Hyperfine interactions in ferrous fluosilicate

The Mössbauer effect spectrum of ferrous fluosilicate has been measured between 1.7 and 4.2 °K in external magnetic fields of up to 30 kG for both single-crystal and polycrystalline specimens. With the field H parallel to the trigonal axis z of the crystal (for which the susceptibility is small) only a small hyperfine splitting was observed, but for H along any axis x in the plane perpendicular to z a large effective field was found. The saturation values deduced for the components of the hyperfine field tensor were H n x (0) = -248 kG and H n z (0) = -550 kG. The spectrum for polycrystalline specimens was almost identical with that for the single crystal with the field perpendicular to z . The data are discussed in terms of the theory of hyperfine fields, and an estimate of the effective value of < r -3 > for the 3d-electron spin is made. The value of the electric quadrupole moment of the excited state of 57 Fe deduced from the quadrupole splitting is re-assessed at +0.18 bn.