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I.R.-IRRADIATION ENHANCED EFFECTS IN
TOURMALINE
M. Dambly, H. Pollak, R. Quartier, W. Bruyneel
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 37, DCcembre 1976, page C6-807
1.R.-IRRADIATION ENHANCED EFFECTS
IN TOURMALINE
M. DAMBLY, H. POLLAK (*), R. QUARTIER and W. BRUYNEEL
Departement de Physique, Groupe de 1'Etat Solide, Universite Nationale du Zaire, Campus de Kinshasa Zaire
Resume.
-
Des monocristaux de tourmaline irradih aux infrarouges ont kt6 ktudiks par spectro- mktrie Mossbauer. Les spectres sont des doublets quadrupolaires correspondant a 4 sites diffkrents :Fez+ el; position cis, Fez+ et Fe3+ en position trans, et un site de valence intermkdiaire. L'irra- diation entraine un klargissement considkrable des raies, ceci pourrait &tre dQ a une transition cis-trans par kchange de proton et saut d'klectron entre Fe3+ et Fez+ en position cis.
Abstract.
-
Single crystals of tourmaline have been investigated by Mossbauer spectroscopy under the influence of infrared irradiation. The spectra consist of a quadrupole doublet which has been fitted with four different iron sites corresponding to a &-Fez+, trans- Fez+, Fe3+ and an intermediate valence site. A considerable broadening of the absorption lines is observed upon irradiation which is attributed to a cis-trans transition through proton exchange and electron hoping between Fe3+ and cis-Fez+ sites.1. Structure and Properties.
-
Tourmalines are silicates of general composition Na(Mg, Fe, Mn),A1,B,Si602,(0H, F), which have a very complex structure [I]. They crystallize in the point group R3m, with unit cell parameter a between 15.84 and 16.03A
and b between 7.10 and 7.25A.
Optical absorption affects only two of the octahedral sites, i. e. b-sites which can contain Mg, Fe, and Mn, and the c-sites which always contain A1 [2]. Since we are investigating by Mossbauer Fe5, absorption, we are only dealing with the b-sites. These sites lie in tri- gonal planes to the c-axis. The central ions are octahedraly coordinated to four oxygen ions and to two hydroxyl groups. The D,, sites share two edges with adjacent octahedra in the trigonal plane and two edges with the c-site octahedra.The connected octahedra build up a spiral chain turning around the c-axis, giving alternatively rise to a cis and trans site (formed by the four oxygen ions and the two hydroxyl groups). Figure 1 represents a cis and a trans site. From the intensive study of Annersten et al. [3,4] on biotite, which presents the same octahe- dral arrangement, it is known that the highest quadru- pole splitting arises from iron in the cis site. They have also pointed out that the cis-trans occupation ratio, in a completely iron filled structure, should be 2 ; we found this ratio to be 1.73, which for an incomple- tely iron substituted structure such as ours, confirms the cis-trans assignment. Moreover, the octahedra are not perfectly regular ; in fact the b-site symmetry is C , with bond distances Mg-(OH), 2.063
A,
Mg-(OH), 2.116A,
Mg-0 2.032A
and 2.023A,
and(*) Centre Regional d'Etudes Nucleaires de Kinshasa, Zalre.
FIG. 1. - Schematic representation of a cis (A) and a trans (B) site in Tourmaline. Small open circles represent oxygen, small crossed circles hydroxyl groups and big double crossed circles
iron ions.
bond angles varying from 910 to 970 [5]. Substitution of Mg by Fe may slightly alter these parameters. The major distortion is a tetragonal elongation, while the central ion is displaced in the direction of the tetragonal axis, which is tilted about 63O with regard to the crystallographic c-axis [2]. In a search for transfer phenomena detected by NGR we may limit ourselves t o one of the trigonal planes, which in fact contains all possible Fe-sites, linked together with a common edge. The crystal may then be looked as if build up by a sequence of such planes. Since we are only considering local effects between adjacent octa- hedra lying in planes, we may expect our conclusions to be valuable for a large class of similar silicates.
Tourmalines are highly dichroic and present a strong absorption band in the infra-red around
C6-808 M. DAMBLY, H. POLLAK, R. QUARTIER AND W. BRUYNEEL
14 500 cm-I 12, 51. This band is assigned to a charge transfer between ferric and ferrous ions in neigh- bouring sites 151. To a very large extent there is a good correlation between pleochroism, I.R. absorp- tion, conductivity and charge transfer in most of the silicates [6].
Ivanov, Kolpakov and Kuz'min [7] studied the effect of illumination on the hyperfine structure of the Mossbauer spectra of paramagnetic ions. It was already pointed out before that a strong illumination can change the electron density at the nucleus and thus result in a shift of the resonance frequencies, and also can lead to an increase in the quadrupole splitting [7]. Our starting point was a little different :
we tried to find an I.R. irradiation stimulated charge transfer. Our reasons were three :
i) as quoted above, illumination may change isomeric shift and quadrupole splitting values,
ii) charge transfer in tourmaline has already been observed by Pollak and Bruyneel using NGR methods 181,
iii) the strong absorption band about 14 500 cm-I is attributed to charge transfer [5].
2. The Experiment. - Two kind of samples were investigated, a pale green one, almost transparent to visible light, and a black opaque one ; The first one contains very little iron ; while the latter has a high iron content. Both samples were cut perpendicular to the c-axis ; the green one having a thickness of 0.8 mm, the black one of 0.5 mm. Specimens were provided by, and are classified at, the MusCe Royal
de 1'Afrique Centrale, Tervuren, Belgium. They were found in Kivu, Zaire. As the samples originate from different mineralogical beds from those previously studied at our laboratory, results are only partly comparable. For the green sample irradiation a commercialy available I.R. bulb of 600 eW was used, giving a flux density of about 3 mW/cm2 at the sample. The black specimen was illuminated by the same lamp, plus another five 200 eW ones, resulting in a flux density of 7 mW/cm2 at the sample. In order to avoid any temperature effect, samples were mounted on a copper disk, refrigerated by a water cooled copper spiral. No higher temperature rise than 3 O was
observed. For each sample three spectra were taken :
before, under, and after switching OK the illumination. Figures 2 and 3 represent the experimental results for the green sample : figure 2 gives the spectrum before and figure 3 displays the spectrum under illumination. A marked difference exists between the two NGR patterns : the depth of the absorption fingers of the irradiated sample is lower than the normal one, while the width has increased. The sum of the areas of all absorption peaks has been conserved. The absorption peak near zero velocity is weaker than the other one ; such an asymmetry is due to the fact that the specimen is a single crystal oriented perpen- dicular to the c-axis, whilst the EFG axis is tilted about 630 off the c-axis.
The experimental spectrum for the normal tourma- line represents one resolved doublet with non-lorent- zian shape. Anyhow, as the well-known crystallo- graphic structure indicates, there should at least be two doublets, corresponding to the two possible sites,
1.R.-IRRADIATION ENHANCED EFFECTS I N TOURMALINE
- 10 0 0 19 2 0 30 rnmls
FIG. 3. - 1.R.-irradiated NGR spectrum of the green Tourmaline (same conventions are taken as in figure 2). It should be noted that the ordinate on figure 3 is extended by a factor 1.3 in comparison to that on figure 2.
even when they are not clearly resolved. In fact computer fitting, with only one doublet has a
x2,
which is much higher than the two doublet fit. This makes the one doublet fit completely unacceptable. In this respect we follow the analysis of materials, such as biotite presenting the same local iron struc- ture [3].Even with this assumption no valuable fit could be obtained, since computed absorption intensities were too low on both flanks of the low energy peak and on the inner flank of the high energy peak. As we are dealing with very thin samples of low iron content, the departure from the fitted Lorentzian shapes due to absorber thickness can not be large enough to account for the discrepancy between experimental and calculated spectra. So we introduced a third doublet corresponding to Fe3+ to take account of one of the dips in the low energy peak ; in fact tour- malines generally contain divalent as well as trivalent ions. As we still are left over with the dip in the second peak, we are forced to introduce a fourth doublet, of intermediate isomeric shift and quadrupole splitting values between those of divalent and trivalent ions. This last one is attributed to iron involved in an exchange process of an electron between neighbouring sites [S].
The four doublet fitting with a
x2
less than 1.2 turned out to be the best one, for the unirradiated and the irradiated sample and also for the sample after return, in spite of the marked difference between the three patterns. As counting rates were greater than 3.6 x lo6 pulses/channel, statistical errors are very small. It is well known that there generally exists more thanone set of values leading to practically identical
xZ
values. We chose initial conditions of our fitting for the three spectra (normal, 1.R.-irradiated and after irradiation) parameters that were comparable among each other for their physical interpretation, i. e. the isomeric shifts and quadrupole splittings for each doublet had neighbouring values for the three cases. Fitted parameters are quoted in table I. AE means half of the total experimental splitting.From our assumption to attribute the fourth doublet to an electronic relaxation, one deduces the activation energy for this process, assuming that relaxation only takes place between the cis site and the Fe3
+.
Following the formula connecting the isomeric shift and the quadrupole splitting of the exchange doublet to its origins (formula 1) [9], one finds by a straight-forward calculation EA = (0.026 # 0.05) eV, a value which is of the same order of magnitude as found earlier [S].P 1 . A ~ + +
+
P , . A E + ' + . ~ - ~ A ' ~ ~ AE,, =PI
+
Pz .ePEAJKT (1)M. DAMBLY, H. POLLAK, R. QUARTIER AND W. BRUYNEEL - 6 - 3+ 6 - 2' cis 6
-
2" trans A E-
2' cis AE-
2' trans Adex r / 2-
3+ r/2 - 2' cis r / 2-
2' trans 'ex12 1- 3+ 1 - 2' cis 1 - 2' trans1
NGR parameters of IR irradiated and non irradiated Tourmaline (isomeric shift referred to metallic iron)
Green Tourmaline
Normal l.R.-irrad. Post-irrad.
-
0.25 f 0.04 0.19+
0.03 0.13+
0.03 1.05+
0.01 1.04+
0.01 1.05 0.01 1.02 f 0.01 1.02+
0.02 0.99+
0.02 0.84 f 0.05 0.86 0.02 0.80+
0.02 0.15+
0.02 0.19+
0.04 0.10+
0.04 1.25 f 0.01 1.34 f 0.02 1.28 0.02 1.13+0.01 1.1250.03 1.14+0.02 0.99 f 0.05 1.04 f 0.04 1.02+
0.03 0.18+
0.04 0.20 f 0.06 0.21+
0.05 0.15 f 0.01 0.16 f 0.02 0.19+
0.02 0.15+
0.01 0.16+
0.02 0.18+
0.02 0.23+
0.07 0.21+
0.04 0.18+
0.03 0.089+
0.020 0.092+
0.020 0.141+
0.020 0.509 f 0.050 0.321+
0.030 0.413+
0.030 0.295+
0.030 0.426 _f 0.010 0.339+
0.040 0.107 f 0.020 0.161+
0.15 0.047+
0.020 Black Tourmaline Normal l.R.-irrad.-
-
0.30 4 0.02 0.24+
0.05 1.27 L- 0.01 1.25+
0.01 1.28+
0.04 1.26+
0.03 1.04 0.05 1.16 f: 0.06 0.13 f 0.03 0.10+
0.03 1.24 f (3.03 1.25 0.03 1.10+
0.02 1.23+
0.03 0.89 f 0.03 1.03+
0.03 0.21 f 0.03 0.26+
0.04 0.18 f 0.01 0.17+
0.02 0.16 f 0.02 0.17+
0.03 0.28+
0.02 0.29+
0.02 0.094+
0.020 0.092 $. 0.020 0.495+
0.040 0.335 f 0.030 0.302+
0.030 0.325 f 0.030 0.109+
0.030 0.258 f 0.030 3. Conclusion.-
Intrinsic parameters, isomericshift, quadrupole splitting and line width, remain almost constant troughout the whole experiment, although a weak increase in quadrupole splitting and line width under irradiation is observed, in accordance with the experimental study of Ivanov et
al. [7]. The major change under irradiation is found
in the intensities : the trans doublet has its intensity largely enhanced, especially for the green single- crystal specimen. The exchange doublet also increase under irradiation, and this effect is more important for the black tourmaline, as expected, because the high iron content favours electron charge transfer, since the number of iron-linked octahedra is related t o the darkness of the sample [lo]. It may be suggested that we simultaneously have two effects :
i) a cis-trans transition or a hydrogen hopping which modifies the original cisltrans population ratio, ii) an enhancement of the fast electron hopping process Fe2+ -t Fe3+.
After switching off the light the intensity of the
trans doublet decreases in favour of the cis one,
whilst the initial ratio is approached. The number
of Fe3+ ions seems not to vary during the experiment. The cis-tran9-cis transition has to be considered as a relaxation process where one hydrogen (H') jumps from an oxygen (0--) to another one of the same iron coordination octahedron. Summarizing, we propose that the irradiation effects in tourmaline observed in Mossbauer spectra may be explained as arising (a) from a cis-trans transition due to proton hopping between two oxy-anions at the same iron site and (b) from electron relaxation between Fe2' and Fe3" in two neighbouring sites. Cleary, further experimental work is necessary to substantiate this interpretation, especially in establishing the differences between NGR parameters for the cis and trans sites in silicates. This would remove much of the ambiguity involved in the computer analysis of the poorly resolved components of Mossbauer spectra.
Acknowledgments.
-
We are very grateful to the MusCe Royal de 1'Afrique Centrale, Tervuren, Bel- gium, and especially to Mr Deliens who provided and cut the samples. We also wish to thank Mr J. Golds- tein without whose technical assistance we could never have accomplished these experiments.References
[I] BUERGER, M. J., BURNHAM, C. W. and PEACOR, D. R., [7] IVANOV, A. S., KOLPAKOV, A. V. and KUZ'MIN, R. N., SOY.
Acta Crystallogr. 15 (1962) 583. Phys. Stat. 16 (1974) 794.
L2] W1LK1Ns, R. T-, FARELL, E. F. and NAIMAN, C. S., J. [8] POLLAK, H. and BRUYNEEL, W., Proc. ZnZe?. Con$ OB
Phys. Chem. Sol. 30 (1969) 43.
[3] ANNERSTEN, H., Lithos 33 (1968) 246. Mossbauer Spec. Cracow 1 (1975) 427.
[4] ~ ; i ~L., w;iPPLING, ~ ~ ~ ~R. 6and A~ , ~H., [91 ~ GERARD, ~A. and GRANDJEAN, ~ ~F., J. P h ~ s - ~ Chem. Sol. ~ 36 ~ ,
Phys. Stat. Sol. 33 (1969) 741. (1967) 1365.
[5] TOWNSEND, M. G., J. Phys. Chem. Sol. 31 (1970) 2481. [lo] ALLEN, G. C. and HUSH, N- S.2 PJ'ogre~~ Inorg. Chem 8 [6] POLLAK, H. and HERINCKX, C., Proc. Inter. ConJ on Moss- (1967) 357.
