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