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

The possible occurrence of static/dynamic disorder at the Mg site in pyrope(Mg3Al2Si3O12), with or without anharmonic contribution to the thermalvibrations even at low temperatures, has been largely debated but conclusionswere contrasting. Here a report is given on the experimental charge densitydistribution, rhoEXP, of synthetic pyrope at T = 30 K, built through a Stewartmultipolar expansion up to l = 5 and based on a very precise and accurate set ofin-home measured single-crystal X-ray diffraction amplitudes with a maximumresolution of 0.44 A? . Local and integral topological properties of rhoEXP are insubstantial agreement with those of rhoTHEO, the corresponding DFT-gradequantum charge density of an ideal pyrope crystal, and those derived fromsynchrotron investigations of chemical bonding in olivines. Relevant thermalatomic displacements, probably anharmonic in nature, clearly affect the wholestructure down to 30 K. No significant (> 2.5sigma) residual Fourier peaks aredetectable from the rhoEXP distribution around Mg, after least-squares refinementof a multipole model with anharmonic thermal motion at the Mg site.Experimental findings were confirmed by a full analysis of normal vibrationmodes of the DFT-optimized structure of the perfect pyrope crystal. Mgundergoes wide displacements from its equilibrium position even at very lowtemperatures, as it is allocated in a 4.5 A? large dodecahedral cavity andinvolved in several soft phonon modes. Implications on the interplay amongstatic/dynamic disorder of Mg and lattice vibrational degrees of freedom arediscussed