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Anharmonic motions versus dynamic disorder at the Mg ion from the charge densities in pyrope (Mg3Al2Si3O12) crystals at 30 K: six of one, half a dozen of the other.

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posted on 2019-02-01, 09:45 authored by R. Destro, R. Ruffo, P. Roversi, R. Soave, L. Loconte, L. Lo Presti
The possible occurrence of static/dynamic disorder at the Mg site in pyrope (Mg3Al2Si3O12), with or without anharmonic contribution to the thermal vibrations even at low temperatures, has been largely debated but conclusions were contrasting. Here a report is given on the experimental charge density distribution, ρEXP, of synthetic pyrope at T = 30 K, built through a Stewart multipolar expansion up to l = 5 and based on a very precise and accurate set of in-home measured single-crystal X-ray diffraction amplitudes with a maximum resolution of 0.44 Å. Local and integral topological properties of ρEXP are in substantial agreement with those of ρTHEO, the corresponding DFT-grade quantum charge density of an ideal pyrope crystal, and those derived from synchrotron investigations of chemical bonding in olivines. Relevant thermal atomic displacements, probably anharmonic in nature, clearly affect the whole structure down to 30 K. No significant (> 2.5σ) residual Fourier peaks are detectable from the ρEXP distribution around Mg, after least-squares refinement of a multipole model with anharmonic thermal motion at the Mg site. Experimental findings were confirmed by a full analysis of normal vibration modes of the DFT-optimized structure of the perfect pyrope crystal. Mg undergoes wide displacements from its equilibrium position even at very low temperatures, as it is allocated in a ∼ 4.5 Å large dodecahedral cavity and involved in several soft phonon modes. Implications on the interplay among static/dynamic disorder of Mg and lattice vibrational degrees of freedom are discussed.

Funding

One of us (LLP) thanks the Italian Supercomputing Centre CINECA for providing CPU time through the LISA initiative (SURGREEN grant, 2014-2015). LLP also acknowledges partial funding from the Centre of Materials Crystallography (CMC) at A˚ rhus (DK).

History

Citation

Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials, 2017, 73 (Pt 4), pp. 722-736

Author affiliation

/Organisation/COLLEGE OF LIFE SCIENCES/Biological Sciences/Molecular & Cell Biology

Version

  • VoR (Version of Record)

Published in

Acta Crystallographica Section B: Structural Science

Publisher

International Union of Crystallography, Wiley

eissn

2052-5206

Acceptance date

2017-04-22

Copyright date

2017

Available date

2019-02-01

Publisher version

http://scripts.iucr.org/cgi-bin/paper?S2052520617006102

Notes

CCDC reference: 1545443 Supporting information: this article has supporting information at journals.iucr.org/b

Language

en

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