Charge density in pyroelectric lithium sulfate monohydrate at 80 and 298 K

Abstract

Lithium sulfate monohydrate has been studied at 80 and 298 K by x-ray diffraction. The monoclinic crystal with space group P21 has lattice dimensions at 298 K of a=5.4553(1), b=4.8690(1), c=8.1761(2) Å, and β=107.337(2)°; lattice dimensions at 80 K were reported in our neutron study [J. Chem. Phys. 80, 423 (1984)]. Least-squares refinement based on 3486 (80 K) and 3390 (298 K) independent reflections, assuming a spherical atom model, results in final R( F 2) values of 0.024 (80 K) and 0.026 (298 K). Static deformation and charge density model refinement, based on Hirshfeld-type multipole functions, greatly reduces the residual electron density and gives R( F 2) values of 0.017 (80 K) and 0.016 (298 K). Refinement of the resulting multipole parameters within the Gaussian radial dependence model allows a qualitative estimation of the differences in electron densities between 80 and 298 K. A change of about 0.2 e Å−3 in the deformation density of the O–H bonds in the water molecule over this temperature range is related to the contraction in hydrogen bonding at 80 K. Changes in the sulfate oxygen atom lone-pair deformation densities, caused by contractions in the SO2−4–Li+ contacts between 298 and 80 K, are also found between the two temperatures. X-ray–neutron diffraction deformation density calculations indicate a substantial electron deficiency in the sulfur valence shell. Kappa refinement leads to a point charge model with positive atomic charge close to 1.8 on sulfur, about −1.0 on each sulfate oxygen atom, and −0.8 on the water oxygen atom. Interatomic distances at 80 and 298 K, particularly those based on a Gaussian radial dependence for the deformation density function, are in excellent agreement with the corresponding neutron diffraction values.