Relative energetics and structural properties of zirconia using a self-consistent tight-binding model

We describe an empirical, self-consistent, orthogonal tight-binding model for zirconia, which allows for the polarizability of the anions at dipole and quadrupole levels and for crystal field splitting of the cation d orbitals. This is achieved by mixing the orbitals of different symmetry on a site with coupling coefficients driven by the Coulomb potentials up to octapole level. The additional forces on atoms due to the self-consistency and polarizabilities are exactly obtained by straightforward electrostatics, by analogy with the Hellmann-Feynman theorem as applied in first-principles calculations. The model correctly orders the zero temperature energies of all zirconia polymorphs. The Zr-O matrix elements of the Hamiltonian, which measure covalency, make a greater contribution than the polarizability to the energy differences between phases. Results for elastic constants of the cubic and tetragonal phases and phonon frequencies of the cubic phase are also presented and compared with some experimental data and first-principles calculations. We suggest that the model will be useful for studying finite temperature effects by means of molecular dynamics.Comment: to be published in Physical Review B (1 march 2000

Free energy and molecular dynamics calculations for the cubic-tetragonal phase transition in zirconia

The high-temperature cubic-tetragonal phase transition of pure stoichiometric zirconia is studied by molecular dynamics (MD) simulations and within the framework of the Landau theory of phase transformations. The interatomic forces are calculated using an empirical, self-consistent, orthogonal tight-binding (SC-TB) model, which includes atomic polarizabilities up to the quadrupolar level. A first set of standard MD calculations shows that, on increasing temperature, one particular vibrational frequency softens. The temperature evolution of the free energy surfaces around the phase transition is then studied with a second set of calculations. These combine the thermodynamic integration technique with constrained MD simulations. The results seem to support the thesis of a second-order phase transition but with unusual, very anharmonic behaviour above the transition temperature

The phase transition sequence in the relaxor ferroelectric PZN-8% PT

Crystal structures and phase transitions in the giant piezoelectric effect material lead zinc niobate-8% lead titanate (PZN-8% PT) between 4.2 and 455 K have been determined using very high resolution powder neutron diffraction. The structure at 4.2 K is monoclinic (Cm). On heating, the monoclinic phase transforms first into tetragonal (P4mm) and then cubic (Pm3m) structures via first-order transitions with wide two-phase regions

Crystal structure of the relaxor ferroelectric PZN: demise of the 'X-phase' (letter)

The crystal structure of lead zinc niobate (PZN) was studied by very high-resolution neutron powder diffraction using both small (<143 ?m) and large (1-2 mm) crystals. No evidence was found for the rhombohedral exterior/cubic interior 'X-phase' structure reported by Xu et al (2004 Appl. Phys. Lett. 84 3975-7). The structure is confirmed to be a rhombohedral perovskite in space group R3m. It is concluded that the X-phase observations were due to a rhombohedral domain size and population gradient within the crystals sampled by a small x-ray beam. © 2005 IOP Publishing Ltd