Stiff host lattice

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.

Nevertheless, there is also an influence of the host lattice. For example, Te + in Cs2SnClA shows an emission spectrum in which vibrational structure can be observed fFig. 3.3). In ZrPjOy Te +, however, this structure has disappeared and the emission band width has more than doubled. The ion shows vibrational structure in its emission band in ScBOj, but not in LaBO [3J. The disappearance of the vibratioita). structure points to an increase of the value of S or of the offset AR. It has been shown that a stiff surroundings of the luminescent center restricts the value of AR and of S. so that the Stokes shift becomes smaller and vibrational structure may appear in... 

Symmetric polymer blends do not exist in reality. A host erf asymmetries are present in real chemical alloys of interest These include attractive potential asymmetries (present even for isotopic blends) and specific interactions, molecular weight asymmetries and polydispersity, and single chain structural differences between the blend components (e.g., monomer shape and volume, backbone stiffness, and tacticity). Realistic accounting for most of these effects would seem to require an off-lattice description which includes local interchain density and concentration correlations, and compressibility effects [1, 2, 63, 66, 67, 80]. 

---