Lithium molybdenum purple bronze is a
chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
with formula , that is, a
mixed oxide
In chemistry, a mixed oxide is a somewhat informal name for an oxide that contains cations of more than one chemical element or cations of a single element in several states of oxidation.Advanced Inorganic Chemistry, F. A. Cotton, G. Wilkinson ...
of
molybdenum
Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lea ...
and
lithium
Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid el ...
. It can be obtained as flat
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
s with a purple-red color and metallic sheen (hence the "purple bronze" name).
[
This compound is one of several molybdenum bronzes with general formula where A is an ]alkali metal
The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
or thallium
Thallium is a chemical element with the Symbol (chemistry), symbol Tl and atomic number 81. It is a gray post-transition metal that is not found free in nature. When isolated, thallium resembles tin, but discolors when exposed to air. Chemists W ...
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
Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physic ...
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 , and 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
Potassium carbonate is the inorganic compound with the formula K2 CO3. It is a white salt, which is soluble in water. It is deliquescent, often appearing as a damp or wet solid. Potassium carbonate is mainly used in the production of soap and gl ...
solution releasing metallic-purple plate-like crystals, a couple mm wide and less than a mm thick.[
]
Structure
The crystal structure of was determined by Onoda and others through single-crystal X-ray diffraction
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
. The crystal system is monoclinic
In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic s ...
, with approximate unit cell
In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector, for example) does not necessaril ...
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 , except that both are organized in layers. The difference may be explained by the relative sizes of the and ions.[
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 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.][
]
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[ Its properties are most spectacular below 5 meV. The Tomonaga-Luttinger liquid theory has been invoked to explain its anomalous behavior.][
]
Electrical conductivity
At room temperature, Greenblatt and others (in 1984) measured the resistivity
Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...
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.[ The conductivities would be in the ratio 1:250:10,][ 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,][ 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,][ which correspond to conductivity ratios of 1:53:13.][
This anisotropy has been attributed to the crystal structure, specifically to the zig-zag chains of molybdenum and oxygen atoms ][
]
Resistivity and temperature
The resistivity along all three axes increases linearly with temperature from about 30 K to 300 K, as in a metal.[ This is anomalous since such a law is expected above the Debye temperature (= 400 K for this compound)][ The resistivity ratios along the three axes are preserved in that range.][
]
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.[ The transition may be related to the onset of a ]charge density wave
A charge density wave (CDW) is an ordered quantum fluid of electrons in a linear chain compound or layered crystal. The electrons within a CDW form a standing wave pattern and sometimes collectively carry an electric current. The electrons in such ...
.[ 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.][
]
Superconducting state
Lithium molybdenum purple bronze becomes superconductor between 1 and 2 K.[
]
Thermal conductivity
Li0.9Mo6O17, due to spin–charge separation
In condensed matter physics, spin–charge separation is an unusual behavior of electrons in some materials in which they 'split' into three independent particles, the spinon, the orbiton and the holon (or chargon). The electron can always be the ...
, can have a much higher thermal conductivity
The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa.
Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
than predicted by the Wiedemann-Franz law.
[''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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Magnetoresistance
The magnetoresistance
Magnetoresistance is the tendency of a material (often ferromagnetic) to change the value of its electrical resistance in an externally-applied magnetic field. There are a variety of effects that can be called magnetoresistance. Some occur in bulk ...
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.[
]
See also
* Sodium tungsten bronze
Sodium tungsten bronze is a form of insertion compound with the formula Na''x''WO3, where ''x'' is equal to or less than 1. So named because of its metallic lustre, its electrical properties range from semiconducting to metallic depending on the ...
, a golden to purple metallic-looking compound.
References
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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.
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{{Lithium compounds
Molybdenum compounds
Lithium compounds
Oxides