Compressibility of NaMnSi2O6: The role of electronic isovalency for the validity of bulk-modulus–volume relationship

A natural crystal with NaMnSi2O6 composition (space group C2/c) belonging to the family of clinopyroxene structures was investigated by in-situ high-pressure single-crystal X-ray diffraction up to 9 GPa in order to verify if the compressibility can be predicted from the unit-cell volume V0 at 1 bar applying an established volume–compressibility relationship. Together with data for isostructural analogue phases the equation-of-state parameters of NaMnSi2O6 showed that such simplified relationship can not be longer applied. The findings demonstrate that prediction models for isostructural series are consistent not only for the aspect of structural isotopy, and valid relationships presuppose similar character of the electronic structure. In case of pyroxenes containing transition elements the established relationship is only valid for series of compounds with cations of identical oxidation state according to isovalent electronic character.

In this work we definitively demonstrate that isostructural compounds obey to the empirical relationship bulk modulus × volume = constant only for series of compounds with cations of identical oxidation state according to isovalent electronic character This can help in predicting physical properties like compressibility only knowing the unit-cell volume at room pressure. Figure optionsDownload as PowerPoint slideHighlights
► HP in situ measurement of NaMnSi2O6 by mean of XRD and a diamond-anvil cell.
► Prediction models for isostructural series for volume compression.
► Determination of two different compressibility trends based on composition.