Phase transitions in ilvaite,a mixed-valence iron silicate

Ilvaite, Ca(Fe²⁺, Fe³⁺) Fe²⁺Si₂O₇O(OH), a mixed-valence iron silicate shows an insulator-semimetal transition with a band gap of 0.13 eV due to thermally induced charge delocalization between Fe²⁺ and Fe³⁺ ions (A sites) in double octahedral chains. The charge delocalization induces a second order crystallographic phase transition on heating from monoclinic (P2₁/a) to orthorhombic (Pnam) symmetry at 346 K. The unit cell dimensions within the 295–420 K range and the crystal structures at 295, 320, 340, 360, 380 and 400 K have been determined by high temperature single crystal X-ray diffraction. The degree of charge delocalization determined from the sizes of the Fe(Ao) and Fe(Am) octahedra is the primary order parameter, Q which couples linearly with the spontaneous strain component, ₁₃. The order parameter coupling and the associated free energy expression is given. The calculated normal modes of the space group symmetry change are consistent with the experimentally observed atomic displacements, which are parallel and antiparallel to c. Formation of antiphase lamellar twin domains parallel to (001) in the monoclinic phase is predicted to occur as a result of the phase transition. Above T_c (= 346 K), the slow asymptotic decrease of ₁₃ attaining a zero value at 380 K indicates the presence of fluctuating precursor clusters with considerable short-range order above T_c. A peak in the specific heat (C_p) measurements coincides with the onset of longrange order at 380 K, whereas ⁵⁷Fe Mössbauer measurements indicate the onset of charge localization at a considerbly higher temperature (470 K). The coupling of the d⁶ electron of the Fe²⁺ (A) ion with a longitudinal optic phonon with the polarization vector along c ^* is the likely mechanism to drive the phase transition. The electronphonon coupling also provides a charge conduction mechanism through electron hopping, whereby the short-bonded Fe²⁺-Fe³⁺ pair containing the d⁶ electron (intermediate polaron) will break up and re-form, thereby propagating the electron one step along the c axis.