The crystal structure and thermal expansion tensor of MgSO4-11D2O(meridianiite) determined by neutron powder diffraction

We have collected high-resolution neutron powder diffraction patterns from MgSO(4)center dot 11D(2)O over the temperature range 4.2-250 K. The crystal is triclinic, space-group P (1) over bar (Z = 2) with a = 6.72746(6) angstrom, b = 6.78141(6) angstrom, c = 17.31803(13) angstrom, alpha = 88.2062(6)degrees, beta = 89.4473(8)degrees, gamma = 62.6075(5)degrees, and V = 701.140(6) angstrom(3) at 4.2 K, and a = 6.75081(3) angstrom, b = 6.81463(3) angstrom, c = 17.29241(6) angstrom, alpha = 88.1183(3)degrees, beta = 89.4808(3)degrees, gamma = 62.6891(3)degrees, and V = 706.450(3) angstrom(3) at 250 K. Structures were refined to wRp = 3.99 and 2.84% at 4.2 and 250 K, respectively. The temperature dependence of the lattice parameters over the intervening range have been fitted with a modified Einstein oscillator model which was used to obtain the coefficients of the thermal expansion tensor. The volume thermal expansion, alpha(V), is considerably smaller than ice Ih at all temperatures, and smaller even than MgSO(4)center dot 7D(2)O (although partial derivative alpha(V)/partial derivative T is very similar for both sulfates); MgSO(4)center dot 11D(2)O exhibits negative alpha(V) below 55 K (compared to 70 K in D(2)O ice Ih and 20 K in MgSO(4)center dot 7D(2)O) The relationship between the magnitude and orientation of the principal axes of the expansion tensor and the main structural elements are discussed.