infrared study of goethites of varying crystallinity

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.
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