Fortes, AD

Knight, KS

UCL Discovery

ISIS Experimental Report  RB Number: 1010078 Rutherford Appleton Laboratory  Date of Report: 24/05/2010   Title of Experiment: Thermal expansion and phase transition in magnesium sulfate trihydrate  Local Contact: K. S. Knight   Principal Proposer:   Affiliation: A.D. Fortes University College London  Instrument: HRPD   Experimental Team: A.D. Fortes, K.S. Knight  Date of Experiment: 4-7/03/2010  Introduction:  The complete crystal structures of MgSO4-hydrates with n = 1, 4, 5, 6, 7, and 11 have each been refined from single-crystal data; the the heavy-atom structures of MgSO42H2O (sanderite) and MgSO42½H2O have been determined recently by X-ray powder diffraction methods [1,2]. In August 2009 I carried out a single-crystal neutron diffraction study of MgSO43D2O at the ILL; the data was used to solve the crystal structure at room temperature, and to obtain information concerning its behaviour as a function of temperature [3]. It was found that a first-order phase transition occurred upon cooling through 245 K; attempts to derive the low-T unit-cell were unsuccessful. However, the identification of disordered and bifurcated hydrogen bonds in the structure at high temperature suggests that the phase transition is due to ordering of these hydrogen bonds on cooling. The objective of this proposed work is (i) to make precise measurements of the unit-cell parameters of MgSO43D2O as a function of temperature, and (ii) to characterise the change in symmetry that occurs on passing through the first-order phase transition at 245 K. Experimental method  Crystals of MgSO43D2O were prepared by gentle evaporation of a 25 wt. % MgSO4 solution at 110°C for ~ 24 hours without stirring. After extraction from the mother liquor the crystals were ground to a powder under a helium atmosphere. Both the pestle and mortar, and the aluminium sample can were chilled on a bed of dry ice snow inside the helium-filled glove bag. The sample was then mounted in a CCR at 250 K and data were collected in the 100-200ms time-of-flight window. Initial inspection of the data revealed a mixture of phases to be present, MgSO43D2O, MgSO46D2O and MgSO47D2O, in the approximate proportions 50:30:20. The specimen was then cooled to 230 K, where the splitting of certain MgSO43D2O reflections became evident (e.g., (221), Fig. 1), before being cooled down to 8 K for a long data collection in the 30-130 ms t-o-f window (400 µAhr). A series of shorter (40 µAhr) integrations were made upon warming to 120 K, 273 K, and 295 K in the 100-200 ms t-o-f window, followed by another series of measurements upon cooling from 245K down to 160 K. Results The unit-cell parameters of all three components in the specimen were refined using the Rietveld method in GSAS. We established that the orthorhombic unit cell of MgSO43D2O above 245 K experiences a modest distortion; in the Pbca setting of the high-T cell, the angle γ jumps discontinuously from 90.0º to 90.2º and this angle increases to ~ 91.1º as the temperature is reduced to limiting low temperature. Permutation of the cell axes in order to locate the 2-fold axis of the low-T structure along the monoclinic b-axis results in a space-group assignment of P21/c. The data for the low temperature phase were fitted to simple saturation functions of the form x=x0+yTS coth(TS/T) in order to quantify the strain due to the phase transition, where TS is a ‘saturation’ temperature.  Table 1. Parameters obtained by fitting the saturation function to the low-T P21/c unit-cell axes and volume.   b-axis (a-axis high-T) c-axis (b-axis high-T) a-axis (c-axis high-T) Unit-cell volume x0 8.119(2) Å 10.729(8) Å 12.31(3) Å 1073(1) Å3 y 2.12(6)x10-4 6.2(2)x10-4 2.2(7)x10-4 1.08(3)x10-1 TS 105(6) K 169(8) K 243(57) K 154(6) K    Figure 1. Splitting of the (221) reflection as the high-T Pbca phase of MgSO43D2O is cooled below 245 K.  References [1] Ma et al. (2009): Am. Min. 94, 622-625. [2] Ma et al. (2009): Am. Min. 94, 1071-1074. [3] Fortes & Lemée-Cailleau, ILL Experimental report number 5-11-360.

Thermal expansion and phase transition in magnesium sulfate trihydrate

http://discovery.ucl.ac.uk/1008249/2/1010078.pdf

Thermal expansion and phase transition in magnesium sulfate trihydrate

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