Amorphous/crystalline (A/C) thermodynamic 'rules of thumb': Estimating standard thermodynamic data for amorphous materials using standard data for their crystalline counterparts

摘要

Standard thermochemical data (in the form of Δ _fH° and Δ _fG°) are available for crystalline (c) materials but rarely for their corresponding amorphous (a) counterparts. This paper establishes correlations between the sets of data for the two material forms (where known), which can then be used as a guideline for estimation of missing data. Accordingly, Δ _fH°(a)/kJ mol ~(-1) ≈ 0.993Δ _fH°(c)/kJ mol ~(-1) + 12.52 (R ~2 = 0.9999; n = 50) and Δ _fG°/kJ mol ~(-1) ≈ 0.988Δ _fH°(c)/kJ mol ~(-1) + 0.70 (R ~2 = 0.9999; n = 10). Much more tentatively, we propose that S° 298(c)/ J K ~(-1) mol ~(-1) ≈ 1.084S° _(298)(c)/J K ~(-1) mol ~(-1) + 6.54 (R ~2 = 0.9873; n = 11). An amorphous hydrate enthalpic version of the Difference Rule is also proposed (and tested) in the form [Δ _fH°(M _pX _q?nH _2O,a) - Δ _fH°(M _pX _q,a)]/kJ mol ~(-1) ≈ Θ _(Hfn) ≈ -302.0n, where M _pX _q?nH _2O represents an amorphous hydrate and M _pX _q the corresponding amorphous anhydrous parent salt.