Thermodynamic and structural variations along the olivenite–libethenite solid solution

Abstract

Many natural secondary arsenates contain a small fraction of phosphate. In this work, we investigated the olivenite–libethenite (Cu2(AsO4)(OH)–Cu2(PO4)(OH)) solid solution as a model system for the P–As substitution in secondary minerals. The synthetic samples spanned the entire range from pure olivenite (Xlib=0) to libethenite (Xlib=1). Acid-solution calorimetry determined that the excess enthalpies are non-ideal, with a maximum at Xlib=0.6 of +1.6 kJ mol−1. This asymmetry can be described by the Redlich–Kister equation of Hex= Xoli⋅Xlib [A+B(Xoli−Xlib)], with A=6.27 ± 0.16 and B=2.9 ± 0.5 kJ mol−1. Three-dimensional electron diffraction analysis on the intermediate member with Xlib=0.5 showed that there is no P–As ordering, meaning that the configurational entropy (Sconf) can be calculated as -R(Xoliln⁡Xoli+Xlibln⁡Xlib). The excess vibrational entropies (Svibex), determined by relaxation calorimetry, are small and negative. The entropies of mixing (Sconf+Svibex) also show asymmetry, with a maximum near Xlib=0.6. Autocorrelation analysis of infrared spectra suggests local heterogeneity that arises from strain relaxation around cations with different sizes (As5+ / P5+) in the intermediate members and explains the positive enthalpies of mixing. The length scale of this strain is around 5 Å, limited to the vicinity of the tetrahedra in the structure. At longer length scales (≈15 Å), the strain is partially compensated by the monoclinic–orthorhombic transformation. The volume of mixing shows complex behavior, determined by P–As substitution and symmetry change. A small (0.9 kJ mol−1) drop in enthalpies of mixing in the region of Xlib=0.7–0.8 confirms the change from monoclinic to orthorhombic symmetry.