A First‐Principles Study on the Electronic, Vibrational, and Thermodynamic Properties of Jadeite and its Tentative Low‐Density Polymorph

Abstract

The high-pressure clinopyroxene mineral jadeite (chemical composition NaAlSi 2 O 6 ) was studied by density-functional theory with respect to its electronic, vibrational, and thermodynamic properties, correctly reproducing the available experimental data. At a larger-thannormal volume, however, a low-density alumosilicate phase with tetrahedral instead of octahedral Al-O coordination was identified. This low-density phase was investigated theoretically, too, and the results were compared to jadeite and experimentally observed properties of what has been dubbed jadeite glass. It turned out that the theoretically * Prof. Dr. R. Dronskowski Fax: +49 241 80 92642 E-Mail: drons@HAL9000.ac.rwth-aachen.de [a] Chair 590 obtained properties of this hypothetical polymorph, such as the bulk modulus, the molar volume and the vibrational frequencies, agree well with the corresponding properties of the glass phase. Hence, from an inverse structure-property relationship we propose to model jadeite glass with the aforementioned low-density alumosilicate phase and tetrahedral Al-O motifs, as suggested from first principles and corroborated from experiment. The possibilities as well as the limitations of electronic, phonon and thermodynamic properties calculations applied to such a polymorph are also discussed.

Experimental and Theoretical Investigation of the Elastic Moduli of Silicate Glasses and Crystals

“…As expected, only at quite small molar volumes, which correspond to higher pressure, these modifications become energetically favorable. A similar observation as for the enstatite polymorphs has recently been made for the case of jadeite (Stoffel et al, 2016) where such a low-density polymorph was found to be similar to the isochemical glass phase with jadeite composition in terms of its macroscopic properties.…”

“…Using Helgeson's (Helgeson and Kirkham, 1976) value of the partial molar volume of H 2 O of approximately 8 cm 3 /mol, this yields a molar volume for dehydrated analcime of approximately 88.9 cm 3 /mol, which is in fair agreement with the value of the hypothetical structure identified; this also matches with the experimentally determined molar volume of the isochemical glass. Previous DFT calculations (Stoffel et al, 2016) as well as calorimetric values showed that both dehydrated analcime and the isochemical glass display nearly identical heat capacity c p .…”