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3.2 f. Short-range order parameter associated with the P21/m <-> C2/m phase transition in cummingtonite (T. Boffa-Ballaran and C.A. McCammon, in collaboration with M.A. Carpenter/Cambridge, UK)

Since the discovery of the Mössbauer effect, numerous applications in a wide variety of scientific disciplines have been described. The hyperfine parameters, which can be determined experimentally from the line positions in a Mössbauer spectrum, provide information on the local atomic environment around the nuclei. In particular, the quadrupole splitting (QS) depends on the valence and spin state of the absorber atoms, as well as the coordination and degree of distortion of the crystallographic sites that such atoms occupy. Therefore, this parameter can potentially be used to describe the variation of structural distortion associated with phase transitions.

In Mg-rich cummingtonites [(Mg,Fe)7Si8O22(OH)2] a displacive phase transition from P21/m to C2/m occurs with increasing temperature. The difference between low and high-symmetry phases is mainly in the degree of distortion of the tetrahedral chains. Whereas in the C2/m structure there is only one crystallographic distinct chain, in the P21/m phase there are two non-equivalent chains, designed as A and B, with different elongation angles. The lowering in symmetry causes a change in the coordination of the M4 sites from 4+2 with four atoms (two O2 and two O4) at ~ 2 Å and two atoms (O6) at ~ 2.7 Å to 4+1+1 with one of the O6 atoms now closer to the M4 cation at ~ 2.5 Å. This change in the M4 coordination may, therefore, affect the Mössbauer parameters.

Mössbauer spectra of a cummingtonite with xFe = 0.37 were collected in the temperature range between 100 and 550 K. The spectra show only four peaks, where the peaks correspond to two different types of iron. The inner two peaks have been assigned as in previous studies to the M4 position due to the higher distortion of the M4 octahedra in the amphibole structure, whereas the outer two peaks have been assigned to the M1, M2 and M3 positions. The M1, M2 and M3 doublets overlap at all the temperatures studied. The QS of the M4 doublet is shown in Figure 3.2-5 as a function of temperature. A clear change in slope is visible at the critical temperature of the P21/m to C2/m phase transition. If we assume that the valence

Fig. 3.2-5. Temperature variation of the quadrupole splitting QS of Fe2+ at the M4 sites. Open squares: P21/m phase, open circles: C2/m phase. The temperature variation within a single structure is approximately linear in the vicinity of the phase transition, and can be used to extrapolate the high-symmetry QS data to low temperature (solid line).

contribution to the QS is similar for both P21/m and C2/m phases, the most likely effect to cause the observed variation is a change in the lattice contribution related to the distortion of the M4 sites. The difference between the QS data of the low-symmetry phase and the values obtained at the same temperatures by extrapolating the QS data of the C2/m phase could be used as a measure of the short-range order parameter associated with the phase transition. Further work is underway to obtain a long-range order parameter described in terms of lattice strains to compare with the short-range order parameter.

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