Clay Minerals (1986) 21, 191-200 191 INFRARED STUDY OF GOETHITES OF VARYING CRYSTALLINITY AND PARTICLE S I Z E : I. I N T E R P R E T A T I O N OF OH AND LATTICE VIBRATION FREQUENCIES P. C A M B I E R Station de Science du Sol, INRA, Route de St Cyr, 78000 Versailles, France (Received 2 December 1985) A B S T R A C T : A detailed study of the IR spectrum of goethite is given with the aim of relating variations to crystalline order and particle size. The OH stretching vibrations are split into two active components at high frequency, plus two inactive ones at low frequency. Two different bending modes exist from site group splitting. Their active modes from factor group splitting are at lower frequencies than the uncoupled ones. The lattice bands at 630 and 400 cm-~ correspond to Fe-O or Fe--OH stretching, approximately parallel to a and c, and thus respectively sensitive and not sensitive to the particle shape, as long as it remains elongated along c. The crystalline structure and properties of goethite, a mineral occurring widely in soils, are well known. They are influenced by factors such as AI/Fe substitution (Thiel, 1963). When this substitution rate varies, however, it is clear that other properties, such as particle size and 'crystallinity', also vary. Neglecting this fact together with misinterpretation of some features of the IR spectra lead to apparently contradictory conclusions: viz. A I / F e substitutions strengthen (Fey & Dixon, 1981) or weaken (Mendelovici et al., 1979) inner H-bonds. Schulze & Schwertmann (1985) were the first to show clearly that properties such as I R absorption frequencies varied considerably even for samples with the same Al-substitution ratio (including zero). A detailed study of some new series of unsubstituted goethites (Schwertmann et al., 1985) thus appeared warranted. However, before considering the effect of crystallinity a re-examination of the goethite I R spectrum proved necessary. In the present paper the O H bands particularly will be considered since they are known to be useful probes for their environment; however, their frequency variations can result either from modifications in the O H sites, or from coupling effects. Another band at ~630 cm -1 has also been recognized as a sensitive probe for crystaUinity (Sato et al., 1969) and this is examined more thoroughly. A subsequent paper (Cambier, 1986) will describe variations in I R spectra of a series of synthetic goethites. MATERIALS AND METHODS Samples From a series of synthetic goethites of varying crystaUinity and size (Cambier, 1986), the sample of largest surface area ( 3 9 / 4 ~ was subjected to hydrothermal treatments in 1986 The Mineralogical Society 192 P. Cambier either light or heavy water, or a mixture of both. After one night at 210~ in a Teflon bomb with pure (,,,98%) D20 and subsequent drying at room atmosphere at 60~ the ratio of deuteration determined from the integrated intensities of OH and OD stretching bands, 2 • I(OD)/2 x I(OD) + / ( O H ) (Rouxhet et al., 1977) was 86%, and was stable for weeks. Thus it could be assumed that the inner H / D ratio was roughly determined by the composition of the treatment water, but surface and other readily accessible groups were always OH. The goethites were highly crystalline, and identical except for the replacement H/D. Their dimensions were about 40, 80, 500 nm along the a, b and c directions, respectively (Schwertmann et aL, 1985) so that the plane of a deposit of these particles was roughly perpendicular to a. Another sample of this series with larger particles (39/80~ was also used for preparing pellets and oriented deposits. Techniques for IR spectrometry Oriented deposits on Irtran or KRS5 allowed dichroism studies that revealed vibrations parallel to a. Numerous other spectra were obtained using the KBr pellet technique. By regrinding weighted parts of pellets with suitable amounts of KBr, some differential IR spectra were also obtained (Wada & Greenland, 1970). Also, some spectra of T1Br pellets gave information on the direction of the transition moment for some bands sensitive to surface effects, i.e. to the particle shape and surrounding medium (Ruppin & Englman, 1970; Hayashi & Kanamori, 1980). Each IR absorption band of small particles has its maximum in a range between the transverse mode frequency (COT)and the longitudinal mode frequency (COL).The frequency of this maximum decreases with increasing dielectric constant of the surrounding medium but COT and COL are constants for each solid. Thus, replacing KBr by T1Br will lower each absorption maximum frequency, unless it is already near the lowest limit, the crystal mode frequency toT, i.e. if it corresponds either to a weak absorption or to a transition moment parallel to a large dimension of the particles. In the first case, the range is narrow (COT~ COL),and in the second case the maximum frequency is close to COT" A Beckman 4250 spectrophotometer was used, at 150 cm-1/min with a suitable lower speed near absorption maxima (sensitivity 2). For detailed studies in a limited range, expanded spectra were recorded at 20 cm-~/min. RESULTS A 4000-200 cm -~ spectrum of the H-goethite is given in Fig. 1. It should be noted that only one OH stretching mode is clearly resolved at ~3150 cm -1. An OH bending mode appears at 892 cm -1, the transition moment lying in the a-b plane; another one at 795 cm -~ corresponds to a moment parallel to c (Scharzmann & Sparr, 1969). The three OH bands are designated VorI, t~oHand YoHrespectively. After almost complete deuteration, the narrower OD band structure revealed two stretching modes (Fig. 2). These have already been shown in a published spectrum (Schwarzmann & Sparr, 1969), but were not discussed. A dichroism result was obtained (Fig. 2, insert), which indicates that the higher-frequency band has a transition moment parallel to a. IR ~0 . O , . , 4ooo , . . l , . , . l . , 3000 . of goethite 193 . i i 2O0O , i I000 .1500 Wovenumber cm -t 500 F]o, 1. Typical goethite spectrum (a-FeOOH). 100. 3 4 / 4 210 D 5 % ( c o n c e n t r e t e d ,o pellet) o K B r pellet bulk OH 60_ u~ ~ 40. 20. 3 9 / 4 2'10 D deposff on IRTRAN Ot increasing inci_ dence 4000 I 3600 I 3200 2800 W o v e n u m b e r c m -1 2400 2000 FIG. 2. Spectra of goethites deuterated and weakly deuteratedin OH-OD stretchingregion. KBr pellets and, inset, deposit on an IRTRAN window. * = H20 species. Another interesting result from this sample is the resolution of uncoupled OH bending bands (Fig. 3). The frequency values are confirmed by the corresponding OD frequencies that were revealed by differential spectrometry using the H-sample and the sample deuterated at 5% (Fig. 4; Table 1). 194 P. Cambier I , 05 ' ~ ~ g ~ -- E, *, -*-,go _~ ~ -~ ~ g lOO , ~O v ~ ' I J o I ~ i rn Io l I' I -~ I _- I ~ I gl o ol ./ I J I l I I / / I l / / / I I 5- 0- 0- / v cnl--'9 FIG. 3. Spectrum of a deuterated goethite in the OH-OD bending region. KBr pellet. For the other bands, comparison of frequencies for KBr and TIBr pellets (Table 2) should give information on the orientation of the transition moments. For example, with a hematite obtained by heating a goethite at 320~ Rendon & Serna (1981) showed that the vibrations at 630 and 390 c m -1 were lowered in T1Br because they are parallel to the thinner dimension of the particles (c) in hematite, derived from a in goethite. The other frequencies are less sensitive to the surrounding medium because they are already low and they correspond to transition moments lying in the plane of the platy crystals. With goethite, the frequency of the intense m a x i m u m around 400 cm - I is not affected by the alkali halide medium and so should be parallel to c. 6on and 7on are not very sensitive (Table 2). They correspond to more covalent bonds with lower transition moments and 7on lies parallel to the long c-axis of the particles. In contrast, the lattice band around 630 cm - t corresponds to a transition moment parallel to a. The dichroism for this region of the spectrum is given in Fig. 5. The band for a deposit becomes more complex but its two components appear above the KBr value as would be expected with the new surrounding medium, air. From examination of Fig. 5 at ~655 cm -~, the T1Br result for the direction of the transition moment is confirmed. IR of goethite 195 ' I 80 ~6c E o OD uncoupled i I 20 I I J 4000 I 500 W a v e n u m b e r crn-1 FIG. 4. Differential IR spectrum. Sample beam = goethite with 5% D; reference beam = goethite H. TABLE 1. OH and OD bending maxima. OH OD ratio uncoupled d~ ), 938 850 690 620 1.36 1.37 coupled t~ 892 795 685 573 1.30 1.39 TABLE 2. Frequencies of absorption maxima of goethite (39/80~ pressed in salt pellets. Intense lattice band OH bending Goethite KBr TIBr Hematite KBr TIBr 890 884 792 790 640 625 and hematite (39/80~ heated at 320~ Group of bands Shoulder ~495 492 449 448 528 524 436 437 //ca* 636 627 * c in hematite corresponds to a in goethite. Main max Intense sharp 397 396 263 263 295 295 -220 _Lc~ 391 386 P. Cambier 196 [ I I I I [ 1 / ,,',, J // \ ,Y';'--.-.,7 I000 I 5 0 0 c m -1 i , , , I FIG. 5.39/80~ spectra. Deposit on a KRS5 windowat 0 ~ incidence(continuousline) and 40~ incidence (dottedline). DISCUSSION The goethite unit cell is figured in the second paper of this series (Cambier, 1986), after the parameters determined by Forsyth et al. (1968). Mirror planes z = c/4 and z = 3c/4 contain all atoms. The origin is a centre of symmetry (group D,~, or Pbnm). This structure (diaspore or goethite) allows two OH stretching modes and three bending ones to be active in IR (Stegmann et al., 1973; Ryskin, in Farmer, 1974, p. 150). All these bands appear for diaspore although they are not all reported by the authors, and they are not clearly attributed (Cabannes-Ott, 1957; Kolesova & Ryskin, 1962; Schwartzmann & Sparr, 1969; Isetti & Penco, 1969; Stegmann et al., 1973). With goethite, only one stretching band and two bending ones have been reported and the transition moment direction of the latter two determined (see RESULTS). Since they are in the a-b plane and parallel to c, respectively, their existence is necessarily due to the site-group splitting. OH stretching vibrations Even for diaspore, whose two predicted modes are resolved, an attribution has not been given. For goethite, it is clear from the dichroism study (Fig. 2, insert) that the higher IR of goethite 197 frequency corresponds to the B3u mode (transition moment parallel to a) and the lower one to the B2u mode (factor group analysis from Stegmann et aL, 1973). To further understand variations in the OH stretching frequencies, the simple model of mechanically bound masses has been considered (Fig. 6). As an isolated vibrator, OD oscillates at 2325 cm -~ (spectrum of a lightly deuterated goethite in Fig. 2). The stronger coupling that can be expected between OD in the unit cell of a completely deuterated system is between OD 1 and OD 2 (Fig. 6). The model indicates (Fig. 6a) that the higher frequency mode is IR active and the lower frequency one is IR inactive. To obtain the two observed bands, a second coupling between OD 1 and OD 3 must be considered (or one may consider that it acts between a double row of octahedra that contain OD~ and OD 2, and another row that contains OD 3 and OD4). It would be expected to be weaker than the first coupling and calculations based on the two simple models of Fig. 6b give the same result--both modes are IR active, the higher frequency mode with a large transition moment parallel to a, and the lower frequency mode with a moment less intense and parallel to b. This is consistent with the results. Of course, it is physically meaningless to treat the two couplings successively. However, adding perturbations is a usual method. The interaction between OH vibrations can be due to factors other than bonds between adjacent masses (Farmer, 1974, p. 294). But the simple model presented schematically in Fig. 6 explains the results qualitatively, and simple calculations show that predicted and observed shifts are of the same order of magnitude: with the formula (FeOOD)2, m = 2 and M = 176 (Fig. 6a), thus the calculated shift between w u and o9a is about 1%, as is the experimental shift between 2325 and (2365 + 2340)/2. The comparison is sfightly less satisfactory for the second coupling. On the other hand, the influence of surface effects is negligible since the frequencies are no different with KBr pellets and deposits. The validity of the model is reinforced by comparison with isostructural diaspore. The resolution of two OH stretching and three bending bands is related to an enhancement of the resonance phenomena, as would be expected with this m o d e by lowering the cation mass. OH bending vibrations The same m o d e (Fig. 6c) explains qualitatively that, from the isolated OH bending frequencies (938, 850 cm-1), the coupling between OH 1 and O H 2 involves new IR active modes at lower frequencies, and IR inactive ones at higher frequencies. However, the calculation using moments of inertia predicts lower shifts than the observed ones (0-1% and 5% respectively). The second coupling between OH1 and OH 3 should give two absorption maxima for the in-plane bending mode. This does not appear and this last interaction may be enough to produce a weak splitting in the OD stretching region but not in the bending one. Thus, the ~oH band consists of a B3u and BEupair, of which B2u (parallel to b) should be more intense. The out-of-plane bending vibration can produce only one IR active mode because of the mirror plane. Goethite lattice vibrations Recently, Verdonck et aL (1982) attempted a normal coordinate analysis of the goethite IR spectrum using a molecular approach. They considered that isolated molecules Fe3OH 198 OH STRETCHING HODES f_N GOETHITE MODELS 0D vibrations in the goethite structure b 2325 cm-1 uncoupled I. vr~ / uncoupled IR coupled active COU '~ IR unactive Coa : i I 6a : ~AIN COUPLING / COs = i I I 2~ r~, ~) m1,1: m2 antisymmetric symmetric < M Coa > Cou > Cos I ~ active , a n t i s. 3,~:..... t t--.C~s' 9 I I i I e I / I I it I 23 ~s65 ~antis. ~sym. l lhigher frel0uency intense-di~hro~c b sym. antis. I I 234I i / lower f r e q u . less intense coupled, I R active higher frequency large variation in dipolar moment lower frequency lower A~' 6b : SECONDCOUPLING //g IBm : OH B E N D I N G M O D E S IN G O E T H I T E ~ (b 938 CM- I !, WCmo +' :' Co1 low frequency = IR act.~ve Col = ~ ~% ",, :" < Cou 892 CM- 3 i = I~To2 + ~ ) > ~u ! I , Co2 high frequency = IR unactive FIG. 6. Diagrams for the OH spectrum interpretation. (a) and (c) Stronger coupling inside the unit cell (for stretching and bending modes respectively). (b) Second coupling acting on the stretching frequencies. I R o f goethite 199 and F%O vibrate and fitted their strength parameters to the experimental data. Their results agree with those presented above: 400 cm -~ for an antisyrnmetric Fe--OH stretching parallel to c and 630 cm -~ for a symmetric F e - O stretching in the (a-b) plane. By varying the pellet medium, it was deduced that the transition moment for this last band must be parallel to a short dimension of the particles, and by analogy with the particles heated at 320~ (Table 2), a was chosen. The dichroism study (Fig. 5) should distinguish between a and b, but unfortunately the deposit gives at least two components around 670 and 650 cm -1. This complexity also appears with ground natural samples. But with alkali halide pellets of these synthetic goethites it was found that the width of this band is correlated with other band widths (Cambier, 1986). Thus it is not plausible to suspect that several vibration modes fall in this range, and the maximum frequency is known to be influenced by the surrounding medium. The complexity is consistently related to the multiplicity of surface modes, which is due to variability in size and shape of the particles with ground samples, and to aggregation effects with deposits (Clippe et al., 1976). In this sense, the dichroic component at 655 cm -~ should be attributed to isolated and oriented particles. Thus the transition moment is parallel to a. CONCLUSION The present study has reviewed previous investigations on the IR spectra of tt-hydroxides isostructural with goethite. Using a simple mechanical model and new results ofdeuteration and dichroism, it has clarified the attribution of the two O H stretching modes and showed the resonance phenomena that influence the frequencies of all O H bands for this mineral. The coupling inside the unit cell lowers the O H bending frequencies by ~50 cm -1, the out-of-plane being affected a little more. The corresponding high-frequency modes are IR inactive. On the other hand, the absolute frequencies must also be influenced by the O H site, e.g. covalency of the c a t i o n - O - H bond and H-bond strength. The lattice band around 630 cm -1 is affected by particle shape and apparently has a transition moment parallel to a. Conversely, the intense 400 cm -1 band corresponds to a vibration parallel to c and its frequency is not influenced by surface effects for goethite particles elongated along c. REFERENCES CABANNES-OTTC. (1957) Sur la structure de quelques hydroxydes naturels: diaspore, manganite, goethite, 16pidocrocite. C.R. Acad. Sci., Paris 244, 2491-2495. CAMBIERP. (1986) IR study of goethites of varying crystallinity and particle size. II--Crystallographic and morphologic changes among the goethites of two synthetic series. Clay Miner. 21, 201-210. CLIPPEP., EVRARDR. & LUCASA.A. (1976) Aggregation effect on the infrared absorption spectrum of small ionic crystals. Phys. Rev. B. 14, 1715-1721. FARMERV.C. (1974) The Infrared Spectra of Minerals. MineralogicalSociety, London. FEY M.V. & DIXONJ.B. (1981) Synthesis and properties of poorly crystalline hydrated aluminous goethites. Clays Clay Miner. 29, 91-100. FORSYTHJ.B., HEDLEYI.G. & JOHNSONC.E. (1968) The magnetic structure and hyperfine field of goethite (or-FeOOH).J. Phys. C. (Proc. Phys. Soc.) 2, 179-188. HAYASm S & KANAMORIH. (1980) Infrared study of surface phonon modes in cr-Fe203 microcrystals. J. Phys. C: Solid StatePhys. 13, 1529-1538. ISETn G. & PENCOA.M. (1969) Studio sul pleocroismoinfrarosso del diasporo. Periodico Mineral. Roma 38, 31-43. 200 P. Cambier KOLESOVA V.A. & RYSKIN YA.I. (1962) IR absorption spectroscopy of aAIOOH, 7AIOOH and GaOOH. Zhurnal struktumoi Khimii 3, 680-684. MENDELOVICI E., YARIV S & VILLALBAR. (1979) Aluminium bearing goethite in Venezuelan laterites. Clays Clay Miner. 27, 368-372. RENDON J.L. & SERNA C.J. (1981) IR spectra of powder hematite: effects of particle size and shape. Clay Miner. 16, 375-381. ROUXHET P.G., SAMUDACHEATAN., JACOBS H & ANTON O. (1977) Attribution of the OH stretching bands of kaolinite. Clay Miner. 12, 171-179. RUppIN R. & ENGLMANR. (1970) Optical phonons of small crystals. Rep. Prog. Phys. 33, 149-196. SATO K., SUDO T., KUROSAWA F. & KAMMORI O. (1969) The influence of crystallization on the infrared spectra of a- and ?-ferric oxyhydroxides. Nippon Kinzoku Gakkaishi, J. Japan Inst. of Metals 33, 1371-1376 (in Japanese). SCHULZE D. & SCHWERXMANNU. (1984) The influence of aluminum on iron oxides X. The properties of Al-substituted goethites. ClayMiner. 19, 521-539. SCHWARZMANNE. & SPARR H. (1969) Die Wasserstoffbriickenbindung in Hydroxiden mit Diasporstruktur. Z. Naturforschg. 24b, 8-11. SCHWERTMANNU., CAMBIER P. & MURAD E. (1985) Properties of goethites of varying crystallinity. Clays Clay Miner. 33, 369-378. STEGMANN M.G., VIVmN D. & MAZIERES C. (1973) Etudes des modes de vibration infrarouge dans les oxyhydroxydes d'aluminium boehmite et diaspore. SpectrochimicaA cta 29A, 1653-1663. THIEL R. (1963) Zum system aFeOOH-ttA1OOH. Zeitsch.fdr anorg, und allg. Chemie 326, 70-78. VERDONCK L., HOSTE S., ROELANDT F.F. & VAN DER CELEN G.P. (1982) Normal coordinate analysis of cr-FeOOH. A molecular approach. J. Mol. Structure 79, 273-279. WADA K. & GREENLAND D.J. (1970) Selective dissolution and differential infrared spectroscopy for characterization of amorphous constituents in soil clays. Clay Miner. 8, 241-254.