First-principles Landau potential for the rocksalt to KOH to TlI-type phase transitions of AgI

Two high-pressure displacive phase transformations of AgI (from cubic Fm (3) over barm to monoclinic P2(1)/m to orthorhombic Cmcm symmetry) were studied by periodic ab initio calculations, employing a B3LYP hybrid density functional theory-Hartree-Fock functional (CRYSTAL code). The order parameter was represented as a distortion of the monoclinic lattice, eta=-(2c cos beta)/a, and enthalpy H=E+pV was the appropriate Landau potential at the athermal limit. Constant-pressure structural optimizations were performed at different eta and p values, obtaining a set of H(eta,p) curves. A study of their behavior in the neighborhood of eta=0 (rocksalt type) and eta=1 (TlI-type phase) allowed us to establish unambiguously the weak first-order character of the cubic-monoclinic transition, with an enthalpy barrier of 0.010 eV at the equilibrium pressure of 19.3 GPa. For the monoclinic-orthorhombic transformation, not observed experimentally yet, an activation barrier of only 0.001 eV was determined at p(t)=41.7 GPa, so as to indicate an effective second-order behavior at nonvanishing temperatures. Expansions of the Landau potential around eta=1 showed that the numerical curves obtained can be fitted by eighth-order even polynomials, with a consistent pressure dependence of the coefficients.