Crystal and Magnetic Structure of Cupric Fluoride Dihydrate at 4.2°K

Abstract

A neutron diffraction study of CuF₂·2H₂O below the Néel point has determined the crystal and magnetic structure. As compared with this crystal at 298°K, the long Cu–F axial bonds of the distorted octahedron around the Cu²⁺ ion are reduced in length from 2.465±6 to 2.391±4 A: the short Cu–F bond of 1.899±6 A and the Cu–O bond of 1.945±4 A are unchanged. The H–O–H angle is reduced from 115.5° at 298°K to 110.1±0.4° at 4.2°K, and the O–H distance from 0.980±7 to 0.959±5 A. The amplitudes of vibrational displacement of the Cu, O, and F atoms at 4.2°K are about one‐half that at 298°K: The hydrogen atoms have about the same motion, due to zero‐point energy. The magnetic unit cell is identical with the nuclear cell, but the spin on the body‐centered copper atom is antiparallel to that at the origin. The spin direction is very nearly along the c axis, although an alternative model with the spin along the long octahedral direction fits the magnetic scattering almost as well. The difference between the Néel temperature of 10.9° and that of 26°K at which the magnetic susceptibility is a maximum is consistent with the observed magnetic structure. The revised nuclear scattering length for fluorine of 0.529×10^—12 cm found in the analysis of CuF₂·2H₂O at 298°K is in excellent agreement with the experimental value of 0.534±12 obtained in the present study.