W.H. McCarroll

Synthesis of reduced complex oxides of molybdenum by fused salt electrolysis

Abstract Fused salt electrolysis has been used to prepare a number of reduced oxides of molybdenum with lanthanum, neodymium, and yttrium in single crystal or oriented polycrystalline form. The average valence of molybdenum in the various compounds ranged from 5.67 to 3.50. Previously unreported compounds include La 5 Mo 4 O 16 (triclinic a = 5.64 A, b = 20.7 0A, c = 5.64 A, α = 86.55°, β = 90.0°, γ = 93.45°); La 2 Mo 2 O 7 (orthorhombic, a = 12.19 A, b = 6.05 A, c = 3.87 A); LaMo 2 O 5 (hexagonal, a = 8.378 A, c = 19.26 A). In addition, single crystal specimens have been prepared of Y 2 MoO 5 , Ln 5 Mo 3 O 16 ( Ln =La, Nd) and metal atom cluster compounds of the A 2 Mo 3 O 8 type ( A = Mg, Co, Ni, Zn).

Structure and properties of La2Mo2O7: A quasi-two-dimensional metallic oxide with strong MoMo bonds

Single crystals of La/sub 2/Mo/sub 2/O/sub 7/, prepared by fused salt electrolysis, were used for structural and electronic characterization. La/sub 2/Mo/sub 2/O/sub 7/ is orthorhombic with a = 6.034 A, b = 12.236 A, and c = 3.888 A. The dominant feature of the structure, which was refined in space group Pnnm, is Mo/sub 2/O/sub 10/ units formed by edge-sharing MoO/sub 6/ octahedra which contain Mo-Mo distances of only 2.478 A. These groups then share corners in two dimensions to give rise to Mo-O layers which are held together by the lanthanum ions. The relationship of the La/sub 2/Mo/sub 2/O/sub 7/ structure to those of other reduced oxides is discussed. La/sub 2/Mo/sub 2/O/sub 7/ is a metallic conductor down to 125 K where a phase transition takes place. A similar transition is seen in the magnetic susceptibility. The anomalous electric and magnetic behavior of this compound may be associated with a charge density wave instability of the type often found in quasi-two-dimensional materials.

Structure and properties of Y5Mo2O12 and Gd5Mo2O12: Mixed valence oxides with structurally equivalent molybdenum atoms

Abstract Crystals of Ln 5 Mo 2 O 12 ( Ln = Y, Gd) were grown by electrochemical reduction of alkali-molybdate/rare-earth oxide melts at 1075–1100°C. A single crystal of Y 5 Mo 2 O 12 , used for structure determination, was found to be monoclinic with a = 12.2376(7) A, b = 5.7177(8) A, c = 7.4835(5) A, β = 108.034(5)°, and Z = 2. Although the structure was refined in space group C2 m , the true space group appears to be P2 1 m . In Y 5 Mo 2 O 12 , rutile-like sheets of edge-shared MoO 6 chains linked by YO 6 octahedra are interconnected with YO 7 monocapped trigonal prisms. The Mo atoms in the chains have alternating distances of 2.496 and 3.221 A and in that respect are similar to MoO 2 . However, in contrast to metallic MoO 2 both the Y and Gd compounds are n -type semiconductors with room temperature resistivities of the order of 10 3 ohm-cm. Magnetic susceptibility measurements confirm the presence of one unpaired electron per Mo 2 unit. The semiconducting behavior can be explained in terms of an unfavorable bridging oxygen coordination which prevents electron delocalization through metal-oxygen pi bonding as in MoO 2 .

Anomalous electrical and magnetic properties of a new quasi-low-dimensional mixed-valent molybdenum-cluster compound

Abstract Large plate-like single crystals of La 5 Mo 4 O 16 were prepared by electrolyzing molten mixtures of La 2 O 3 , MoO 3 and Na 2 MoO 4 . The temperature dependence of electrical resistivity in the ab plane and parallel to the c-axis of the single crystal shows semiconducting behavior in the temperature range 180–300 K and a sharp upturn in the resistivity below ∼180 K. The temperature variation of the susceptibility in the temperature range 200–300 K follows Curie-Weiss behavior with an observed moment ∼1.66 B.M. corresponding to one localized electron per formula unit. The magnetization study on a batch of randomly oriented single crystals showed a sharp decrease in the susceptibility at ∼180 K indicative of a long-range antiferromagnetic order. Directional magnetization studies on La 5 Mo 4 O 16 indicated that the magnetic ordering is confined to the ab plane. The anomalous electrical and magnetic properties are discussed in terms of the unique structure of La 5 Mo 4 O 16 .

Variable-temperature infrared spectroscopic study of some molybdenum bronzes: evidence for electron–phonon coupling

An infrared vibrational spectroscopic study of some molybdenum oxide bronzes, commonly termed the ‘blue’ bronzes, the ‘red’ bronzes and the ‘purple’ bronzes has been made. The charge density wave driven metal-to-semiconducting phase transition, mediated through electron–phonon coupling, is evidence in the infrared spectra of the blue bronze studied as anomalous intensity behaviour on some of the internal modes, specifically in the lower-frequency Mo—O—Mo stretching region. Significant intensity differences are observed between these different classes of bronzes: a qualitative explanation for these differences is given.

Quasi-one-dimensionality in the new bronze-like compound La2Mo2O7

Abstract Directional, four-probe electrical resistivity and magnetic susceptibility measurements were carried out on the bronze-like, purple compound, La 2 Mo 2 O 7 , between room temperature and 1.5 and 4.2 K, respectively. The resistivity is anisotropic with ϱ(300 K) = 6.4 × 10 −2 , 5.9 × 10 −2 , and 4.0 × 10 −3 (Ω cm) along the a , b , and c directions of the orthorhombic unit cell, respectively. There is an increase in the resistivity near 125 K, and a drop in the susceptibility seen with H parallel to the c axis but not perpendicular to it. These observations are indicative of a charge-density-wave (CDW) transition.

Synthesis of LnMn2O5 (Ln = Nd, Pr) crystals using fused salt electrolysis

Abstract A simple method of synthesis of LnMn 2 O 5 (Ln = Nd, Pr) is presented. Mixtures of sodium molybdate, molybdenum(VI) oxide, MnCO 3 , and Nd 2 O 3 or Pr 6 O 11 were electrolyzed for 3–24 h at 930–1020°C with platinum electrodes. Single crystals of LnMn 2 O 5 (Ln = Nd, Pr) with an edge up to 1.5 mm formed at the anode. The crystal structures of NdMn 2 O 5 and PrMn 2 O 5 were refined by Rietveld profile analysis in space group Pbam (Z = 4); the lattice parameters are a = 7.4970(2) A, b = 8.6086(2) A, c = 5.6963(1) A and a = 7.5384(2) A, b = 8.6319(2) A, c = 5.7022(1) A, respectively. The magnetic susceptibility measurements show an antiferromagnetic transition at 25 K for PrMn 2 O 5 . Both the NdMn 2 O 5 and PrMn 2 O 5 crystals are insulating.

The crystal structure of La3Mo4.33Al0.67O14 and the electronic structure of La3Mo4XO14 (X = Si; Mo13Al23; Al12V12)

Abstract Black needles of La3Mo4.33Al0.67O14 were prepared by electrolytic reduction at 1100°C of a melt containing Na2MoO4, MoO3, and La2O3 held in an alumina crucible. The compound is isomorphous with La3Mo4SiO14, crystallizing in space group Pnma with a = 17.750(3) A, b = 5.6600(9) A, c = 11.070(2)A, V = 1112.1(6) A3, and Z = 4. The structure was refined by full-matrix least squares to RF = 3.43%, RWF = 3.45%, with 1912 independent reflections. As in La3Mo4SiO14, the structure contains Mo3O13 clusters and chains of edge-sharing MoO6 octahedra with alternately short (2.535 A) and long (3.167 A) MoMo distances, but unlike La3Mo4SiO14, the tetrahedral silicon is replaced by a random distribution of 1 3 Mo and 2 3 Al . Electrical conductivity measurements show that La3Mo4SiO14 and La 3 Mo 4 Al 2 3 Mo 1 3 O 14 are both highly anisotropic semiconductors with the easy direction of conduction being parallel to the chain cluster units. Semiempirical bond length-bond strength calculations lead to the conclusion that the valences of the molybdenums in both the chains and the clusters are remarkably similar. This coupled with the observed diamagnetism of the silicon compound can best be explained in terms of an electronic band structure rather than a localized model.

Substitutional studies on the anisotropic, semiconducting, molybdenum bronze, Li0.33MoO3

Abstract Pure Li0.33MoO3 and vanadium-doped Li0.33Mo1−xVxO3 crystals were grown by a temperature gradient flux technique. Directional electrical resistivity measurements have been made on both the pure and doped crystals. The electrical properties of Li0.33MoO3 are highly anisotropic with resistivity values at room temperature of 1.8 × 10−1 and 6.0 × 102 Ω cm along the c and b∗ directions, respectively. Li0.33 MoO3 shows complex semiconducting behavior and an anomalously low paramagnetic signal. The effects of vanadium substitution on the structure and transport properties of Li0.33MoO3 are dramatic. Attempts are made to correlate the observed electrical and magnetic behavior with the structure of Li0.33MoO3.

Substitutional effects on the electrical properties of the purple bronze Li0.9Mo6O17

Abstract Dopant-level substitution of tungsten for molybdenum in Li 0.9 Mo 6− x W x O 17 solid solutions is shown to have a large effect on the electrical transport properties, which might be attributed to W interfering with the overlap of t 2g 4d Mo and π 2p orbitals forming the conduction band. With increasing concentration of W, the metal-to-metal transition broadens and shifts to higher temperature. At ∼6 at.% W content the material becomes semiconducting. Effects on the electronic properties are much less dramatic by alkali metal substitution in (Na 1− x Li x ) 0.9 Mo 6 O 17 alloys, since the alkali metals are only indirectly involved in the formation of the band structure. With x = 0, 0.25 and 0.5 the electrical resistivity behaves similar to Na 0.9 Mo 6 O 17 . At x = 0.75 the electrical transport properties are nearly identical to that Li 0.9 Mo 6 O 17 and the supercond uctivity at 2 K is still observed.