‘Soft’ phonon modes, structured diffuse scattering and the crystal chemistry of Fe-bearing sphalerites
✍ Scribed by Ray L. Withers; T.R. Welberry; Allan Pring; Cristophe Tenailleau; Yun Liu
- Publisher
- Elsevier Science
- Year
- 2005
- Tongue
- English
- Weight
- 475 KB
- Volume
- 178
- Category
- Article
- ISSN
- 0022-4596
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✦ Synopsis
Electron diffraction has been used to carefully investigate the reciprocal lattices of a range of iron-bearing sphalerites looking for evidence of Fe clustering and/or Fe/Zn ordering in the form of either additional satellite reflections or a structured diffuse intensity distribution accompanying the strong Bragg reflections of the underlying sphalerite-type average structure. While a highly structured diffuse intensity distribution in the form of transverse polarized f110g à sheets of diffuse intensity has been detected and found to be characteristic of all compositions, it does not appear to arise from Fe clustering and/or Fe/Zn ordering. Rather inherently low frequency, and therefore strongly thermally excited, phonon modes propagating along reciprocal space directions perpendicular to each of the six /110S real space directions of the average structure are suggested to be responsible for these f110g à sheets of diffuse intensity. Monte Carlo simulation (for a range of Zn-S, Zn-Zn and S-S interaction strengths) and subsequent Fourier transformation is used to confirm the existence of these low-frequency phonon modes of distortion as well as to show that they are an intrinsic, predictable property of the corner-connected tetrahedral structure of sphalerite. The low-frequency phonon modes involve coupled (Zn, Fe) and S motion in one-dimensional strings along /110S real space directions.