Structures of potassium sodium sulphate and tripotassium sodium disulphate
1980; International Union of Crystallography; Volume: 36; Issue: 4 Linguagem: Inglês
10.1107/s0567740880004852
ISSN1600-8650
Autores Tópico(s)X-ray Diffraction in Crystallography
ResumoKNaSO4: trigonal, P3ml , a = 5.6066 (7), c = 7.177(1) A, Z = 2, D x = 2.69 Mg m -a. K3Na(SO4)2: trigonal, P3ml, a = 5.6801 (6), c 7.309 (3) A, Z = 1, D x = 2.70 Mg m -3. Crystals of both compounds were synthesized by heating a mixture of K2SO 4 and Na2SO4 in a Pt crucible at 1273 K for 2 h. Colourless transparent prismatic crystals were obtained. Both structures are built up of SO 4 tetrahedra, M(1)O 6 octahedra, M(2)O~0 and M(3)O~2 polyhedra. In KNaSO4, Na atoms occupy the M(1) and M(2) and K atoms the M(2) and M(3) sites. In K3Na(SO4) 2, Na atoms occupy the M(1) and K atoms the M(2) and M(3) sites. Introduction. From Weissenberg photographs, the crystals of KNaSO 4 were found to have trigonal Laue symmetry 3m and those of KaNa(SO4) 2 6/mmm. Systematic absences were not observed for either substance. The possible space groups of KNaSO 4 were restricted to P3ml, P321 or P3ml. The crystals of K3Na(SO4) 2 were considered to be merohedral twins formed from crystals having space groups P3ml, P321 or P3ml in two orientations related by a rotation around the c axis. P3ml and P3ml gave satisfactory results for the structure refinements of KNaSO 4 and K3Na(SO4) 2 respectively. Intensity collections were carried out using for KNaSO 4 a prismatic crystal with dimensions about 0.15 x 0.05 x 0.05 mm and for K3Na(SO4) 2 a nearly cubic crystal with dimensions about 0.12 x 0.12 x 0.10 mm. The to-28 scan technique was used to measure intensities on a Philips PW 1100 automated four-circle diffractometer, with graphite-monochromatized Mo Ka radiation, up to 28 = 75 o for KNaSO 4 and 28 = 70 ° for K3Na(SO4) 2. The scan speed was 4 ° min -1 in o9 and scanning was repeated twice when the total counts were less than 3000. The scan width was determined according to (1.4 + 0 . 2 t a n 0 ) °. Intensities were corrected for Lorentz-polarization factors. No absorption corrections were made, since the #R values of both crystals were less than 0.1 [p(Mo Ka) = 0.346 for KNaSO 4 and 0.392 mm -1 for KaNa(SO4)2]. 373 and 317 independent reflexions with IFI > 3e(IFI) were obtained and used for the structure determinations of KNaSO4 and K3Na(SO4) 2 respectively. 0567-7408/80/040919-03501.00 The positions of all atoms of KNaSO4 were obtained from Patterson maps. The structure was refined with the full-matrix least-squares program LINUS (Coppens & Hamilton, 1970) and anisotropic thermal parameters for all atoms. The site populations of the Na and K atoms in the K(2) and Na(2) sites were also varied, but the existence of neither the Na atom in the K(2) site nor the K atom in the Na(2) site could be detected. The final value of R was 0.046. The crystal of K3Na(SO4) 2 used for the structure determination was assumed to be a merohedral twin formed from crystals with space group P3ml in two orientations. Thus, the structure determination was carried out by a method similar to that used for K2W4013 (Okada, Marumo & Iwai, 1978). Crystals in one orientation were related to those in the other by a rotation of 180 ° about the c axis. In this case, the hkl reflexions of one component crystal coincide with the khl reflexions of the other in reciprocal space. The volume ratio between the crystals in the two orientations was assumed to be 1:1 because all the specimens examined always gave diffraction patterns with hexagonal Laue symmetry 6/mmm. The parameters were refined by the least-squares procedure, using hkO and hhl data of 57 reflexions which were not Table 1. Finalpositionalparameters
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