Bond lengths oxyhalides

Fig. 17. Atomization energies, De, (experimental for the 4d and 5d elements and extrapolated from the DFT and RECP calculations for the 6d elements) and optimized bond lengths, Rc, for various halides, oxides and oxyhalides of group-4 through 8 elements [113-115,122-123].

Several studies of the effect of pressure on the radial Bkq crystal field parameters have been reported. Lanthanide oxyhalides (REOX RE = La, Gd, Y X = Cl, Br) doped with Eu + have been extensively studied. The REOX lattices are iso-structural and incorporate Eu + in bonding sites with 4 symmetry. In the oxybromides, Eu " is coordinated to four oxygens and four bromides. The RE-0 bond lengths ( 2.3 - 2.4 A) are significantly shorter than the RE-Br bond lengths ( 3.2-3.3 A). A fifth, more distant bromide ( 3.5-3.9 A) is located on the C4 axis [191]. Eu + in the oxychlorides is coordinated similarly (RE-0 2.3-2.4 A, RE-Cl 3.0-3.2 A), but differs with respect to the position of the fifth chloride. The smaller size of the chlorides allows the fifth, axial chloride to approach more closely Eu " and enter the coordination sphere at a distance of 3.0 - 3.2 A [ 192]. 

From a fundamental point of view, in hosts with multiple centers one can typically assign the various emission/excitation bands to specific sites from bond lengths and/or coordination arguments. However, the variety of stmctures and coordination environments in oxyhalide can make it difihcult to draw further stmcture-property conclusions toward the emission, excitation, and thermal quenching of these materials. 

Table 8 Bond lengths (in A) of halides, oxyhalides and oxides of group 4—8 elements as a result of the 4c-DFT [159-170] and RECP CCSD(T) [171] calculations in comparison with experiment...

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