Crystal structures and thermal expansion of α-MgSO4and β-MgSO4from 4.2 to 300 K by neutron powder diffraction

Abstract

Detailed neutron powder diffraction measurements have been carried out on two polymorphs of anhydrous magnesium sulfate, α-MgSO4and β-MgSO4. α-MgSO4is orthorhombic, space groupCmcm(Z= 4); at 4.2 K the unit-cell dimensions area= 5.16863 (3),b= 7.86781 (5),c= 6.46674 (5) Å,V= 262.975 (2) Å3[ρcalc= 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[ρcalc= 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 toRP< 3%; less precise structural parameters were determined during warming from 4.2 to 300 K. β-MgSO4has a more complex structure, crystallizing in space groupPbnm(Z= 4); the unit-cell dimensions at 4.2 K area = 4.73431 (8),b= 8.58170 (12),c= 6.67266 (11) Å,V= 271.100 (5) Å3[ρcalc= 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[ρcalc= 2925.42 (4) kg m−3]. The thermal expansivities of theaandcaxes, and the volumetric thermal expansion coefficient, are positive at all temperatures and normally behaved. However, the thermal expansion of thebaxis is both very small and negative below ∼125 K. Structural and thermal motion parameters for β-MgSO4as a function of temperature are also reported.