A paramagnetic resonance study of Cr3+ and Fe3+ in sillimanite

Abstract

The paramagnetic resonance spectra of W+ and Fe3+ in natural crystals of the mineral sillimanite have been investigated. The chromium results can be interpreted with a spin Hamiltonian having axial and rhombic terms. Four magnetically inequivalent (directionally) sites (Site I) are observed, with parameters g 1 = 1.975, 911 = 1.963, B: = 6.91, and Bi = 3.77 GHz. The iron results have been interpreted on the assumption that Fe3+ occupies the two crystallographically inequivalent A13+ sites in the sillimanite, Site I1 having tetrahedral coordination and Site 111 octahedral coordination. The constants of the conventional orthorhombic spin Hamiltonian are as follows, Site 11: B, O = 1.74 GHz, B; = = 1.62 GHz, Bi = 0.0 GHz, B: = 0.0 GHz, B: = 0.0 GHz, and g, = gar = gz = 2.004; Site 111: B: = 11.8 GHz, B; = 4.04 GHz, B: = 0.37 GHz, S i = 0.09 GHz, and g2 = g, = = g, = 2.004. A comparison of the results for the octahedrally coordinated sites in the three polymorphs kyanite, andalusite, and sillimanite is made. Die EPR-Spektren von W + und Fe3+ in naturlichen Kristallen des Minerals Sillimanit aurden untersucht. Die Ergebnisse an Chrom konnen mit einem Spin-Hamiltonian mit axialen und rhombischen Termen interpretiert werden. Vier magnetisch nichtaquivalente (Richtung) Pliitze (Platz I) werden beobachtet, mit Parametern g 1 == 1,975, 911 = 1,963, Bi = 6,91 und B; = 3,77 GHz. Die Ergebnisse a n Eisen werden mit der Annahme erklart, daB Fe3+ die beiden kristallographisch inaquivalenten A13 k-P1atze in Sillimanit -Platz I1 hat tetraedrische und Platz 111 oktaedrische Koordinationbesetzt. Die Konstanten des konventionellen rhombischen Spin-Hamiltonians sind fur Platz I1 : Bl = = 1,74 GHz, BZ = 1,62 GHz, B: = 0,O GRz, B4 = 0,O GHz, Bi = 0,O GHz und gz = = g, = gr = 2,004; fur PlatzIII: B, O = 11,s GHz, Bi = 4,04 GHz, Bi = 0,37 GHz, B: = = 0,09 GHz und g , = gy = gz = 2,004. Ein Vergleich der Ergebnisse fur die oktaedrisch koordinierten Platze wurde fiir die drei Polymorphe Cyanit, Andalusit und Sillimanit durchgefiihrt.

Correlation of spectroscopic properties and substitutional sites of Cr3+ in aluminosilicates: II. Andalusite and sillimanite

“…There is also only one set of the ZFS parameters (b2 ~ b~) available for Cr 3 § at the A1 sites in sillimanite (Marshall et al 1971), which yields the ZFS value of 1.449 cm-< In Table 3, we obtain (with Rmz,= 1.89 ~) the theoretical value of ZFS as 1.44 cm-1 for Cr 3 + (AlJ. For Cr 3 § (AlJ an apparent ~ which has a different nature than the usual ZFS of an orbital singlet ground state (here 4A2), is obtained as 41.6 cm -1.…”

Role of Structural Fe(III) and Iron Oxide Nanophases in Mullite Coloration

“…EPR studies 2,5 have established that iron, as Fe 3ϩ substitutes for A1 mainly in the octahedral sites, chains of which extend along the c-axis of the structure. The intensities of EPR signals at g eff ϭ 9 and g eff ϭ 4, corresponding, respectively, to octahedral and tetrahedral iron, 2,5 by analogy with an EPR study on Fe 3ϩ center in sillimanite 18 which has a structure similar to that of mullite, were used to monitor Fe 3ϩ content in the investigated samples. Figure 1(a) shows the EPR sensitivity to Fe(III) content which increases from reference to A-88 mullite.…”

Combustion synthesized MgAl2O4:Cr phosphors—An EPR and optical study

“…If there is a large separation between these two doublets then the resonance at g ¼ 2.6 can also be observed [16]. Le Marshall et al [17] reported the Cr 3+ distribution in sillimanite by the EPR which gave evidence for a strong orthorhombic crystal field for the Cr 3+ centre located in a slightly distorted octahedral environment. The signals at low field region is normally attributed to isolated Cr 3+ ions, whereas the signal in the high field region near gE2 is an indication of Cr 3+ ions may occur in pairs [18].…”