Lithium molybdenum purple bronze

Lithium molybdenum purple bronze is a chemical compound with formula Li
0.9Mo
6O
17, that is, a mixed oxide of molybdenum and lithium. It can be obtained as flat crystals with a purple-red color and metallic sheen (hence the "purple bronze" name).[1][2]

This compound is one of several molybdenum bronzes with general formula A
xMo
yO
z where A is an alkali metal or thallium Tl. It stands out among them (and also among the sub-class of "purple" molybdenum bronzes) for its peculiar electrical properties, including a marked anisotropy that makes it a "quasi-1D" conductor, and a metal-to-insulator transition as it is cooled below 30 K.

Preparation

The compound was first obtained by Martha Greenblatt and others by a temperature gradient flux technique. In a typical preparation, a stoichometric melt of Li
2MoO
4, MoO
2 and MoO
3 is maintained in a temperature gradient from 490 to 640 °C over 15 cm in vacuum over several days. Excess reagents are dissolved with a hot potassium carbonate solution releasing metallic-purple plate-like crystals, a couple mm wide and less than a mm thick.[1][3]

Structure

The crystal structure of Li
0.9Mo
6O
17 was determined by Onoda and others through single-crystal X-ray diffraction. The crystal system is monoclinic, with approximate unit cell dimensions a = 1.2762 nm, b = 0.5523 nm, and c = 0.9499 nm, with angle β = 90.61°, volume V = 0.6695 nm3 and Z = 2. In typical crystals, a is the shortest dimension (perpendicular to the plates) and b the longest. The density is 4.24 g/cm3. The structure is rather different from that of potassium molybdenum purple bronze K
0.9Mo
6O
17, except that both are organized in layers. The difference may be explained by the relative sizes of the K+
 and Li+
 ions.[1][2]

The unit cell contains six crystallographically independent molybdenum sites. One-third of the molybdenum atoms are surrounded by four oxygens, two thirds are surrounded by six oxygens. The crystal is a stack of slabs; each slab consists of three layers of distorted MoO
6 octahedra sharing corners. The lithium ions are inserted in the large vacant sites between the slabs. There are zigzag chains of alternating molybdenum and oxygen atoms extending along the b axis.[2]

Properties

Lithium molybdenum purple bronze is quite different than the sodium, potassium and thallium analogs. It has a three-dimensional crystal structure, but a pseudo-one-dimensional (1D) metallic character, eventually becoming a superconductor at about 2 K[4] Its properties are most spectacular below 5 meV. The Tomonaga-Luttinger liquid theory has been invoked to explain its anomalous behavior.[5]

Electrical conductivity

At room temperature, Greenblatt and others (in 1984) measured the resistivity of lithium purple bronze along the a, b and c axes as 2.47 Ω cm, 0.0095 Ω cm, and on the order of 0.25 Ω cm, respectively.[1] The conductivities would be in the ratio 1:250:10,[2][6] which would make this compound an almost one-dimensional conductor. However, Da Luz and others (2007) measured 0.079, 0.018, and 0.050 Ω cm, respectively,[7] which corresponds to conductivity ratios 1:6:2.4 for a:b:c; whereas H. Chen and others (2010) measured 0.854, 0.016, and 0.0645 Ω cm, respectively,[3] which correspond to conductivity ratios of 1:53:13.[3]

This anisotropy has been attributed to the crystal structure, specifically to the zig-zag chains of molybdenum and oxygen atoms [2]

Resistivity and temperature

The resistivity along all three axes increases linearly with temperature from about 30 K to 300 K, as in a metal.[3] This is anomalous since such a law is expected above the Debye temperature (= 400 K for this compound)[8] The resistivity ratios along the three axes are preserved in that range.[3]

Metal-insulator transition

As the lithium purple bronze is cooled from 30 K to 20, it changes abruptly to an insulator. After reaching a minimum at about 24 K, the resistivity increases 10-fold and becomes somewhat more isotropic, with conductivities 1:25:14. The anisotropy is partially restored if a magnetic field is applied perpendicular to the b axis.[3] The transition may be related to the onset of a charge density wave.[1] Santos and others have observed that the thermal expansion coefficient is largest along the a axis, so cooling will bring the conducting chains closer together, leading to a dimensional cross-over.[9] The theory of Luttinger liquids then predicts such behavior. Anyway, as of 2010 there was no consensus explanation for this transition.[3]

Superconducting state

Lithium molybdenum purple bronze becomes superconductor between 1 and 2 K.[1]

Magnetoresistance

The magnetoresistance of lithium purple bronze is negative when the magnetic field is applied along the b-axis, but large and positive when the field is applied along the a-axis and the c-axis.[3]

See also

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 M. Greenblatt, W. H. McCarroll, R. Neifeld, M. Croft, J. V. Waszczak (1984), "Quasi two-dimensional electronic properties of the lithium molybdenum bronze, Li
    0.9    Mo
    6    O17    ". Solid State Communications, volume 51, issue 9, pages 671–674. doi:10.1016/0038-1098(84)90944-X
  2. ↑ 2.0 2.1 2.2 2.3 2.4 M. Onoda, K. Toriumi, Y. Matsuda, M. Sato "Crystal structure of lithium molybdenum purple bronze Li
    0.9    Mo
    6    O
    17    " Journal of Solid State Chemistry, volume 66, issue 1, pages 163–170 doi:10.1016/0022-4596(87)90231-3
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 H. Chen, J. J. Ying, Y. L. Xie, G. Wu, T. Wu and X. H. Chen (2010) "Magnetotransport properties in purple bronze Li
    0.9    Mo
    6    O
    17     single crystal". Europhysics Letters, volume 89, issue 6, article 67010 doi:10.1209/0295-5075/89/67010
  4. ↑ Myung Hwan Whangbo, Enric Canadell (1988), "Band electronic structure of the lithium molybdenum purple bronze Li
    0.9    Mo
    6    O17    ". Journal of the American Chemical Society, volume 110, issue 2, pages 358–363. doi:10.1021/ja00210a006
  5. ↑ P. Chudzinski, T. Jarlborg, and T. Giamarchi (2012), "Luttinger-liquid theory of purple bronze Li
    0.9    Mo
    6    O17     in the charge regime". Physical Review B, volume 86, paper 075147 (19 pages). doi:10.1103/PhysRevB.86.075147
  6. ↑ Martha Greenblatt (1996), "Molybdenum and tungsten bronzes: Low-dimensional metals with unusual properties". In C. Schlenker ed., "Physics and Chemistry of Low-Dimensional Inorganic Conductors" Book, Springer, 481 pages. ISBN 9780306453045
  7. ↑ M. S. da Luz, C. A. M. dos Santos, J. Moreno, B. D. White, and J. J. Neumeier (2007), "Anisotropic electrical resistivity of quasi-one-dimensional Li
    0.9    Mo
    6    O
    17     determined by the Montgomery method" Physical Review B, volume 76, issue 23, paper 233105 (3 pages) doi:10.1103/PhysRevB.76.233105
  8. ↑ Mohamed Boujida, Claude Escribe-Filippini, Jacques Marcus, Claire Schlenker (1988), "Superconducting properties of the low dimensional lithium molybdenum purple bronze Li
    0.9    Mo
    6    O17    ". Physica C: Superconductivity, volumes 153–155, part 1, pages 465–466. doi:10.1016/0921-4534(88)90685-5
  9. ↑ C. A. M. dos Santos, B. D. White, Yi-Kuo Yu, J. J. Neumeier, and J. A. Souza (2007), "Dimensional crossover in the purple bronze Li
    0.9    Mo
    6    O17    " Physical Review Letters, volume 98, issue 26, paper 266405 (4 pages) doi:10.1103/PhysRevLett.98.266405
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