J. Chem. SOC., Faraday Trans. 1 , 1982, 78, 3177-3185 Coordination and Dispersion of Co2+ Ions in Coo-MgO Solid Solutions BY KRYSTYNA DYREK AND ZBIGNIEW SOJKA Institute of Chemistry, Jagiellonian University, Cracow, Poland Received 30th November, 198 1 COO-MgO solid solutions of COO concentrations 0.25-1 5.3 mol % have been investigated by e.s.r. spectroscopy in the temperature range 4-77 K. The distribution of isolated Co2+ ions between octahedral and tetrahedral sites has been estimated quantitatively. In the most dilute solid solutions (up to 3.00 mol % COO) Co2+ ions occupy predominantly trigonally distorted tetrahedral sites. The exchange interactions extend up to the 3rd coordination sphere in CoSMgO. The dependence of the number of paramagnetic 0; radicals formed in the course of oxygen adsorption on COO-MgO at room temperature is correlated with the analogous dependence of the number of isolated Co2+ ions of tetrahedral symmetry with trigonal distortion.Two kinds of adsorption centres are postulated in CoCMgO solid solutions: tetrahedrally coordinated Co2+ ions with trigonal distortion isolated or coupled to the paramagnetic neighbours. The occupation by Co2+ ions of both octahedral and tetrahedral sites in Coo-MgO solid solutions was postulated by one of us on the basis of magnetic- susceptibility measurements and reflectance spectra. This interpretation was also made by Stone et aL39 in thecase of the dilute COO-MgO solid solutions. The non-octahedral coordination of Co2+ in MgO was also postulated by Krylov et aL5 Stone et aL3v4 represent the opinion that Co2+ in tetrahedral coordination appears only in the surface layers of the Coo-MgO crystallites whereas, according to our data, in the dilute solid solutions (containing 30.9 mol % COO or less) tetrahedrally coordinated Co2+ is also present in the bulk crystal.To clarify this problem we have attempted in the present work to determine quantitatively the distribution of Co2+ ions between octahedral and tetrahedral sites in dilute Coo-MgO solid solutions. A more precise determination of the symmetry of Co2+ ions in tetrahedral surroundings was another object of our investigations, together with the problem of cobalt dispersion in the MgO matrix. EXPERIMENTAL MATERIALS Coo-MgO solid solutions were obtained by the thermal decomposition in U ~ C U O (1 0-3- 1 OP4 Pa) at 773 K of coprecipitated carbonates of Co2+ and Mg2+ as described previously.' The only difference in the procedure was a shortening of the calcination time from 20 to 10 h.The starting materials supplied by Polskie Odczynniki Chemiczne (Gliwice, Poland) were analytically pure. All operations on the samples were carried out in sealed glass ampoules in such a way that the samples were not allowed to come into contact with air. CHEMICAL ANALYSIS The concentration of Co and Mg in the investigated solid solutions was determined by complexometric titration with EDTA after dissolving the samples in HC1.I The absence of Co3+ impurities was checked by an iodometric method.6 103 3177 FAR 783178 COORDINATION AND DISPERSION OF Co2+ I N Coo-MgO E.S.R.SPECTRA E.s.r. spectra were recorded in the temperature range 4-300K using a Bruker ER200 spectrometer and an X-25 e.s.r. spectrometer (constructed at the Department of Electronics, Technical University, Wrodaw, Poland) both operating in the X-band mode with modulation frequency 100 kHz. Ultramarine and Co2+ in MgO (0.39 mol % COO), calcined at 1273 K, were used as standards of spin concentration. ADSORPTION Oxygen adsorption was carried out at room temperature under a pressure of 5 hPa. After 10 min of adsorption any excess oxygen was removed by evacuation. RESULTS AND DISCUSSION two kinds of e.s.r. signals were expected for COO-MgO: those of Co2+ ions in octahedral and tetrahedral surroundings. In an octahedral crystal field the ground state 4qg of Co2+ is split by spin-orbit coupling to yield a Kramers doublet between which the e.s.r.transitions are observed. The resulting g factor is given by the formula' On the basis of our previous g = 1O/3 + k, - 15/2 (L/A) (1) where k, is the parameter of covalency (0.85-0.90 for CoO-Mg07), 13. is the spin-orbit coupling constant ( - 178 cm-l for Co2+ 8, and A is the octahedral crystal-field splitting (ca. 9500 cm-l for COO-MgOg). The g value calculated from formula (1) is equal to ca. 4.3. The lowest excited state has an energy only 305 cm-l higher than the Kramers doublet,1° and so the spin-lattice relaxation time for Co2+ in octahedral surroundings is very short and the corresponding e.s.r. signal may be observed exclusively at low temperatures. In fact the signal of CO;&~ has been observed by several authors at liquid-helium and liquid-nitrogen temperatures.'* At 77 K the hyperfine splitting resulting from the nuclear spin of Co ( I = 7/2) is unresolved, and only a broad single line is observed.In a tetrahedral crystal field the ground-state level 4A2 of Co2+ is separated from the lowest excited level by ca. 4200 cm-l l4 and so the relaxation time is long enough to observe the e.s.r. signal of tetrahedral cobalt at room temperature. The g factor is given by the formula g = 2-811/A (2) which leads to a g value of 2.34. In the e.s.r. spectra of dilute Coo-MgO solid solutions (containing 15.3 mol % of COO or less) recorded at 4 K two signals are observed (fig. 1); a broad line with g = 4.23, which may be attributed to isolated Co2+ ions in octahedral coordination, and a sharp symmetric signal of much lower intensity, g = 2.00, characteristic of defects in MgO.I5 The signal ascribed to Co2+ ions vanished at ca.100 K whereas the signal arising from defects was also observed at room temperature. No signal due to tetrahedrally coordinated cobalt was observed even at 4 K, in spite of the fact that the dilute solid solutions according to magnetic-susceptibility data and reflectance spectral? were believed to contain predominantly or even exclusively tetrahedrally coordinated Co2+ ions. The number of spins associated with the signal attributed to the octahedrally coordinated Co2+ ions is considerably lower than the total amount of cobalt calculated from the weight of the sample and known concentration of COO (table 1, columnsK.DYREK AND Z. SOJKA 3179 FIG. 1.-E.s.r. spectrum of COO-MgO (0.39 mol % COO) at 4 K. TABLE 1 .-DISTRIBUTION OF Co2+ IONS IN Coo-MgO SOLID SOLUTIONS~ no. of isolated no. of isolated total no. of CoEzta ions (from CO;;~, ions (from Co2+ ions e.s.r.) before e.s.r.) after isolated ions in clusters mol % in the sample calcination at calcination at Co,Z,',,,. undetectable in COO x 10-19 1273 K x 1273 K x 10-19 ions x e.s.r. x no. of no. of Co2+ (1) (2) (3) (4) ( 5 ) (6) 0.25 3.7 1.4 & 0.2 3.5k 1.4 2.1 0.2 0.39 5.8 3.4 f. 1.2 5.6 1.7 2.2 0.2 2.14 31.4 8.5+ 1.0 19.2 f. 3.2 10.7 12.2 3.00 52.3 10.4& 1.5 25.6 f 2.3 15.2 26.7 10.39 142.5 4.1 & 1.1 5.9 & 2.4 1.8 136.6 15.30 202.0 4.4f0.1 8.4 k 3.2 4.0 193.6 a All the values in table 1 are calculated from three independent experiments.3 and 2, respectively). After the samples had been heated in vaczio at 1273 K for 7 h the intensity of the signal due to the octahedrally coordinated Co2+ increased considerably (table I , column 4, and fig. 2). Simultaneously the samples changed colour from blue to pink. These results indicate the presence in the dilute solid solutions of two kinds of Co2+ paramagnetic species, those detectable and not detectable by e.s.r. As has been shown already the e.s.r. signal of g value 4.23 observed in the temperature range 4- 100 K may be attributed to octahedrally coordinated Co2+ ions. The species not detectable by e.s.r. predominate in the dilute solid solutions of high specific surface area,' i.e.in crystallites revealing a high concentration of Schottky defects.16 The magnetic properties and optical absorption of the Co2+ ions in these solid solutions are typical of tetrahedrally coordinated cobalt, which strongly suggests that the deficit in the number of spins determined by e.s.r. spectroscopy corresponds to Co2+ ions in tetrahedral coordination. The change of colour from blue to pink and simultaneous increase in intensity of the signal attributed to octahedrally coordinated Co2+ ions which occur upon heating the samples in vacuo at 1273 K indicate an ordering of the lattice and migration of Co2+ ions from tetrahedral to octahedral sites typical of the B, type crystal lattice. These results agree with those of previous investigations of reflectance spectra2 which showed that the change in Co coordination in the COO-MgO solid solutions is a result of calcination.103-23180 COORDINATION A N D DISPERSION OF Co2+ I N Coo-MgO 0 L 8 12 16 niol ? COO FIG. 2.-Plot of e.s.r. signal intensity of isolated CoE&,, ions as a function of Coo concentration: 1, before calcination; 2, after calcination at 1273 K for 7 h. The lack of an e.s.r. signal from tetrahedrally coordinated cobalt may be explained by the assumption of a trigonal distortion of the tetrahedron, as shown by Kazansky et a[." In the case of a shortening of the tetrahedron along the trigonal axis the e.s.r. signal may be described by a spin Hamiltonian of the form H = Bkil Sz f f z + gdSy H y + s, w1+ m% + S(S + 111 (3) with a negative D value and with gll > gl > 2.At liquid-helium temperature (kT = 3 cm-l) for D < 0 and 101 $= kT (strong trigonal distortion) the population of the doublet 1/2 taking part in the resonance is very small and so observation of the e.s.r. signal for polycrystalline samples is difficult. With increasing temperature the total population of the doublet increases but simultaneously the difference in the population of levels of the doublet decreases. Competition between these two effects precludes the observation of the e.s.r. signal of Co2+ ions in tetrahedral coordination having a sufficiently strong trigonal distortion (at both low and high temperatures). The lack of an e.s.r. signal for Co2+ in tetrahedral surrounditlgs has been observed by Graber et for spinel CdIn2S, and by Kazansky et all9 for Co-exchanged zeolites, in spite of the observation of tetrahedral Coz+ in the reflectance spectra.The width of the e.s.r. signal for g = 4.23 attributed to octahedrally coordinated Co2+ ions changes with Co concentration, as shown in fig. 3. We suppose that two main factors control the width of the e.s.r. line: dipole-dipole interactions, which increase with increasing Co concentration, and imperfections of the crystal lattice, which increase as the Co concentration decreases. The important contribution of the lattice imperfections to the total linewidth is indicated by a reduction in width of the e.s.r. signal upon calcination of the sample. On the other hand in the concentration range up to 15.3 mol % COO the exchange interactions do not narrow the signal significant 1 y , The dependence of signal intensity of CO:&~ on the temperature at which measure-K.D Y R E K A N D Z. SOJKA 3181 I 0 4 8 12 16 mol % COO FIG. 3.-Dependence of the width of the Co& e.s.r. line on COO concentration at 77 K. 0 20 4 0 60 80 100 temperature/K FIG. 4.-E.s.r. signal intensity of Co2+ in COO-MgO plotted against temperature: 1, 0.39; 2, 3.00; 3, 15.3 mol % COO. ments are made is shown in fig. 4. The intensity of the signal for a given sample increases on lowering the temperature down to a temperature just above the Neel point. Around the Neel temperature and below it a decrease in the signal intensity owing to increasing antiferromagnetic interactions is observed. The Neel temperature decreases with decreasing cobalt concentration, and so for 15.3 mol % COO a maximum occurs at 20 K, whereas for more dilute solid solutions a continuous increase in the signal intensity is observed, indicating that the Neel temperature is beyond the range in which the measurements were carried out.Solid solutions containing > 15.3 mol % COO do not give an e.s.r. signal at low temperatures because the Neel point of these preparations lies above 77 K.' At temperatures > 77 K there is also no e.s.r. signal owing to the short relaxation time of the Co2+ ions in octahedral surroundings. The dependence of the signal intensity on Co concentration at 77 K is shown in fig. 2. The shape of this dependence suggests that it is determined by two opposing factors.An increase in the Co concentration (i.e. an increase in the total number of paramagnetic centres) causes an increase in the signal intensity. However, a simulta- neous increase in metal-metal exchange interactions leads to a decrease in the number of isolated Co2+ ions and hence a decrease in their e.s.r. signal intensity. These two3182 COORDINATION AND DISPERSION OF C02+ IN COO-MgO opposing effects cause the appearance of a maximum in the plot of I = f ( x ) (where x is the mole fraction of COO) at x = 0.03. As shown by Gesmundo20 the value of x corresponding to the maximum e.s.r. signal intensity can be used to estimate the distance within which the exchange interactions occur. The number of isolated Co2+ ions distributed randomly in a solid solution is proportional to x( 1 - x ) ~ . The value of rn equal to the number of cationic sites around a given ion which should not be occupied by other Co2+ ions in order that the given ion is isolated is related to the mole fraction of COO corresponding to the e.s.r.signal of maximum intensity by the formula m = (1 --xmax)/~,,,. In the MgO crystal lattice (B, type) there are 18 cationic sites around a given cation including the next-nearest neighbours and 42 including the third-nearest neighbours. A value of rn = 32 was found from the experimental dependence of the e.s.r. signal intensity on Co concen- tration. The experimental data shown in fig. 5 fit the curve corresponding to m = 42 better than that corresponding to rn = 18, and so we conclude that the exchange interactions extend to the third-nearest neighbours. 0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 x (mole fraction of Coo) FIG. 5.-Comparison of the experimental (solid line) and calculated (dashed line) data for extent of exchange interactions: 1, up to the second coordination sphere (rn = 18); 2, up to the third coordination sphere (rn = 42); 3, experimental data.As has already been shown the number of spins associated with the signal attributed to the octahedrally coordinated isolated Co2+ ions is considerably lower than the total amount of cobalt in the sample (table 1 and fig. 2). We assume that the deficit in the number of Co2+ ions determined by e.s.r. spectroscopy corresponds to the tetrahedrally coordinated cobalt with trigonal distortion not observable in the e.s.r.spectra under the conditions in which the measurements were carried out and/or to the Co2+ in clusters coupled by exchange interactions. The difference between the number of spins corresponding to the signal due to isolated Co2+ ions in octahedral surroundings before and after heating of the sample at 1273 K gives the number of isolated Co2+ ions tetrahedrally coordinated with trigonal distortion. The amount of Co2+ in clusters not detectable by e.s.r. spectroscopy can then be estimated as the difference between the total content of cobalt, calculated from the weight of a given sample and known concentration of COO, and the total number of isolated Co2+ ions, as found in the sample after additional calcination. The results presented in table 1 reveal that in mostK.DYREK AND Z. SOJKA 3183 of the dilute solid solutions isolated Co2+ ions predominate in distorted tetrahedral coordination. Evidently cobalt having this coordination also enters the crystal bulk and is not restricted to the surface, as postulated by other author^.^^^ The number of Co2+ ions in clusters was calculated assuming that they do not contribute to the e.s.r. signal. Indeed, the Co2+-Co2+ pairs exhibit resonance outside the signal due to isolated Co2+ ions and may contribute only to the tails of the line.20 However, the clusters consisting of several Co2+ ions give a signal of Lorenzian shape and with a g factor close to that of the isolated ions.21 Such clusters may contribute to the total intensity of the signal at g = 4.23, which reduces the accuracy of the estimation.This effect may be considerable in the case of the solid solutions containing 10.4 and 15.3 mol % COO, which explains the anomalous intensity of their e.s.r. signals as compared with that calculated on the basis of the isolated-ion approximation. mol % COO FIG. 6.-E.s.r. signal intensity of 0; species plotted against concentration. Adsorption at room temperature. Spectra registered at 77 K. As shown previously1 the mechanism of oxygen adsorption is different for solid solutions of various COO concentrations; paramagnetic 0; species appear in the dilute solid solutions whereas diamagnetic 02- ions are formed in the concentrated solid solutions. In order to check the role of cobalt dispersion in the determination of the adsorptive properties of the solid solutions we determined quantitatively the amount of oxygen adsorbed in a paramagnetic form.The plot of the e.s.r. signal intensity of 0; radical as a function of COO concentration (fig. 6 ) passes through a maximum at 3.00 mol % COO; this is undoubtedly correlated to the maximum concentration of isolated Co2+ ions. Similar behaviour is observed for dilute solid solutions of Mn0-Mg0.22 On all the samples showing hyperfine splitting, i.e. containing isolated Mn2+ ions, oxygen is adsorbed in the form of paramagnetic species giving a signal in the centre of the sextet hyperfine structure. The intensity of this signal increases monotonically with Mn concentration up to ca. 3 mol % MnO. For the more concentrated solid solutions giving only a single e.s.r.line from coupled Mn2+ ions oxygen is adsorbed in form of diamagnetic 02- ions. The formation of paramagnetic species of adsorbed oxygen 0- and 0; requires the3184 COORDINATION AND DISPERSION OF Co2+ I N Coo-MgO transfer of only one electron per atom or molecule, respectively. The conditions for such a transfer are fulfilled only in the case of isolated Co2+ ions. The exchange interactions in COO-MgO are extended up to the third coordination sphere. There are three principal contributions to the exchange interaction : correlation, polarisation and delo~alisation.~~ In the latter effect the electron is assumed to drift from one cation to another and so enables the transfer of more than one electron from other cations to the cation which plays the role of adsorption centre.The distance from which the next electron may be supplied via delocalisation effects is determined by the extent of the exchange interactions. In the Coo-MgO solid solutions the Co2+ adsorption centre may be considered as isolated if the three neighbouring spheres are unoccupied by other Co2+ cations. However, if this condition is not satisfied, i.e. if there are other cobalt cations in the vicinity of the Co2+ ion, transfer of more than one electron to the adsorption centre is possible and oxygen is adsorbed in a diamagnetic form. CONCLUSIONS The mechanism of oxygen adsorption on solid solutions of transition-metal oxides in MgO is determined by two factors: coordination of the transition-metal ion and metal-metal interactions. The change of coordination from octahedral to tetrahedral may influence significantly the adsorption properties of the solid solutions, as the metal ions of reduced coordination number show enhanced ability for adsorption in order to complete their coordination spheres.On the other hand the metal-metal interactions control the form in which oxygen is adsorbed. The existence of two kinds of adsorption centres in COO-MgO solid solutions is postulated, both containing tetrahedrally coordinated cobalt with trigonal distortion. One of them is isolated, the other coupled to its paramagnetic neighbours. The authors are greatly indebted to Prof. A. Bielanski, Jagiellonian University, for helpful discussions. We also thank Prof. P. Hagenmuller and Prof. M. 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