by heat treatment at 650 °C. X-ray powder diffraction analysis confirmed that the bronze structure changed with the increase of the incorporated lithium content. These bronzes were investigated from the aspect of lithiation/delithiation kinetics in an aqueous saturated LiNO3 electrolyte solution. By cyclic voltammetry, both the kinetics of intercalation/deintercalation and the coulombic capacity were found to decrease with the increase of initial lithium content, in agreement with literature data for electrochemically lithiated tungstophosphate bronzes. Monoclinic lithium-free bronze PW8O26, as the most promising material, was subjected to a further detailed galvanostatic investigation coupled with the LiFe0.95V0.05PO4/C composite as a cathode material in an aqueous battery. In comparison to its behavior in an organic electrolyte, a considerably lower initial capacity of the bronze electrode was measured. However, its cyclic stability was much better in an aqueous than in an organic electrolyte.

High-resolution angle-resolved photoemission investigation of potassium and phosphate tungsten bronzes

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) and density functional ab initio theoretical calculation to study the electronic structure of potassium (K0.25WO3) and phosphate (P4W12O44) tungsten bronzes. We have experimentally determined the band dispersions and Fermi surface topology of these bronzes and compared with our theoretical calculations and a fair agreement has been seen between them. Our experimental as well as theoretical investigation elucidates the origin of transport anomalies in these bronzes. The Fermi surfaces of these bronzes consist of flat patches, which can be connected with each other by a constant nesting wave vector, q. The scattering wave vectors found from diffraction measurements match with these nesting vectors and the anomalies in the transport properties of these bronzes can be well explained by the evolution of charge-density wave with a partial nesting between the flat segments of the Fermi surfaces.

Phase transitions toward charge density wave in the m=11.5 Mono phosphate bronze

Mono Phosphate Tungsten Bronzes (MPTB), (PO2)4(WO3)2m (m ranging from 4 to 14), are a large family of conductors of low dimensionality[1]. Their structure may be described as a regular stacking of WO3-type slabs with a thickness function of m, joined by slices of tetrahedral PO4 phosphate. Successive Peierls transitions towards charge-density wave (CDW) or spin-density wave (SDW) states are observed below different critical temperatures (TC1, TC2 ...). Structural transitions toward incommensurate modulated phases are associated with these complex electronic states. Recently an original member of MPTB, the member m=11.5 with formula (PO2)4(WO3)11(WO3)12, resulting in a regular intergrowth between m=11 and m=12 member, has been synthetized. An accurate investigation of the reciprocal space versus the temperature using single crystal X-ray diffraction shows the existence of two phase transitions at Tc1=4300C and Tc2=2800C. The ground state structure may be described with the following cell parameters a=5.3431(7) b=6.5901(9) c=40.884(6) α=93.096(2) β=93.734(2) γ=90.000(2) and the SG P-1. Both transitions associated with the occurrence of CDW are characterized by the appearance of modulation vectors (q1 and q2 associated to Tc1 and Tc2 respectively) and an increasing of the dimension of the superspace group (see figure 1). A structural study of the m=11.5 member is performed above Tc1 and below Tc1 and Tc2; an interpretation of the CDW state is then proposed. The evolution of the intensity of the satellite and main reflections around Tc1 and Tc2 is analyzed for characterizing the order parameter of the transitions. Finally, resistivity and magnetoresitance measurements of a large single crystal sample of m=11.5 MPTB are performed in a temperature interval from 2K to 300K. These measures are revealing at ~50K a possible SDW transition due to electron-electron interactions playing significant role in that material[2]. Further neutron diffraction experiment at very low temperature shall be performed to clarify this point.

Electronic band structure and Fermi surfaces of the quasi–two-dimensional monophosphate tungsten bronze, P4W12O44

The electronic structure of quasi-two-dimensional monophosphate tungsten bronze, P4W12O44, has been investigated by high-resolution angle-resolved photoemission spectroscopy and density functional theoretical calculations. Experimental electron-like bands around point and Fermi surfaces have similar shapes as predicted by calculations. Fermi surface mapping at different temperatures shows a depletion of density of states at low temperature in certain flat portions of the Fermi surfaces. These flat portions of the Fermi surfaces satisfy the partial nesting condition with incommensurate nesting vectors and , which leads to the formation of charge density waves in this phosphate tungsten bronzes. The setting up of charge density wave in these bronzes can well explain the anomaly observed in its transport properties.

Nano-designing of Mg doped phosphate tungsten bronzes and SiO 2 composite obtained by ultrasonic spray pyrolysis method

In this study, the structure and substructure of SiO 2–Mg phosphate tungsten bronzes, MgPTB, (MgHPW 12O 40 · 29H 2O) obtained by ultrasonic spray pyrolysis method from a silica sol, and a MgPTB solution, obtained by the ion exchange method, as precursors were investigated. The mechanism of the formation of aerosol droplets is discussed. Phase composition, structure and substructure of SiO 2–MgPTB particles were investigated by X-ray powder diffraction (XRPD) analysis, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Good agreement between the theoretically predicted values for the mean diameters of particles and subparticles (1.27 μm and 75.4 nm, respectively) and the experimentally obtained ones (1.17 μm and 65–90 nm) was found. This agreement confirms the applicability of the model to get a satisfactory prediction of the most important data related to the nano-structural design of SiO 2–MgPTB powders.

Phosphate tungsten bronze series: crystallographic and structural properties of low-dimensional conductors.

Phosphate tungsten bronzes have been shown to be conductors of low dimensionality. A review of the crystallographic and structural properties of this huge series of compounds is given here, corresponding to the present knowledge of the different X-ray studies and electron microscopy investigations. Three main families are described, monophosphate tungsten bronzes, Ax(PO2)4(WO3)2m, either with pentagonal tunnels (MPTBp) or with hexagonal tunnels (MPTBh), and diphosphate tungsten bronzes, Ax(P2O4)2(WO3)2m, mainly with hexagonal tunnels (DPTBh). The general aspect of these crystal structures may be described as a building of polyhedra sharing oxygen corners made of regular stacking of WO3-type slabs with a thickness function of m, joined by slices of tetrahedral PO4 phosphate or P2O7 diphosphate groups. The relations of the different slabs with respect to the basic perovskite structure are mentioned. The structural description is focused on the tilt phenomenon of the WO6 octahedra inside a slab of WO3-type. In this respect, a comparison with the different phases of the WO3 crystal structures is established. The various modes of tilting and the different possible connections between two adjacent WO3-type slabs involve a great variety of structures with different symmetries, as well as the existence of numerous twins in MPTBp's. Several phase transitions, with the appearance of diffuse scattering and modulation phenomena, were analysed by X-ray scattering measurements and through the temperature dependence of various physical properties for the MPTBp's. The role of the W displacements within the WO3-type slabs, in two modulated structures (m = 4 and m = 10), already solved, is discussed. Finally, the complexity of the structural aspects of DPTBh's is explained on the basis of the average structures which are the only ones solved.

A Monophosphate Molybdenum Bronze Built up from ReO3-Type Slabs: Ag0.7Mo3O7(PO4)

A monophosphate molybdenum bronze Ag2.8(PO2)4(MoO3)12has been synthesized for the first time. This monoclinic bronze (a=23.857(3) Å,b=5.2990(6) Å,c=6.5659(4) Å,β=93.950(6)°), represents them=6 member of the Ax(PO2)4(MO3)2mfamily (M=W, Mo). Its structure, which consists of ReO3-type slabs interconnected with monophosphate groups, is very similar to those of them=6 members Kx(PO2)4(WO3)12and Mo4O11, forming hexagonal tunnels where the Ag+cations are located. The distribtion of Ag+in the tunnels is however different from that of K+in the phosphate tungsten bronze: two sites are partially occupied close to the walls of the tunnels so that Ag+exhibits a ninefold and a tetrahedral coordination. The resistivity measurements, although performed on polyphasic samples, clearly show a semiconducting behavior. The possibility of charge density wave properties is considered.

The crystal chemistry of the higher tungsten oxides

The crystal chemistry of binary and ternary tungsten oxides with overall compositions between MO 3 and approximately MO 2 is reviewed. The chapter focuses primarily upon the reduced oxides and their fully oxidised analogues and descriptions of the structures have been employed which stress the linkage of component metal oxygen polyhedra. The stable form of the parent oxide WO 3 consists of corner-linked WO 6 octahedra. Reduction by way of the addition of metal atoms which can occupy the cage sites between these octahedra leads to the formation of the perovskite structure tungsten bronzes. Slight reduction of the binary oxide with medium sized metal atoms produces compounds containing units of edge-shared octahedra. These materials form the crystallographic shear (CS) phases. At greater degrees of reduction with similar metals pentagonal bipyramidal WO 7 polyhedra are utilised to form a variety of pentagonal column (PC) phases. This group includes the tetragonal tungsten bronze (TTB) structure and derivatives thereof. Low temperature preparations have lead to the formation of two other WO 3 structure types based upon rings of corner-sharing octahedra, one of which is the hexagonal tungsten bronze framework and the other the pyrochlore framework. These can be considered as parent structures for the high temperature hexagonal tungsten bronze (HTB) phases, the intergrowth tungsten bronze (ITB) phases and a number of ternary phases with structures related to that of pyrochlore. An extensive group of phases, the so-called ‘phosphate tungsten bronzes’, are ternary oxides containing phosphate groups, rather than bronzes. They are described in terms of the octahedral and tetrahedral polyhedra from which they are built up and their relationship to Mo 4O 11 structure indicated. A concluding section considers some aspects of the stability and reactivity of these phases.

PHOSPHATE TUNGSTEN BRONZES — A NEW FAMILY OF QUASI-LOW-DIMENSIONAL METALLIC OXIDES

A brief introduction is presented on transition metal oxide bronzes and their relationship to the phosphate tungsten bronzes; the latter compounds are the major focus of this review. The phosphate tungsten bronzes (PTB) are a new class of quasi-low-dimensional materials which exhibit charge density wave (CDW) instabilities. The growth of single crystals and investigation of the physical properties including the temperature dependence of the electrical resistivity and magnetic susceptibility on oriented single crystals are discussed for selected members of the major families in the PTB’s. Correlation of the physical measurement data with structural properties, X-ray diffraction data and results of the theoretical band structure calculations are also presented.

Specific heat at CDW transitions in bronzes

We have measured the specific heat of single crystals of P 4 W 8 O 32 , P 4 W 12 O 44 , and K 0.3 MoO 3 at their CDW transitions. For K 0.3 MoO 3 , the anomaly is fit to a 3d-XY crossover mill, and compared to the derivative of the susceptibility. For the phosphate tungsten bronzes, only the anomaly at the upper CDW transition in P 4 W 12 O 44 is resolved

Magnetotransport and charge density wave instabilities in the quasi-two-dimensional phosphate tungsten bronzes (PO 2) 4(WO 3) 2m (m = 4,6,7)

Electrical resistivity has been measured between 4.2 K and 300 K in fields up to 6 Tesla on the monophosphate tungsten bronzes P 4W 8O 32, P 4W 12O 44 and P 4W 14O 50. These compounds are anisotropic quasi-two-dimensional metals showing several charge density wave instabilities. For m = 4 and m = 6, a large anisotropic magnetoresistance is found in the lowest temperature charge density wave state, as expected, with small carrier pockets left on the Fermi surface by gap openings. For m = 7, several instabilities are found by the transport properties together with a small magnetoresistance. These results are partly consistent with X-ray diffuse scattering studies.

KMoWP3O12, a tunnel structure of the KMo2P3O12-type

Potassium molybdotungstotriphos- phate(V), KMoWP3OI2, Mr = 603-80, orthorhombic, Pbcm, a = 88180 (6), b = 9 1574 (8), c = 12.3836 (8) A, V= 1000.0 (2) A 3, Z = 4, Dx = 4.01 Mg m -3, ,t(Mo Ka) = 0.71073 A, /z = 13.9 mm- 1, T = 294 K, F(000) = 276, R = 0.035 and wR=0-042 for 2291 observed reflections. The framework is built up from MoO6 octahedra and PO4 tetrahedra which delimit tunnels running along b, where the K ions are located. The structure leads to the formula KMoWO(PO4)(P207). Introduction. Molybdenum and tungsten exhibit very different behavior in spite of their similar electronic structure. Recent studies of the phosphates of these elements support this point of view, as shown for instance by the existence of phosphate tungsten bronzes (see Raveau, 1986, for review), and the synthesis of original frameworks of phosphates of