The monophosphate tungsten bronzes (PO2)(4)(WO3)(2m) form family of two-dimensional metals which exhibit charge density wave (CDW) instabilities. These materials are generally built by the regular stacking of (a,b) layers in which chains made of segments of m WO6 octahedra directed along the a and a+/-b directions are delimited. Their electronic structure thus originates from quasi-one-dimensional (1D) bands located on these chains. As a consequence their Fermi surface (FS) exhibits large flat portions whose nesting gives rise to successive CDW instabilities. Here we present a structural study of the CDW instability of the (PO2)(4)(WO3)(10) member formed by the alternate stacking of layers built with segments of m=4 and m=6 WO6 octahedra. Its ab initio electronic structure calculation shows that the FS of this member exhibits large flat portions which can be extremely well nested. Its best nesting wave vector accounts for the modulation wave vector stabilized by the CDW transition which occurs at 156 K. Because of the regular stacking of layers of different m values the FS is slightly split. The unusual thermal dependence of the x-ray satellite intensity provides evidence that the two types of layers become modulated at different temperature. This also leads to a slight thermal sliding of the CDW-nesting modulation wave vector, which can be accounted for within the framework of a Landau-Ginzburg theory. In addition, the observation of a global hysteresis in the thermal cycling of the satellite intensity, as well as the degradation of the interlayer order upon cooling, suggest the formation of a disordered lattice of dilute solitons. Such solitons allow to accomodate the charge transferred between the two types of layer. Finally the relevance of local charge transfers, at intergrowth defects, for example, to create pinned discommensurations that break the CDW coherence is emphasized in this whole family of bronzes.

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.

The charge-density-wave (CDW) structure of the monophosphate tungsten bronze P4W20O68, the m = 10 member of the (PO2)(4)(WO3)(2m) series has been solved at room temperature from single-crystal x-ray-diffraction data. In agreement with previous x-ray diffuse scattering experiments, intense first-order satellite reflections at +/- 3/7a* reduced wave vectors as well as second-order satellite reflections at +/- 1/7a* were observed. The CDW structure was refined in a four-dimensional (4D) formalism and found to have the superspace group P2(1) (alpha 00) 0. The modulation, of the displacive type, involves mainly the tungsten atoms inside the different WO6 octahedra of the WO3-type slab. The W displacements, which are mostly oriented in the direction of the segment of 10 WO6 octahedra building the slab, tends to be oriented in opposite directions between neighboring segments. It is suggested that for the large in members these features represent a good compromise between the CDW instability of the metallic bronzes, related to their quasi-1D electronic structure, and the incipient antiferroelectric lattice distortion of the insulating oxide WO3, which corresponds to the limit m-->infinity of this series. These structural features play a crucial role in setting the coupling between the differently oriented 1D portions of the Fermi surface required for the hidden nesting mechanism at the basis of the stabilization of a CDW ground state in the ReO3-type metallic W and Mo bronzes and oxides.

The monophosphate tungsten bronzes (PO2)4(WO3)2m with pentagonal tunnels are quasi-two-dimensional conductors that show charge density wave type electronic instabilities. These series of compounds provide a model system where the low-dimensional character and the average electron concentration are functions of the m parameter. The low m compounds (m=4, 6) show conventional charge density wave instabilities. The m=5 compound exists with two different crystal structures and shows instabilities with slightly different properties. We report measurements of transport properties for the compounds m=5, 7, 8, 9. We show that, for m>7, these compounds exhibit an upturn of resistivity and field dependence of the magnetoresistance characteristic of quantum interference effects. We also report transport properties of the compounds KxP4W8O32 with pseudo-hexagonal tunnels that show electronic instabilities with critical temperatures depending on x.

The association of an inorganic/organic composite matrix with a permselective poly(4-vinylpyridine-co-styrene) membrane allows to obtain a selective glucose biosensor, allowing a sensitivity of 30 mA M−1 cm−2, for a linear dynamic range from 10−5 M to 1.5 mM. Moreover, the combined use of such permselective membranes with a bienzymatic system, glucose oxidase/polyphenol oxidase, has demonstrated that the effect of electroactive interferent species such as ascorbate, urate and p-acetaminophen, can be entirely removed.

Phase relations in the system Na-K-P-W-O have been studied for two values m = 4 and m = 6 of the relevant parameter m which characterizes the thickness of the perovskite WO3-type slabs within the series of the low dimensional tungsten phosphate bronzes (K-x-Na-y)(PO2)(4)(WO3)(2m). New NayP4W12O44 bronzes (0 less than or equal to y less than or equal to 1) belonging to the monophosphate tungsten bronzes with pentagonal tunnels family have been isolated and studied by single-crystal X-ray diffraction and resistivity measurements. The crystal structure of Na0.96P4W12O44 has been solved and refined to conventional R = 0.0289 (wR = 0.0414) with 2924 reflections with I greater than or equal to 3 sigma(I). Like that of P4W12O44, the unit cell is orthorhombic (space group P2(1)2(1)2(1)) with a = 5.3083(3) Angstrom, b = 6.5790(7) Angstrom, and c = 23.6380(10) Angstrom. The location of Na+ cations in the O-18 cages with pentagonal windows is compared to that of Na+ cations in the O-18 cages with hexagonal windows of the monophosphate tungsten bronzes with hexagonal tunnels Na1.7P4W12O44. Electron transport properties of single crystals of both monophosphate tungsten bronzes with pentagonal and hexagonal tunnels are described and discussed in terms of possible charge density wave (CDW) instabilities.

The conductors (PO2)4(WO3)2m show CDW type electronic instabilities. We report electrical properties of the m = 5 crystal variety made of a regular 5/5/5 stacking of WO3 layers. An anomaly at T~60K is observed in the electrical resistivity, Hall coefficient and thermoelectric power. The origin of the anomaly is discussed.

Resistivity and magnetoresistance have been measured on single crystals of the monophosphate tungsten bronzes (PO 1 ) 4 (WO 3 ) 2m for m=5. The m = 5 compound exists in two structural varieties which lead to different resistivity and magnetoresistance properties. Shubnikov de Haas oscillations with large amplitude are observed in one of the variety. The origin of the observed anomalies in the resistivity vs temperature curves is discussed.

A new member, m = 6, Of the series K-x(PO2)(4)(WO3)(2m) has been isolated and studied by single crystal X-ray diffraction and electrical transport measurements. The structure corresponds to that of the MonoPhosphates Tungsten Bronzes with hexagonal tunnels (MPTBh's). It has been solved and refined to conventional R = 0.0268 with 2342 independent reflections with I> 3 sigma(I). The unit cell is monoclinic (space group P2(1)/m) with a = 6.6736(2), b=5.3543(3), c=11.9005(5) Angstrom, beta = 92.615(3)degrees. The oxygen surroundings of K; P and W atoms are described and compared with those of other members of the series including Pb-, Na- and K-based analogues. Electrical properties and correlations with structural data are discussed. Large magnetoresistance effects have been observed at low temperature and assigned to possible CDW instabilities.

A new variety of P 4 W 10 O 38 , m = 5 member of the monophosphate tungsten-bronze family with pentagonal tunnels (MPTBp), \(\), has been isolated and studied by X-ray diffraction measurements. The structure has been solved by direct methods from single crystal X-ray data. The monoclinic unit cell corresponds to a regular stacking of WO 3 -type slabs which are all five-WO 6 -octahedra thick and connected through PO 4 tetrahedra. This structure is comparable to that previously described for the MPTBp'series with m = 4 (P 4 W 8 O 32 ) and m = 6 (P 4 W 12 O 44 ). An X-ray diffuse scattering investigation has revealed that this member is subjected to charge density wave instabilities (CDW) located on chains running along the \(\) directions. Two CDW transitions have been observed at \(\) and \(\), bearing some resemblance with those exhibited by the m = 4 member. The corresponding modulation wave vectors can be accounted for by a hidden nesting mechanism which connects the crossing points of differently oriented quasi-planar Fermi surfaces, as found for the low m (4 and 6) members of the MPTB's.

A feasibility study in view of a future industrial development of a phenol biosensor is presented. The biosensor construction is based on the electropolymerization of a pyrrole amphiphilic monomer–tyrosinase (EC.1.14.18.1) mixture previously adsorbed on a glassy carbon electrode surface. The optimized biosensor provides a low detection limit (10 nM) and a linear response up to 10 μM of phenol. The procedure allows the construction of biosensors exhibiting very reproducible characteristics. These bioelectrodes are characterized by exceptional long-term stability since they could be stored for 1 year without significant loss of their electroenzymatic activity.

The crystallographic study of the tungsten phosphate bronze P4W24O80 was performed from three-dimensional single-crystal X-ray diffraction data and electron microscopy. This compound crystallizes in the orthorhombic system with unit-cell dimensions a = 5.312 (1), b = 6.5557 (8), c = 42.196 (8) Å and space group P212121. The crystal structure was solved by direct methods and Fourier techniques, and refined to the reliability factor R = 0.0284 (wR = 0.0272). Its determination confirms that it belongs to the monophosphate tungsten bronze family of the general formula (PO2)4(WO3)2m with the value m = 12. Large empty cages surrounded by 18 O atoms are built up of eight WO6 octahedra and four PO4 tetrahedra sharing corners. They are located between two WO3-type slabs forming pentagonal-shaped tunnels running in the a direction. Electron microscopy investigations confirm that the studied crystal does not imply modulation phenomena when other crystals of the same composition exhibit satellite reflections with a modulation vector q* involving a doubling of a. The observations also reveal the existence of a monoclinic form of the m = 12 compound, which is a regular intergrowth of m = 11 and m = 13 members in a similar way to the m = 5 member of the series where the same feature has already been observed. On each edge of a WO3-type slab, a large variation (from 1.73 to 2.09 Å) of the six W—O distances within the WO6 octahedron is noted, which yields an oxidation state of W near 6, whereas for the WO6 octahedra located in the middle part of the slab the six W—O distances are gathered about their mean value (1.92 Å), which involves a more important electronic delocalization. The thermal motion of the W atoms is described. The absolute structural configuration is tested on the basis of some calculated structure factors, which are more sensitive to the x, y, z → −x, −y, −z change of atomic positions.

A new series of lead-based monophosphate tungsten bronzes (MPTBh), Pbx(PO2)4(WO3)2m, was isolated for 6≤m≤10 and characterized by X-ray diffraction, electron microscopy, and resistivity and magnetic susceptibility measurements. The crystal structure was solved form=7 from three-dimensional single-crystal X-ray data [Pb0.66P4W14O50: triclinic,A1,a=6.6015(3) Å,b=5.3156(4) Å,c=27.039(2) Å,α=90.208(6)°,β=96.757(5)°,γ=89.867(5)°,Z=1, finalR=2.16% for 4462 independent reflections withI>3.0σ(I)]. Lead atoms are distributed in cages bounded by 18 oxygen atoms, in a part that favors a PO4tetrahedron neighbor rather than a WO6octahedron neighbor. Consequently, Pb atoms exhibit an (3+5) eightfold coordination that takes into account the 6s2lone-pair stereoactivity. Resistivity measurements performed on oriented single crystals reveal quasi-two-dimensional metallic behavior for the entire temperature range investigated, 4.2 to 300 K. Magnetic susceptibility data indicate Pauli paramagnetic behavior. The results are compared with those of alkaline-based counterparts,Ax(PO2)4(WO3)2m(A=Na, K).

A new mixed membrane material for amperometric glucose biosensors, based on immobilization of glucose oxidase (GOD) in inorganic laponite gels followed by an enzyme crosslinking through glutaraldehyde (GA) is investigated. This method gives the possibility to modulate the enzyme loading in the biomembrane, the corresponding sensitivities obtained being directly proportional to the enzyme content in the membrane. Thus, with a GOD/laponite ratio of 3.3, sensitivities reach up to 132mAM−1cm−2 for a linear dymamic range from 10−2 to 20mM. Effects of oxygen and temperature have been investigated. The analysis of the bioelectrode characteristics shows a good permeability of this inorganic/organic enzymatic membrane. A specific design, combining a Nafion membrane and incorporation of polyphenol oxydase, allows to reduce the effects of interfering species such as ascorbate, urate and acetaminophen by a factor 4.

Monoenzyme (HRP) and bilayer/monolayer bienzyme (HRP-GOX) bioelectrodes were realized bu the oxidative electropolymerization of amphiphilic pyrrole monomer 1 enzyme(s) mixtures, previously adsorbed on the surface of a glassy carbon electrode. Cyclic voltammetry measurements, carried out on poly 1-HRP modified electrodes showed that the electrocatalytical reduction of H2O2, in the presence of K4Fe(Cn)(6) as mediator, occurs at applied potentials well placed in the optimal potential range for amperometric detection. An optimization study concerning the mediator concentration the amount of the immobilized enzyme(s), the electrode material and the matrix structure (mono-or bilayer) was performed in order to obtain the glucose biosensor, At an applied potential of -0.1 V (vs. SCE), batch amperometric response to H2O2 for the HRP modified electrodes gave a sensitivity of fa. 280 mAM(-1) cm(-2) (up to 0.65 mM). For monolayer biosensor the sensitivity io glucose was ca. 170 mAM(-1) cm(-2) (up to 1.9 mM). The interference of ascorbate, urate and acetaminophen was found almost negligible.

A new mixed-valent monophosphate Sr(2)V(2)O(PO(4))(3) has been synthesized. Its 4-fold superstructure has been refined using the 4D formalism for modulated structures and using X-ray single-crystal diffraction data. The average structure can be described as a stacking of [V(2)P(3)O(13)](infinity) layers and [Sr(2)](infinity) layers. In these layers, the VO(6) octahedra, occupied by V(III), the VO(5) trigonal bipyramids, occupied by V(IV), and the PO(4) tetrahedra form chains running along b. The 4D description allows us to give a better description of the measured intensities in reciprocal space (main and first order satellite reflections) and particularly to explain why the second order satellite reflections are not observed using a classical X-ray source. The corresponding structural mechanism consists of an alternation of disordered ribbons of VO(5) bipyramids and PO(4) tetrahedra in two enantiomorphic configurations, separated by displacively modulated ribbons of VO(6) octahedra and PO(4) tetrahedra. Crystal data: monoclinic, superstructure space group P2(1)/c, Z = 8, a = 17.389(1) Å, b = 5.094(1) Å, c = 30.032(4) Å, beta = 132.17(1) degrees; superspace group P2(1)/m(0,0,(1)/(4))0s, Z = 2, c' = c/4.

Large single crystals of with x = 1.00(5), x = 1.05(5) and x = 1.30(9) were prepared by the chemical vapour transport technique and electron transport properties were investigated. Thermal variation of the resistivity between 300 K and 4.2 K shows metallic behaviour with anomalies whose onset temperature depends on the potassium content x: K for x = 1.00, for x = 1.05 and for x = 1.30. Anomalies can also be observed in the thermal dependence of the thermopower. A conventional metal behaviour is thus observed above while a strong deviation from linearity is observed below with a change of sign from negative to positive which depends on the x value. The magnetic field dependence of the resistivity has been studied in the low-temperature state for x = 1.30 and shown to be strongly correlated with the orientation of the magnetic field with respect to the conducting plane. The results are explained on the basis of Peierls instabilities and charge density waves (CDW) gap openings with electron and hole pockets which compete on the Fermi surface.

A structural analysis of the double-collapsed-phase Bi6+xSr9-xFe5O26 is presented in light of the modulated-phase Bi2.4Sr2.6Fe2O9+delta previously investigated [Y. Lepage, W. R. McKinnon, J. M. Tarascon, and P. Bar boux, Phys. Rev. B 40, 6810 (1989); O. Perez, W. LeIigny, D. Grebille, J. M. Greneche, Ph. Labbe, D. Groult, and B. Raveau, Phys. Rev. B 55, 1236 (1997)]. The study was carried out on a single crystal, using synchrotron radiation (lambda = 0.326 Angstrom). The crystal is monoclinic with cell parameters a = 16.491(9) Angstrom, b = 5.481(3) Angstrom, c = 30.086(16) Angstrom, beta = 91.39(2)degrees. The real structure of symmetry P2(1)/n appears as a small perturbation of the ideal structure with higher symmetry B2/m. The undulating (001)(m) layers of the modulated (m) phase built from one kind of cation (Bi, Sr, or Fe) are replaced by mixed layers. The waving of these layers is less regular than in the modulated phase because interrupted layers are connected. One of the most interesting results relates to the interrupted Bi layers: the isolated blocks composed of two adjacent infinite [010] ribbons, six Bi atoms wide, are similar to the ordered part (condensed zones) of the Bi-2212 modulated phase. The Bi disordered regions (diluted zones) are missing in the double-collapsed phase. An original octahedral coordination is implied for the Bi atoms located near the Fe atoms in the same undulated mixed layer. Disorder phenomena have been observed in a complementary HREM study and appear as localized faults modifying mainly the length of the Bi ribbons.

The refinement of the modulated structure of Bi2+xSr3-xFe2O9+delta, using the our-dimensional formalism and single-crystal x-ray-diffraction data, has been performed. Different models, already used for the copper compound, have been tested to describe the [BiO] layers. A discussion pointing out the qualities and the failings of each one led us to consider a model assuming a static disorder both on the bismuth and oxygen atom positions. Then, an extra oxygen atom has been located within the [BiO] layers and its occupancy probability has been refined. Because of this model, two disordered regions, governing the behavior of all the atoms in the crystal, have been evidenced. A comparison with the supercell description previously reported shows the interest of the four-dimensional formalism in this case. Finally, a Mossbauer study of Bi2+xSr3-xFe2O9+delta specifies the trivalent state of iron and confirms the various surroundings in oxygen atoms of the iron atoms. The correlation between the incommensurate character of the structure and the Mossbauer spectroscopy data is discussed.