- Email: [email protected]
Combined vibrational spectra of natural wardite
Journal of Molecular Structure 706 (2004) 95–99 www.elsevier.com/locate/molstruc
Combined vibrational spectra of natural wardite D.K. Breitingera,*, H.-H. Belzb, L. Hajbac, V. Komlo´sic, J. Minkc, G. Brehmd, D. Colognesie, S.F. Parkere, R.G. Schwabf a
Institute of Inorganic Chemistry, University of Erlangen-Nuernberg, Egerlandstrasse 1, D-91058 Erlangen, Germany b Thermo Electron, Im Steingrund 4-6, D-63303 Dreieich, Germany c Chemical Research Center of the Hungarian Academy of Sciences, Molecular Spectroscopy Department, P O Box 17, H-1525 Budapest, Hungary d Institute of Physical Chemistry, Egerlandstrasse 3, D-91058 Erlangen, Germany e Rutherford Appleton Laboratory, Chilton, Didcot, OX11 OQX, UK f Institute of Geology and Mineralogy, Schlossgarten 5a, D-91054 Erlangen, Germany Received 5 January 2004; revised 5 January 2004; accepted 29 January 2004 Dedicated to Professor H.D. Lutz, Siegen, Germany, on the occasion of his 70th birthday Available online 9 April 2004
Abstract Vibrational spectra (IR, Raman, inelastic neutron scattering) were measured of natural wardite (ideal formula NaAl3(OH)4(PO4)2·2 H2O) from Trauira, Brazil, with the main impurities Fe and Ca. The spectra are discussed on the basis of a symmetry analysis restricted to one layer in the four-layer structure. The band pattern in the n(OH) region is due to two different Al2OH groups and their correlation coupling; their deformations d and g are assigned based on IR and INS spectra. Contributions of the hydrogen-bonded H2O molecules are discussed, as are the vibrations of the AlO6 octahedra dominating the Raman spectrum. From the fundamentals n(OH) of the OH groups and their overtones anharmonicity constants have been estimated. q 2004 Elsevier B.V. All rights reserved. Keywords: Wardite; IR and NIR spectra; Raman spectra; Inelastic neutron scattering spectra; Anharmonicity
1. Introduction In our spectroscopic studies of minerals associated in occurrence and paragenesis to the alunite group (see e.g. Ref. [1]) we turned our attention to wardite-type minerals (wardite end-member NaAl3(OH)4(PO4)2·2 H2O) and started with natural samples from Trauira/Maranha˜o, Brazil, containing some Fe3þ (6.7 mol-%) in place of Al3þ. The aim of this study is to understand the vibrational dynamics of this type of minerals on the basis of their structures, using our experiences with alunites [2,3]. For this purpose wide-range IR spectra (near-, mid- and far-IR, 7500 –150 cm21), Raman and inelastic neutron scattering spectra (INS) were measured (Table 1). As a basis for the discussion of these spectra symmetry analyses were * Corresponding author. Tel.: þ 49-9131-8527352; fax: þ 49-91318527387. E-mail address: [email protected] (D.K. Breitinger). 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2004.01.039
performed. A preliminary presentation of part of the results has already been given [4].
2. Experimental The material was characterized by X-ray powder diffraction (tetragonal; a ¼ 704, c ¼ 1888 pm) and analyzed by atomic absorption spectrometry for Na (3.6%), Mg (0.15%), Ca (3.6%), Sr (0.93%), Al (17.5%), and Fe (2.6%); P was determined photometrically as the vanadate – molybdate complex (12.66%). Weight-loss on heating (18.2%) gave the water content. K, Ti and Si were present at the trace level. The pure compound NaAl3(OH)4(PO4)2·2 H2O requires Na 5.78%, Al 20.34%, P 15.57%, H2O 18.11%. NIR spectra of the sample were measured with a Nicolet Nexus 870 spectrometer (CaF2 beam splitter, uncooled PbSe detector) using the DRIFT accessory; FTRaman spectra (excitation by Nd-YAG laser 1064 nm)
96
D.K. Breitinger et al. / Journal of Molecular Structure 706 (2004) 95–99
Table 1 Wave numbers in IR, Raman and INS spectra of wardite and their assignments IR
Raman
INS
n(H2O) 2d(H2O) 2130 m 1695 m 1522 m 1186 m 1121 s 1057 s
1650 m 1559 mw 1168 sh 1129 sh 1058 vs 992 m 898 m 797 mw 666 sh 636 s 588 mw
Assignments
n(OH)
3613 m 3580 sh 3545 s 3520 vw 3404 s,br 3306 m,br 3154 w,br 2967 w,br
1033 m,br 999 m ,800 vw,br
636 vs
990 m 896 vs 797 s 708 w 673 m 626 w 595 mw 576 m 553 mw 535 w
397 s
502 w 452 m 415 w 393 w
336 vw
326 ms
253 sh 238 m 224 sh
258 w 243 sh 219 vw
191 w 169 w 156 w
197 w
446 vw
2d(Al2OH) d(H2O)
d(Al2OH) n3(PO4)/d(Al2OH) n3(PO4) n1(PO4)/d(Al2OH) v/r(H2O) r/v(H2O) n(Al2OH) n(Al(O/OH)6) g(Al2OH)
n(Al(O/OH)6)
515 s 498 mw 468 sh 427 m 393 w 360 mw 337 m 307 sh
278 mw 250 mw
All raw spectral data were processed and presented with 6.0.
ORIGIN
n(AlOH2) d(Al(O/OH)6) d(AlOH2Na(Ca))
skeletal def./T(Na(Ca))
222 m 210 mw
were obtained with the same instrument with Raman accessory. For comparison Raman spectra were also measured with Biorad and Bruker: RFS 100/s FT-Raman spectrometers:(Ge detector, 93 K) Fall-off of detector sensitivity prevented registration of Raman spectra above 3300 cm21 in all cases. Due to strong fluorescence of our samples excitation of Raman spectra with lasers in the visible range was not possible. Mid-IR spectra of KBr and CsI pellets were scanned with the spectrometers Mattson Infinity TM and Biorad FTS 40, far-IR spectra (PE pellets) with the Biorad FTS 175. INS spectra (sample at 10 K) were measured with the inverted time-of-flight spectrometer TOSCA at the pulsed spallation neutron source ISIS (Rutherford Appleton Laboratory, Chilton, UK).
3. Structure The crystal structures of natural wardite and of the isomorphous cyrilovite NaFe3(OH)4(PO4)2·2 H2O were solved (space group P41212, Z ¼ 4Þ [5,6]; recently, the structure of the latter was further refined [7]. Hydrogen atoms were not located, but reasonable positions can be estimated. The structures contain layers of two kinds of corner-linked m2 – OH bridged MO6 octahedra (M ¼ Al, Fe), stacked along the tetragonal c-axis in a four-layer sequence and linked by PO4 groups. Within a layer, e.g. around the (001) plane, two independent pairs of symmetry-correlated m2 – OH groups are arranged in the equatorial pseudo-planes of one kind of MO6 octahedra (around M(2), atom numbering scheme as in Refs. [5,6]) with the OH bonds almost perpendicular to the (001) plane. The distances between these OH groups along the octahedron edges are short (r(O· · ·O) 266 and 272 pm in wardite; M ¼ Al). Also the H2O molecules as ligands in the second kind of Al(1)O6 octahedra (r(Al(1) – O(6)) 202 pm) have short intermolecular distances (r(O· · ·O) 311 pm). This point is relevant in the discussion of the vibrations of the OH and H2O particles. The H2O molecules are engaged in medium– strong hydrogen bonds to one oxygen atom O(2) of the PO4 groups and to one of the bridging O(5)H groups as acceptors (r(O– H· · ·O) 264 and 283 pm, respectively). Obviously, none of the O(5)H and O(7)H groups is involved in hydrogen bonds as donor. In the present sample Fe3þ replaces 6.7% of the Al3þ assuming statistical distribution in the two different sites of Al3þ. An EPR study of iron-containing wardite suggests a preference in substitution of Fe3þ for Al(2)3þ in the Wyckoff site 4a (symmetry 2-C2 Þ [8]; if so, Al(2)3þ in our sample can be replaced by Fe3þ up to 20% giving rise to local formation of Al –OH – Fe groups. Naþ in an unusual distorted tetragonal-prismatic eightcoordination with two equivalent H2O ligands (r(Na –O(6)) 257 pm) and O-atoms of PO4 groups can be replaced to some extent by Ca2þ and even Sr2þ, which fit better into this site ð4aÞ: It is not clear how the charge-balance is achieved; options are under-occupation of this site and/or partial reduction of Fe3þ to Fe2þ and partial substitution of Al3þ by Mg2þ.
4. Results and discussion 4.1. Symmetry analyses Principally, a symmetry analysis of the whole unit cell can be performed, e.g. using the Adams – Newton Tables [9],
D.K. Breitinger et al. / Journal of Molecular Structure 706 (2004) 95–99
but it does not make much sense to classify all 333 degrees of vibrational freedom of the unit cell. Since the distances between the layers are around 500 pm their dynamical interaction is expected to be low, and hence to a good approximation the symmetry analysis can be restricted to one layer as an independently vibrating system; of course, couplings of some skeletal vibrations in adjacent layers occur via the linking PO4 groups. The layer symmetry is only p121 or ða00 : a0 Þ·2 (Shubnikov notation [10]) with the transformed axes a0 ¼ a1 þ a2 and a00 ¼ 2a1 þ a2 : Then the vibrations are classified into the species a and b of 2-C2 ; where a means in-phase and b counter-phase vibrations in symmetry-correlated pairs of groups in the layer; both species are IR and Raman active. Under the layer symmetry all overtones (species a) and combinations (species a and b) are principally active in both IR and Raman spectra, but from experience these secondorder excitations are usually only observable in IR spectra.
4.2. Mid-IR and far-IR spectra The n(OH) modes in the two independent pairs of symmetry-correlated OH groups classify as 2a þ 2b; with the correlation splitting between a and b species depending on the distances in each of the pairs. The n(OH) region of IR (and Raman) spectra of wardite (see Fig. 1 and Ref. [11]) shows two sharp bands (3613 and 3545 cm21) with two weak shoulders or satellites (, 3580 and 3520 cm21). The strong IR bands are most probably the counter-phase b species with an amplified resultant transition moment; in the a species the oscillating dipole moments in neighbouring OH groups nearly compensate mutually giving weak IR absorptions. The satellites could also originate from the n(OH) modes in the Al –OH – Fe groups (cf. Section 3, see the discussion in Refs. [6,11]). It is not possible to assign unambiguously the two main n(OH) bands to the two independent pairs of OH groups because of their different
Fig. 1. Mid-IR spectrum of natural wardite from Trauira/Maranha˜o, Brazil.
97
bonding situation with counteracting effects (Al –O bond lengths, acceptor for hydrogen bonds). The system of broad overlapping bands in the region 3500 – 3000 cm 21 is due to the valence vibrations of the hydrogen-bonded H2O molecules and the resonance-enhanced overtones of their deformation mode d(H2O). The latter clearly show correlation splitting (1650/1560 cm21) as a consequence of the short distance and orientation of the H2O molecules. The deceptivly simple strong IR band centered at 1059 cm21 contains at least four components of n(PO4) generated by lifting of the originally threefold degeneracy of n3(PO4) ðt2 Þ and activation of n1(PO4) ða1 Þ due to the general position of PO4 and again at least four components ð2a þ 2bÞ of the deformation modes d(Al2OH) involving the two pairs of unequal OH groups. The region near the minimum of transmittance is dominated by the components of n3(PO4) and the band at 992 cm21 by n1(PO4), as suggested by the Raman spectrum. The latter vibration and one of the d(Al2OH) modes are obviously accidentally degenerate (see Section 4.5). Other d(Al2OH) components appear as weak bands/shoulders at 1168 and 1129 cm21. The possible d modes of the Al –OH – Fe groups also have to be taken into account. The neatly separated bands at 898 and 797 cm21 are to be assigned to the librational modes v and r of the H2O molecules coordinated to both Al and Na(Ca) and involved in two hydrogen bonds; due to this bonding situation these frequencies are rather high (for the discussion of the librational modes of complexed H2O see Ref. [12]). Strong support of this assignment is provided by the INS spectra (Section 4.5). The band system with minimum at 636 cm21 is again highly complex as it comprises the split n(PO4) mode, at least four g(Al2OH) modes ð2a þ 2bÞ (plus g(AlOHFe) and several valence vibrations of the two distorted AlO6 (and FeO6) octahedra with admixtures of deformations d(PO4). The group of bands below 500 cm21 (Fig. 2) comprises further Al – O valence vibrations, especially n(Al – OH2)
Fig. 2. Far-IR spectrum of wardite.
98
D.K. Breitinger et al. / Journal of Molecular Structure 706 (2004) 95–99
modes (species a þ b), more precisely valence vibrations of the bent Al– OH2 –Na(Ca) subsystem; candidates for these latter vibrations are the bands at 468 and 427 cm21. The next band (393 cm21) coincides with Raman and INS bands (Sections 4.3 and 4.5) assigned to a deformation of the octahedra AlO6. Deformations of the above subsystem (in plane, out of plane with respect to e.g. the (001) plane) could cause bands at 360, 337 and 307 cm21. Finally, the bands at 238 cm21 and below (191, 169, 156 cm21) could be associated to the translational motions of Naþ or Ca2þ in their compliant cages. 4.3. Raman spectra The deceptively simple Raman spectra (Fig. 3) display an unresolved broad n3(PO4) and a n1(PO4) band, a very weak and broad feature around 800 cm21 for v(H2O) or r(H2O) and three strong bands which are characteristic for the Raman active valence and deformation modes n1 ; n2 and n5 of an octahedron AlO6; for comparison with the vibrations of [Al(H2O)6]3þ see e.g. Refs. [13,14]. Of course, these three broadened, partly asymmetric bands hide a great variety of vibrations, as discussed in Section 4.2. The weak bands at 258 cm21 and below probably reflect the vibrations of the cations Naþ and Ca2þ. Remarkably, the FT-Raman spectrum displays two welldefined absorptions (!) in the thermal-emission background, formally with Raman shifts of 2475 and 2340 cm21 (see insert in Fig. 3; these shifts correspond to absolute wavenumbers 6923 and 7058 cm21, which match almost exactly values for two NIR-bands (see Section 4.4). 4.4. NIR spectra These absorption bands in the FT-Raman spectrum appear in the NIR spectrum (Fig. 4) at 7052 and 6914 cm21, and are clearly true overtones or quasi-overtones of
Fig. 4. NIR spectrum of wardite.
the n(OH) modes (cf. [3]), i.e. binary combinations of the same fundamentals in different species (here a þ bÞ: The band system with minimum at 4581 and shoulders at 4665, 4545 and 4503 cm21 is to be assigned to several possible binary combinations n þ d and the broad weak bands around 4050 and 3900 cm21 to combinations n þ g: Obviously, also ternary combinations n þ d þ g appear (, 5000 cm21 and above), as has been also found with augelites [15]. Assuming true overtones for the high-frequency NIR bands anharmonicity parameters (first-order correction for anharmonicity) for the n(OH) vibrations can be estimated using the equation set for fundamentals n01 ¼ ve 2 2xe ve and overtones n02 ¼ 2ve 2 6xe ve given e.g. in Ref. [3]. With the frequencies for fundamentals n01 ¼ 3613/ 3545 cm21 and overtones n02 ¼ 7052=6914 cm21 the parameters ve ¼ 3787=3727 cm21 and xe ve ¼ 87=90 cm21 are obtained. 4.5. Inelastic neutron scattering (INS) spectra
Fig. 3. FT-Raman spectrum (section) of wardite. (insert shows absorptions for 2v(OH); apparent Raman shifts converted to absolute wavenumbers).
With respect to the scattering cross sections for neutrons and the vibrational amplitudes of the constituents of wardite only vibrations involving hydrogen nuclei are expected to be observed in the INS spectra, irrespective of the fact that all vibrations should be observable without symmetry restrictions. In principle, vibrations involving Fe (moderate cross section) could also contribute, but its content is probably too low to give observable signals. In the high-energy transfer part of the INS spectrum of wardite no clear-cut bands are observed, probably due to the energy distribution of the incoming neutrons and the increasing dissipation of intensity into overtones and combinations of fundamentals and their phonon-wings (combinations of internal and lattice vibrations, see the discussion in Ref. [12]). Therefore, only the low-energy
D.K. Breitinger et al. / Journal of Molecular Structure 706 (2004) 95–99
99
5. Conclusions The combination of these complementary methods of vibrational spectrometry fosters the basic understanding of the vibrational dynamics of this natural ‘impure’ material. A more detailed understanding would require syntheses of stoichiometric and also deuterated compounds in order to get simpler spectra and a broader basis for their assignment. Up to now attempts to synthesize this compound under varying conditions always gave mixtures with other aluminium – phosphate phases. The search for phase-pure synthetic wardite will be continued.
Acknowledgements Fig. 5. Inelastic neutron scattering spectrum (section) of wardite (smoothed by the box-car/gliding average method).
transfer range of the INS spectrum of wardite is shown (Fig. 5). Nevertheless, some signals outside this range are shortly discussed. A structured signal peaked at 2130 cm21 comprises overtones and combinations of the different d(Al2OH) modes. Sharp signals at 1695 and 1522 cm21 are d(H2O) modes without doubt, but in the same region weaker combinations of d(Al2OH) and g(Al2OH) appear. A group of four stronger signals (with satellites) at 1186, 1121, 1057 and 990 cm21 (Fig. 5) are the expected d(Al2OH) (and d(AlFeOH)) modes the frequency region of which is well known from alunites (prototype KAl3(OH)6(SO4)2) [2] and augelites (Al2(OH)3(PO4)) [1]. The accidental degeneracy of the lowest d(Al2OH) and the n1(PO4) vibrations has already been adressed in Section 4.2. The prominent bands at 896 and 797 cm21 are the v(H2O) and r(H2O) modes showing-up at the same frequencies in the IR spectrum. The highly complex band system between 700 and 400 cm21 should contain six AlO valence vibrations in the Al2OH fragments (in the 700 –600 cm21 section), four g(Al2OH) modes (around the maximum at 576 down to 500 cm21), two twist modes t(H2O) (below 500 cm21) and two n(Al – OH2) valence vibrations 452 and 415 cm21 (more detailed discussion of this point in Section 4.2). A weak sharp feature at 393 cm21 corresponds to one of the strong Raman bands (derived from n5 of an octahedron AlO6) and to an IR band and is thus a deformation mode involving OH groups. The lowest group of signals starts with deformations of the fragment Al– OH2 –Na(Ca) (326 cm21, strong in IR, very weak in Raman at 337 cm21), followed by skeletal deformations with contributions of OH and H2O induced by and coupled with motions of the Naþ and Ca2þ cations.
Support by the Deutsche Forschungsgemeinschaft, Bonn-Bad Godesberg, (grant Br 422/23-1 þ 2) and by the Fonds der Chemischen Industrie, Frankfurt/Main, is gratefully acknowledged. Thanks are also due to then Thermo Nicolet, NeuIsenburg, for the possiblity to measure NIR and FT-Raman spectra.
References [1] D.K. Breitinger, J. Mohr, D. Colognesi, S.F. Parker, H. Schukow, R.G. Schwab, J. Mol. Struct. 563/564 (2001) 377. [2] D.K. Breitinger, R. Krieglstein, A. Bogner, R.G. Schwab, Th.H. Pimpl, J. Mol. Struct. 408/409 (1997) 287. [3] D.K. Breitinger, H. Schukow, H.-H. Belz, J. Mohr, R.G. Schwab, J. Mol. Struct. 480/481 (1999) 677. [4] D.K. Breitinger, L. Hajba, V. Komlo´si, J. Mink, G. Brehm, D. Colognesi, S.F. Parker, H. Schukow, R.C. Schwab, in: J. Mink, G. Jalsovszki, G. Keresztury (Eds.), XVIIIth International Conference on Raman Spectroscopy (ICORS 2002), Budapest, Hungary, 25– 30 August, Wiley, New York, 2002, p. 921. [5] L. Fanfani, A. Nunzi, P.F. Zanazzi, Min. Mag. 37 (1970) 598. [6] D. Cozzupoli, O. Grubessi, A. Mottana, P.F. Zanazzi, Min. Petr. 37 (1987) 1. [7] M.A. Cooper, F.C. Hawthorne, P. Cerny, J. Czech. Geol. Soc. 45 (2000) 95. [8] A.B. Vassilikou-Dova, Appl. Magn. Resonance 5 (1993) 25. [9] D.M. Adams, D.C. Newton, Tables for Factor Group and Point Group Analysis, Beckman-RIIC, Croydon, 1970. [10] A.V. Shubnikov, V.A. Kopsik, Symmetry in Science and Art, Plenum Press, New York, 1974, Chapter 8, Table 11. [11] P. Tarte, A.M. Fransolet, F. Pillard, Bull. Mine´ral. 107 (1984) 745. [12] S.F. Parker, K. Shankland, J.C. Sprunt, U.A. Jayasooriya, Spectrochim. Acta A53 (1997) 2333. [13] W.W. Rudolph, R. Mason, C.C. Pye, Phys. Chem. Chem. Phys. 2 (2000) 5030. [14] W.W. Rudolph, R. Mason, J. Solution Chem. 30 (2001) 527. [15] D.K. Breitinger, H.-H. Belz, H. Schukow, J. Mohr, R.G. Schwab, J. Mol. Struct. 651– 653 (2003) 177.
Combined vibrational spectra of natural wardite D.K. Breitingera,*, H.-H. Belzb, L. Hajbac, V. Komlo´sic, J. Minkc, G. Brehmd, D. Colognesie, S.F. Parkere, R.G. Schwabf a
Institute of Inorganic Chemistry, University of Erlangen-Nuernberg, Egerlandstrasse 1, D-91058 Erlangen, Germany b Thermo Electron, Im Steingrund 4-6, D-63303 Dreieich, Germany c Chemical Research Center of the Hungarian Academy of Sciences, Molecular Spectroscopy Department, P O Box 17, H-1525 Budapest, Hungary d Institute of Physical Chemistry, Egerlandstrasse 3, D-91058 Erlangen, Germany e Rutherford Appleton Laboratory, Chilton, Didcot, OX11 OQX, UK f Institute of Geology and Mineralogy, Schlossgarten 5a, D-91054 Erlangen, Germany Received 5 January 2004; revised 5 January 2004; accepted 29 January 2004 Dedicated to Professor H.D. Lutz, Siegen, Germany, on the occasion of his 70th birthday Available online 9 April 2004
Abstract Vibrational spectra (IR, Raman, inelastic neutron scattering) were measured of natural wardite (ideal formula NaAl3(OH)4(PO4)2·2 H2O) from Trauira, Brazil, with the main impurities Fe and Ca. The spectra are discussed on the basis of a symmetry analysis restricted to one layer in the four-layer structure. The band pattern in the n(OH) region is due to two different Al2OH groups and their correlation coupling; their deformations d and g are assigned based on IR and INS spectra. Contributions of the hydrogen-bonded H2O molecules are discussed, as are the vibrations of the AlO6 octahedra dominating the Raman spectrum. From the fundamentals n(OH) of the OH groups and their overtones anharmonicity constants have been estimated. q 2004 Elsevier B.V. All rights reserved. Keywords: Wardite; IR and NIR spectra; Raman spectra; Inelastic neutron scattering spectra; Anharmonicity
1. Introduction In our spectroscopic studies of minerals associated in occurrence and paragenesis to the alunite group (see e.g. Ref. [1]) we turned our attention to wardite-type minerals (wardite end-member NaAl3(OH)4(PO4)2·2 H2O) and started with natural samples from Trauira/Maranha˜o, Brazil, containing some Fe3þ (6.7 mol-%) in place of Al3þ. The aim of this study is to understand the vibrational dynamics of this type of minerals on the basis of their structures, using our experiences with alunites [2,3]. For this purpose wide-range IR spectra (near-, mid- and far-IR, 7500 –150 cm21), Raman and inelastic neutron scattering spectra (INS) were measured (Table 1). As a basis for the discussion of these spectra symmetry analyses were * Corresponding author. Tel.: þ 49-9131-8527352; fax: þ 49-91318527387. E-mail address: [email protected] (D.K. Breitinger). 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2004.01.039
performed. A preliminary presentation of part of the results has already been given [4].
2. Experimental The material was characterized by X-ray powder diffraction (tetragonal; a ¼ 704, c ¼ 1888 pm) and analyzed by atomic absorption spectrometry for Na (3.6%), Mg (0.15%), Ca (3.6%), Sr (0.93%), Al (17.5%), and Fe (2.6%); P was determined photometrically as the vanadate – molybdate complex (12.66%). Weight-loss on heating (18.2%) gave the water content. K, Ti and Si were present at the trace level. The pure compound NaAl3(OH)4(PO4)2·2 H2O requires Na 5.78%, Al 20.34%, P 15.57%, H2O 18.11%. NIR spectra of the sample were measured with a Nicolet Nexus 870 spectrometer (CaF2 beam splitter, uncooled PbSe detector) using the DRIFT accessory; FTRaman spectra (excitation by Nd-YAG laser 1064 nm)
96
D.K. Breitinger et al. / Journal of Molecular Structure 706 (2004) 95–99
Table 1 Wave numbers in IR, Raman and INS spectra of wardite and their assignments IR
Raman
INS
n(H2O) 2d(H2O) 2130 m 1695 m 1522 m 1186 m 1121 s 1057 s
1650 m 1559 mw 1168 sh 1129 sh 1058 vs 992 m 898 m 797 mw 666 sh 636 s 588 mw
Assignments
n(OH)
3613 m 3580 sh 3545 s 3520 vw 3404 s,br 3306 m,br 3154 w,br 2967 w,br
1033 m,br 999 m ,800 vw,br
636 vs
990 m 896 vs 797 s 708 w 673 m 626 w 595 mw 576 m 553 mw 535 w
397 s
502 w 452 m 415 w 393 w
336 vw
326 ms
253 sh 238 m 224 sh
258 w 243 sh 219 vw
191 w 169 w 156 w
197 w
446 vw
2d(Al2OH) d(H2O)
d(Al2OH) n3(PO4)/d(Al2OH) n3(PO4) n1(PO4)/d(Al2OH) v/r(H2O) r/v(H2O) n(Al2OH) n(Al(O/OH)6) g(Al2OH)
n(Al(O/OH)6)
515 s 498 mw 468 sh 427 m 393 w 360 mw 337 m 307 sh
278 mw 250 mw
All raw spectral data were processed and presented with 6.0.
ORIGIN
n(AlOH2) d(Al(O/OH)6) d(AlOH2Na(Ca))
skeletal def./T(Na(Ca))
222 m 210 mw
were obtained with the same instrument with Raman accessory. For comparison Raman spectra were also measured with Biorad and Bruker: RFS 100/s FT-Raman spectrometers:(Ge detector, 93 K) Fall-off of detector sensitivity prevented registration of Raman spectra above 3300 cm21 in all cases. Due to strong fluorescence of our samples excitation of Raman spectra with lasers in the visible range was not possible. Mid-IR spectra of KBr and CsI pellets were scanned with the spectrometers Mattson Infinity TM and Biorad FTS 40, far-IR spectra (PE pellets) with the Biorad FTS 175. INS spectra (sample at 10 K) were measured with the inverted time-of-flight spectrometer TOSCA at the pulsed spallation neutron source ISIS (Rutherford Appleton Laboratory, Chilton, UK).
3. Structure The crystal structures of natural wardite and of the isomorphous cyrilovite NaFe3(OH)4(PO4)2·2 H2O were solved (space group P41212, Z ¼ 4Þ [5,6]; recently, the structure of the latter was further refined [7]. Hydrogen atoms were not located, but reasonable positions can be estimated. The structures contain layers of two kinds of corner-linked m2 – OH bridged MO6 octahedra (M ¼ Al, Fe), stacked along the tetragonal c-axis in a four-layer sequence and linked by PO4 groups. Within a layer, e.g. around the (001) plane, two independent pairs of symmetry-correlated m2 – OH groups are arranged in the equatorial pseudo-planes of one kind of MO6 octahedra (around M(2), atom numbering scheme as in Refs. [5,6]) with the OH bonds almost perpendicular to the (001) plane. The distances between these OH groups along the octahedron edges are short (r(O· · ·O) 266 and 272 pm in wardite; M ¼ Al). Also the H2O molecules as ligands in the second kind of Al(1)O6 octahedra (r(Al(1) – O(6)) 202 pm) have short intermolecular distances (r(O· · ·O) 311 pm). This point is relevant in the discussion of the vibrations of the OH and H2O particles. The H2O molecules are engaged in medium– strong hydrogen bonds to one oxygen atom O(2) of the PO4 groups and to one of the bridging O(5)H groups as acceptors (r(O– H· · ·O) 264 and 283 pm, respectively). Obviously, none of the O(5)H and O(7)H groups is involved in hydrogen bonds as donor. In the present sample Fe3þ replaces 6.7% of the Al3þ assuming statistical distribution in the two different sites of Al3þ. An EPR study of iron-containing wardite suggests a preference in substitution of Fe3þ for Al(2)3þ in the Wyckoff site 4a (symmetry 2-C2 Þ [8]; if so, Al(2)3þ in our sample can be replaced by Fe3þ up to 20% giving rise to local formation of Al –OH – Fe groups. Naþ in an unusual distorted tetragonal-prismatic eightcoordination with two equivalent H2O ligands (r(Na –O(6)) 257 pm) and O-atoms of PO4 groups can be replaced to some extent by Ca2þ and even Sr2þ, which fit better into this site ð4aÞ: It is not clear how the charge-balance is achieved; options are under-occupation of this site and/or partial reduction of Fe3þ to Fe2þ and partial substitution of Al3þ by Mg2þ.
4. Results and discussion 4.1. Symmetry analyses Principally, a symmetry analysis of the whole unit cell can be performed, e.g. using the Adams – Newton Tables [9],
D.K. Breitinger et al. / Journal of Molecular Structure 706 (2004) 95–99
but it does not make much sense to classify all 333 degrees of vibrational freedom of the unit cell. Since the distances between the layers are around 500 pm their dynamical interaction is expected to be low, and hence to a good approximation the symmetry analysis can be restricted to one layer as an independently vibrating system; of course, couplings of some skeletal vibrations in adjacent layers occur via the linking PO4 groups. The layer symmetry is only p121 or ða00 : a0 Þ·2 (Shubnikov notation [10]) with the transformed axes a0 ¼ a1 þ a2 and a00 ¼ 2a1 þ a2 : Then the vibrations are classified into the species a and b of 2-C2 ; where a means in-phase and b counter-phase vibrations in symmetry-correlated pairs of groups in the layer; both species are IR and Raman active. Under the layer symmetry all overtones (species a) and combinations (species a and b) are principally active in both IR and Raman spectra, but from experience these secondorder excitations are usually only observable in IR spectra.
4.2. Mid-IR and far-IR spectra The n(OH) modes in the two independent pairs of symmetry-correlated OH groups classify as 2a þ 2b; with the correlation splitting between a and b species depending on the distances in each of the pairs. The n(OH) region of IR (and Raman) spectra of wardite (see Fig. 1 and Ref. [11]) shows two sharp bands (3613 and 3545 cm21) with two weak shoulders or satellites (, 3580 and 3520 cm21). The strong IR bands are most probably the counter-phase b species with an amplified resultant transition moment; in the a species the oscillating dipole moments in neighbouring OH groups nearly compensate mutually giving weak IR absorptions. The satellites could also originate from the n(OH) modes in the Al –OH – Fe groups (cf. Section 3, see the discussion in Refs. [6,11]). It is not possible to assign unambiguously the two main n(OH) bands to the two independent pairs of OH groups because of their different
Fig. 1. Mid-IR spectrum of natural wardite from Trauira/Maranha˜o, Brazil.
97
bonding situation with counteracting effects (Al –O bond lengths, acceptor for hydrogen bonds). The system of broad overlapping bands in the region 3500 – 3000 cm 21 is due to the valence vibrations of the hydrogen-bonded H2O molecules and the resonance-enhanced overtones of their deformation mode d(H2O). The latter clearly show correlation splitting (1650/1560 cm21) as a consequence of the short distance and orientation of the H2O molecules. The deceptivly simple strong IR band centered at 1059 cm21 contains at least four components of n(PO4) generated by lifting of the originally threefold degeneracy of n3(PO4) ðt2 Þ and activation of n1(PO4) ða1 Þ due to the general position of PO4 and again at least four components ð2a þ 2bÞ of the deformation modes d(Al2OH) involving the two pairs of unequal OH groups. The region near the minimum of transmittance is dominated by the components of n3(PO4) and the band at 992 cm21 by n1(PO4), as suggested by the Raman spectrum. The latter vibration and one of the d(Al2OH) modes are obviously accidentally degenerate (see Section 4.5). Other d(Al2OH) components appear as weak bands/shoulders at 1168 and 1129 cm21. The possible d modes of the Al –OH – Fe groups also have to be taken into account. The neatly separated bands at 898 and 797 cm21 are to be assigned to the librational modes v and r of the H2O molecules coordinated to both Al and Na(Ca) and involved in two hydrogen bonds; due to this bonding situation these frequencies are rather high (for the discussion of the librational modes of complexed H2O see Ref. [12]). Strong support of this assignment is provided by the INS spectra (Section 4.5). The band system with minimum at 636 cm21 is again highly complex as it comprises the split n(PO4) mode, at least four g(Al2OH) modes ð2a þ 2bÞ (plus g(AlOHFe) and several valence vibrations of the two distorted AlO6 (and FeO6) octahedra with admixtures of deformations d(PO4). The group of bands below 500 cm21 (Fig. 2) comprises further Al – O valence vibrations, especially n(Al – OH2)
Fig. 2. Far-IR spectrum of wardite.
98
D.K. Breitinger et al. / Journal of Molecular Structure 706 (2004) 95–99
modes (species a þ b), more precisely valence vibrations of the bent Al– OH2 –Na(Ca) subsystem; candidates for these latter vibrations are the bands at 468 and 427 cm21. The next band (393 cm21) coincides with Raman and INS bands (Sections 4.3 and 4.5) assigned to a deformation of the octahedra AlO6. Deformations of the above subsystem (in plane, out of plane with respect to e.g. the (001) plane) could cause bands at 360, 337 and 307 cm21. Finally, the bands at 238 cm21 and below (191, 169, 156 cm21) could be associated to the translational motions of Naþ or Ca2þ in their compliant cages. 4.3. Raman spectra The deceptively simple Raman spectra (Fig. 3) display an unresolved broad n3(PO4) and a n1(PO4) band, a very weak and broad feature around 800 cm21 for v(H2O) or r(H2O) and three strong bands which are characteristic for the Raman active valence and deformation modes n1 ; n2 and n5 of an octahedron AlO6; for comparison with the vibrations of [Al(H2O)6]3þ see e.g. Refs. [13,14]. Of course, these three broadened, partly asymmetric bands hide a great variety of vibrations, as discussed in Section 4.2. The weak bands at 258 cm21 and below probably reflect the vibrations of the cations Naþ and Ca2þ. Remarkably, the FT-Raman spectrum displays two welldefined absorptions (!) in the thermal-emission background, formally with Raman shifts of 2475 and 2340 cm21 (see insert in Fig. 3; these shifts correspond to absolute wavenumbers 6923 and 7058 cm21, which match almost exactly values for two NIR-bands (see Section 4.4). 4.4. NIR spectra These absorption bands in the FT-Raman spectrum appear in the NIR spectrum (Fig. 4) at 7052 and 6914 cm21, and are clearly true overtones or quasi-overtones of
Fig. 4. NIR spectrum of wardite.
the n(OH) modes (cf. [3]), i.e. binary combinations of the same fundamentals in different species (here a þ bÞ: The band system with minimum at 4581 and shoulders at 4665, 4545 and 4503 cm21 is to be assigned to several possible binary combinations n þ d and the broad weak bands around 4050 and 3900 cm21 to combinations n þ g: Obviously, also ternary combinations n þ d þ g appear (, 5000 cm21 and above), as has been also found with augelites [15]. Assuming true overtones for the high-frequency NIR bands anharmonicity parameters (first-order correction for anharmonicity) for the n(OH) vibrations can be estimated using the equation set for fundamentals n01 ¼ ve 2 2xe ve and overtones n02 ¼ 2ve 2 6xe ve given e.g. in Ref. [3]. With the frequencies for fundamentals n01 ¼ 3613/ 3545 cm21 and overtones n02 ¼ 7052=6914 cm21 the parameters ve ¼ 3787=3727 cm21 and xe ve ¼ 87=90 cm21 are obtained. 4.5. Inelastic neutron scattering (INS) spectra
Fig. 3. FT-Raman spectrum (section) of wardite. (insert shows absorptions for 2v(OH); apparent Raman shifts converted to absolute wavenumbers).
With respect to the scattering cross sections for neutrons and the vibrational amplitudes of the constituents of wardite only vibrations involving hydrogen nuclei are expected to be observed in the INS spectra, irrespective of the fact that all vibrations should be observable without symmetry restrictions. In principle, vibrations involving Fe (moderate cross section) could also contribute, but its content is probably too low to give observable signals. In the high-energy transfer part of the INS spectrum of wardite no clear-cut bands are observed, probably due to the energy distribution of the incoming neutrons and the increasing dissipation of intensity into overtones and combinations of fundamentals and their phonon-wings (combinations of internal and lattice vibrations, see the discussion in Ref. [12]). Therefore, only the low-energy
D.K. Breitinger et al. / Journal of Molecular Structure 706 (2004) 95–99
99
5. Conclusions The combination of these complementary methods of vibrational spectrometry fosters the basic understanding of the vibrational dynamics of this natural ‘impure’ material. A more detailed understanding would require syntheses of stoichiometric and also deuterated compounds in order to get simpler spectra and a broader basis for their assignment. Up to now attempts to synthesize this compound under varying conditions always gave mixtures with other aluminium – phosphate phases. The search for phase-pure synthetic wardite will be continued.
Acknowledgements Fig. 5. Inelastic neutron scattering spectrum (section) of wardite (smoothed by the box-car/gliding average method).
transfer range of the INS spectrum of wardite is shown (Fig. 5). Nevertheless, some signals outside this range are shortly discussed. A structured signal peaked at 2130 cm21 comprises overtones and combinations of the different d(Al2OH) modes. Sharp signals at 1695 and 1522 cm21 are d(H2O) modes without doubt, but in the same region weaker combinations of d(Al2OH) and g(Al2OH) appear. A group of four stronger signals (with satellites) at 1186, 1121, 1057 and 990 cm21 (Fig. 5) are the expected d(Al2OH) (and d(AlFeOH)) modes the frequency region of which is well known from alunites (prototype KAl3(OH)6(SO4)2) [2] and augelites (Al2(OH)3(PO4)) [1]. The accidental degeneracy of the lowest d(Al2OH) and the n1(PO4) vibrations has already been adressed in Section 4.2. The prominent bands at 896 and 797 cm21 are the v(H2O) and r(H2O) modes showing-up at the same frequencies in the IR spectrum. The highly complex band system between 700 and 400 cm21 should contain six AlO valence vibrations in the Al2OH fragments (in the 700 –600 cm21 section), four g(Al2OH) modes (around the maximum at 576 down to 500 cm21), two twist modes t(H2O) (below 500 cm21) and two n(Al – OH2) valence vibrations 452 and 415 cm21 (more detailed discussion of this point in Section 4.2). A weak sharp feature at 393 cm21 corresponds to one of the strong Raman bands (derived from n5 of an octahedron AlO6) and to an IR band and is thus a deformation mode involving OH groups. The lowest group of signals starts with deformations of the fragment Al– OH2 –Na(Ca) (326 cm21, strong in IR, very weak in Raman at 337 cm21), followed by skeletal deformations with contributions of OH and H2O induced by and coupled with motions of the Naþ and Ca2þ cations.
Support by the Deutsche Forschungsgemeinschaft, Bonn-Bad Godesberg, (grant Br 422/23-1 þ 2) and by the Fonds der Chemischen Industrie, Frankfurt/Main, is gratefully acknowledged. Thanks are also due to then Thermo Nicolet, NeuIsenburg, for the possiblity to measure NIR and FT-Raman spectra.
References [1] D.K. Breitinger, J. Mohr, D. Colognesi, S.F. Parker, H. Schukow, R.G. Schwab, J. Mol. Struct. 563/564 (2001) 377. [2] D.K. Breitinger, R. Krieglstein, A. Bogner, R.G. Schwab, Th.H. Pimpl, J. Mol. Struct. 408/409 (1997) 287. [3] D.K. Breitinger, H. Schukow, H.-H. Belz, J. Mohr, R.G. Schwab, J. Mol. Struct. 480/481 (1999) 677. [4] D.K. Breitinger, L. Hajba, V. Komlo´si, J. Mink, G. Brehm, D. Colognesi, S.F. Parker, H. Schukow, R.C. Schwab, in: J. Mink, G. Jalsovszki, G. Keresztury (Eds.), XVIIIth International Conference on Raman Spectroscopy (ICORS 2002), Budapest, Hungary, 25– 30 August, Wiley, New York, 2002, p. 921. [5] L. Fanfani, A. Nunzi, P.F. Zanazzi, Min. Mag. 37 (1970) 598. [6] D. Cozzupoli, O. Grubessi, A. Mottana, P.F. Zanazzi, Min. Petr. 37 (1987) 1. [7] M.A. Cooper, F.C. Hawthorne, P. Cerny, J. Czech. Geol. Soc. 45 (2000) 95. [8] A.B. Vassilikou-Dova, Appl. Magn. Resonance 5 (1993) 25. [9] D.M. Adams, D.C. Newton, Tables for Factor Group and Point Group Analysis, Beckman-RIIC, Croydon, 1970. [10] A.V. Shubnikov, V.A. Kopsik, Symmetry in Science and Art, Plenum Press, New York, 1974, Chapter 8, Table 11. [11] P. Tarte, A.M. Fransolet, F. Pillard, Bull. Mine´ral. 107 (1984) 745. [12] S.F. Parker, K. Shankland, J.C. Sprunt, U.A. Jayasooriya, Spectrochim. Acta A53 (1997) 2333. [13] W.W. Rudolph, R. Mason, C.C. Pye, Phys. Chem. Chem. Phys. 2 (2000) 5030. [14] W.W. Rudolph, R. Mason, J. Solution Chem. 30 (2001) 527. [15] D.K. Breitinger, H.-H. Belz, H. Schukow, J. Mohr, R.G. Schwab, J. Mol. Struct. 651– 653 (2003) 177.