Neutron diffraction structural study of pyroelectric Li2SO4⋅H2O at 293, 80, and 20 K

Abstract

Lithium sulfate monohydrate has been studied at 293, 80, and 20 K by neutron diffraction using the same ground spherical crystal. Li2SO4⋅H2O is monoclinic, space group P21, with lattice dimensions at 293 K of a=5.450(3), b=4.872(3), c=8.164(4) Å, β=107.31(3)°; at 80 K: a=5.449(2), b=4.832(2), c=8.141(2) Å, β=107.21(1)°; and at 20 K: a=5.449(2), b=4.832(2), c=8.137(2) Å, β=107.19(1)°. There are two formula units in the unit cell. Least-squares refinement based on 1749 (293 K), 1712 (80 K), and 880 (20 K) reflections gave final R(F) values of 0.022, 0.016, and 0.014, respectively. All structure factors were corrected for thermal diffuse scattering. A Gram–Charlier expansion of the probability density function was used to describe the thermal vibrations of the water molecule which at 293 K are consistent with a previously postulated flipping motion. The O(W)⋅⋅⋅O(W) hydrogen bond distance is 0.091(3) Å shorter and the O(W)⋅⋅⋅O(W)⋅⋅⋅O(W) angle is 3.9(1)° larger at 20 K than at 293 K: the O(W)⋅⋅⋅O(1) distance to a sulfate oxygen is 0.041(3) Å shorter. Both kinds of hydrogen bonds are weak and bent. A slight asymmetry in the SO2−4 ion at all temperatures is related to a difference in the coordination of the four oxygen atoms: that forming the shorter S–O(1) bond (by 0.015 Å) is hydrogen bonded to O(W) and coordinated to one Li+ ion whereas the remaining three with equal S–O distance of 1.475(1) Å at 293 K are each coordinated to two Li+ions.