Host Lattice Contributions

Host properties are extremely influential in most upconversion processes. These influences can be used in advantageous ways, and four examples of very general routes to do so are described here as they apply to specific and unusual upconversion processes. 

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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. 

The standard heat of sorption, Afl°, can then be interpreted in terms of appropriate energy contributions and the corresponding entropy, AS°, in terms of various degrees of freedom of the guest molecules relative to the host lattice (Table II) (10). An example of the use of the above method is given elsewhere in this volume (11). In comparing observed standard entropies with those based on the models of Table II, only empirical methods (12) are available for a priori estimates of the frequencies v. Nevertheless, comparisons have been of considerable interest (10). 

Later, this approach was used for the study of influence of vanadium concentration (from 0.01 to 1.0 mol%) on the EPR linewidth of V4+ ions in Ti02 (rutile) host lattice. Our calculation of values from the dipolar AH[Pg.220]

Host lattices may also contribute to upconversion in more direct ways. A dramatic example of this is in the exchange-induced upconversion phenomenon in 0.1% Yb " RbMnCl3 [77]. The lowest excited state of this host lattice is the Mn " ligand-field dependent spin-forbidden Tj excited state with an origin at ca. 

For most insertion compounds, the interaction of intercalated ions with each other in the host lattice is not negligible. In order to simply consider the contribution of ionic interaction in Equation (5.8), it is often assumed that each ion experiences a mean interaction or energy field from its neighboring ions, based on a mean-field theory [10]. According to this approximation, the contribution to the chemical potential is proportional to the fraction of sites occupied by the ions 5, and hence the interaction term is introduced into Equation (5.8) as... 

Neodymium. Neodymium can be present in relatively high concentrations in fluorapatites. Gaft et al. (2001a) lists Nd analyses for several natural apatites that are higher than any other REE except Ce, and at a concentration level of about 40% of the Ce value. Nd emission is well into the IR, and it is not sensitized by most of the other REE. Hence, Nd emission is expected to be relatively independent of other impurities, and will not contribute to visible luminescence. However, Nd -doped synthetic apatites are excellent laser materials, due to several physical attributes of the Nd electronic structure in the host lattice. Detailed evaluation of the optical properties of Nd in Ba fluorapatite... 

An important contribution to the theoretical model formation originates from the use of vacuum-ultraviolet radiation from a synchrotron source [6). It was shown that photostimulable centers in BaFBr Fu can be created by irradiation into the vacuum-ultraviolet region (> 6.7 eV), i.e. in the excitonic and interband region of the host lattice. 

A better X-ray phosphor with related characteristics is LaOBr Tm. This host has the same crystal structure as BaFCl (Fig. 8.9). However, the density of LaOBr is considerably higher 6.13 g. cm. Rabatin has contributed extensive work on phosphors based on this host lattice. It has been described in several patents [1]. Its emission spectrum is given in Pig. 8.15. It consists of intraconfigurational line transitions of Tm (4/ ) in the near ultraviolet and blue spectral r on (see Sect. 3.3.2). This phosphor is nowadays a commercial product because of its superb properties. 

Fig. 3.3. Illustration of the main proton transfer mechanisms (a) defect mechanism in a densely packed structure (b) loosely packed structure with a high concentration of mobile species (c) quasi-liquid state with a proton jump contribution In (a) the conductivity is favoured by intrinsic (interstitial rabbits) or extrinsic (impurity elephant) point defects. An orientation defect (hippopotamus in the wrong orientation) can also favour disorder of rabbits (Oj for Zr02 CaO, H for KHSO4) (b) the tree sublattice is a perfectly stable loosely packed structure and a high rabbit disorder can exist without affecting the host lattice (e.g. NH4 in p-AljOj) (c) only the mobile species sublattice is considered here these entities are moving with different speeds in different directions and some are hopping such may be the image of a quasi liquid or surface liquid (V205.nH20, HUP).

Similarly to the ESR of the lanthanide ions in insulators, in metallic systems ESR contributes to understanding of the spectroscopic state of the ion in the host lattice and the symmetry and magnitude of the crystalline electric field at the lanthanide site. The g-shift of the resonance may be related to the sf exchange interaction and the spin polarization of the conduction electrons, and the temperature and concentration dependence of the g-shift and resonance linewidth relate to the bottleneck effect in the spin relaxation process. These relationships have been outlined in section 3.5. 

Inclusion polymerization of conducting polymers such as polyaniline or polypynole in layer structures such as FeOCl or V2O5 opens the way to new classes of hybrid systems (M. Kanatzidis). If the host lattice involves redox reactions such as in V2O5. mobile carriers are creat on the host network which contribute to the overall conductivity of the system, to v ing degrees such that n-type, p-type, and metallic behavior can be observed. Aniline could also be oxidatively intercalated into the layered proton conductor HFe(S04>2 XH2O (D. J. Jones). 

The host lattice (//) keeps this integrity through the intercalation reaction, fixing the concentration of sites available for A ions. The chemical potential of the ionic contribution is ... 

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