Polarized single crystal absorption spectroscopy of the Pnam-P2 1/a transition of Ilvaite Ca(Fe2+, Fe3+)Fe2+Si2O8(OH) as measured between 300 K and 450 K

The temperature dependence of the absorption spectra of ilvaite, Ca(Fe²⁺,Fe³⁺)Fe²⁺Si₂O₈(OH), shows strongly one dimensional transport behaviour with no singularity at the Pnam-P2₁/a phase transition point near 335 K. Polarized single crystal transmission measurements were carried out between 300 K and 450 K in a frequency range between 600 and 23 000 cm⁻¹. No Drude —absorption at low energies was found at any temperature. A macroscopic, thermodynamic model based on Landau-Ginzburg theory is given which accounts for the observed macroscopic properties of the structural phase transition and its coupling with the Fe²⁺-Fe³⁺ ordering. This ordering scheme is discussed on an atomistic level and compared with the behaviour of magnetite and trans-(CH) _x .