Silvia Tinte

Atomistic modelling of BaTiO3 based on first-principles calculations

Interatomic potentials are determined in the framework of a shell model used to simulate the structural instabilities, dynamical properties, and phase transition sequence of BaTiO3. The model is developed from first-principles calculations by mapping the potential energy surface for various ferroelectric distortions. The parameters are obtained by performing a fit of interatomic potentials to this energy surface. Several zero-temperature properties of BaTiO3, which are of central importance, are correctly simulated in the framework of our model. The phase diagram as a function of temperature is obtained through constant-pressure molecular dynamics simulations, showing that the non-trivial phase transition sequence of BaTiO3 is correctly reproduced. The lattice parameters and expansion coefficients for the different phases are in good agreement with experimental data, while the theoretically determined transition temperatures tend to be too small.

First-principles-based simulations of relaxor ferroelectrics

The phenomenology of Pb(B,B′)O3 perovskite-based relaxor ferroelectrics (RFE) is reviewed, with emphasis on the relationship between chemical short-range order and the formation of polar nanoregions in the temperature range between the “freezing” temperature, T f, and the Burns temperature, T B. Results are presented for first-principles-based effective Hamiltonian simulations of (PSN), and simulations that were done with empirically modified variants of the PSN Hamiltonian. Arbitrarily increasing the magnitudes of local electric fields, caused by an increase in chemical disorder, broadens the dielectric peak, and reduces the ferroelectric (FE) transition temperature; and sufficiently strong local fields suppress the transition. Similar, but more dramatically glassy results are obtained by using the PSN dielectric model with a distribution of local fields that is appropriate for (PMN). The results of these simulations, and reviewed experimental data, strongly support the view that within the range , polar nanoregions are essentially the same as chemically ordered regions. In PSN a ferroelectric phase transition occurs, but in PMN, a combination of experimental and computational results indicate that pinning from local fields is strong enough to suppress the transition and glassy freezing is observed.

Vibrational effects on SrTiO3 Ti 1s absorption spectra studied using first-principles methods

We analyze the vibrational effects on the Ti 1s excited states in cubic SrTiO3 and related pre-edge x-ray absorption fine structure using first-principles methods. Ground-state, total-energy and electron–core hole Bethe–Salpeter calculations are performed for different atomic configurations related to eg-symmetry distortions of SrTiO3. From these, we can obtain normal-mode gradients of the electronic excited-state energy, i.e., of the excited-state Born–Oppenheimer surface. This yields the corresponding electron–phonon coupling coefficients that allow us to predict the spectral broadening induced by those vibrational modes.

THE EFFECT OF NEAREST NEIGHBOR [Pb─O] DIVACANCY PAIRS ON THE FERROELECTRIC-RELAXOR TRANSITION IN NANO-ORDERED Pb(Sc1/2Nb1/2)O3

ABSTRACT Molecular dynamics simulations were performed on a first-principles-based effective Hamiltonians for chemically short-range ordered Pb(Sc1/2Nb1/2) O3 with nearest neighbor [Pb─O] divacancy pairs. The divacancy-concentration (X[Pb-O]) vs. temperature phase diagram was calculated, and it is topologically equivalent to the hydrostatic pressure (P) vs. temperature diagram: a ferroelectric ground-state phase at low X[Pb - O] (P); that transforms to a relaxor paraelectric phase at moderate X[Pb - O] (P); followed by a crossover to a normal paraelectric phase at high X[Pb - O](P).