OPTICAL ABSORPTION IN THE DIVALENT OXIDES OF COBALT AND NICKEL BY W. P. DOYLE AND G. A. LONERGAN Dept. of Chemistry, University College, Upper Merrion Street, Dublin Received 9th July, 1958 The absorption spectra of cobaltous and nickelous oxides have been determined between 200 and 1000 mp by transmission through thin films and between 360 and 1000 mp by the technique of pressed potassium bromide discs. The first maxima in the thin film spectra, at 5.9 eV for cobaltous oxide and 4.4 eV for nickelous oxide, are interpreted as exciton transitions and the second maximum in nickelous oxide, at 5.2 eV, is attributed to the transition to the conduction band. The pressed disc spectra are discussed in terms of crystal field theory. It was not found possible to prepare thin films of ferrous oxide.During the past few years there has been considerable development in the interpretation of the optical properties of transition metal complexes. Ab- sorption in the visible and near ultra-violet has been discussed in terms of crystal field theory 1 and that in the further ultra-violet has been attributed to electron transfer processes.2 Interest has largely centred on octahedral complexes in solution and there is little data on solid compounds of the transition metals. In solids with the sodium chloride lattice the metal ion is octahedrally surrounded by negative ions so that it is in an environment analogous to that in an octahedral complex. Thus absorption data on such solids may be of interest and the object of the present investigation was to determine the absorption spectra between 200 and 1OOOmp of some transition metal compounds having the sodium chloride lattice.EXPERIMENTAL THIN FEM SPECTRA Thin films of the oxides studied were prepared by oxidation of evaporated metallic films. The evaporation chamber consisted of a glass bell-jar vacuum-sealed to a steel base-plate by an L-gasket. The base-plate was connected to the pumping-system and carried the leads for the heating element. The latter was a small boat of 0.002 in. thick tungsten strip and was cleaned before use by heating in hydrogen. Evaporation was carried out at a pressure of less than 10-4 mm of mercury and the evaporated layers were deposited on amorphous quartz discs supported vertically over the heating element at a distance sufficient to ensure uniformity of film thickness.For the evaporation of nickel, cobalt or iron, the metal is electroplated on to the heating element.3 Cobalt films were oxidized by heating in a stream of steam at 300"C, preheating and cooling being carried out in nitrogen. The films so obtained were grey in colour, uniform and free from turbidity. Three typical films were identified by electron diffraction as the divalent oxide. The normal COO diffraction pattern was obtained from one film while the patterns from the other two films, although certainly of COO, were somewhat dis- torted. Two of the films were oriented with the (100) plane parallel with the surface while in the third preferred orientation could just be detected. In the pattern from one film a few faint additional lines were present ; these could be due to a higher oxide and might represent approximately 5 % of the oxide.Impurity concentrations up to about 10 % are not detectable in the transmission spectra of thin films 4 and the spectrum of the film containing the higher oxide was identical with that of the films containing no higher oxide. The differing degrees of distortion and preferred orientation observed by electron diffrac- tion caused no differences in the absorption spectra. Films prepared by heating the metal in oxygen at 400°C and then heating in carbon dioxide at 950°C to dissociate any higher oxide 5 always contained signiscant quantities of higher oxides. 2728 OPTICAL ABSORPTION I N coo A N D NiO Nickel films were oxidized by heating in a stream of oxygen at 300°C. The films so obtained were grey-green in colour, uniform and free from turbidity. Two typical films were examined by electron diffraction.The diffraction pattern from one film, although badly distorted, was recognizable as that of the divalent oxide. With the other film a weak, but sharp, pattern was obtained which was unlike that of either NiO or nickel. Distortion prevented accurate calculation of lattice spacings and the film could not be identified. However, the absorption spectra of the two films were the same. The reason for this apparent discrepancy between the electron diffraction results and the absorption results is not known. The unidentified film was about one-third as thick as the NiO film but it is believed that this fact has not influenced the identification except in so far as it introduced greater accumulation of charge due to the insulating nature of the quartz mount.Ferrous oxide can be prepared by oxidation of iron at temperatures above 575°C at low oxygen pressures ; 6 at room temperature it is metastable with respect to higher oxides but the rate of transformation is slow.7 Iron films were heated in an evacuated tube to 9OO"C, oxygen at a pressure of 0.01 mm of mercury was admitted for 15 min, the tube was then re-evacuated and rapidly cooled. Films prepared in this way were always orange-brown in colour indicating that they consisted largely of higher oxides and not the divalent oxide which is black. Presumably in thin films the rate of transformation is greater than in the bulk material.The absorption of a film was determined in a calibrated Beckmann model DU spectro- photometer and the absorption coefficient calculated from the thickness determined by weighing as described previously.8 For each oxide, the absorption of at least three films covering a variation in thickness of a factor of at least two was measured and the ab- sorption curves were found to be reproducible. PRESSED DISC SPECTRA The transmission spectra of pressed potassium bromide discs containing 0.1, 0.2 and 0.5 % NiO and 0-15 and 0.3 % COO were measured, relative to that of a blank pressed disc, from 360 to 1OOOmp. NiO was prepared from the nitrate9 and the product was light green in colour and therefore reasonably stoichiometric.10 COO was prepared 5 by ignition of Co304.RESULTS The absorption spectra, from 200 to lo00 mp of typical films of COO and NiO are shown in fig. 1 and 2 respectively. In COO, the absorption increases rapidly from about 500 mp h c., .w 0.4 'd d a 3 .3 c 0 . 2 2 0 0 2 5 0 300 5 0 0 1000 wavelength in mp FIG. 1.-Absorption spectrum of COO film.29 (2-5 ev) and rises to a maximum at 210 mp (5.9 ev). The main features of the spectrum of NiO are the threshold at 600 mp (2.1 eV), the broad maximum at 280 mp (4.4 eV) W. P. DOYLE AND G . A . LONERGAN wavelength in mp FIG. 2.-Absorption spectrum of NiO film. 0 . 4 0 .d 8 a - 8 ._ Y a 0 0.35 0 . 3 3 0 0 400 6 0 0 1000 wavelength in mp FIG. 3.-Absorption spectrum of pressed KBr disc containing 0-1 % NiO. and the peak at 240 mp (5-2 eV). For both oxides the absorption coefficient in the region of maximum absorption is of the order of 106 cm-1.The absorption spectrum of the pressed potassium bromide disc containing 0.1 % NiO is shown in fig. 3. The steps in the range 550-36Omp are considered to be absorption30 OPTICAL ABSORPTION IN COO AND NiO maxima which appear flattened due to the underlying continuous rise in absorption which is probably du0 to the tail of the fundamental band. The only feature observed in the pressed disc spectrum of COO was a broad maximum extending from about 480 to 360 mp. DISCUSSION FIRST MAXIMUM IN THIN FILM SPECTRA For both COO and NiO the absorption coefficient in the region of maximum absorption is of the order of 106cm-1, which can occur only if the oscillator strength is very close to unity.Thus the absorption cannot be due to defects or impurities, nor can it be attributed to transitions in the cation which are forbidden in the free ion but become allowed with a small transition probability in the lattice as a result either of thermal oscillations or the symmetry-type of the lattice.11 Bands of such intensity must correspond either to fully permitted transitions in the cation or to electron transfer processes.12 For the Co2+ ion the first allowed transition (3d7 + 3d64p) is at 12.2 eV and for the Ni2+ ion the first allowed transi- tion (3d8 -+ 3d74p) is at 13.7 eV.13 In the crystal, the energy of transition may be expected to be somewhat reduced,ll by about 2eV, because the electrons in the excited state are able to employ the crystalline field to advantage.However, interpretation of the first maxima in COO and NiO as allowed cation transitions would require that the energy of transition be reduced by as much as 6 to 9 eV which appears unlikely. It is probable therefore that the maxima should be attributed to an electron transfer process. It has often been suggested that in binary ionic inorganic solids the first ab- sorption maximum corresponds to the transfer of an electron from the negative to the positive ion. For the alkali halides the energy of this process has been calculated 14 using the following cycle-a negative ion is removed from the crystal, converted to an atom and the atom replaced; then a positive ion is removed, converted to an atom and the atom replaced; the values so calculated agree well with the experimental data.For COO and NiO, in which both cation and anion are doubly charged, the cycle gives the energy of transfer of an electron from a negative to a positive ion as where CCM is the Madelung constant for the crystal, e, the electronic charge, Y, the interatomic distance, E2, the electron affinity of the 0- ion, 12, the second ioniza- tion potential of the metal. w, the energy of polarization resulting from the absorption act, is given by the expression 15 where a is the sum of the polarkabilities of the ions. For NiO, u was calculated from the refractive index using the Lorentz-Lorenz formula. For COO, whose refractive index has not been reported, the value of o was taken to be the same as for NiO. For the alkali halides, an additional term was introduced to allow for the fact that energy is required to insert an alkali atom in the lattice compared with the smaller positive ion which was extracted.For both nickel and cobalt, the singly charged ion resulting from the absorption act differs from the doubly charged ion in having an extra electron in the 3d shell ; thus there is little difference in size between the singly and doubly charged ions so that the interaction energy of the singly charged ion with the lattice is small and may be neglected. The quantities necessary for the cycle calculations and the resulting values of E are given in table 1. Electron transfer from negative to positive ion corresponds in the zone theory of solids to transitions from the highest full band to the first exciton level so that it may be necessary to consider band width.E calculated as described presumably E = (4uM - l)e2/r + E2 - I2 - W, o = 2.03 e2u/r4,W. P. DOYLE AND G. A . LONERGAN 31 corresponds with the separation between the peak:of the full band and the exciton level. Thus the energy of transitions from near the top of the full band will be less than E by an amount depending on the band width. In the alkali halides the full band is narrow and thus satisfactory agreement with experiment is obtained without considering band width; however, the bands for oxides are much wider and must have a considerable effect. There is no experimental evidence on the width of the highest full band for COO or NiO but soft X-ray emission spectra TABLE 1 COO ( 4 a ~ - l)e2/r, eV 408 12, ev 17.1 rr 8, 2.12 E2, ev - 9.1 nred tc W, eV I 3.7 NiO 208 415 - 9.1 18.2 2.18 2.36 X 10-24 3.7 E, eV 10.9 10.5 have shown 16 that in FeO the width of the oxide ion band is 19 eV, a large part of the width being due to ends where the electron density is very low.It appears that the band ends have not sufficient density to affect the optical behaviour of the crystal so that the effective optical width is the width without the low density ends.17 For FeO the effective width is 8 eV and the top of the band is then 4 eV above the peak. It is assumed that for COO and NiO the band width is the same as for FeO because the width is determined by the extent of overlapping of the oxygen ions and since there is little difference between the radii of the ferrous, cobaltous and nickelous ions, the extent of overlapping, and hence the band width, will be much the same.Thus the expected energy of the exciton transition is 4 eV less than E, that is 6.9 eV for COO and 6.5 eV for NiO. Considering the uncertainties of calculations of this type, these values agree sufficiently well with the experimental values of 5.9 eV and 4.4 eV respectively to confirm that the first absorption maxima are associated with electron transfer from negative to positive ion. For NiO, this interpretation is supported by an analysis of the conductivity, Seebeck effect and optical transmission between 440 and 2500mp from which it has been suggested 18 that the absorption band extending from about 460 mp to shorter wavelengths is probably associated with excitation of electrons from the top full band.SECOND MAXIMUM IN THIN FILM SPECTRA For the alkali halides, the second maximum in the absorption spectrum has been interpreted 19 as the series limit, that is as a transition from the full band to the conduction band and it seems probable that the second peak in NiO may be interpreted in the same way. It has been pointed out 20 that for these excited states of higher quantum number it is preferable to regard the electron in the field of the positive hole as a hydrogen-like system in a medium of dielectric constant KO, so that the separation between the energies of the higher states should be similar to those for the hydrogen atom decreased by a factor ~ 0 2 . Thus if the separation between the exciton level and the series limit is known for one crystal, it can be estimated 17 for others of known KO. Here, KO, the dielectric constant at infra-red frequencies, is estimated from observed indices of refraction.For NaCI, the energy separation between the first and second absorption maxima is 1-9 eV,4 the refractive index is 1.54 and that of NiO is 2-18. Thus, for NiO the32 OPTICAL ABSORPTION I N COO AND NiO expected energy separation is 0.5 eV which is sufficiently close to the experimental value, 0.8 eV, to suggest that in NiO also the second absorption maximum cor- responds to transitions from the full band to the conduction band. NiO does not show photoconductivity between 250 and 800 mp.21 Thus interpretation of the maximum at 280mp as an exciton transition requires that the exciton is not thermally dissociated at room temperature.Further, if the peak at 240 mp is associated with transitions to the conduction band, NiO should be photoconducting below 240 mp. However, no experimental evidence on this point is available. PRESSED DISC SPECTRA The maxima observed in the spectrum of NiO by the technique of pressed potassium bromide discs are given in table 2 (column 3) together with those observed in the transmission spectrum 18 of a 14 p thick film of NiO (column 1) and in the diffuse reflection spectra 22 of NiO + MgO mixed crystals (column 2). In column 4 of table 2 are given the maxima observed in the hydrated Ni2+ ion in aqueous solution which have been assigned 1 to the transitions predicted (columns 5 and 6) using a slightly modified crystal field theory.TABLE 2 observed maxima (cm-1) predicted transitions NiO film 8,070 15,400 From table containing the NiO + Mg0 NiO + K33r Nii$ frequency (cm-1) assignment 8,500 8,500 3A2g-3T2g 13,900 14,500 14,000 14,000 3A2g-3 TIg 21,500 22,200 22,500 22,500 3A2g-1 T2g 25,000 25,000 25,600 3A~g-1A lg 26,600 26,000 27,000 3A2g-3 Tlg 14,900 15,000 3A2g-1Eg 2 it can be seen that the transitions observed in the solid systems Ni2+ ion correspond well with one another and with both the experimental and predicted transitions for the aquo-complex. Thus the spectral data on solids may be interpreted in the same way as those for octahedral nickelous complexes in solution in terms of the transitions within the Ni2+ ion predicted from the crystal field theory.1 This correspondence may be expected as in all the solid systems studied the Ni2+ ion will be octahedrally surrounded by negative ions since NiO, MgO and KBr all possess the rock-salt lattice.The closeness in the frequencies observed for the solid systems and those of the aquo-complex indicate that the appropriate value of Dq for the Ni2+ ion in the solids studied is practically the same as that for the aquo-complex,l 850cm-1. A few mis- cellaneous points arise in connection with the data of table 2. First, the maximum observed at 25,000 cm-1 in both the NiO + MgO mixed crystals and the pressed disc corresponds quite well to the transition to the 1A1, state; this transition has not been observed in the aquo-complex. Secondly, the band at 8070 cm-1 in the NiO film has been interpreted 18 as due to excess oxygen.The correspondence of this band with the 8,500 cm-1 band in the aquo-complex would suggest that it also may be associated with the transition to the 3T2= state of the Ni2+ ion. Thirdly, the aquo-complex band at 14,000 cm-1 which has been assigned 1 to the transition to the 3Tlg state consists of two bands 23 with maxima at 13,800 and 15,100 cm-1. These two maxima correspond well to the transitions to the 3T1, state at 14,000 cm-1 and that to the 1E' state at 15,OOO cm-1. The broad band from about 21,000 cm-1 to about 26,000 cm-1 observed in the pressed disc spectrum of COO appears to correspond to the broad complexW. P. DOYLE A N D G . A . LONERGAN 33 band of the hydrated Co2+ ion which stretches from about 15,000 cm-1 to about 28,000 cm-1 and whose position i s in agreement 1 with the transitions expected from the crystal field theory.The electron diffraction studies were made by Miss M. A. Barrett in the Research Department, Imperial Chemical Industries, Ltd., Metals Division, by courtesy of Dr. N. P. Inglis. One of us (G. A. L.) is indebted to the Department of Education, Republic of Ireland, for a maintenance grant. We wish to thank Prof. T. S. Wheeler for his continued support and encouragement. 1 Orgel, J. Chem. Physics, 1955, 23, 1004. 2 Orgel, Quart. Rev., 1954, 8, 422. 3 Olsen, Smith and Crittenden, J. Appl. Physics, 1945,16,425. 4 Schneider and O’Bryan, Physic. Rev., 1937, 51,293. 5 Chufarov, Zhuravleva and Tatievskaya, Doklady Akad. Nauk S.S.S.R., 1950, 73, 6 Iimori, Nature, 1937, 140, 278. 7 Forestier, Ann. Chim., 1928, 9, 316. 8 Doyle, J. Physics Chem. Solids, 1958, 4, 144. 9 Cairns and Ott, J. Amer. Chem. SOC., 1933, 55, 527. 10 Le Blanc and Sachse, Z. Elektrochem., 1926, 32, 58, 204. 11 Seitz, Rev. Mod. Physics, 1951, 23, 328. 12 Rabinowitch, Rev. Mod. Physics, 1942, 14, 112. 13 Moore, Atomic Energy Levels, vol. I1 (Natl. Bur. Stand Circ. 467), 1952, 85, 103. 14 von Hippel, Z. Physik, 1936, 101, 680. 15 de Boer, Electron Emission and Adsorption Phenomena (University Press, Cam- 16 O’Bryan and Skinner, Proc. Roy. SOC. A, 1940, 176, 229. 17 Wright, Proc. Physic. SOC., 1948, 60, 13. 18 Morin, Physic. Rev., 1954, 93, 1199. 19 Mott, Trans. Faraday Soc., 1938, 34, 500. 20 Mott and Gurney, Electronic Processes in Ionic Crystals (University Press, Oxford, 21 de Boer and Verwey, Proc. Physic. SOC., 1937, 49, suppl. 59. 22 Kroger, Vink and van der Boomgaard, Physica, 1952, 18, 77. 23 Roberts and Field, J. Amer. Chem. SOC., 1950, 72, 4232. 1209. bridge, 1939, p. 241. 1940), p. 99.