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 Fm3̄m to monoclinic P2₁∕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, η=-(2c0.2em0excos0.2em0exβ)∕a, and enthalpy $H=E+pV$ was the appropriate Landau potential at the athermal limit. Constant-pressure structural optimizations were performed at different η and p values, obtaining a set of H(η,p) curves. A study of their behavior in the neighborhood of η=0 (rocksalt type) and η=1 (TlI-type phase) allowed us to establish unambiguously the weak first-order character of the cubic-monoclinic transition, with an enthalpy barrier of 0.0100.3em0exeV at the equilibrium pressure of 19.30.3em0exGPa. For the monoclinic-orthorhombic transformation, not observed experimentally yet, an activation barrier of only 0.0010.3em0exeV was determined at pₜ=41.70.3em0exGPa, so as to indicate an effective second-order behavior at nonvanishing temperatures. Expansions of the Landau potential around η=1 showed that the numerical curves obtained can be fitted by eighth-order even polynomials, with a consistent pressure dependence of the coefficients.