Conference Proceedings Vol. 50
,,ICAME-95“
I. Ortaili (Ed.)
SIF, Bologna,1996

U. Hiller^a), M. K. Krauseb), C.A. Kleint^b), W. Potzel^a), H, Karzel^a), M. Steiner^a), W. Schiessl^a), G.M. Kalvius.^a), R. Sonntag^c), and E. Rönsch^d)

^a) Physik-Department E15, Technische Universität Miinchen, D-85747 Garching, Germany
sup>b) Fakultät für Physik und Geowissenschaften, Universität Leipzig, D-04103 Leipzig, Germany
^c) Hahn-Meitner-lnstitut, D-14109 Berlin and Institut für Kristallographie, Universität Tiibingen,
Tübingen, Germany
^d) Institut für Anorganische Chemie, Technische Universität Dresden, D-01062 Dresden. Germany

The compounds ZnxFe2-xSiO4 (x=2 and x=1.2) which crystallize with willemite structure have been investigated by 67Zn-Mössbauer spectroscopy and by neutron diffraction. The willemite structure provides two non-equivalent lattice sites for zinc, both being tetrahedrally coordinated by oxygens. In ZnSiO 4 both sites are characterized by surprisingly large asymmetry parameters of the electric field gradient tensor (EFG) at the 67Zn nucleus: h=0.58(2) (Zn(l)) and h=0.99(1) (Zn(2)). For the main components we obtain: VZZ=+1.62(16)×1017V/cm2 (Zn(l)) and VZZ=±2.27(23)×1017V/cm2 (Zn(2)). Zn1.2Fe0.8SiO4 orders antiferromagnetically below TN=16K with antiparallel orientation of neighboring Fe spins along the crystallographic c axis. Fe atoms favor the slightly larger Zn(2) sites. We find the occupation numbers (XFe/XZn) »l:3 and »11:9 for the Zn(l) and Zn(2) sites, respectively. Below TN we observe magnetic hyperfine fields of »0.5T at the 67Zn nuclei at both Zn sites. Above TN we measure VZZ=+1.43(15)×1017V/cm2, h=0.70(10) and VZZ=+2.67(28)×1017V/cm2, h =0.80(8) for the Zn(l) and Zn(2) sites, respectively. A simple point charge model is totally insufficient to explain the EFG tensors at the Zn sites.