Crystal field transitions

Consider first blue sapphire Al203 Ti(III), Fe(III) (Fig. 2). In the absence of Fe(III) the absorption spectrum is easy to interpret. The weak band with a maximum at about 500 nm is due to the t2 —> e crystal-field transition on Ti(III) (3d ), the strong band at 2<280nm is due to a Ti(III)-0( — II) LMCT transition. The absorption band in the region around 700 nm in the case of the codoped crystal cannot be due to Fe(III). It has been ascribed to MMCT, i.e. to a transition within an iron-titanium pair ... 

An interesting case is the optical absorption of M(II)-doped MgTi205 [33]. The spectra of interest are given in Fig. 3. The undoped MgTi205 shows a strong optical absorption which starts at about 320 nm. This is due to the 0( - II)-Ti(IV) LMCT transition. The spectra of MgTijOj doped with Mn(II), Fe(II), Co(II) and Ni(II) show considerable additional absorption in the visible. Only Co(II) and Ni(II) are expected to show spin-allowed crystal-field transitions in this spectral range [14]. These are in fact observed (see Fig. 3) ... 

The important feature of magnetic insulators is that, being nonmetallic, they have a band gap and possess unpaired electrons. They show crystal-field transitions due to the presence of open-shell (d") ions. Mott proposed that electron repulsion can be responsible for the breakdown of the normal band properties of transition-metal... 

Consider first of all the so-called crystal field transitions these involve moving the b electron to the e, bi, and Iai M.O. s, which are of course essentially the 3d metal orbitals, resulting in E(I), Bi, and Ai exdted states, respectively. To a good approximation dectron repulsion... 

The vibronic transitions in the intraconfigurational dn (crystal-field) transitions have been discussed at length elsewhere (see e.g. Ref. [1]). We concentrate here on new emissions situated in the infrared spectral region and on information from vibronic spectra on distortions in the excited state. 

On the other hand, crystal-field transitions take place between a pair of orbitals and so they are forbidden. Why they occur at all, although with diminished intensity, is just what I want to discuss after I have finished with the charge transfer states. 

With this set of identities, we may express the ath vector component of the vibronic crystal field transition dipole moment as given below ... 

Figure 3.16 Energy level diagram for ferric iron matched to spin-forbidden crystal field transitions within Fe3+ ions, which are portrayed by the polarized absorption spectra of yellow sapphire (adapted from Ferguson Fielding, 1972 Sherman, 1985a). Note that the unassigned band at -17,600 cm-1 represents a paired transition within magnetically coupled Fe3+ ions located in adjacent face-shared octahedra in the corundum structure.

Taking into account all of the factors influencing intensities of crystal field spectra discussed so far, the following generalizations may be made. Transitions of 3d electrons within cations in octahedral coordination are expected to result in relatively weak absorption bands. Intensification occurs if the cation is not centrally located in its coordination site. In tetrahedral coordination, the intensities of crystal field transitions should be at least one-hundred times larger than those in octahedrally coordinated cations. Spin-forbidden transitions are usually about one-hundred times weaker than spin-allowed transitions in centrosymmetric, octahedrally coordinated cations, but become... 

January garnet e.g. rhodolite (Mg,Fe)3Al2Si30I2 red Fe2+ Crystal field transition in Fe2+ in distorted cubic... 

CajfFe. AlljSijOu (eight-coordinated) site. Crystal field transition in Fe3+ in octahedral site... 

May emerald Be3Al2Si60js green Cr3- Crystal field transitions in... 

June alexandrite (chrysoberyl) Al2Be04 red/green Cr3+ Crystal field transitions in Cr+ concentrated in non-centrosymmetric distorted six-coordinated site. 

August peridot (olivine) (Mg.Fe SiO, yellow-green Fe2 Crystal field transitions in Fe" in two distorted six-coordinated sites. 

Figure 4.10 Polarized absorption spectra of ruby (from Bums, 1984). The ruby formula is (AIq 99gCr0 002)203, and the spectra originate from crystal field transitions within Cr3+ replacing Al3+ ions in trigonally distorted octahedral sites in the trigonal corundum structure. Consequently, the spectra differ slightly for light polarized (a) parallel (Ellc) and (b) perpendicular (E c) to the c crystallographic axis. The group theoretical assignments of the absorption bands are also indicated. [Reproduced from Chemistry in Britain, 1984, p. 1004]...

The absorption bands at 18,450 cm-1 and 20,300 cm-1 (fig. 4.16c) represent crystal field transitions within Ti3+ ions, and the weaker band near 12,500 cm-1 may represent a Ti3+ - Ti4+ IVCT transition between cations in face-shared octahedra. The peaks in the spectra of the yellow and blue sapphires clustered at 22,200 cm-1 and near 26,000 cm-1 represent spin-forbidden 6A, - 4AxfE G) and 6A[ — 4A2,4E(D) transitions in octahedrally coordinated Fe3+ ions (fig. 3.10), intensified by exchange interactions between adjacent Fe3+ ion pairs in the corundum structure ( 3.7.3). Other spin-forbidden Fe3+ bands occur at... 

Laporte and spin-multiplicity selection rules ( 3.7) and have intensities 103 to 104 times higher than those of crystal field transitions (table 3.6), their absorption edges may extend well into the visible region and overlap crystal field spin-allowed and spin-forbidden peaks. 

Crystal field spectra of a chromium-bearing forsterite yielded bands at 23,500 and 16,900 cm"1 (Scheetz and White, 1972), indicating a CFSE of 20,280 cm-1 for Cr3+ in the olivine structure. Additional broad, asymmetric bands centred at 11,800 and 6,400 to 6,700 cm"1 were attributed to crystal field transitions in Cr2+ (Scheetz and White, 1972 Bums, 1974). Although EPR measurements of forsterite show slight enrichments of Cr3 in the Ml sites (Rager, 1977), polar-... 

In optical spectra of emerald (Neuhaus, 1960 Poole, 1964 Wood and Nassau, 1968 Schmetzer and Bank, 1981), Cr3+ CF bands are located near 16,130 cm-1 and 23,530 cm-1 and are assigned to cations in octahedral sites. Similar bands for octahedral V3+ ions in beryl occur around at 16,000 cm-1 and 23,800 cm-1 (Beckwith and Troup, 1973 Schmetzer, 1982 Ghent and Lucchesi, 1987). Spectral features of pink and red beryls in the region 18,000-20,000 cm-1 (Wood and Nassau, 1968) may originate from crystal field transitions in Mn3+ ions in morganite. 

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