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'''Lithium molybdenum purple bronze''' is a [[chemical compound]] with formula {{chem|Li|0.9|Mo|6|O|17}}, that is, a [[mixed oxide]] of [[molybdenum]] and [[lithium]]. It can be obtained as flat [[crystal]]s with a purple-red color and metallic sheen (hence the "purple bronze" name).<ref name=Greenblatt/><ref name=Onoda/>

This compound is one of several [[molybdenum bronze]]s with general formula {{chem|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 [[isotropic|anisotropy]] that makes it a "quasi-1D" conductor, and a metal-to-insulator transition as it is cooled below 30 [[kelvin|K]].

==Preparation==
The compound was first obtained by Greenblatt and others by a temperature gradient flux technique. In a typical preparation, a soichometric melt of {{chem|Li|2|MoO|4}}, {{chem|MoO|2}} and {{chem|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 relasing metallic-purple plate-like crystals, a couple [[millimetre|mm]] wide and less than a mm thick.<ref name=Greenblatt/><ref name=Chen/> 

==Structure ==
<!--NOTE: Greenblatt and Chen swap the ''a'' and ''c'' axes relative to Onoda ->
The crystal structure of {{chem|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 [[nanometre|nm]], ''b'' = 0.5523 nm, and ''c'' = 0.9499 nm, with angle β = 90.61°, volume ''V'' = 0.6695 nm<sup>3</sup> and Z = 2.  In typical crystals, ''a'' is the shortest dimension (perpendicular to the plates) and ''b'' the longest. The density is 4.24 [[gram|g]/[[centimetr|cm]]<sup>3</sup>.  The structure is rather different from that of [[potassium molybdenum purple bronze]] {{chem|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 {{chem|K|+}}  and {{chem|Li|+}} ions.<ref name=Onoda/><ref name=Greenblatt/>

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 {{chem|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.<ref name=Onoda/>

==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<ref name=Whangbo88/> Its properties are most spectacular below 5 meV.  The [[Tomonaga-Luttinger liquid]] theory has been invoked to explain its anomalous behavior. <ref name=Chudz/>

=== 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 [[ohm centimetre|Ω cm]], 0.0095 Ω cm, and on the order of 0.25 Ω cm, respectively.<ref name=Greenblatt/> The [[conductivity|conductivities]] would be in the ratio 1:250:10,<ref name=GreenMoTu/><ref name=Onoda/> which would make this compound an almost one-dimensional conductor.  However, Da Luz and others (2007) measured 0.079, 0.050, and 0.018 Ω cm, respectively,<ref name=daLuz> which corrresponds 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,<ref name=Chen/> which correspond to coductivity ratios of 1:53:13.<ref name=Chen/> 

This anisotropy has been attributed to the crystal structure, specifically to the zig-zag chains of molybdenum and oxygen atoms <ref name=Onoda/>

=== Resistivity and temperature ===

The resistivity along all three axes increases linearly with temperature from about 30 K to 300 K, as in a metal.<ref name=Chen/> This is anomalous since such a law is expected above the Debye temperature (= 400 K for Li0.9Mo6017)<ref name=Boujida/>  The resistivity ratios along the three axes are preserved in that range.<ref name=Chen/> 

=== 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 anisotrpoy is partially restored if a magnetic field is applied perpendicular to the ''b'' axis.<ref name=Chen/> The transition may be related to the onset of a [[charge density wave]].<ref name=Greenblatt/>  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.  The theory of Luttinger liquids then predicts such behavior.  Anyway, as of 2010 there was no consensus explanation for this transition.<ref name=Chen/>

=== Superconducting state === 
Lithium molybdenum purple bronze becomes [[superconductor]] between 1 and 2 [[kelvin|K]].<ref name=Greenblatt/> 

=== 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.<ref name=Chen/>


==See also==
* [[Sodium tungsten bronze]] {{chem|Na|''x''|WO|3}}, a golden to purple metallic-looking compound.

==References==
<references>

<ref name=Greenblatt>
 M. Greenblatt, W. H. McCarroll, R. Neifeld, M. Croft, J. V. Waszczak (1984), "Quasi two-dimensional electronic properties of the lithium molybdenum bronze, {{chem|Li|0.9|Mo|6|O17}}". Solid State Communications, volume 51, issue 9, pages 671–674. {{doi|10.1016/0038-1098(84)90944-X}}
</ref>

<ref name=Whangbo88>
  Myung Hwan Whangbo, Enric Canadell (1988), "Band electronic structure of the lithium molybdenum purple bronze {{chem|Li|0.9|Mo|6|O17}}". Journal of the American Chemical Society, volume 110, issue 2, pages 358–363. {{doi|10.1021/ja00210a006}}
</ref>

<ref name=GreenMoTu>
  Martha Greenblatt (), "Molybdenum and tungsten bronzes: Low-dimensional metals with unisial properties". In C. Schlenker ed. (1996), "Physics and Chemistry of Low-Dimensional Inorganic Conductors" Book, Springer, 481 pages. ISBN 9780306453045
</ref>

<ref name=Boujida>
  Mohamed Boujida, Claude Escribe-Filippini, Jacques Marcus, Claire Schlenker (1988), "Superconducting properties of the low dimensional lithium molybdenum purple bronze {{chem|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}}
</ref>

<ref name=Santos>
  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 {{chem|Li|0.9|Mo|6|O17}}" Physical Review Letters, volume 98, issue 26, paper 266405 (4 pages) {{doi|10.1103/PhysRevLett.98.266405}}
</ref>

<ref name=Chudz>
  P. Chudzinski, T. Jarlborg, and T. Giamarchi (2012), "Luttinger-liquid theory of purple bronze {{chem|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}}
</ref>

<ref name=Onoda>
 M. Onoda, K. Toriumi, Y. Matsuda, M. Sato "Crystal structure of lithium molybdenum purple bronze {{chem|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}}
</ref>

<ref name=Chen>
 H. Chen, J. J. Ying, Y. L. Xie, G. Wu, T. Wu and X. H. Chen (2010) "Magnetotransport properties in purple bronze {{chem|Li|0.9|Mo|6|O|17}} single crystal". Europhysics Letters, volume 89, issue 6, article 67010 {{doi|10.1209/0295-5075/89/67010}}
</ref>

<ref name=daLuz>
 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 {{chem|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}}
</ref>

</references>