Rare earths energy exchange between

B. Energy Exchange between Rare-Earth Ions 211... 

It appears at this time that one of the most important mechanisms involved in the luminescence of rare earth ions is energy exchange between them. One may clearly differentiate between two distinct mechanisms (a) radiative exchange and (b) nonradiative exchange. In the radiative mechanism, a photon emitted by ion A is captured by ion B. Since the photon has left the A system, the capture of it by B cannot decrease the lifetime of A. However, f the photon is shuttled back and forth between similar or dissimilar ions, the fluorescent lifetime could well be increased by radiation trapping. This is an interesting phenomenon and warrants further discussion. 

In the nonradiative-resonance-exchange process an excited ion transfers its energy to a second ion without the emission of a photon, and with only a very minimal amount of energy lost to vibrations. For this process to occur, some coupling must, of course, exist between the ions. There is some evidence that this may be electrostatic (44,52). It appears that in most cases the ion-ion interaction is sufficiently weak that the energy levels of the rare earths are only slightly altered from their free ion positions. In this regard, one finds it convenient to talk about the system in terms of the states of the uncoupled ions. 

Ananias et al. reported NPs of layered Ln2(Si04H)(0H)2(H20)Cl (where Ln = Eu, Gd, and Tb) and their microcrystalline correspondence with mixed Eu/Gd or Tb/Gd rare earths (Ananias et al., 2008). These materials display energy transfer between different pairs Eu / Gd and Tb /Gd. The PL properties of the mixed Eu /Gd sample change upon E for Cl ion exchange, with potential application for sensing. 

Another characteristic of rare earth ions (except for Ce + and Yb +) in absorption spectra are their linear-like behavior. This comes from f-f transitions where 4f electrons exchange between different 4f energy levels. However, no f-f transition is allowed for Ce +(4f ) or Yb + (4f ). The broad absorption bands observed originates from configuration transitions, for example 4f" to 4f" 5d ... 

Among the resonance mechanisms due to various electrostatic multipole interactions, the dipole-quadrupole interaction gave the best fit between theory and experiment. The transfer due to the exchange interaction was inferred not to be operative. It was finally concluded that the mechanism of the energy transfer between unlike trivalent rare earth ions in ionorganic solids is predominantly governed by the dipole-quadrupole interaction. 

It can be seen from the comparison of the known Neel temperatures between R2C3 and RC2 (Atoji 1978) that in the light rare earth compounds the crystal field effect is often predominant, as exemplified by the fact that praseodymium carbides have exceptionally low values of and strongly suppressed ordered moments (1.14/1b in PrC2). In the heavy lanthanide compounds, the exchange interaction and anisotropy energy become the major factors in the magnetization. 

The rare earth metals exhibit a variety of ordered states from ferromagnetic to complicated antiferromagnetic structures collinear, spiral, helical, conical and fan structures, which can be altered by temperature, magnetic fields or by the application of pressure (Nikitin et al., 1972). These complicated arrangements of spins result from the balance in energy between magnetocrystalline anisotropy and exchange forces (Elliott, 1965). Much pressure work has been done on pure rare earth metals and alloys in the last decade especially, and measurements... 

State (Thole et al. 1985) for all the rare-earth ions and for all ionization states known to be relevant to the solid state. The electrostatic and exchange parameters were all scaled down to 80% of their Hartree-Fock (HF) values. The spin-orbit parameter was adjusted by a factor to correct the energy splitting between 3d /2 and 3d3/2 peaks. The resulting line intensities were broadened by a life-time broadening function for comparison with the observed 3d-4f spectra from metallic rare-earth samples. We omit here details of the experimental and theoretical procedures which can be foimd in the paper by Thole et al. (1985). 

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