Monophosphate Tungsten Bronzes Research Articles

Layered Monophosphate Tungsten Bronzes [Ba(PO4)2]WmO3m−3: 2D Metals with Locked Charge‐Density‐Wave Instabilities

AbstractPhosphate 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][WmO3m−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.

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.

On the stability of MOPO4 structure types with M: V, Mo, Nb, W, Ta, Sb

Transition metal oxide phosphates MOPO4 have recently attracted interest due to their catalytic and electronic properties, and as electrode material. In order to predict possible polymorphs that have hitherto not been prepared, we present a systematic theoretical study on the stability of four known MOPO4 structure types, β-VOSO4, α-MoOPO4, SbOPO4 and monophosphate tungsten bronzes (MPTB) with six transition and main group metals that are interesting due to their catalytic activity (M: V, Mo, Nb, W, Ta, Sb). Harmonic frequencies and Crystal Orbital Hamilton Populations (COHP) were computed to verify and predict the stability and interpret the bond strengths, respectively, of the optimized structure models. The M-O bond strength has only a minor influence of the polymorph formation, but its vibrational contribution to the thermodynamic functions has a substantial effect. Based on the electronic energy calculated with the hybrid density functional PW1PW, every thermodynamical stable polymorph could be identified. The predictions of polymorph formation are in agreement with the experimental observations of VOPO4, NbOPO4 and TaOPO4, while the theoretical results for MoOPO4 and SbOPO4 show some deviations from the experimentally observed stability order.

Polymorphism and sodium-ion conductivity of NaTa2PO8 synthesized via the Li+/Na+ ion-exchange reaction of LiTa2PO8

Ion exchange reaction is a promising method to explore metastable compounds that could not be synthesized by conventional high-temperature solid-phase reactions. Herein, a new sodium tantalum phosphate, NaTa2PO8 was synthesized via the Li+/Na+ ion-exchange reaction of the parent compound, LiTa2PO8 in molten NaNO3 medium. NaTa2PO8 underwent an irreversible phase transition from the low- (LT-NaTa2PO8) to the high-temperature polymorph (HT-NaTa2PO8) at approximately 1000 °C. The crystal structures were solved using an ab initio structural determination method based on synchrotron X-ray powder diffraction data. The LT-NaTa2PO8 presented an orthorhombic structure, closely related to that of the parent LiTa2PO8 structure. In contrast, the HT-NaTa2PO8 was found to adopt a monoclinic structure, belonging to a family of monophosphate tungsten bronzes with pentagonal tunnels. The ionic conductivities of LT-NaTa2PO8 (σ = 5 × 10−5 S/cm at 309 °C) and HT-NaTa2PO8 (σ = 2 × 10−7 S/cm at 300 °C) exhibited Arrhenius behavior with activation energies of 0.49 and 0.79 eV, respectively. Bond valence energy landscape (BVEL) calculations indicated that a three-dimensional (3D) conduction pathway is formed in LT-NaTa2PO8 structure, while the conduction pathway in HT-NaTa2PO8 shows a two-dimensional (2D) character.

Tailoring pulsed laser deposition of phosphorus doped WOx films from (PO2)4(WO3)4 target by space-resolved optical emission spectroscopy.

Phosphorus doped WOx films were deposited on fused silica substrates from a target of monophosphate tungsten bronze (PO2)4(WO3)2 m with m = 2. Different conditions of O2 pressures and substrate temperature depositions were performed in parallel to plasma investigations during the depositions step. Changes were observed in the film stoichiometries affecting the film properties while the increase of the O2 pressure. Optical bandgap was shifted from 3.39 eV to 3.86 eV as a function of the deposition pressure from 7.5 Pa to 20 Pa. In situ optical emission spectroscopy was carried out in all the O2 atmosphere conditions. With an increase in the oxygen pressure to 5 Pa, a change in the spectral composition of the plasma with a deficit of the phosphorus lines was observed. A complex plasma containing atoms and multiple ionized ions attributed to each composing species was observed at high O2 pressures of deposition. The increase in O2 pressure led to strong thermalization of the plasma and strong scattering of the plasma particle which created a favourable environment for the WO3 formation.

The interplay of framework instability and electron–phonon coupling in a CDW system, the monophosphate tungsten bronze family

The interplay of framework instability and electron–phonon coupling in a CDW system, the monophosphate tungsten bronze family

Highly robust, novel aluminum counter cation-based monophosphate tungsten bronze electro-catalysts for oxygen evolution in acidic solution.

This study describes the successful synthesis of novel bronze with a low tungsten oxidation state for the efficient electro-catalytic oxidation of water. An extraordinarily robust monophosphate tungsten bronze (MPTB)-modified graphite anode was successfully fabricated for the oxygen evolution reaction (OER) at a thermodynamic potential of 1.23 V in H2SO4 acidic solution. Several Al, Cr and Fe counter-cation-based MPTBs were synthesized by the solution combustion method. Novel Al-based MPTBs calcined at 700 °C in O2 (AlO7) showed almost zero onset overpotential, high current density, high turnover frequency for OER and steady catalysis in repeated use even after 30 weeks. The orthorhombic AlO7 comprising crystallites of 9.89 nm and an indirect band gap (1.89 eV), is an unusually stable MPTB that contains 98% W5+ state stabilized with the Al3+ counter cation. The catalysis decreases as the ratio of W5+ : W6+ in MPTBs decreases and [410] and [601] facets play main roles in the first H2O association and nucleophilic attack of the second H2O molecule on the catalyst surface. Thus, MPTBs can be non-noble metal anode materials for robust acidic H2O electrolyzers.

Cascading transitions toward unconventional charge density wave states in the quasi-two-dimensional monophosphate tungsten bronze P4W16O56.

Single crystals of the m = 8 member of the low-dimensional monophosphate tungsten bronzes (PO2)4(WO3)2m family were grown by chemical vapour transport technique and the high crystalline quality obtained allowed a reinvestigation of the physical and structural properties. Resistivity measurements revealed three anomalies at T C1 = 258 K, T C2 = 245 K and T C3 = 140 K, never observed until now. Parallel X-ray diffraction investigations showed a specific signature associated with three structural transitions, i.e. the appearance of different sets of satellite reflections below T C1, T C2 and T C3. Several harmonics of intense satellite reflections were observed, reflecting the non-sinusoidal nature of the structural modulations and a strong electron-phonon coupling in the material. These transitions could be associated with the formation of three successive unconventional charge density wave states.

Evolution of coherent collective modes through consecutive charge-density-wave transitions in the(PO2)4(WO3)12monophosphate tungsten bronze

All optical femtosecond relaxation dynamics in a single crystal of mono-phosphate tungsten bronze (PO$_{2}$)$_{4}$(WO$_{3}$)$_{2m}$ with alternate stacking m=6 of WO$_{3}$ layers was studied through the three consequent charge density wave (CDW) transitions. Several transient coherent collective modes associated to the different CDW transitions were observed and analyzed in the framework of the time dependent Ginzburg-Landau theory. Remarkably, the interference of the modes leads to an apparent rectification effect in the transient reflectivity response. A saturation of the coherent-mode amplitudes with increasing pump fluence well below the CDWs destruction threshold fluence indicates a decoupling of the electronic and lattice parts of the order parameter under strong optical drive.

Detailed investigation of the phase transition inKxP4W8O32and experimental arguments for a charge density wave due to hidden nesting

Detailed structural and magnetotransport properties of the monophosphate tungsten bronze K$_{x}$P$_4$W$_{8}$O$_{32}$ single crystals are reported. Both galvanomagnetic and thermal properties are shown to be consistent with a charge density wave electronic transition due to hidden nesting of quasi - 1D portion of the Fermi surface. We also observe the enhancement of electronic anisotropy due to reconstruction of the Fermi surface at the Peierls transition. The resistivity presents a thermal hysteresis suggesting a first order nature characteristics of a strong coupling scenario. However, other measurements such as the change of carriers density demonstrate a second order Peierls scenario with weak coupling features. We suggest that the structural transition driven by the residual strain in the K - P - O environment is responsible for the resistivity hysteresis and modifes the Fermi surface which then helps the rise to the second order Peierls instability.

Substitution of W5+in monophosphate tungsten bronzes by combinationsMn+/W6+

AbstractA series of hitherto unknown mixed-metal phosphates of the monophosphate tungsten bronze structure family [MPTB; (WO3)2m(PO2)4] have been obtained by solution combustion synthesis followed by annealing (ϑ= 850°C) at appropriate oxygen pressures. These new phosphates show substitution of W5+by eitherM3+1/3W6+2/3(M: V, Cr, Fe, Mo) or Ti4+1/2W6+1/2. Members of the MPTB structural series withm= 2 [e.g. CrIII4/3WVI8/3O12(PO2)4; TiIV6/3WVI6/3O12(PO2)4] andm= 4 [e.g. Cr4/3W20/3O24(PO2)4] have been obtained. In the course of our investigation the crystal structure of WOPO4(MPTB withm= 2: W4O12(PO2)4) has been re-determined from X-ray single-crystal data, showing monoclinic instead of the orthorhombic symmetry reported in literature (P21/m,Z= 1, 80 parameters, 1832 independent reflectionsR1= 0.027,wR2= 0.063). The crystal structures of MoIII4/3WVI8/3O12(PO2)4and CrIII4/3WVI8/3O12(PO2)4(MPTBs withm= 2) were also refined from single-crystal data {(Mo/W (Cr/W):P21/m,Z= 1, 80(86) parameters, 1782(1769) independent reflections,R1= 0.035(0.059),wR2= 0.081(0.146)}. These refinements indicate statistical distribution ofMIIIand WVIover the metal sites. By selected area electron diffraction the unit cell dimensions of CrIII4/3WVI8/3O12(PO2)4and CrIII4/3WVI20/3O24(PO2)4derived from XRPD and SXRD are confirmed. HRTEM images of Cr4/3W20/3O24(PO2)4are in agreement with its assumed close structural relation to W8O24(PO2)4and show an highly ordered atomic arrangement.

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.

Monophosphate tungsten bronzes with pentagonal tunnels: reinvestigation through the peephole of the superspace

The large family of monophosphate tungsten bronzes with pentagonal tunnels (MPTBp), (PO2)4(WO3)2m with m ranging from 4 to 14, can be considered as modular structures via a description with (PO2)2(WO3)m modules related together by a symmetry operation and alternating along the z axis. Following the success of the application of the superspace for the description of the lillianites homologous series, a (3+1)-dimensional superspace model is efficiently defined to unify the structural analysis of the MPTBp. The (3+1)-dimensional model reveals hidden common characteristics such as the symmetry. An evaluation of the model for six well known members of the series was carried out from experimental data collected to this aim.

Monophosphate tungsten bronzes with pentagonal tunnels: reinvestigation through the peephole of the superspace

The large family of monophosphate tungsten bronzes with pentagonal tunnels (MPTBp), (PO2)4(WO3)2m with m ranging from 4 to 14, can be considered as modular structures via a description with (PO2)2(WO3)m modules related together by a symmetry operation and alternating along the z axis. Following the success of the application of the superspace for the description of the lillianites homologous series, a (3 + 1)-dimensional superspace model is efficiently defined to unify the structural analysis of the MPTBp. The (3 + 1)-dimensional model reveals hidden common characteristics such as the symmetry. An evaluation of the model for six well known members of the series was carried out from experimental data collected to this aim.

A density-functional theory investigation of the structural and spin properties of (PO2)4(WO3)8 model bronzes

Plane-wave density-functional theory calculations of P4W8O32 mono-phosphate tungsten bronzes with pentagonal tunnels demonstrate that (1) these materials possess a ground (singlet) state structure of orthorombic symmetry, as observed by X-ray diffraction crystallography at room temperature; (2) open-shell states lie very close in energy to the ground state and are significantly populated at room temperature; and (3) these low-lying states are likely to be implicated in the formation of a superstructure due to the tilting of WO6 octahedra (Peierls-like structural transformations) upon temperature decrease. The latter finding is consistent with the existence of a low-temperature incommensurate structure, as evidenced by the satellite peaks observed in X-ray diffraction patterns.

Electrochemical lithium insertion in (PO 2) 4(WO 3) 2 m (2 ≤ m ≤ 10): Relation among the electrochemical insertion process and structural features

In this work it is presented a review of the main results obtained during the electrochemical lithium insertion in the family of monophosphate tungsten bronzes (PO 2) 4(WO 3) 2 m (2 ≤ m ≤ 10). This family of oxides is a good system in order to study the relation among the electrochemical processes observed in the course of lithium insertion and the changes of bronzes structures. By means of X-ray diffraction experiments, the nature of Li x (PO 2) 4(WO 3) 2 m phases has been elucidated and a correlation with the reversible/irreversible processes observed during the electrochemical insertion has been established. The electrical properties of the inserted Li x (PO 2) 4(WO 3) 2 m phases were measured and a relation with the amount of lithium inserted and m was also found.

Features of the electrochemical lithium insertion into monophosphate tungsten bronzes (PO 2) 4(WO 3) 2m ( m=9 and 10)

Electrochemical lithium insertion into (PO 2) 4(WO 3) 2 m , where m=9 and 10, has allowed the determination of several phases Li x (PO 2) 4(WO 3) 2 m between 3.4 and 0.01 V vs Li +/Li 0. After the first cycle the electrochemical system was unable to maintain the high specific capacity of the cells (∼540 Ah/kg) due to irreversible processes. Nevertheless at high voltage values, above 1.4 V vs Li +/Li 0, the lithium insertion proceeded through a reversible mechanism. By means of X-ray diffraction experiments we have detected the nature of different phases Li x (PO 2) 4(WO 3) 2 m formed and we have established a correlation with the reversible/irreversible processes detected during the electrochemical insertion.

Behavior of the monophosphate tungsten bronzes (PO 2) 4(WO 3) 2 m ( m = 7 and 8) in the course of electrochemical lithium insertion

The electrochemical lithium insertion process has been studied in the family of monophosphate tungsten bronzes (PO 2) 4(WO 3) 2 m , where m = 7 and 8. Structural changes in the pristine oxides were followed as lithium insertion proceeded. Through potentiostatic intermittent technique the different processes which take place in the cathode during the discharge of the cell were analyzed. The nature of the bronzes Li x (PO 2) 4(WO 3) 2 m formed was determined by in situ X-ray diffraction experiments. These results have allowed establishing a correlation with the reversible/irreversible processes detected during the electrochemical lithium insertion.

Behavior of the monophosphate tungsten bronzes (PO 2) 4(WO 3) 2 m ( m = 4 and 6) in electrochemical lithium insertion

The electrochemical lithium insertion process has been studied in the family of monophosphate tungsten bronzes (PO 2) 4(WO 3) 2 m , where m = 4 and 6. Structural changes in the pristine oxides were followed as lithium insertion proceeded. Through potentiostatic intermittent technique, the different processes which take place in the cathode during the discharge of the cell were analysed. The nature of the bronzes Li x (PO 2) 4(WO 3) 2 m formed was determined by in situ X-ray diffraction experiments. These results have allowed establishment of a correlation with the reversible/irreversible processes detected during the electrochemical lithium insertion. Measurements of resistivity showed that upon lithium insertion, the metallic pristine oxides become insulating.