Skutterudite

Binary skutterudites

Crystalline structure

Binary skutterudites of the MX₃ form (with M a Metal of transition from the column of the Cobalt, and X a Pnictogène: Phosphorus, Arsenic or Antimoine) derives directly from the natural mineral CoAs₃. Their crystallographic structure, defining a new type, was established in 1928 by Oftedal. They crystallize in the Groupe of space Im3 (n°204 group). The crystalline structure makes up of under simple network cubic of atoms M occupying the crystallographic positions 8c of the elementary mesh (1/4, 1/4, 1/4), atoms X occupant the crystallographic positions 24g (0, there, Z). The cubic mesh centered contains 32 Atome S as shown in the figure opposite.

Knowing the Group of space, the crystallographic structure is completely determined by the knowledge of the three parameters there, Z (positions of the Atome S X in the elementary mesh) and has (cell parameter). It can then be described in two complementary ways:

the figure of left shows the structure skutterudite while placing an atom M (site 8c) in the beginning of the elementary mesh. In this representation, the whole of the positions taken by the Métal of transition M constitutes a simple network cubic. In the center of six cubes out of eight is a rectangular ring of Pnictogène S. These rings are perpendicular to the three crystallographic directions (100), (010) and (001). The last two cubes are unoccupied and correspond to the crystallographic site 2a. This representation makes it possible to highlight the chemical bonds X-X.

the figure of right-hand side shows the structure skutterudite while placing the site 2a (unoccupied) in the beginning of the mesh. This representation can be described like a simple network cubic metal of transitions, each one being located at the center of a Octaèdre deformed pnictogene. It thus highlights the chemical bonds MX, and one then sees clearly appearing a bulky cage between eight octahedral MX₆, centered in 2a.

This second representation makes it possible to obtain a “genealogy” of the structure skutterudite which can be conceived like resultant of a loss of symmetry of the structure Perovskite ReO₃ (Groupe of space Pm3m, n°221). This one consists of atoms of Rhénium located at the center of Octaèdre S regular, two octahedral close relations close sharing a top with an angle Re-O-Re to 180° (figure below). The passage of the cubic structure lacunar ReO₃ with the lacunar cubic structure of CoX₃ is done in " tiltant" octahedral MX₆, with the passage a doubling in each direction of the size of the elementary mesh. Same manner as the structure Perovskite, the structure skutterudite then reveals bulky cages corresponding to the sites 2a of the crystallographic structure.

Electric and magnetic properties

Binary skutterudites not doped, in the absence of on-stoichiometry or of under-stoichiometry in Antimony, are Semi-conducteur S of the type p (conduction by holes). Their gap (or forbidden band) is very weak, about 0.05 eV with 0.2 eV according to the composition. Property more remarkable of skutterudites binary, which was at the origin of the interest which is carried to them in the field of the Thermoélectricité, is the exceptionally high value of the Mobilité of the charge carrier which reaches 2000 cm ². V^-1.s^-1 in CoSb₃ and even 1.104 cm ². V^-1.s^-1 in RhSb₃. This very large Mobility of the charge carrier with for corollary a electric Conductivity σ very high for Semiconductor S (Resistivity lower than 1 mΩ.cm). Binary skutterudites have moreover values of thermoelectric capacity S (or Seebeck coefficient) raised, higher than 100 µV.K^-1, in agreement with their state Semi-conducteur. They thus have thermoelectric power-factors σS ² very high. Binary skutterudites are diamagnetic compounds (magnetic absence of moment). It is possible to carry out dopings on the site of the Métal of transition by elements from the column of the Fer or Nickel or on the site of the Pnictogène by elements of the column of the tin or Tellure in order to modify the electronic properties and to obtain compounds of the type N (conduction by electron S) like type p (conduction by holes). For example in the case of CoSb₃, it is possible to obtain a doping of the type p in substituent Co by Fe. The doping of the type N can be obtained by substitution of Co by Nor, Pd or Pt or of Sb by Te or . Doping tends to decrease at the same time the electric Résistivité and the coefficient Seebeck.

Thermal properties

The compounds of the family of binary skutterudites have thermal conductivities relatively low. This is due to the great number of Atome S in the elementary Maille as with the fact that this one is mainly made up of heavy atoms, in particular for skutterudites containing Antimoine. She is about 100 mW.cm ^-1.K^-1, that is to say approximately 40 times lower than that of the Cuivre. The greatest part of this thermal Conductivité is due to the vibrations of the crystal lattice (Phonon S), for approximately 90%, and approximately 10% with the charge carriers (electrons and holes). She is very definitely reduced in the solid solutions such as for example Ir_0.5Rh_0.5Sb₃.

Skutterudites filled

The Chemist S Jeitschko and Braun showed in 1977 that it is possible to insert a ground-rare in the cage of the site 2a of binary skutterudite (between octahedral MX₆) to form a ternary skutterudite of " type; skutterudite filled " or " filled skutterudite". This filling of the empty cage is made possible by the concomitant substitution of the Métal of transition by a element having a electron from less (Fer, Ruthénium or Osmium) to compensate for the electrons brought by the electropositive Ion . Since then, it was shown that many other elements can be inserted in the structure: alkaline-earth, Thorium, Uranium, Sodium or Potassium, Thallium, etc

Crystalline structure

Skutterudites filled have a crystallographic structure similar to that of binary skutterudites (see figure): they crystallize in the Groupe of space Im3, with the electropositive ion in site 2a (0, 0,0) of the group of space, the Métal of transition in site 8c (1/4, 1/4, 1/4) and the Pnictogène in site 24g (0, there, Z). The general formula of a skutterudite filled is thus R₂M₈X₂₄, or in a more common way RM₄X₁₂ by considering the unit mesh Rhombohédrique. The size of the cage is large compared to the rays of the electropositive Ion S being able to be inserted, and this particularly in skutterudites containing Antimoine. There is thus no sharp variation of the Cell parameter with the filling (it passes from 9,035 Å in CoSb₃ to 9,135 Å in CeFe₄Sb₁₂, that is to say an increase about the percent), and the electropositive ion is slightly related to its environment. This results in an amplitude of important vibration of the ion around its position of balance. It is thus not possible to insert too small ions, they would not be related enough to the cage. Although there is one crystallographic site for the ground-rare Atome of , the rate of filling of the cage is never equal to one, and the structure should rather be described by the general formula R_yM₄X₁₂. The cause of this fractional filling is not known at present. The rate of filling depends there on the chemical composition and the technique of synthesis of the Cristal.

Electric properties

We saw that binary skutterudites are compounds Semi-conducteur S. Any variation of the number of electron S in the system will thus involve an evolution towards a state more Métal lic. Let us take the example of a skutterudite filled RFe₄Sb₁₂. The Iron having a electron of valence of less than the Cobalt, the passage of CoSb₃ (or Co₄Sb₁₂) with corresponds to the loss of four electron S. the electropositive Ion inserted in the cage being in a state of valence n+, N electron S are provided to the system to compensate for the electronic holes. Thus in the case of ground-rare trivalent (3+), it will remain an electronic hole by formula unit in the system, which will be thus metal. It is in fact the case of the majority of skutterudites filled, the atoms being able to be inserted in the cage being for the majority divalent (2+) or trivalent (3+). Certain skutterudites filled present even a superconductive state at low temperature.

Magnetic properties

Skutterudites filled have a very large variety of magnetic properties according to the chemical composition. Their Magnétisme is consisted of the sum of two contributions: a contribution of the network, and a contribution of the electropositive Ion . These two contributions can be independent or coupled according to the composition. There exist thus skutterudites filled:

* paramagnetic

* ferromagnetic

* antiferromagnetic

* heavy fermions (Effect Kondo)

* superconductive

Thermal properties

Skutterudites filled have a thermal Conductivité appreciably lower than binary skutterudites. Indeed, the electropositive Ion being inserted in a cage much larger than him, it can vibrate in an incoherent way with a great amplitude around its position of balance. These vibrations prevent the propagation of the Phonon S, by a mechanism which is not included/understood at present.

Thermoelectric properties

Skutterudites filled are promising compounds for applications of generation of electricity by Thermoelectric effect. They have high figures of thermoelectric merit indeed. Those can in addition be very clearly to increase by carrying out substitutions (doping) on the sites M and X, which makes it possible to optimize the electric properties (electric Conductivité and coefficient Seebeck. It is also possible to decrease the rate of filling of the cage, which makes it possible to increase the disorder of the structure and to decrease the electric conductivity. These skutterudites, known as then partially filled , appear among the most promising materials studied currently in laboratories for applications in a temperature range of about 300 to 500 °C.

See too

* Thermoelectricity

* Magnetism

* crystalline Structure

External bonds

For more information, it is possible to consult the thesis of doctorate " Study of skutterudites therare ones (R) and metals D (M) of the RM₄Sb₁₂ type: new thermoelectric materials for the generation of electricity. " , available on line on http://tel.ccsd.cnrs.fr/

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