Density functional study of the structural properties of silver halides : LDA vs GGA calculations

We perform first principles total energy calculations to investigate the atomic structure of three silver halides: AgCl, AgBr, and AgI in sodium chloride, cesium chloride, zincblende and wurtzite structures. Calculations are done within the density functional theory. We employ the full potential linearized augmented plane wave method as implemented in the wien2k code. The exchange and correlation potential energies are treated in the generalized gradient approximation (GGA) within the Perdew, Burke, Ernzerhof (PBE) parameterization, and the local density approximation (LDA). Our results show that the LDA calculations correctly predict the ground state structure of all three binary compounds: rocksalt for AgCl and AgBr, and zincblende/wurtzite for AgI, while the PBE calculations always prefer the more covalent zincblende/wurtzite configurations (although the difference between rocksalt and wurtzite energies is very small). We observe that the distribution of electron densities for rocksalt is more homogeneous than that for zincblende. As a consequence, the energy difference between zincblende/wurtzite and rocksalt phases is enhanced in the PBE calculations, predicting the wrong ground state structure for AgCl and AgBr. Assuming the right experimental structure, the PBE parameterization gives lattice parameters, bulk moduli and cohesive energies closer to experimental values.