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

Detailed neutron powder diffraction measurements have been carried out on two polymorphs of anhydrous magnesium sulfate, alpha-MgSO(4) and beta-MgSO(4). alpha-MgSO(4) 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) angstrom, V = 262.975 (2) angstrom(3) [rho(calc) = 3040.16 (2) kg m(-3)], and at 300 K, a = 5.17471 (3), b = 7.87563 (5), c = 6.49517 (5) angstrom, V = 264.705 (2) angstrom(3) [rho(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. beta-MgSO(4) 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) angstrom, V = 271.100 (5) angstrom(3) [rho(calc) = 2949.04 (5) kg m(-3)], and at 300 K, a = 4.74598 (7), b = 8.58310 (10), c = 6.70933 (10) angstrom, V = 273.306 (4) angstrom(3) [rho(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 similar to 125 K. Structural and thermal motion parameters for beta-MgSO(4) as a function of temperature are also reported.