Spin transition

Spin transition

The spin transition is one of the most spectacular example of transitionbetween two electronic states in molecular chemistry. The ability of to evolve from a stable to another stable (or metastable) electronicstate in a reversable and detectable fashion, makes these molecular systems appealing in the field of molecular electronics.When a transition metal ion of configuration d^{n}, n=4 to 7,is in octahedral surroundings, its ground state may be low spin (LS)or high spin (HS), depending to a first approximation on the magnitudeof the Delta energy gap between e_{g} and t_{2g} metal orbitalsrelative to the mean spin pairing energy P (see Crystal field theory). More precisely, forDelta>>P, the ground state arises from the configuration wherethe d electrons occupy first the t_{2g} orbitals of lower energy,and if there are more than six electrons, the e_{g} orbitals ofhigher energy. The ground state is then LS. On the other hand, forDelta<< P, Hund's rule is obeyed. The HS ground state has gotthe same multiplicity as the free metal ion. If the values of Pand Delta are comparable, a LS↔HS transition may occur.

Between all the possible d^{n} configurations of the metal ion,d^{5} and d^{6} are by far the most important. The spin transitionphenomenon, in fact, was first observed in 1930 for tris(dithiocarbamato)iron(III) compounds. On the other hand, the iron(II) spin transitioncomplexes were the most extensively studied: among these twoof them may be considered as archetypes of spin transition systems,namely Fe(NCS)2(bipy)2 and Fe(NCS)2(phen)2(bipy = 2,2'-bypiridine and phen = 1,10-phenanthroline).

We discuss the mechanism of the spin transition by focusing on thespecific case of iron(II) complexes. At the molecular scale the spin transition corresponds to an interionicelectron transfer with spin flip of the transferred electrons. Foran iron(II)compound this transfer involves two electrons and the spinvariations is Delta S=2. The occupancy of the e_{g} orbitalsis higher in the HS state than in the LS state and these orbitalsare more antibonding than the t_{2g}. It follows that the averagemetal-ligand bond length is longer in the HS state than in the LSstate. This difference is in the range 0.14-0.24 Åfor iron(II) compounds.

The most common way to induce a spin transition is to change the temperature of the system: the transition will be then characterized by a ho_{H}=f(T), where ho_{H}is the molar fraction of molecules in high-spin state. Several techniquesare currently used to obtain such curves. The simplest method consistsof measuring the temperature dependence of molar susceptibility. Anyother technique that provides different responses according to whetherthe state is LS or HS may also be used to determine ho_{H}. Amongthese techinques, Mössbauer spectroscopy has been particularly usefulin the case of iron compounds, showing two well resolved quadrupoledoublets. One of these is associated with LS molecules, the otherwith HS molecules: the high-spin molar fraction then may be deducedfrom the relative intensities of the doublets.

Various types of transition have been observed. This may be abrupt,occuring within a few Kelvin range, or smooth, occurring within alarge temperature range. It could also be incomplete both at low temperatureand at high temperature, even if the latter is more often observed.Moreover, the ho_{H}=f(T) curves may be strictly identical inthe cooling or heating modes, or exhibit a hysteresis: in this casethe system could assume two different electronic states in a certainrange of temperature. Finally the transition may occur in two steps.


Wikimedia Foundation. 2010.

Игры ⚽ Нужно решить контрольную?

Look at other dictionaries:

  • TRANSITION (MÉTAUX DE) — Les cinquante six éléments chimiques dits de transition actuellement connus comportent tous, soit à l’état d’élément simple, soit dans un état oxydé stable, une sous couche électronique d ou f partiellement remplie. L’existence de niveaux… …   Encyclopédie Universelle

  • Transition de spin-peierls — La transition de spin Peierls est observée dans certains matériaux antiferromagnétiques quasi unidimensionnels. En dessous de la température de transition TSP, une distorsion du réseau cristallin apparaît, tandis qu un gap Δ(T) se développe dans… …   Wikipédia en Français

  • Spin density wave — (SDW) and charge density wave (CDW) are names for two similar low energy ordered states of solids. Both these states occur at low temperature in anisotropic, low dimensional materials or in metals that have high densities of states at the Fermi… …   Wikipedia

  • Transition berezinsky-kosterlitz-thouless — En physique théorique la transition de Berezinky Kosterlitz Thouless est une transition de phase obtenue dans le modèle XY à deux dimensions (ou modèle planaire) en variant la température. Sommaire 1 La transition BKT 2 Autres modèles et… …   Wikipédia en Français

  • Transition isomerique — Transition isomérique La transition isomérique est un mode de désintégration radioactive par lequel un isomère nucléaire libère tout ou partie de son énergie d excitation sans subir de transmutation : les nucléons se réorganisent au sein du… …   Wikipédia en Français

  • transition element — Chem. any element in any of the series of elements with atomic numbers 21 29, 39 47, 57 79, and 89 107, that in a given inner orbital has less than a full quota of electrons. Also called transition metal. [1920 25] * * * Any chemical element with …   Universalium

  • Transition metal — The term transition metal (sometimes also called a transition element) has two possible meanings: The IUPAC definition[1] states that a transition metal is an element whose atom has an incomplete d sub shell, or which can give rise to cations… …   Wikipedia

  • Spin structure — In differential geometry, a spin structure on an orientable Riemannian manifold allows one to define associated spinor bundles, giving rise to the notion of a spinor in differential geometry. Spin structures have wide applications to mathematical …   Wikipedia

  • Spin quantum number — In atomic physics, the spin quantum number is a quantum number that parameterizes the intrinsic angular momentum (or spin angular momentum, or simply spin) of a given particle. The spin quantum number is the fourth of a set of quantum numbers… …   Wikipedia

  • Transition de spin-Peierls — La transition de spin Peierls est observée dans certains matériaux antiferromagnétiques quasi unidimensionnels. En dessous de la température de transition TSP, une distorsion du réseau cristallin apparaît, tandis qu un gap Δ(T) se développe dans… …   Wikipédia en Français

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”