electronic reprint Acta Crystallographica Section C
Crystal Structure Communications ISSN 0108-2701
Editor: Anthony Linden
Monoclinic structure and nonstoichiometry of ’KAlSiO4-O1 ’ Aleksandar Kremenovi´c, Biljana Lazic, Hannes Kruger, ¨ Martina Tribus and Predrag Vuli´c
Acta Cryst. (2013). C69, 334–336
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Crystallography Journals Online is available from journals.iucr.org Acta Cryst. (2013). C69, 334–336
Kremenovi´c et al. · KAlSiO4
inorganic compounds Acta Crystallographica Section C
Crystal Structure Communications ISSN 0108-2701
Monoclinic structure and nonstoichiometry of ‘KAlSiO4-O1’ c Aleksandar Kremenovic´,a* Biljana Lazic,b Hannes Kru ¨ ger, c a Martina Tribus and Predrag Vulic´ a
Laboratory of Crystallography, Faculty of Mining and Geology, University of Belgrade, Ðusˇina 7, 11000 Belgrade, Serbia, bMineralogical Crystallography, Institute of Geological Sciences, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland, and cInstitute of Mineralogy and Petrography, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria Correspondence e-mail:
[email protected]
Figure 1 A polyhedral representation of KAlSiO4-O1, projected along c, with AlO4 (blue in the electronic version of the paper) and SiO4 (red) tetrahedra. K atoms are represented as ellipsoids (drawn at the 50% probability level).
Received 19 November 2012 Accepted 3 March 2013
Crystals of KAlSiO4-O1 (potassium aluminium silicate) were synthesized using a flux method and analysed utilizing singlecrystal X-ray diffraction and electron microprobe analysis. Both methods confirm that the crystals are nonstoichiometric according to K1 xAl1 xSi1+xO4 with x = 0.04 (1). KAlSiO4-O1 is closely related to the stuffed derivatives of tridymite, although the topology of the Si/Al-ordered framework is different. Six-membered rings of UUDDUD and UUUDDD (U = up and D = down; ratio 2:1) configurations are present in layers parallel to the ab plane. In contrast, the framework of tridymite exhibits UDUDUD rings. The crystals are affected by inversion, pseudo-orthorhombic and pseudo-hexagonal twinning.
Comment Within the K2O–Al2O3–2SiO2 system several polymorphs of KAlSiO4 are known to exist. The so-called O1 phase is one of the polymorphs for which no good single-crystal data were available. According to Smith (1977), its framework topology should be a variant of the tridymite structure. This polymorph was identified as ‘orthorhombic KAlSiO4-O1’ by Smith & Tuttle (1957) and as ‘orthorhombic KAlSiO4-O1 (low T)’ by Cook et al. (1977). So far, this phase has not been found in nature. However, it has been found in blast-furnace linings (Rigby & Richardson, 1947) and as part of magnetohydrodynamic generators (Cook et al., 1977). The crystal structure of this KAlSiO4 polymorph remained unresolved for a long time due to serious problems with pseudosymmetry and twinning. Furthermore, no single crystals of sufficient quality have been synthesized and used for structure refinement until now. Recently, the crystal structure has been solved from X-ray powder diffraction data and refined using the Rietveld method (Gregorkiewitz et al., 2008). The accuracy of the obtained structure and geometric data is limited, because restraints (for interatomic distances) had to be used during the
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refinement. Evidently, for more reliable structure and geometry information there is a need for single-crystal data of KAlSiO4-O1. We obtained KAlSiO4-O1 single crystals by the flux method and used them for X-ray structure determination. The new structure refinement basically confirms the findings of Gregorkiewitz et al. (2008). However, the structural parameters and geometric data are of higher quality than previous results derived from powder data. The crystal structure of KAlSiO4-O1 (Fig. 1) can be described as a variant of tridymite, i.e. a stuffed derivative of it (Buerger, 1954). The K+ cations are situated between sheets of (Al,Si)O4 tetrahedra. The sheets are parallel to the ab plane and are assembled from six-membered rings of (Al,Si)O4 tetrahedra. Compared with tridymite, where the sequence of the directedness [free apex pointing up (U) or down (D)] of the tetrahedra within one ring is UDUDUD, the O1 phase exhibits a different framework topology, namely that two
Figure 2 An isolated layer of six-membered rings of tetrahedra of KAlSiO4-O1, parallel to the ab plane. The directedness of the tetrahedra is denoted as U = apex up and D = apex down.
doi:10.1107/S0108270113006069
electronic reprint
Acta Cryst. (2013). C69, 334–336
inorganic compounds different types of six-membered oval rings (occurring in a ratio of 2:1) have to be distinguished. They show ring configurations of UUDDUD and UUUDDD. Adjacent sheets of tetrahedra are connected by bridging apical O atoms. It should be noted that megakalsilite (KAlSiO4; Khomyakov et al., 2002) is a topological variant of tridymite, with UDUDUD and UUUDDD rings in the ratio 1:3. In contrast with tridymite, which contains only six-membered rings, the framework topology of KAlSiO4-O1 includes four-, six-, eight- and ten-membered rings of tetrahedra. Our new structure refinement results show that the O1 phase has an Si/Al-ordered framework structure. Si and Al are distributed on alternating tetrahedra [average values for all ˚ and Al—O = 1.74 A ˚ , minimum tetrahedra are Si—O = 1.62 A ˚ , respectively, and maximum Si—O = 1.590 (2) and 1.646 (3) A and minimum and maximum Al—O = 1.713 (2) and ˚ , respectively], so that each SiO4 tetrahedron is 1.754 (2) A coordinated to four AlO4 tetrahedra and vice versa (Figs. 1 and 2). On the basis of MAS NMR results, an ordered distribution of Si and Al in the framework structure was also reported by Becerro et al. (2009), Gregorkiewitz et al. (2008) and Stebbins et al. (1986). The crystals synthesized in this work show a small excess of silicon. Charge balance of the structure is achieved by a deficiency of potassium, according to the formula K1 xAl1 xSi1+xO4. The excess silicon may be distributed evenly on the Al sites, or it may be associated with faults as suggested by Gregorkiewitz et al. (2008).
Experimental The flux technique, using homemade KVO3 as the high-temperature solvent, was applied to obtain single crystals of the O1 phase. The starting materials were K2CO3 (Merck), Al2O3 (Merck), SiO2 (Merck) and KVO3. The nutrient reagents were mixed carefully in an agate mortar with a K2CO3:Al2O3:SiO2 molar ratio of 1:1:2. KVO3 flux was added and the compounds were mixed again, resulting in a sample of 1 g with a nutrient-to-flux weight ratio of 1:9, which was placed in a 25 ml platinum crucible with a platinum lid. The mixture was heated from 473 to 1473 K (at a rate of 50 K h 1) in a resistanceheated Carbolite furnace; it was held at this temperature for 24 h to homogenize the melt and then cooled to 873 K at a rate of 5 K h 1 for crystallization. Subsequently, it was cooled to room temperature at a rate of 50 K h 1. The crystals obtained were removed from the polycrystalline potassium vanadate matrix by dissolving it in hot water. In addition to the title compound, the experiment produced leucite and kalsilite crystals. Leucite crystallized in characteristic pseudoicosadodecahedral forms of different sizes varying from 0.1 to 1 mm. The small ( 3(I) Rint = 0.018
Refinement R[I 2 > 3(I 2)] = 0.040 wR(F ) = 0.055 S = 1.44 20167 reflections 389 parameters
˚ 3 max = 1.82 e A ˚ 3 min = 0.63 e A Absolute structure: Flack (1983), with 8056 Friedel pairs Flack parameter: 0.32 (2)
The diffraction pattern can be indexed with the given pseudoorthorhombic lattice. The geometry of the unit cell indicates a b, possible transformation to a (pseudo)hexagonal cell: ahex = a bhex = 2b and chex = c. This hexagonal cell requires additional reflections in positions +(a*+b*)/2, relative to the Bragg spots of the pseudo-orthorhombic lattice. Such additional reflections are actually present but they are very weak, indicating the presence of pseudohexagonal twinning. Furthermore, weak one-dimensional streaking can be observed parallel to b* (and even weaker along the directions related to b* by hexagonal pseudosymmetry). The diffuse scattering in this compound was also noticed by Gregorkiewitz et al. (2008) using electron microscopy. Merging the data in Laue group mmm revealed an Rint value of 0.0245. Using monoclinic symmetry, the a, b and c settings give Rint values of 0.0227, 0.0203 and 0.0230, respectively. The structure was solved in the space group P21 and subsequently refined (JANA2006; Petrˇı´cˇek et al., 2006) to an Robs value of 0.075. As the analysis of merging R values suggested that pseudoorthorhombic twinning is present, twinning was added (twin element m[100]) to the refinement and consequently the Robs value dropped to 0.045. Additional inversion-twinning and pseudo-hexagonal twinning improved the Robs value to 0.040, using five twin individuals. It has to be noted that the structure can be refined in the incorrect space group Pn21m. The result (with an almost satisfactory Robs value of 0.07) is a structural model which exhibits complete disorder of Al and Si on the tetrahedral sites. As the electron microprobe analysis revealed that vacancies on potassium sites have to be present, the occupancy factors of all K atoms were refined. The result is in very good agreement with the value x = 0.04 as determined by the chemical analysis. Data collection: APEX2 (Bruker, 2011); cell refinement: SAINT (Bruker, 2011); data reduction: SAINT; program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: JANA2006 (Petrˇı´cˇek et al., 2006); molecular graphics: DIAMOND (Brandenburg & Putz, 2005); software used to prepare material for publication: publCIF (Westrip, 2010).
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inorganic compounds The SNF supported this work financially under the SCOPES project (grant No. IZ73Z0_1 27961). The Ministry of Education, Science and Technological Development of the Republic of Serbia supported this work financially through grant Nos. 176016 and 172035. Supplementary data for this paper are available from the IUCr electronic archives (Reference: QS3022). Services for accessing these data are described at the back of the journal.
References Altomare, A., Burla, M. C., Camalli, M., Cascarano, G. L., Giacovazzo, C., Guagliardi, A., Moliterni, A. G. G., Polidori, G. & Spagna, R. (1999). J. Appl. Cryst. 32, 115–119. Becerro, A. I., Escudero, A. & Mantovani, M. (2009). Am. Mineral. 94, 1672– 1678.
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Brandenburg, K. & Putz, H. (2005). DIAMOND. Crystal Impact GbR, Bonn, Germany. Bruker (2011). APEX, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA. Buerger, M. J. (1954). Am. Mineral. 39, 600–614. Cook, L. P., Roth, R. S., Parker, H. S. & Negas, T. (1977). Am. Mineral. 62, 1180–1190. Flack, H. D. (1983). Acta Cryst. A39, 876–881. Gregorkiewitz, M., Li, Y., White, T. J., Withers, R. L. & Sobrados, I. (2008). Can. Mineral. 46, 1511–1526. Khomyakov, A. P., Nechelyustov, G. N., Sokolova, E., Bonaccorsi, E., Merlino, S. & Pasero, M. (2002). Can. Mineral. 40, 961–970. Petrˇı´cˇek, V., Dusˇek, M. & Palatinus, L. (2006). JANA2006. Institute of Physics, Prague, Czech Republic. Rigby, G. R. & Richardson, H. M. (1947). Mineral. Mag. 28, 75–88. Smith, J. V. (1977). Am. Mineral. 62, 703–709. Smith, J. V. & Tuttle, O. F. (1957). Am. J. Sci. 255, 282–305. Stebbins, J. F., Murdoch, J. B., Carmichael, I. S. E. & Pines, A. (1986). Phys. Chem. Miner. 13, 371–381. Westrip, S. P. (2010). J. Appl. Cryst. 43, 920–925.
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Acta Cryst. (2013). C69, 334–336
supplementary materials
supplementary materials Acta Cryst. (2013). C69, 334-336
[doi:10.1107/S0108270113006069]
Monoclinic structure and nonstoichiometry of `KAlSiO4-O1′ Aleksandar Kremenović, Biljana Lazic, Hannes Krüger, Martina Tribus and Predrag Vulić Potassium aluminium silicate Crystal data KAlSiO4 Mr = 158.2 Monoclinic, P21 Hall symbol: P 2yb a = 15.6553 (2) Å b = 9.0565 (1) Å c = 8.5568 (1) Å β = 90.017 (1)° V = 1213.20 (2) Å3 Z = 12
F(000) = 936 Dx = 2.597 Mg m−3 Mo Kα radiation, λ = 0.71073 Å Cell parameters from 31861 reflections θ = 2.4–47.5° µ = 1.7 mm−1 T = 293 K Plate, colourless 0.5 × 0.4 × 0.1 mm
Data collection Bruker APEXII area-detector diffractometer Radiation source: X-ray tube Graphite monochromator ω scans Absorption correction: multi-scan (SADABS; Bruker, 2011) Tmin = 0.604, Tmax = 0.749
35192 measured reflections 20167 independent reflections 18282 reflections with I > 3σ(I) Rint = 0.018 θmax = 47.5°, θmin = 2.4° h = −25→32 k = −18→18 l = −17→17
Refinement Refinement on F R[F2 > 2σ(F2)] = 0.040 wR(F2) = 0.055 S = 1.44 20167 reflections 389 parameters 0 restraints 1 constraint
Weighting scheme based on measured s.u.'s w = 1/(σ2(F) + 0.000625F2) (Δ/σ)max = 0.017 Δρmax = 1.82 e Å−3 Δρmin = −0.63 e Å−3 Absolute structure: Flack (1983), with 8056 Friedel pairs Flack parameter: 0.32 (2)
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2)
K1 K2 K3 K4 K5
x
y
z
Uiso*/Ueq
Occ. (
