Electron delocalization and magnetic behavior in a single crystal of ilvaite, a mixed valence iron silicate

Abstract

Ilvaite, Ca(Fe2+, Fe3+)Fe2+Si2O8(OH), a black mixed valence iron silicate shows considerable Fe2+⇄Fe3+ electron delocalization above 400 K, reminiscent of magnetite. A crystallographic phase transition from orthorhombic (Pnam) to monoclinic (P21/a) symmetry takes place on cooling at 343 K induced by electron ordering. In both phases, Fe2+ and Fe3+ occur in double octahedral chains parallel to the c axis. The thermal characteristics of the magnetic susceptibilities and their anisotropies in different crystallographic planes have been measured in the temperature range 400−21 K. Below 343±1K, a continuous rotation of the molar susceptibility K∥ in the ab plane down to 90±2 K is observed, where the symmetry of the magnetic ellipsoid remains unchanged. Xa, Xb and Xc increase abruptly below 123±0.5 K, although antiferromagnetic ordering of Fe2+ and Fe3+ spins on A sites was suggested in previous Mossbauer and neutron powder diffraction studies. In addition, 1/Xa shows an antiferromagnetic maximum at 50±3 K, whereas 1/Xb and 1/Xc at first increase sharply below 123 K, followed by antiferromagnetic curvatures in the lowest temperature region. This behavior is consistent with the antiferromagnetic ordering of Fe2+ spins in the B sites. The observed magnetic phenomena suggest charge delocatization effects between adjacent Fe2+(A)-Fe3+(A) pairs not only along c, but also along a and b directions. The negative sign of the molar anisotropy (K∥-K⊥) suggests a singlet ground State 5A1 for the Fe2+ ions, in agreement with previous Mossbauer studies.