Journal of Applied Crystallography

Volume 40, Part 4 (August 2007)

research papers

 

Crystal structures and thermal expansion of -MgSO₄ and -MgSO₄ from 4.2 to 300 K by neutron powder diffraction

A. D. Fortes, I. G. Wood, L. Vocadlo, H. E. A. Brand and K. S. Knight

Abstract: Detailed neutron powder diffraction measurements have been carried out on two polymorphs of anhydrous magnesium sulfate, -MgSO₄ and -MgSO₄. -MgSO₄ is orthorhombic, space group Cmcm (Z = 4); at 4.2 K the unit-cell dimensions are a = 5.16863 (3), b = 7.86781 (5), c = 6.46674 (5) Å, V = 262.975 (2) Å³ [_calc = 3040.16 (2) kg m^-3], and at 300 K, a = 5.17471 (3), b = 7.87563 (5), c = 6.49517 (5) Å, V = 264.705 (2) Å³ [_calc = 3020.29 (2) kg m^-3]. The axial and volumetric thermal expansion coefficients are positive at all temperatures and exhibit no unusual behaviour. Structures were refined at 4.2 and 300 K to R_P < 3%; less precise structural parameters were determined during warming from 4.2 to 300 K. -MgSO₄ has a more complex structure, crystallizing in space group Pbnm (Z = 4); the unit-cell dimensions at 4.2 K are a = 4.73431 (8), b = 8.58170 (12), c = 6.67266 (11) Å, V = 271.100 (5) Å³ [_calc = 2949.04 (5) kg m^-3], and at 300 K, a = 4.74598 (7), b = 8.58310 (10), c = 6.70933 (10) Å, V = 273.306 (4) Å³ [_calc = 2925.42 (4) kg m^-3]. The thermal expansivities of the a and c axes, and the volumetric thermal expansion coefficient, are positive at all temperatures and normally behaved. However, the thermal expansion of the b axis is both very small and negative below 125 K. Structural and thermal motion parameters for -MgSO₄ as a function of temperature are also reported.

Keywords: magnesium sulfate; polymorphism; thermal expansion; neutron powder diffraction.

	Zip compressed file (3792.4 kbytes) [ doi:10.1107/S0021889807029937/db5013sup2.zip ] DAT files and GSAS refinement files for -MgSO<inf>4</inf>
	Zip compressed file (1908.1 kbytes) [ doi:10.1107/S0021889807029937/db5013sup3.zip ] DAT files and GSAS refinement files for -MgSO<inf>4</inf>

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