Lithium molybdenum purple bronze

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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]

Contents

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/cm 3. 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

Li0.9Mo6O17, 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

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References

  1. 1 2 3 4 5 6 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. 51 (9). Elsevier BV: 671–674. Bibcode:1984SSCom..51..671G. doi:10.1016/0038-1098(84)90944-x. ISSN   0038-1098.
  2. 1 2 3 4 5 Onoda, M.; Toriumi, K.; Matsuda, Y.; Sato, M. (1987). "Crystal structure of lithium molybdenum purple bronze Li0.9Mo6O17". Journal of Solid State Chemistry. 66 (1). Elsevier BV: 163–170. Bibcode:1987JSSCh..66..163O. doi:10.1016/0022-4596(87)90231-3. ISSN   0022-4596.
  3. 1 2 3 4 5 6 7 8 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). 89 (6). IOP Publishing: 67010. arXiv: 0906.3855 . Bibcode:2010EL.....8967010C. doi:10.1209/0295-5075/89/67010. ISSN   0295-5075. S2CID   122903841.
  4. ↑ Whangbo, Myung Hwan.; Canadell, Enric. (1988). "Band electronic structure of the lithium molybdenum purple bronze Li0.9Mo6O17". Journal of the American Chemical Society. 110 (2). American Chemical Society (ACS): 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. 86 (7): 075147. arXiv: 1205.0239 . Bibcode:2012PhRvB..86g5147C. doi:10.1103/physrevb.86.075147. ISSN   1098-0121. S2CID   53396531.
  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. 76 (23). American Physical Society (APS): 233105. Bibcode:2007PhRvB..76w3105D. 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. 153–155. Elsevier BV: 465–466. Bibcode:1988PhyC..153..465B. 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. 98 (26). American Physical Society (APS): 266405. Bibcode:2007PhRvL..98z6405D. doi:10.1103/physrevlett.98.266405. ISSN   0031-9007. PMID   17678113.
  10. ↑ Chudzinski, P.; Berben, M.; Xu, Xiaofeng; Wakeham, N.; Bernáth, B.; Duffy, C.; Hinlopen, R. D. H.; Hsu, Yu-Te; Wiedmann, S.; Tinnemans, P.; Jin, Rongying; Greenblatt, M.; Hussey, N. E. (2023-11-17). "Emergent symmetry in a low-dimensional superconductor on the edge of Mottness". Science. 382 (6672): 792–796. doi:10.1126/science.abp8948. ISSN   0036-8075.
  11. ↑ Tomé, César (2023-11-21). "Purple bronze, from insulator to superconductor and back". Mapping Ignorance. Retrieved 2023-12-02.
  12. ↑ 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
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