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

作者：Destro Riccardo; Ruffo Riccardo; Roversi Pietro; Soave Raffaella; Loconte Laura; Lo Presti Leonardo

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

DOI：10.1107/S2052520617006102

• pyrope garnet
• experimental charge density
• cryocrystallography
• disorder
• anharmonic motion
• quantum theory of atoms in molecules

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, rho(EXP), 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 angstrom. Local and integral topological properties of rho(EXP) are in substantial agreement with those of rho(THEO), 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 (> 2.5 sigma) residual Fourier peaks are detectable from the rho(EXP) 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 similar to 4.5 angstrom 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.

• 出版日期2017-8