Thermodynamic and structural variations along theolivenite-libethenite solid solution

Many natural secondary arsenates contain a small fraction of phosphate. In this work, we investigated the olivenite-libethenite (Cu-2(AsO4)(OH)-Cu-2(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 (X-lib=0) to libethenite (X-lib=1). Acid-solution calorimetry determined that the excess enthalpies are non-ideal, with a maximum at X-lib=0.6 of +1.6 kJ mol(-1). This asymmetry can be described by the Redlich-Kister equation of H-ex= X-oli.X-lib [A+B(X-oli-X-lib)], 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 X-lib=0.5 showed that there is no P-As ordering, meaning that the configurational entropy (S-conf) can be calculated as -R(X(oli)lnX(oli)+X(lib)lnX(lib)). The excess vibrational entropies (S-vib(ex)), determined by relaxation calorimetry, are small and negative. The entropies of mixing (S-conf+S-vib(ex)) also show asymmetry, with a maximum near X-lib=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 angstrom, limited to the vicinity of the tetrahedra in the structure. At longer length scales (approximate to 15 angstrom), 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 X-lib=0.7-0.8 confirms the change from monoclinic to orthorhombic symmetry.