Lithium molybdenum purple bronze is a chemical compound with formula Li
0.9
Mo
6
O
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
x
Mo
y
O
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
2
MoO
4
, MoO
2
and MoO
3
is maintained in a temperature gradient from 490 to 640 °C oven 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
0.9
Mo
6
O
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
0.9
Mo
6
O
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]

Thermal conductivity

Li0.9Mo6O17, due to spin–charge separation, can have a much higher thermal conductivity than predicted by the Wiedemann-Franz law. [10]

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. ^ a b c d e f Greenblatt, M.; McCarroll, W.H.; Neifeld, R.; Croft, M.; Waszczak, J.V. (1984). "Quasi two-dimensional electronic properties of the lithium molybdenum bronze, Li0.9Mo6O17". Solid State Communications. Elsevier BV. 51 (9): 671–674. doi:10.1016/0038-1098(84)90944-x. ISSN 0038-1098.
  2. ^ a b c d e Onoda, M.; Toriumi, K.; Matsuda, Y.; Sato, M. (1987). "Crystal structure of lithium molybdenum purple bronze Li0.9Mo6O17". Journal of Solid State Chemistry. Elsevier BV. 66 (1): 163–170. doi:10.1016/0022-4596(87)90231-3. ISSN 0022-4596.
  3. ^ a b c d e f g h Chen, H.; Ying, J. J.; Xie, Y. L.; Wu, G.; Wu, T.; Chen, X. H. (2010-03-01). "Magnetotransport properties in purple bronze Li0.9Mo6O17 single crystal". EPL (Europhysics Letters). IOP Publishing. 89 (6): 67010. doi:10.1209/0295-5075/89/67010. ISSN 0295-5075.
  4. ^ Whangbo, Myung Hwan.; Canadell, Enric. (1988). "Band electronic structure of the lithium molybdenum purple bronze Li0.9Mo6O17". Journal of the American Chemical Society. American Chemical Society (ACS). 110 (2): 358–363. doi:10.1021/ja00210a006. ISSN 0002-7863.
  5. ^ Chudzinski, P.; Jarlborg, T.; Giamarchi, T. (2012-08-27). "Luttinger-liquid theory of purple bronze Li0.9Mo6O17 in the charge regime". Physical Review B. American Physical Society (APS). 86 (7): 075147. doi:10.1103/physrevb.86.075147. ISSN 1098-0121.
  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. ^ da Luz, M. S.; dos Santos, C. A. M.; Moreno, J.; White, B. D.; Neumeier, J. J. (2007-12-21). "Anisotropic electrical resistivity of quasi-one-dimensional Li0.9Mo6O17 determined by the Montgomery method". Physical Review B. American Physical Society (APS). 76 (23): 233105. doi:10.1103/physrevb.76.233105. ISSN 1098-0121.
  8. ^ Boujida, Mohamed; Escribe-Filippini, Claude; Marcus, Jacques; Schlenker, Claire (1988). "Superconducting properties of the low dimensional lithium molybdenum purple bronze Li0.9Mo6O17". Physica C: Superconductivity. Elsevier BV. 153–155: 465–466. doi:10.1016/0921-4534(88)90685-5. ISSN 0921-4534.
  9. ^ dos Santos, C. A. M.; White, B. D.; Yu, Yi-Kuo; Neumeier, J. J.; Souza, J. A. (2007-06-28). "Dimensional Crossover in the Purple Bronze Li0.9Mo6O17". Physical Review Letters. American Physical Society (APS). 98 (26): 266405. doi:10.1103/physrevlett.98.266405. ISSN 0031-9007.
  10. ^ Wiedemann-Franz Law: Physicists break 150-year-old empirical laws of physics,Gross violation of the Wiedemann–Franz law in a quasi-one-dimensional conductor Wakeham et al. 2011