The Phase Transformation in Ferreous Fluosilicate Hexahydrate

Abstract

The struct,ural change of FeSiF, . 6 H,O at T, = 325 K (Pgl/C to P3ml above T,) is well known from X-ray measurements. The temperature dependence of the Mossbauer parameters in the neighbourhood of the phase transformation (200 K 2 T 5 250 K) is studied. At T, the quadrupole splitting AEQ changes with increasing temperature by (+0.009 f 0.002) mm/s, the Lamb factorf slightly decreases by Sf = -0.011 f 0.004, while the anisotropy off changes remarkably: S(f11 -fl) = -0.18. From the relaxation broadening of the absorption lines below and above T, i t is concluded that the non-axial terms of the ligand field a t the Fez+ ion do not vanish above the transition temperature. Die Strukturanderung von FeSiF, . 6 H,O bei T, = 225 K (P2,/C zu P3m 1 oberhalb T,) ist aus Rontgenstrukturuntersuchungen wohl bekannt. Die Temperaturabhiingigkeit der MoBbauerparameter in der Umgebung des Phasenubergangs (300 K 5 T 5 250 K) wird untersucht. Bei T, andert sich die Quadrupolaufspaltung AEQ um (+0,009 & 0,002) mm/s mit wachsender Temperatur, der Lamb-MoBbauer-Faktor f fallt ein wenig, um S,f = -0.011 & 0,004, wahrend die Anisotropie vonf sich betrichtlich andert : S(flifl) = -0,18. Die Relaxationsverbreiterung der Absorptionslinien linter und oberhalb T , lassen den Schlu13 zu, daB die nichtaxialen Terme des Ligandenfeldes am Fe"-Ort oberhalb T, nicht verschwinden.

Isotropy of the Lamb-Mössbauer factor in ferrous fluosilicate single crystals (Debye model)

“…The Debye-Waller factor does not show any sizeable anomaly when passing through the P3ml +-+ P21/c transition : neither in the average value nor in the axial anisotropy of f, in conflict with the conclusions of [1]. Therefore a f anisotropy cannot be associated with the sharp change in the quadrupolar spectrum asymmetry at Te.…”

“…Introduction. -Recently, the Goldanskii-Karyagin effect was proposed [1] as one of the phenomena associated with the asymmetrical shape of the quadrupolar spectra of ferrous fluosilicate powder samples [2].…”

“…Results. -In order to determine /;j and fl in ferrous fluosilicate, we have used : -a platelet, cut perpendicularly to c-axis, 1.5 mm ± 0.1 mm thick (fl), -a platelet, containing c-axis, 0.3 mm + 0.05 mm thick, obtained by cleaving, (/J.…”