Molybdenum Bronze Research Articles

The composition and proposed structure of the alkali metal layered molybdenum bronzes

The chemistry of the layered molybdenum bronzes with alkali metals as the guest cations was investigated. The reaction of MoO 3 and dithionete ion, S 2O 4 2−, in neutral, buffered solutions resulted in the formation of bronzes, [A +(H 2O) n] xMoO 3 x− where A = Li, Na, K, Rb or Cs, and eliminated the problem of the co-intercalation of adventitious protons. Chemical analysis of these bronzes shoed x to be ∼ 0.25 and thermogravimetric analysis was used to determine the number of intercalated water molecules, n. The interlayer spacings of both air-dried ( AD) and vacuum dried ( VD) products were measured, and models to account for these spacings were proposed. Electron diffraction from the ( hOl) planes of [Na(H 2O) 2] 0.25MoO 3 ( VD) allowed the powder X-ray diffraction pattern to be indexed on an orthorhombic unit cell with a = 3.876(2), b = 19.093(7) and c = 3.733(3) A ̊ . The systematic absences were consistant with space group Ammm. The powder pattern of the more fully hydrated bronze [Na(H 2O) 5] o.25MoO 3 ( AD) was also indexed on a related orthorhombic cell with a = 3.884(1), b = 22.618(8) and c = 3.751(2) A ̊ . The systematic absences in this case are compatable with Cmcm symmetry.

ChemInform Abstract: Structure and Motion of Hydrogen in Molybdenum Bronzes HxMoO3 as Studied by NMR.

AbstractBonding properties, location and mobility of hydrogen in molybdenum bronzes HxMoO3 with hydrogen contents in the range x = 0.2‐2.0 have been studied using 1H NMR solid state techniques.

Decomposition of hydrogen molybdenum bronze, H0.30MoO3, upon increasing temperature in various gases

When H0.30MoO3 was heated at increasing temperature in H2, N2, H2O, and in vacuum, the total weight loss just corresponded to the weight of 0.15 H2O in H0.30MoO3. On the other hand, when the bronze was heated in CH3OH vapor, one of the lattice oxygens was removed and the bronze was partly reduced to MoO2.

アモルファス三酸化モリブデン薄膜のイオンレジスト効果

Ion-beam modification of thin amorphous films of molybdenum trioxide (a-MoO3) was studied using thermionic sodium ions with an accelerating voltage of 4-12 kV. The films become abruptly insoluble in alkaline solutions at ion doses beyond a threshold value Dth. The a-MoO3 film is potentially useful as a negative-type inorganic ion resist with extremely high contrast. From the analysis of the voltage dependence of Dth, it is suggested that the resistive property is a result of the insulator-metal transition of sodium molybdenum bronze (NaxMoO3) produced by ion implantation. This hypothesis is also supported by a significant decrease in the sheet resistance of the film with increasing ion dose.

2H NMR lineshapes in deuterium molybdenum bronze 2H 1.66MoO 3

2H NMR lineshapes for the deuterium molybdenum bronze H 1.66MoO 3 were measured over the temperature range 20–410 K using the quadrupolar echo technique. The low temperature lineshape demonstrates the existence of two equally populated deuterium sites in the lattice. At 200 K, a motionally narrowed, completely asymmetric pattern is observed. The width is substantially less than that expected for simple flips of OH 2 moieties or quasi one-dimensional motion.

Temperature-programmed decomposition studies of a hydrogen molybdenum bronze, H 0.83MoO 3

A temperature-programmed decomposition (TPD) study of a hydrogen molybdenum bronze, H 0.83MoO 3, is reported. TPD spectra obtained from changes in pressure showed four bands (PI, PII, PIII and PIV); activation energies, E a , determined from the shifts in maximum peak temperatures were 23.8 (PI), 106.7 (PII), 118.6 (PIII) and 164.3 (PIV) kJ mol −1. E a s obtained from thermogravimetric data varied from 52.7 to 153.2 kJ mol −1 as the fraction reacted increased. E a s obtained from both methods were in good agreement.

Structural and compositional changes in a hydrogen molybdenum bronze, H 0.83MoO 3, by vacuum heating

H 0.83MoO 3 was a mixture of blue orthorhombic and blue monoclinic phases and decomposed, accompanied by structural and compositional changes, as follows: ▪.

Superconducting properties of the low dimensional purple bronze Li 0.9Mo 6O 17

The purple bronze Li 0.9Mo 6O 17 has recently been found to be a quasi two dimensional metal showing an instability towards a semiconducting type behaviour near 25 K and a superconducting transition at T c = 1.9 K. We report results of ac-susceptibility obtained on single crystals grown by the electrocrystallization technique. The transition at T ≅ 25 K is attributed to a charge density wave instability, as established for the neighbouring compound K 0.9Mo 6O 17. The superconducting transition temperature is found to be sample dependent ( T c = 1.1. to 1.6 K probably in relation with the crystal stoichiometry. The critical field H c is anisotropic with H c2|;/ H c2⊥ ≈- and temperature independent ( H c2| corresponds to H| layers), H c2|;) ≈- 20 kOe at 0.3 K. Specific heat data corroborate the existence of both transitions. These results are discussed in relation with the anisotropy of the Fermi surface and with the Ginzburg-Landau theory with anisotropic effective masses.

Energy bands and final-state effects in K0.30MoO3.

An x-ray photoemission spectroscopy study of the blue molybdenum bronze ${\mathrm{K}}_{0.30}$${\mathrm{MoO}}_{3}$ shows a conduction band of Mo 4d character extending 2 eV below the Fermi level. The valence band lies between 2 and 10 eV. The Mo 3d core-electron spectrum has components at binding energies previously identified with ${\mathrm{Mo}}^{5+}$ and ${\mathrm{Mo}}^{6+}$ ions in related molybdenum oxide compounds. In this metallic system these two states are, however, of final-state origin, representing alternate screening channels and do not indicate that discrete valences are present in the initial state.

Charge-density-wave instabilities in the low-dimensional molybdenum bronzes and oxides

Abstract The molybdenum bronzes A0·30MoO3, A0·9Mo6O17 and Mo4O11 oxides all show low-dimensional electronic properties, due to anisotropic crystal structures, which lead to charge-density-wave (CDW) instabilities. In the CDW states of the quasi-one-dimensional blue bronzes, non-linear transport properties are attributed to the sliding of the CDW. A lock-in transition from an incommensurate to a commensurate CDW is found in the best crystals. The non-linear properties are associated with noise voltage at both low (around 1 Hz) and high (around 10 kHz) frequencies as well as with hysteresis and time-dependent phenomena, showing the importance of metastability. The purple bronzes A0·9Mo6O17 and the oxides η-MO4O11 and γ-Mo4O11 have quasi-two-dimensional properties and quasi-cylindrical Fermi surfaces. In the CDW state, galvano-magnetic studies indicate the presence of small electron and/or hole pockets induced by the CDW gap openings. In the bronze Li0·9Mo6O17, the low-temperature properties reflect the competition between the CDW instability and superconductivity.

Quasi-two-dimensional electronic properties of the sodium molybdenum bronze, Na 0.9Mo 6O 17

Four probe electrical resistivity measurements between 1.5 and 300 K were made on single crystals of the violet-red bronze Na 0.9Mo 6O 17 grown by a temperature gradient flux technique. The temperature variation of the resistivity shows metallic conductivity and highly anisotropic behavior similar to K 0.9Mo 6O 17 and Li 0.9Mo 6O 17. The room-temperature resistivity, measured in the direction parallel to the plate axis, is 3.0 × 10 −3 Ω cm and perpendicular to that axis it is 0.21 Ω cm. A transition observed at ∼88 K is possibly related to the onset of a charge density wave. The temperature variation of the susceptibility show Pauli paramagnetic behavior at high temperature, and highly anomalous behavior in the vicinity of the transition at low temperatures.

Structural and Compositional Changes of Blue Orthorhombic Hydrogen Molybdenum Bronze, H0.30MoO3, by Vacuum Heating and Catalytic Activity for 1-Butene Isomerization

Abstract Orthorhombic bronze, H0.30MoO3, was decomposed by vacuum heating to MoO2.86 With a rhombic MoO3 structure. The surface area increased slightly with the increase in the rhombic structure in which vacancies were produced. An approximate linear relationship was found between the catalytic activity of 1-butene isomerization and the hydrogen content(x) in the samples which were treated in air at 65 °C or in vacuo below 220 °C. On the other hand, the activity above 220 °C increased with the formation of vacancies.

ChemInform Abstract: DECOMPOSITION OF BLUE ORTHORHOMBIC HYDROGEN MOLYBDENUM BRONZE, H0.30MOO3, BY HEATING IN VACUO AND IN HELIUM GAS

AbstractBei der (temp‐programmierten) Zers. (TPD) wurde außer H2O kein anderes Gas nachgewiesen.

Mobile Bipolaron In Quasi-1-D Conductor Vanadium Bronzes

Abstract The existence of the characteristic bound state of BIPOLARON (a dimerized electron pair of up and down spins on the near neighbour sites) was established by NMR and X ray diffraction measurements in the quasi-1-d conductor MxV205(M= Na,Ag,Li,Cu, and Pb). On the basis of the results of EPR and NMR relaxation, the observed high electrical conductivity along the 1-d axis is interpreted as the COLLECTIVE MOTION of BIPOLARONS. These characteristic properties are discussed with the strong electron-phonon interaction of the rather well localized small polarons in the 3d vanadium bronzes, which is different from the 4d. molybdenum bronzes.

Crystal growth of alkali metal molybdenum bronzes by a temperature gradient flux technique

Single crystals of alkali metal molybdenum bronzes of the type: A 0.9Mo 6O 17 with A = Na, K and A 0.3MoO 3 where A = K, Rb, Cs have been grown by the temperature gradient flux technique. Impure polycrystalline specimens of K 0.33MoO 3 and MoO 3 like phases which may be sodium and potassium intercalates respectively, have also been observed. Optimal growth conditions determined for each of the phases are reported. Na 0.9Mo 6O 17 is monoclinic and a quasi two-dimensional metallic conductor at room temperature, similar to Li 0.9Mo 6O 17 and K 0.9Mo 6O 17. A mechanism for the formation of bronze phases is proposed based on X-ray powder diffraction and DTA results.

Electrochemical crystal growth in the cesium molybdate-molybdenum trioxide system

A systematic investigation of crystal growth in the cesium molybdate/molybdenum trioxide system is described. A previously unknown blue cesium molybdenum bronze phase has been prepared as well as the known red bronze, Cs 0.33MoO 3, and high-quality crystals of the Magneli-phase compound, γ-Mo 4O 11. This new blue bronze, with empirical formula, Cs 0.19MoO 2.85, is monoclinic with cell constants, a = 19.198(4) Å, b = 5.519(2) Å, c = 12.213(2) Å, and β = 119.44(2)°. Measurements of the susceptibility and of the resistivity vs temperature are reported. As is the case for other alkali molybdenum bronzes, the product formed is determined by the molar ratio of alkali molybdate to molybdenum trioxide and the melt temperature.

Decomposition of Blue Orthorhombic Hydrogen Molybdenum Bronze, H0.30MoO3, by Heating in vacuo and in Helium Gas

Abstract The decomposition process of orthorhombic hydrogen molybdenum bronze, H0.30MoO3, was studied by the temperature programmed decomposition (TPD) technique in vacuo and in helium gas. The only gas liberated during the decomposition was H2O and no other gas was detected. The TPD spectrum above 150 °C was composed of three bands (PII, PIII, and PIV). The PII band corresponded to the process from H0.30MoO3 to H0.21MoO2.95 without any structural change. The PIII band corresponded to the process from H0.21MoO2.95 to H0.06MoO2.88 with the structural change and the PIV band, to the process from H0.06MoO2.88 to MoO2.85.

Dramatic impurity effects on the charge-density wave in potassium molybdenum bronze

Potassium molybdenum bronze, ${\mathrm{K}}_{0.30}$Mo${\mathrm{O}}_{3}$, undergoes a charge-density-wave (CDW-) driven phase transition at 180 K, below which nonlinear current-voltage characteristics are observed. Behavior is similar to that observed in Nb${\mathrm{Se}}_{3}$ and is currently understood based on models which involve charge transport by the CDW. The effects of impurities on CDW phenomena has been a subject of considerable interest. Two types of substitutional doping are possible in the ${\mathrm{K}}_{0.30}$Mo${\mathrm{O}}_{3}$ structure. In the alloy system, ${\mathrm{K}}_{0.30\ensuremath{-}x}{\mathrm{Rb}}_{x}\mathrm{Mo}{\mathrm{O}}_{3}$, disorder is introduced on the alkali sublattice while in the alloy system, ${\mathrm{K}}_{0.30}{\mathrm{Mo}}_{1\ensuremath{-}x}{\mathrm{W}}_{x}{\mathrm{O}}_{3}$, the isoelectronic element tungsten is substituted for molybdenum. We show that the effects of tungsten substitution are very large, larger than such doping effects in other CDW systems, whereas alkali substitution produces only small effects. The data obtained on the ${\mathrm{K}}_{0.30}{\mathrm{Mo}}_{1\ensuremath{-}x}{\mathrm{W}}_{x}{\mathrm{O}}_{3}$ system suggest that the low-temperature CDW coherence length becomes very short at low tungsten concentrations.

Sliding charge-density wave conductivity in potassium molybdenum bronze

The monoclinic blue-bronze, K 0.30MoO 3, is an anisotropic metal that undergoes a charge density wave (CDW)-driven phase transition at 180 K, below which nonlinear current-voltage characteristics are observed. In this paper, we examine the anisotropy of the electric and magnetic properties of K 0.30MoO 3 and their relationship to observed CDW phenomena. We show that the conductivity is highest along the monoclinic b axis and 10 and 100 times lower in two directions perpendicular to b (in the ( 2 0 1 plane and perpendicular to these planes, respectively). In the CDW state, the conductivity along the b axis is nonlinear when the applied electric field exceeds a small threshold on the order of 50 mV/cm, but is field-independent in perpendicular directions. A periodic response to a dc bias was observed in one small K 0.30MoO 3 sample, however, the observation of an NbSe 3-like response in K 0.30MoO 3 may depend on sample size or the quality of the electrical contacts to the sample.

Evidence for a charge-density-wave instability in the purple bronze K0·9Mo6O17

Abstract The purple molybdenum bronze K0–9Mo6O17 is a trigonal, quasi-two-dimensional metal which shows an electronic transition at 120 K. In this paper are reported specific-heat data which provide evidence that a second-order phase transition takes place at this temperature, with an entropy change of the order of 0·5 J mol−1 K−1. Both X-ray diffuse scattering and electron diffraction studies have established that this transition is accompanied by a structural transition towards a commensurate 2a × 2a × c state. All the results are consistent with a model of a Fermi-surface-driven instability. The temperature dependence of the charge-density-wave gaps, as well as the nature of the nesting at the Fermi surface, are also discussed.