Disordered kalsilite KAlSiO₄

The X-ray powder diffraction pattern that corresponds to the disordered state of kalsilite (potassium aluminium orthosilicate), KAlSiO₄, is investigated. The directionality of (Al,Si)O₄ tetra­hedra within single six-membered tetra­hedral ring building units (S6R) could not be defined. With equal probability for the directionality of each tetra­hedra within one S6R [free apex pointing up (U) or down (D)], an undefined sequence of U and D directionalities is needed to describe the S6R building units. The extinction conditions of disordered kalsilite are also different compared to ordered kalsilite within the space group P6₃. In disordered kalsilite, h0l and hhl reflections with l = 2n + 1 are systematically absent.

S1. Comment

Kalsilite, a feldspatoid group mineral, can be found in basic plutonic rocks rich in potassium, as well as in metamorphic rocks (Woolley et al., 1996). In addition, it is a well-known ceramic material. Claringbull & Bannister (1948) carried out the first structural investigations and suggested a tridymite structural type for kalsilite. Perrotta & Smith (1965) proved their conclusion by giving a precise structure description: potassium ions coordinate with nine nine O atoms (O^2-), three apical O atoms connect neighbouring tetra­hedral layers and two groups of three basal O atoms form two different tetra­hedral layers. The average K—O distance is 2.90 Å. In addition, they also found disorder of apical O atoms which are shifted \sim 0.25 Å from the threefold axis. Consequently, the 180° T—O—T (T = Si, Al) angle is reduced to a more energetically favourable value of 163°. The tetra­hedral layers are perpendicular to the c axis and are assembled from six-membered rings (S6R) of (Al,Si)O₄ tetra­hedra in which the sequence of the directionality [free apex pointing up (U) or down (D)] of the tetra­hedra within one ring is UDUDUD.

The changes of diffraction intensities or reflection extinction conditions in kalsilite patterns are explained by twinning (double or triple), a polydomain structure or modulations of the structure. Andou & Kawahara (1982) and Kawahara et al. (1987) reported different intensities and the disappearance of hhl (l = 2n+1) reflections, which are associated with different ratio of twinning domains in a crystalline phase described by the space group P6₃mc. Dollase & Freeborn (1977) reported the same phenomenon for 11l (l = 2n+1) reflections found in phase–anti­phase boundaries on the domain contacts. They suggested that a great number of nuclei of both kinds are formed with equal probability during fast nucleation. Abbot (1984) pointed out the great influence of domain structure on the diffraction pattern in all KAlSiO₄ polymorphs and suggested that synthetic kalsilite reported by Tuttle & Smith (1958) is probably the so-called inter­mediary kalsilite phase with two domains that may be described in P6₃/mmc and P6₃/mc. Xu & Veblen (1996) investigated the superstructure reflections and domain structure in synthetic and natural (with 0.5–5 atomic% Na) kalsilites. In natural kalsilite, with more than 2.5% Na, they found superstructure reflections. These superstructure reflections correspond to the three orthorhombic domains (P2₁) which are rotated 120° around the c axis of unit cell. In synthetic kalsilite, they found a complex microstructure with large number of `fine' twinning lamellas parallel to c axis. They concluded that twin domains correspond to P6₃ and P31c polymorph modifications. Further studies of kalsilite involving temperature-induced changes and the use of transmission electron microscope (TEM) and microstructure analysis have revealed the true complexity of this problem (Henderson & Taylor, 1988; Andou & Kawahara, 1982; Kawahara et al., 1987; Capobianco & Carpenter, 1989; Carpenter & Cellai, 1996; Cellai et al., 1992, 1997, 1999; Dollase & Freeborn, 1977; Abbot 1984; Artioli & Kvick, 1990; Kosanovic et al., 1997, Dimitrijevic & Dondur, 1995; Barbier & Fleet, 1988; Xu & Veblen, 1996).

Polymorphism of phases at the KAlO₂–SiO₂ join of the K₂O—Al₂O₃–SiO₂ phase diagram was investigated by the ZTIT (zeolite thermally induced phase transformation) method by Dimitrijevic & Dondur (1995). They reported five KAlSiO₄ polymorphs stable at room temperature, two of them having powder patterns close to kalsilite framework topology. In contrast to known kalsilite (space group P6₃) synthesized at 1373 K [a = 5.160 (1) Å and c = 8.632 (6) Å], the X-ray powder diffraction (XRPD) pattern of a new polymorph synthesized at 1273 K [a = 5.197 (1) Å and c = 8.583 (5) Å] is characterized by systematic disappearance of h0l and hhl reflections with l = 2n+1. The latter we will hereafter denote as disordered kalsilite.

In ordered kalsilite, the layers of tetra­hedra are perpendicular to the c axis and are assembled from six-membered rings (S6R) of (Al,Si)O₄ tetra­hedra, where the sequence of the directionality [free apex pointing up (U) or down (D)] of the tetra­hedra within one ring is UDUDUD. However, in the disordered kalsilite, the directionality of (Al,Si)O₄ tetra­hedra within one S6R could not be defined. With equal probability for the directionality of each tetra­hedra within one S6R, an undefined sequence of letters U and D are needed to describe the S6R building units. Such disorder structure model is characterized by systematic disappearance of h0l and hhl reflections with l=2n+1 (Fig. 1). The disordered kalsilite structure model could be described as two substructures, denoted a and b, each with 50% population (Fig. 2). In Fig. 3, simulated XRPD patterns for models composed from substructrures are presented: 100% a, 90% a and 10% b, 75% a and 25% b, and 50% a and 50% b. Evidently only the model composed of 50% a and 50% b substructures results in systematic disappearance of (h0l) and (hhl) reflections with l = 2n+1.

Inter­atomic distances and angles for disordered kalsilite are in agreement with previously published data for known kalsilite structures: space group P6₃ (Andou & Kawahara, 1982), P31c (Cellai et al. 1997) and P6₃mc (Dollase & Freeborn, 1977). The electrostatic valence balance calculated according to the method of Brown & Altermatt (1985) is satisfactory. In the structure, Si⁴⁺ and Al³⁺ are fully ordered, which is consistent with the ²⁹Si and ²⁷Al MAS NMR results obtained by Dimitrijevic & Dondur (1995).

S2. Experimental

Cation exchanged zeolites are confirmed as excellent precursors for the preparation of aluminosilicate ceramics (Dondur & Dimitrijevic, 1986). Numerous phases, such as β-eucriptite, nepheline, carnegite, kalsilite, anorthite, celsian or α-cordierite, were synthesized using the ZTIT method (Dondur & Dimitrijevic, 1986; Norby, 1990; Newsam, 1988). We applied this method in order to produce different phases on the KAlO₂–SiO₂ join of the K₂O–Al₂O₃—SiO₂ system, particularly KAlSiO₄ polymorphs stable at room temperature. As a starting material, the sodium form of synthetic zeolite LTA (Meier & Olson, 1992) manufactured by Union Carbide Co was used. A fully exchanged K⁺ form of LTA zeolite was prepared from KCl solution, using several successive exchanges. The disordered kalsilite was synthesized from K-LTA zeolite by the ZTIT route after heating for 1 h at 1273 K. A detailed procedure of the synthesis is explained by Dimitrijevic & Dondur (1995). Diffraction data were collected on a Phillips PW-1710 diffractometer equipped with a graphite monochromator (Cu Kα) and an Xe-filled proportional counter. Divergence and receiving slits were fixed to 1° and 0.1 mm, and the generator was set at 40 kV and 32 mA. The diffractometer alignment was checked using a reference material of powdered crystalline silicon. Data for Rietveld refinements were collected, in scan-step mode, between 4 and 90° 2θ with 0.02° 2θ step and 15 s per step.

Related literature

For related literature, see: Abbot (1984); Andou & Kawahara (1982); Artioli & Kvick (1990); Barbier & Fleet (1988); Brown & Altermatt (1985); Capobianco & Carpenter (1989); Carpenter & Cellai (1996); Cellai et al. (1992, 1997, 1999); Claringbull & Bannister (1948); Dimitrijevic & Dondur (1995); Dollase & Freeborn (1977); Dondur & Dimitrijevic (1986); Henderson & Taylor (1988); Kawahara et al. (1987); Kosanovic et al. (1997); Meier & Olson (1992); Newsam (1988); Norby (1990); Perrotta & Smith (1965); Tuttle & Smith (1958); Woolley et al. (1996); Xu & Veblen (1996).

Computing details

Data collection: Philips PC-APD PW1877 (Philips, 1989); program(s) used to refine structure: FULLPROF (Rodriguez-Carvajal, 1990); molecular graphics: DIAMOND (Brandenburg & Putz, 2005); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures

	Fig. 1. Final Rietveld plot for disordered KAlSiO4. Observed data points are indicated by circles, and the best-fit profile (upper trace) and the difference pattern (lower trace) are indicated by solid lines. The vertical bars indicate the positions of the Bragg peaks.
	Fig. 2. Polyhedral representation of KAlSiO4 substructures a (upper row) and b (lower row). Single tetrahedral layers (built-up from the S6R units) are projected along c with AlO4 (dark grey) and SiO4 (light grey) tetrahedra. K atoms are represented with grey spheres. For both substructures, the stacking of the layers with respect to the unit cell is represented projected along a.
	Fig. 3. Simulated XRPD patterns of KAlSiO4 composed of different ratios of substructures a and b: (a) 100% a; (b) 90% a and 10% b; (c) 75% a and 25% b; and (d) 50% a and 50% b. The arrows are pointing out the gradual decrease of the intensity of h0l and hhl reflections with l = 2n+1 that completely disappear in fully disordered model. XRPD patterns of substructure mixtures with: 100% b; 90% b and 10% a; 75% b and 25% a have been omitted because they correspond to (a), (b), (c) and (d), respectively. Note that intensities over 38° 2θ are multiplied by 5 to emphasize the details.

Potassium aluminium orthosilicate

Crystal data

KAlSiO4	Z = 2
Mr = 158.16	Dx = 2.614 Mg m^−3
Hexagonal, P63	Cu Kα1, Cu Kα2 radiation, λ = 1.540562, 1.544390 Å
Hall symbol: P 6c	T = 295 K
a = 5.19817 (15) Å	white
c = 8.5865 (3) Å	flat sheet, 25 × 25 mm
V = 200.93 (1) Å^3

Data collection

Philips PW1710 diffractometer	Data collection mode: reflection
Equatorial mounted graphite monochromator	Scan method: step
Specimen mounting: packed powder pellet	2θmin = 4.001°, 2θmax = 89.961°, 2θstep = 0.020°

Refinement

Rp = 7.001	Profile function: pseudo-Voigt
Rwp = 9.735	45 parameters
Rexp = 4.507	6 restraints
RBragg = 8.182
χ^2 = 4.666	Background function: linear, extrapolation,, points, were, determined, by, visual, estimation, and, refined
4299 data points

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Sia	0.33333	0.66666	0.45815 (8)	0.04750 (2)*	0.50000
Ala	0.33333	0.66666	0.07424 (8)	0.042 (3)*	0.50000
O2a	0.61394 (13)	0.019 (2)	0.02309 (8)	0.0214 (18)*	0.50000
O1a	0.33333	0.66667	0.27785 (8)	0.060 (3)*	0.50000
Sib	0.33333	0.66666	0.95815 (8)	0.04750 (2)*	0.50000
Alb	0.33333	0.66666	0.57425 (8)	0.042 (3)*	0.50000
O2b	0.61394 (13)	0.019 (2)	0.52310 (8)	0.0214 (18)*	0.50000
O1b	0.33333	0.66667	0.77785 (8)	0.060 (3)*	0.50000
K	0.00000	0.00000	0.27354	0.048 (2)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
K	0.067 (3)	0.067 (3)	0.0099 (18)	0.033 (3)	0.00000	0.00000

Bond lengths (Å)

Sia—O2a^i	1.614 (6)	K—O2a^viii	2.971 (3)
Sia—O2a^ii	1.614 (10)	K—O2a^ix	2.977 (7)
Sia—O2a^iii	1.614 (4)	K—O2a^x	2.971 (4)
Sia—O1a	1.5481 (10)	K—O2a^vi	2.977 (3)
Ala—O2a^iv	1.733 (7)	K—O2a^iii	2.971 (7)
Ala—O2a^v	1.733 (3)	K—O1a^xi	3.0014 (1)
Ala—O2a^vi	1.733 (10)	K—O1a^xii	3.0014 (1)
Ala—O1a	1.7483 (10)	K—O1a	3.0014 (1)
Sib—O2b^i	1.614 (6)	K—O2b^vii	2.971 (4)
Sib—O2b^ii	1.614 (10)	K—O2b^xiii	2.977 (3)
Sib—O2b^iii	1.614 (4)	K—O2b^ix	2.971 (7)
Sib—O1b	1.5481 (10)	K—O2b^xiv	2.977 (4)
Alb—O2b^iv	1.733 (7)	K—O2b^vi	2.971 (3)
Alb—O2b^v	1.733 (3)	K—O2b^xv	2.977 (7)
Alb—O2b^vi	1.733 (10)	K—O1b^xvi	3.0014 (1)
Alb—O1b	1.7482 (10)	K—O1b^xvii	3.0014 (1)
K—O2a^vii	2.977 (4)	K—O1b^xviii	3.0014 (1)

Symmetry codes: (i) x−y, x, z+1/2; (ii) −x+1, −y+1, z+1/2; (iii) y, −x+y+1, z+1/2; (iv) x, y+1, z; (v) −y, x−y, z; (vi) −x+y+1, −x+1, z; (vii) x−1, y, z; (viii) x−y−1, x−1, z+1/2; (ix) −y, x−y−1, z; (x) −x+1, −y, z+1/2; (xi) x−1, y−1, z; (xii) x, y−1, z; (xiii) x−y−1, x−1, z−1/2; (xiv) −x+1, −y, z−1/2; (xv) y, −x+y+1, z−1/2; (xvi) x−y, x−1, z−1/2; (xvii) x−y, x, z−1/2; (xviii) x−y+1, x, z−1/2.