Co ( II ) OPTICAL ABSORPTION IN SPINELS : INFRARED AND LIGAND-FIELD SPECTROSCOPIC STUDY OF THE IONICITY OF THE BOND . MAGNETIC STRUCTURE AND Coz + +-Fe 3 MMCT INFERRITES . CORRELATION WITH THE MAGNETO-OPTICAL PROPERTIES

The analysis of the infrared and ligand field spectra of COM2O4 spinels reveals that the ionicity of these compounds varies in the following order aluminate > gallate > ferrite and chromite > rhodite and cobaltite. A linear relation has been established between the Δ(LO-TO)1 splitting, Racah parameter and the ionic-covalent parameter SSp=ΣICP

INTRODUCTION Application of cobalt ferrite as high density magneto-optical recording media has been demonstrated since about 15 years.Nevertheless, the interpretation of the Faraday rotation spectra remains uncertain.Consequently, in this paper, we present a reinvestigation of the vibrational and optical properties of cobalt(II) spinels in correlation with a new parameter introduced by Portier et al. [1]: the ionic- covalent parameter of cations based on polarizing power and electronegativity (this parameter accounts for the iono-covalent nature of chemical bonds permitting coherent interpretation of electronic properties).Another purpose of this paper is the study of the influence of superexchange interactions on the optical spectra of substituted cobalt(II) ferrites and more precisely on the Co2+--Fe 3+ metal- metal charge transfer (MMCT) located at 1.7 eV.The compounds chosen to illustrate the optical properties of the (CoFeO10) 15-cluster   belong to the CoFez_xGaxO4 system.The cation distribution is deduced from EXAFS and XANES measurements.The analysis of the iron(III) electronic spectra in ferrimagnetic spinels like MgFe204 and Li0.sFe2.O4 and the study of the optical properties of Co(II) spinels allow a quantitative evaluation of the contribution of both chargetransfer and crystal-field transitions to the magneto-optical properties of different substituted cobalt ferrites in the range 0.5-2.5 eV.

EXPERIMENTAL
Different series of cobalt spinels CoFe2_xGaxO4, CoFe2_xCrxO4 and CoGaz_xCrxO4 were prepared by means of a simple ceramic method by mixing suitable proportions of CoCO3 and mixtures of appropriate trivalent oxides and twofold sintering at 800C and l00C in air atmosphere.The samples slowly cooled at 10C.hr -1 were checked by X-ray diffraction.X-ray absorption spectra of polycrystalline samples have been recorded at room temperature using the radiation emitted at the DCI synchrotron (LURE, Orsay, France) running at 1.85 GeV.For the data collection which has been performed in transmission mode over 6900-7900 eV and 7500-8500 eV, respectively for Fe and Co absorptions, the white radiation was monochromatized by the EXAFS IV double crystal (Si 311) spectrophotometer working in the stepping mode (0.25 eV for XANES and 2 eV for EXAFS).The coordination of cobalt may be determined by measuring the intensity of the pre-edge peak on normalized XANES spectra.
UV-Vis-NIR diffuse reflectance spectra were obtained using a Perkin-Elmer lambda 9 spectrophotometer equipped with an integrat- ing sphere accessory and a 7300 computer.The spectra are expressed in absorbance or converted to the Kubelka-Munk remission function.
The calculated infrared phonon modes for II-III cobalt spinels have been determined by Kramers-Kronig analysis of the transmission spectra using a method previously published [2].The crystallographic data and cation distribution are listed in Table I.EXAFS measure- ments of the Co2+-O 2-distances in A and B sites confirms the deviation from Vegard's relationship observed by Lensen [3] for the variation of the lattice parameter with increasing gallium content in the CoFe2_xGaxO4 system.In ferrimagnetic compounds (x < 1), Ga 3 + ions show a marked Td site preference.

RESULTS AND INTERPRETATION
3.1.Spectroscopic Study of the lonicity of Cobalt(ll) Spinels The ground state of tetrahedrally coordinated Co 2+ is 4A2 and there are three excited quartet states 4T2(F),4 T(F)and 4TI(P).In the absence of spin-orbit coupling only the 4A9.--+4T transitions are allowed and these dominate the absorption spectrum.
The optical absorption of tetrahedral cobalt(II) in spinel-type oxides has been reinvestigated in order to analyze the relationship between the ionic-covalent parameter ICP and the Racah B value.An assignment of peak positions is presented in Table II.The bandwidths observed for the two-low energy transitions are large and may be accounted for by spin orbit coupling (theoretically, to first order, 4A and 6A for 4T 2 and 4Tl(4F)).Low temperature studies suggest that low symmetry fields and vibrational structure may contribute to the band widening [4,5].Moreover in the visible region there are many spin forbidden quartet-doublet transitions which can gain intensity by interaction with the spin allowed band.For computational purposes, the transition energies to calculate Oq and B, are based on the centers of intensity of these bands.The spin-orbit coupling constant has been calculated from the 4T (4F) band: the values obtained are similar to those reported previously.
4A2--4Tl(4F The Racah parameter B for a specific ion is well known to vary as a function of the ligands bound to that ion.The value of this parameter is always reduced from that observed with the free spherical ion.The reduction ofB has been explained by certain covalency effects, which are called "central-field covalency" and "symmetry restricted covalency" [7].The first of these is a spherically depressing symmetric effect on the Racah parameter caused by an expansion of the central-ion radial functions as the consequence of a lower effective charge compared to the ionic charge of the free ion.The second one takes into account the additional effect that the eg and t2g-electrons are not exclusively d- character but contain ligand contributions.Because the eg-orbitals are cr-antibonding and the t2g-orbitals only 7r-antibonding, different depressing effects on the interelectronic repulsion parameters are to be n expected depending on the specific electronic configuration t'ge g.
When, for a given cation, ligands are ordered according to the value of B in the corresponding complex, one obtains the nephelauxetic series.Expressed in terms of donor atoms, this series is approximately in order of the polarisability of the ligand atom.If the ligand is maintained constant a nephelauxetic series of metal ions can be established.This will vary according to the polarizing power of cations.The value of B, for a given metal and ligand, tends to decrease with decreasing coordination number, i.e., with a decrease in metal- ligand bond length and a concomitant increase in covalency.
The ligand-field parameter A and the Racah parameter B of Cr 3 + [8] and Ni 2 + [9, 10] ions in a number of oxidic structures have been analyzed with respect to their validity as indicators for certain properties of the metal-oxygen bond.In particular, information about the cationic environment beyond the first anionic coordination sphere of six oxygens-ligands could be obtained from the spectroscopic data: the variation of the ligand-field parameter A of Ni + ions in different oxidic lattices with the structure and the chemical constitution of the host lattices investigated by Reinen [9] reveals that A is not only a function of the Ni2+-O 2-distances, but is strongly influenced by polarization effects and the kind of cationic coordination of the 0 2. ion as well.
No general theory is available at the present time for explaining phenomena of this kind, which are usually summarized under the term "cooperative effects".
Similar observations may be deduced from the study of optical properties of Co(II) direct or partially inverse spinels.
From crystallographic data, one may estimate that the increase of the ionic-covalent parameter of the trivalent associated cation in direct spinels (ICPM3/) induces an increasing effect of covalency in short- ening Co 2 / -0 2. bonds.However, an opposite variation of A and B is observed (Tab.III).A plot of the experimental values of the Racah parameter versus the ionic-covalent parameter Ssp of the compounds COM204 demonstrates that all the data including those for direct or partially inverse spinels fall on the same straight line (Fig. 1).The present data fit the equation: A study of infrared reflection spectra of the II-III spinels has been undertaken [16].The transverse and longitudinal optical phonon frequencies were determined by Kramers-Kronig analysis.It is well established that (TO-LO)I of the two high-energy modes supplies a criterion of the ionicity of oxidic spinels [13,14,[16][17][18][19].A significant linear relationship between the (TO-LO)I splittings of direct and partially inverse cobalt(II) spinels and the corresponding Ssp is observed (Fig. 2): A(TO-LO) 114.6 SSp 148.5 R 2 0.92   In conclusion, the analysis of the infrared and ligand field spectra of cobalt (II) spinels reveals that the ionicity of these compounds varies in the following order: aluminate > gallate > ferrite and chromite > rhodite and cobaltite.The Racah parameter of the Co 2+ tetrahedrally coordinated ion reflects the ionicity of the spinel and, consequently, is mainly influenced by the nature, the electronic configuration and spin state of the trivalent associated cation: the highest value of B is observed for magnesium aluminate, i.e., in spinel formed with "hard" acids (Mg 2 +, A13+) and the lowest one for cobalt rhodite (the LS Rh 3+ has a relatively large electronegativity).In CoFe204, B cannot be deter- mined because of the MMCT C02+ Fe 3+ band which hides the 4A2 ___+4 Tl(4p) transition of tetrahedral Co 2+ ions (see next section).
A reinvestigation of the absorption specturm of Co304 has shown that the optical transitions of this compound in the range 0.45-5 eV are due to intense ligand field absorptions of Co 2+ and LS Co 3+ in tetrahedral and octahedral sites and to charge transfer between Co 2 + and Co 3+ and from oxygen ligands to Co + ions [20].The optical spectra of Fe 3+ systems have been poorly understood.Often different ligand field states of the Tanabe-Sugano diagram are obscured by the higher energy LMCT transitions.All of the transitions of the 6A1(6S) ground state to the excited ligand field states are, in principle both spin and parity forbidden.Or, in a number of iron (III) oxides, these transitions are found instead to be quite intense: the apparent relaxation of the spin selection rule results from the magnetic coupling of next-nearest-neighbour Fe 3 + cations in the crystal structure [21, 22].An additional phenomenon resulting from the magnetic coupling is the presence of new absorption features or pair excitations (i.e., the simultaneous excitation of two Fe 3 + centers by a single photon) which are also spin-allowed and occur at energies given approximately by the sum of two single ions Fe 3 + ligand field transitions [26].
The electronic structures of Fe + coordination sites in iron oxides have been obtained from self-consistent field Xc scattered wave (SCF-Xc-SW) molecular calculations on an octahedral (FeO6) 9-cluster, a trigonally distorted (FeO6) 9-cluster and a tetrahedral (FeO4) 5-cluster [27].Multiplet theroy has been used to relate the one-electron molecular orbital energies to the ligand field spectra of Fe 3+ in oxides.The calculated optical spectra Fe 3+ oxides summarized hereafter are compared with the experimental data relative to reference systems and have been used to interpret the NIR-Visible-UV spectra presented in Table IV Calculated energies of the Fe 3 + ligand field transitions and ligand to metal-charge transfer transitions are in good agreement with experi- mental data corresponding to spectra of dilute Fe 3 + cations in oxide host phases.The theoretical results have been used to relate chemical bonding to the physical properties (i.e., magnetic structures) and crystal chemistry of iron (III) oxides and silicates [32, 33].Face-sharing antiferromagnetic interactions in corundum structure and edgesharing antiferromagnetic interactions in spinel structure enhance Fe 3 + ligand field transitions and Fe 3 +-Fe 3 + pair transitions (both types of transitions are Laporte and spin-allowed via the magnetic coupling of adjacent Fe 3+ cations).Consequently, the visible region absorption edge which gives their red or brown colors to the iron oxides does not result from LMCT transitions but is a consequence of the strong enhancement of ligand field and pair transitions.
The substitution of the tetrahedral and octahedral Fe 3+ by non- magnetic Ga 3+ in ferrites provides a possibility for experimentally distinguishing the different transitions (Tab.IV).In pure ferrites: Lio.sFe2.504,MgFe204 and NiFe204, the band at 19000 cmincludes the 6A 4 -T2 transition of the tetrahedral iron(III) and the 2(6Ag) --+ 24Tlg(4G) pair transition.The gallium substitution in these ferrites influences the nature of superexchange interactions: as the gallium content is increased, the intersublattice interactions weaken and the intrasublattice interaction become stronger facilitating a canted spin alignment on the octahedral sites.This evolution induces a consider- able decrease of the Fe 3+ octahedral ligand field 4Tlg(4G), 4T2g(4G transitions and 24Tlg(4G) pair transition.Thus, the absorption edge is shifted to higher energies (i.e., from 1.5 to 2.5-3 eV in paramagnetic compounds).The paramagnetic compounds at room temperature present a spectrum similar to that of ZnFe.O4 characterized by a 4 A strong intensification of the 2 6Alg--+4Elg lg(4G)+4TIg(4G) transi- tion in the range 28000--29000 cm-1.A similar behavior has been observed in oxides of corundum type (Tab.IV).
3.2.2.Antiferromagnetic Co 2 + -Fe 3+ Interactions and Optical Spectra of Spinel Oxides Figure 3 shows the absorption spectra of some cobalt ferrigallates.The spectra of the ferrimagnetic compounds are characterized by an absorption edge near 1.4 eV.The absorption edge is shifted to higher energies on the spectra of materials having a lower Curie temperature.
The spectra of paramagnetic compounds are dominated by the 4A2--+ 4T1Co2+ transitions near 0.85 and 2 eV.The Figure 4 present the difference spectra CoFez_xGaxO4-CoGaO4(a) and CoFe_x GaxO4-MgFez_xGaxO4(b).The difference spectra of ferrimagnetic materials reveal an intense band near.1.7 eV assigned to the Co 2 + + Fe +--, Co + + Fe + metal-metal charge transfer (MMCT).This band disappears on the spectra of paramagnetic samples (Curve E, Fig. 4a and curve C, Fig. 4b).A tentative assignment of the different bands observed on the difference spectra is presented in Table V.
The metal-metal charge transfer involving one cloud-shell transi- tion metal ion oxide has been thoroughly investigated but unfortu- nately considerably less is known about MMCT in other mixed oxides.These transitions are of large importance with regards to photoredox, magneto-optical processes and are also responsible for the color of many inorganic compounds and minerals and for the presence or absence of certain luminescence processes.According to Blasse [34], the black color of certain ferrites like MnFe204, CoFe204 and NiFe204 is undoubtedly due to a MMCT of the type: M(II) + Fe(I!I) -, M(III) + Fe(II) in the near infrared.The energy of the absorption for the (MFeO0) 15-clusters in spinel oxides increases in the sequence Fe(II), Co(II) and Ni(II) as that relative to the M(II)+ Ti(IV) MMCT in MgTi.O5 [35]:  [37] and 0.62 [38] in Fe304 1.65-1.70 2.85 [40]   value 1.05 in Fe.sAl.504 [39] (this study) No evidence for absorption bands corresponding to possible MMCT transitions between octahedral Fe 3+ and Mn + ions of the (FeMnO10) 5-cluster has been observed in minerals [39] and MnGaz_xFexO4 spinels.The quantitative evaluation of both the charge transfer and crystal field transitions in CoFeO4 and other cobalt spinels allows the interpretation of magneto-optical properties of substituted cobalt ferrites.
The magneto-optical properties of cobalt ferrites have been extensively studied [41 to 46].The polar Kerr rotation of a CoFe204 single crystal measured in the range 0.6-5.5 eV reveals a strong dispersive transition at 0.8 eV and a broad transition with a dispersive transition superimposed around 2 eV.At higher energies, the polar Kerr rotation is similar to most of the iron containing spinels and garnets: the transitions at 4 and 5 eV with a diamagnetic line shape are O2--Fe3+LMCT transitions.The transition around 0.8 eV is identified as the 4A:z---4T(F) transition of Co2+(Td).transition and the second one being the Co2+(Td)4A2--4TI(P) transition with diamagnetic line shape.
The absorption coefficients and the Faraday rotation spectra of nanocrystalline cobalt ferrite thin films have been determined in the visible-near infrared range [48].The Faraday rotation shows similar features to that observed previously [47], with two mean negative peak located near 6300 cm-(0.78eV) and 13100 cm -(1.62 eV).Their energies are 6300 and 12500 cmin ref. [47].According to Stichauer et al. [48] the local symmetry of the ions in octahedral coordination is broken and the three symmetry point groups 4, 422 and 32 are suggested to explain the behavior of the absorption coefficient.The main features of the Faraday rotation spectra may be explained by the crystal field transitions of the Co 2+ and Fe 3+ ions similarly as on magnetic garnets.Influence of charge transfer cannot be disregarded around 13000 cm-1.
The experimental results issued from the study of the Co 2 + optical properties in spinels allow a quantitative evaluation of the contribu- tion of both the charge-transfer and crystal field transitions to the Faraday rotation in the range 1.  [41] of the negative 1.97 1.93 1.70 [43] rotation peak 1.95-2.30[42] 1.70 [45] % Co2+in Td 45 [44] 100149] 45 [46] 45 [ In substituted cobalt ferrites the shift of the negative peak near 2 eV on the polar Kerr rotation spectra to higher energies may be assigned to a major contribution of the 4Az--4Tl(P Co 2+ tetrahedral transition.The shift to lower energies as in CoFe204 and CoMnFeO4 indicates that the MMCT transition is predominating.The Co2+-Fe 3+ MMCT energy may explain the Faraday rotations calculated Marents [46] at 1.55 eV photon energy (800 nm), for various substituted CoFez_xMxO4(M Rh 3 + Mn 3 + Ti 4 + + Co +).Conse- quently, CoFeRhO4 is not a direct spinel [46] but a partially inverse one as CoGaRhO4 [50].

FIGURE 2
FIGUREPlot of Racah parameters of Co tetrahedral ions (Tab.II) as a function of SSp.

FIGURE 3 FIGURE 4
FIGURE 3 Diffuse reflectance spectra of CoFe2_xGaxO4 solid solutions recorded at room temperature.

TABLE II Co
tetrahedral ligand field parameters in spinels 3.2.Magnetic Structure and Co 2 +-Fe 3+ Charge Transfer in Cobalt Ferrites Correlation with Magneto-Optical Properties 3.2.1.Fe Optical Spectra and Magnetism in Oxides .
5-2eV for different bulk samples of