PHASE TRANSITIONS IN CESIUM SULFAMATE, Cs(NH2SO3)
M. Kresse1, L. Bohatı1, U. Peuchert1 and J. Schneider2
1 Institute of Crystallography, University of Cologne, Zülpicher Str. 49b, D-50674 Köln, Germany
2 Institute
of Crystallography and Applied Mineralogy, University of Munich,
Theresienstr. 41,
D-80333 München, Germany,
e-mail: kressem@kri.uni-koeln.de
Keywords: Cesium sulfamate, Cs(NH2SO3), phase transition
In this contribution we present the crystal structure of the intermediate phase of cesium sulfamate - which is stable in the temperature range from 80°C to 160°C - and analyse the structural aspects of the transition room temperature (RT) phase intermediate (IT) phase. Additionally, the new results of our investigations of the high temperature phase of Cs(NH2SO3) will be given.
Cesium sulfamate, Cs(NH2SO3), undergoes two structural phase transitions (PT(I) and PT(II)) between room temperature and its melting point at approximately 180°C: PT(I) occurs near 80°C, PT(II) appears at 160°C. Both phase transitions were discovered in 1992 in our group (Bohatı, Rapp [1]) during systematic crystallographic and crystal physical investigations of sulfamates of mono- and divalent cations. Later, PT(I) was found independently by Haussühl and Haussühl [2]. Additionally, Tzolova et al. [3] describes a third anomaly of the thermoanalytical curves (in the temperature range from 52°C to 73°C). Neither by means of thermoanalytical and thermooptical analysis nor X-ray powder diffraction investigations have we observed this third anomaly.
At room temperature Cs(NH2SO3) crystallizes monoclinic (space group P21/c). The structure of the room temperature phase was solved by Schreuer [4] from single crystal X-ray diffraction data. Because of the reconstructive nature of the PT(I) our attempt to prepare crystals of the intermediate phase for single crystal X-ray analysis failed. Passing the PT(I) a single crystal of the RT phase yields a polycrystal; sometimes the sample crumbles.
Thus for a structural analysis of the intermediate phase only powder X-ray diffraction data could be measured. The data were collected at T = 120(2)°C in Debye-Scherrer transmission geometry on a STOE STADI P diffractometer with moving PSD using germanium monochromatized MoKa1 radiation.
The powder pattern was indexed using the program ITO [5] included in the program package GUFI3 [5]. The intermediate phase of cesium sulfamate is orthorhombic (a = 8.4082(3) A, b = 6.779(2) A and c = 8.3163(3) A). Systematic absence of reflexes (0kl) with k+l 2n, (hk0) with h 2n and (h00), (0k0), (00l) with h, k, l 2n allows two possible space groups: Pnma and its polar subgroup Pn21a.
The structure solution was carried out in the centrosymmetric space group Pnma using a combined trial-and-error and difference Fourier strategy, successfully applied recently in the case of HT-K2MgWO2(PO4)2 [7]. The final Rietveld refinements (22 structural and 8 profile parameters) converged to the reliability values of Rwp = 5.93% and RBragg = 4.01%. Subsequent structural calculations in the polar space group Pn21a did not yield better results. For structure solution and Rietveld refinement we used the program package WYRIET version 3 [8].
A comparison of the monoclinic room temperature structure and centrosymmetric structure of the intermediate phase shows that the monoclinic structure can be derived from the orthorhombic structure by sheering.