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 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
_0.9Mo
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] In 2023 it has been suggested that the strange behaviour could be by emergent symmetry (in contrast to symmetry breaking) from interference between the conduction electrons and dark excitons^[10][11]

Superconducting state

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

Thermal conductivity

Li_0.9Mo₆O₁₇, due to spin–charge separation, can have a much higher thermal conductivity than predicted by the Wiedemann-Franz law. ^[12]

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

• Sodium tungsten bronze Na
  _xWO
  ₃, a golden to purple metallic-looking compound.
• Magnetochromism

References