Host lattices, rare earth

The divalent rare-earth ion Eu has the 4f electronic configuration at the ground states and the 4f 5d electronic configuration at the excited states. The broadband absorption and luminescence of Eu are due to 4f - 4 f 5d transitions. The emission of Eu is very strongly dependent on the host lattice. It can vary from the ultraviolet to the red region of the electromagnetic spectrum. Furthermore, the 4f-5d transition of Eu decays relatively fast, less than a few microseconds [33]. 

It has been noted that the conductivity and activation energy can be correlated with the ionic radius of the dopant ions, with a minimum in activation energy occurring for those dopants whose radius most closely matches that of Ce4+. Kilner et al. [83] suggested that it would be more appropriate to evaluate the relative ion mismatch of dopant and host by comparing the cubic lattice parameter of the relevant rare-earth oxide. Kim [84] extended this approach by a systematic analysis of the effect of dopant ionic radius upon the relevant host lattice and gave the following empirical relation between the lattice constant of doped-ceria solid solutions and the ionic radius of the dopants. 

It is difficult to predict the effect of surface functionalization on the optical properties of nanoparticles in general. Surface ligands have only minor influence on the spectroscopic properties of nanoparticles, the properties of which are primarily dominated by the crystal field of the host lattice (e.g., rare-earth doped nanocrystals) or by plasmon resonance (e.g., gold nanoparticles). In the case of QDs, the fluorescence quantum yield and decay behavior respond to surface functionalization and bioconjugation, whereas the spectral position and shape of the absorption and emission are barely affected. 

The materials used as up-converters consist of a rigid host lattice with rare earth dopants. Fluorides and oxides have been used as host lattices, the latter being slightly... 

Rare earth consumption could, however, be affected by a change in host lattice system, with Eu3+ still being retained as activator. Either cost or luminescence efficiency could drive such a change. Costs could be decreased either by eliminating rare earth host lattice cations or by a decrease in the required concentration of Eu. However, in spite of considerable research effort, new host systems that accomplish these objectives have not been found. 

Most of the successful rare earth activated phosphors comprise host lattices in which the host cation is also a rare earth. A principal reason for this relates to the optical inertness of La, Gd, Y, and Lu this is essential to avoid interference with activator emission spectra. Close chemical compatibility including amenability to substitutional Incorporation of rare earth activators are also essential features. Rare earth hosts such as oxides, oxysulfides, phosphates, vanadates and silicates also tend to be rugged materials compatible with high temperature tube processing operations and salvage. 

The most important activators for sulfide phosphors are copper and silver, followed by manganese, gold, rare earths, and zinc. The charge compensation of the host lattice is effected by coupled substitution with mono- or trivalent ions (e.g., Cl or Al3+). In addition, disorders, such as unoccupied sulfur positions, can also contribute to charge compensation. 

Oxyhalides. The oxyhalides of yttrium, lanthanum, and gadolinium are good host lattices for activation with other rare-earth ions such as terbium, cerium, and thulium. The use of LaOCl Tb3+ as the green component in projection-television tubes has been discussed [5.419]. LaOBr Tb3+ and LaOBr Tm3+ exhibit high X-ray absorption, and they are used in X-ray intensifying screens [5.420]. 

Of the various crystal matrices, the cubic calcium fluoride (CaF2), the lanthanum trihalides (LaFs, LaCl3, LaBr3) and ethylsulphate are popular host lattices. The spectra of rare earths with partly filled /-shells in doped crystals consist of very sharp lines, similar to those in atomic spectra, of closely spaced groups. Fig. 20 gives a summary of the crystal... 

Valuable insight has been obtained by measuring the spectra of rare-earth ions in various host lattices. However, with respect to the relation of energy levels and host lattice structure, the presence of different ligands and local distortions around the impurity ion in each host lattice and the limited number of isostructural crystals available for spectroscopic studies restrict the empirical information thus obtainable. 

The aim of this chapter is to present a review of the high pressure optical studies on rare-earth ions in non-metallic compounds. Other methods, as for example neutron scattering, magnetic resonance techniques or MoBbauer spectroscopy will not be considered here, unless they provide additional valuable information to the optical studies. It will be demonstrated that the problem of host lattice structural dependence of 4f/v states can be effectively tackled by high pressure techniques and hopefully the interest for further, more refined high pressure studies of this problem can be stimulated. 

The unique properties of rare earth ions are (i) the spectral positions of the emission lines are independent of the host lattice, (ii) some of the ions, Tb3+, Eu3+ emit at spectral positions, enabling high lumen efficacies along with a very good quality of white light. 

So far neither work of phosphor films via the Pechini-type sol-gel proeess nor their direct patterning via soft lithography have been reported in literature. Yttrium vanadate (YVO4) has been shown to be a useful host lattice for rare earth ions to produce phosphors emitting a variety of coIors[7]. In this paper, we report a Pechini sol-gel synthesis of the nanocrystalline YV04 A (A=Eu, Sm, Er ) thin phosphor films and their... 

A second commonly invoked kinetic model for ETU processes is the so-called Dimer Model [22,23], in which pairs are treated as distinct isolated entities. Such scenarios are often encountered when trivalent rare-earth metal ions are substituted into divalent host lattices of the CsNiClj type. In these hosts, only small concentrations of M + ions can typically be incorporated, and it has been shown that more than 90% of all ions are introduced as (M + - vacancy - M +) pairs to satisfy charge compensation requirements [24, 25]. The ions are thus introduced as isolated pairs. In this model, three excitation populations are con-... 

Table III shows for a series of borates how the Stokes shift (i.e., AQ) increases if the size of the host lattice cation increases (100). In ScBOa the rare-earth ions are strongly compressed and the surroundings are stiff. Small Stokes shifts result for Ce, Pr, and Bi, but not for the smaller Sb . Note, however, that the Stokes shift of the 4/ -5d transitions is less sensitive to the surroundings than that of the 5s-5p transitions. If the data of Table III are extrapolated to, for example, borate glasses, it can be concluded that we find no efficient Sb or Bi emission, but for or Pr this may still be the case. This is what has been observed experimentally.

The magnetic field at Eu + ions measured by the 103-keV resonance in rare-earth garnets doped with 2-5% Sm (M3FesOi2, M = Gd, Tb, Dy, Ho, Er, Eu, Tm, Yb, Lu, Y) increases by only 8% in that order [68]. Identical fields in YIG and LuIG in which the rare earth is diamagnetic show that the rare-earth-iron exchange interaction is insensitive to the lattice parameters which differ by 1%. However, in the series as a whole, the Eu - -/rare-earth exchange interaction does alter and the field shows a linear dependence with the spin moment of the host, = (gj — ). As already seen for the... 

As should be apparent by now, the rare earths are one of the few cases where such Stokes processes can be studied at room temperature. If we were to study phonon processes using other activators, we would find that we must study them at 4.2 K. We would also find that the phonon spectra were rather complex and difficult to interpret, instead of being simple. Now we will address the so-called "Anti-Stokes phosphors where the dominant mechanism is absorption of several phonons to produce a single photon of higher energy. We will find that only certain trivalent lanthanides eire suitable when combined with a selected host lattice if we wish to have high efficiency, i.e.- "brightness". 

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