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Papers by Alberto Perez Garcia

Research paper thumbnail of First-principles study of the ferroelastic phase transition in CaCl2

Physical Review B, 2002

ABSTRACT First-principles density-functional calculations within the local-density approximation ... more ABSTRACT First-principles density-functional calculations within the local-density approximation and the pseudopotential approach are used to study and characterize the ferroelastic phase transition in calcium chloride (CaCl_2). In accord with experiment, the energy map of CaCl_2 has the typical features of a pseudoproper ferroelastic with an optical instability as ultimate origin of the phase transition. This unstable optic mode is close to a pure rigid unit mode of the framework of chlorine atoms and has a negative Gruneisen parameter. The ab-initio ground state agrees fairly well with the experimental low temperature structure extrapolated at 0K. The calculated energy map around the ground state is interpreted as an extrapolated Landau free-energy and is successfully used to explain some of the observed thermal properties. Higher-order anharmonic couplings between the strain and the unstable optic mode, proposed in previous literature as important terms to explain the soft-phonon temperature behavior, are shown to be irrelevant for this purpose. The LAPW method is shown to reproduce the plane-wave results in CaCl_2 within the precision of the calculations, and is used to analyze the relative stability of different phases in CaCl_2 and the chemically similar compound SrCl_2. Comment: 9 pages, 6 figures, uses RevTeX4

Research paper thumbnail of Dynamics of incommensurate structures and inelastic neutron scattering

Physical Review B, 1989

A general analysis of the dynamics of displacive incommensurately modulated structures within the... more A general analysis of the dynamics of displacive incommensurately modulated structures within the harmonic approximation is presented. Superspace formalism is not used. The present approach is compared with the usual Landau model and some recent phenomenological predictions for hard internal modes. The characteristics of coherent inelastic neutron scattering are also analyzed. A compact expression for the one-phonon dynamic structure factor

Research paper thumbnail of Devonshire-Landau free energy of BaTiO3 from first principles

Research paper thumbnail of Optimized local modes for lattice-dynamical applications

Research paper thumbnail of Multiple instabilities in Bi4Ti3O12: A ferroelectric beyond the soft-mode paradigm

Physical Review B, 2008

The Aurivillius compound Bi4Ti3O12 exhibits a single phase transition from a tetragonal high-temp... more The Aurivillius compound Bi4Ti3O12 exhibits a single phase transition from a tetragonal high-temperature phase to a ferroelectric state. By means of first-principles calculations guided by a complete symmetry analysis of the experimental distortion, we confirm B1a1 as the (monoclinic) symmetry of the room-temperature phase but show that the transition involves a complex interplay of six different normal modes belonging to four different irreducible representations. Three of them are necessary for the observed symmetry break. We do not find any feature in the energy landscape that would force the simultaneous condensation of these three order parameters and cause such an avalanche transition, so we suggest that further experimental work might be able to detect intermediate phases. On the other hand, the necessary condensation of pairs of normal modes of the same symmetry already puts Bi4Ti3O12 outside the standard soft-mode paradigm and has important structural implications. In particular, the approximate rigidity of the BiO6 octahedra is due to anharmonic couplings of these modes. In addition, a comparison of the energy calculations and the behavior of the bond-valence global instability index is presented.

Research paper thumbnail of Competing structural instabilities in the ferroelectric Aurivillius compound SrBi2Ta2O9

Physical Review B, 2004

The Aurivillius family includes many compounds with ferroelectric phases. Despite sharing the bas... more The Aurivillius family includes many compounds with ferroelectric phases. Despite sharing the basic perovskite structural block with more common ferroelectric oxides such as BaTiO 3 or PbTiO 3 , their phase-transition behavior is rather more complex. In particular, SrBi 2 Ta 2 O 9 ...

Research paper thumbnail of Superlattice pseudouniform orderings as modulated structures: Stripe and checkerboard arrangements

Research paper thumbnail of Comparison of empirical bond-valence and first-principles energy calculations for a complex structural instability

Physical Review B, 2005

A simple bond-strain parameter, the so-called global-instability index, defined within the bond-v... more A simple bond-strain parameter, the so-called global-instability index, defined within the bond-valence model reproduces with striking accuracy the basic features of the ab initio total-energy surface around the energy minimum of SrBi2Ta2O9 , but only for the degrees of freedom within the subspace of energetically unstable modes. These results imply that ab initio total-energy calculations and bond-valence criteria could be

Research paper thumbnail of Compositional uniformity, domain patterning and the mechanism underlying nano-chessboard arrays

Journal of Physics: Condensed Matter, 2012

Research paper thumbnail of First-principles study of ferroelasticity in CaCl 2 and As 2 O 5

Ferroelectrics, 2000

First-principles calculations are used to characterize ferroelastic phase transitions in calcium ... more First-principles calculations are used to characterize ferroelastic phase transitions in calcium chloride (CaCl2) and arsenic pentoxide (As2O5). We demonstrate that CaCl2, in accord with experiment, behaves as a pseudoproper ferroelastic with an optical instability as ultimate origin of the phase transition. In the case of As205, however, although an elastic constant is essentially unstable, a much more complex situation is

Research paper thumbnail of First-principles study of the structural instabilities in hexagonal barium titanate: Coupling between the soft optical and the acoustic Modes

Ferroelectrics, 2000

Hexagonal BaTiO3 undergoes a structural phase transition to an orthorhombic C2221 phase at To = 2... more Hexagonal BaTiO3 undergoes a structural phase transition to an orthorhombic C2221 phase at To = 222 K. The transition is driven by a soft optical mode with E2u symmetry whose couplings force the appearance of a spontaneous E2g strain (improper ferroelastic character). Staying within the same E2u subspace, the system could in principle settle into a second (not observed) orthorhombic phase (Cmc21). We have carried out a first-principles investigation of these questions, studying the structure of the existing C2221 and the virtual Cmc21 phases, and describing the spontaneous E2g strain in accord with the experimental observations. In addition, we show that the occurrence of C221 instead of Cmc2l cannot be explained by the E2u soft modes themselves and, therefore, must be related to their couplings with secondary order parameters. A more detailed analysis proves that the E2g strains do not account for the experimental preference.

Research paper thumbnail of Displacive vs. order-disorder in structural phase transitions

Ferroelectrics, 2000

The quantitative assessment of the displacive degree of a structural phase transition is discusse... more The quantitative assessment of the displacive degree of a structural phase transition is discussed both from a microscopical and an empirical viewpoint. Using Monte Carlo results for the Φmodel as a qualitative guide, the relation between both perspectives is shown. Based on the general trends observed, an empirical parameter for describing the displacive degree of any structural phase transition is

Research paper thumbnail of Monte Carlo simulation of dielectric response near a phase transition: displacive vs. order—disorder character

Computer Physics Communications, 1999

Research paper thumbnail of Tabla V

Research paper thumbnail of First-principles study of the ferroelastic phase transition in CaCl2

Physical Review B, 2002

ABSTRACT First-principles density-functional calculations within the local-density approximation ... more ABSTRACT First-principles density-functional calculations within the local-density approximation and the pseudopotential approach are used to study and characterize the ferroelastic phase transition in calcium chloride (CaCl_2). In accord with experiment, the energy map of CaCl_2 has the typical features of a pseudoproper ferroelastic with an optical instability as ultimate origin of the phase transition. This unstable optic mode is close to a pure rigid unit mode of the framework of chlorine atoms and has a negative Gruneisen parameter. The ab-initio ground state agrees fairly well with the experimental low temperature structure extrapolated at 0K. The calculated energy map around the ground state is interpreted as an extrapolated Landau free-energy and is successfully used to explain some of the observed thermal properties. Higher-order anharmonic couplings between the strain and the unstable optic mode, proposed in previous literature as important terms to explain the soft-phonon temperature behavior, are shown to be irrelevant for this purpose. The LAPW method is shown to reproduce the plane-wave results in CaCl_2 within the precision of the calculations, and is used to analyze the relative stability of different phases in CaCl_2 and the chemically similar compound SrCl_2. Comment: 9 pages, 6 figures, uses RevTeX4

Research paper thumbnail of Dynamics of incommensurate structures and inelastic neutron scattering

Physical Review B, 1989

A general analysis of the dynamics of displacive incommensurately modulated structures within the... more A general analysis of the dynamics of displacive incommensurately modulated structures within the harmonic approximation is presented. Superspace formalism is not used. The present approach is compared with the usual Landau model and some recent phenomenological predictions for hard internal modes. The characteristics of coherent inelastic neutron scattering are also analyzed. A compact expression for the one-phonon dynamic structure factor

Research paper thumbnail of Devonshire-Landau free energy of BaTiO3 from first principles

Research paper thumbnail of Optimized local modes for lattice-dynamical applications

Research paper thumbnail of Multiple instabilities in Bi4Ti3O12: A ferroelectric beyond the soft-mode paradigm

Physical Review B, 2008

The Aurivillius compound Bi4Ti3O12 exhibits a single phase transition from a tetragonal high-temp... more The Aurivillius compound Bi4Ti3O12 exhibits a single phase transition from a tetragonal high-temperature phase to a ferroelectric state. By means of first-principles calculations guided by a complete symmetry analysis of the experimental distortion, we confirm B1a1 as the (monoclinic) symmetry of the room-temperature phase but show that the transition involves a complex interplay of six different normal modes belonging to four different irreducible representations. Three of them are necessary for the observed symmetry break. We do not find any feature in the energy landscape that would force the simultaneous condensation of these three order parameters and cause such an avalanche transition, so we suggest that further experimental work might be able to detect intermediate phases. On the other hand, the necessary condensation of pairs of normal modes of the same symmetry already puts Bi4Ti3O12 outside the standard soft-mode paradigm and has important structural implications. In particular, the approximate rigidity of the BiO6 octahedra is due to anharmonic couplings of these modes. In addition, a comparison of the energy calculations and the behavior of the bond-valence global instability index is presented.

Research paper thumbnail of Competing structural instabilities in the ferroelectric Aurivillius compound SrBi2Ta2O9

Physical Review B, 2004

The Aurivillius family includes many compounds with ferroelectric phases. Despite sharing the bas... more The Aurivillius family includes many compounds with ferroelectric phases. Despite sharing the basic perovskite structural block with more common ferroelectric oxides such as BaTiO 3 or PbTiO 3 , their phase-transition behavior is rather more complex. In particular, SrBi 2 Ta 2 O 9 ...

Research paper thumbnail of Superlattice pseudouniform orderings as modulated structures: Stripe and checkerboard arrangements

Research paper thumbnail of Comparison of empirical bond-valence and first-principles energy calculations for a complex structural instability

Physical Review B, 2005

A simple bond-strain parameter, the so-called global-instability index, defined within the bond-v... more A simple bond-strain parameter, the so-called global-instability index, defined within the bond-valence model reproduces with striking accuracy the basic features of the ab initio total-energy surface around the energy minimum of SrBi2Ta2O9 , but only for the degrees of freedom within the subspace of energetically unstable modes. These results imply that ab initio total-energy calculations and bond-valence criteria could be

Research paper thumbnail of Compositional uniformity, domain patterning and the mechanism underlying nano-chessboard arrays

Journal of Physics: Condensed Matter, 2012

Research paper thumbnail of First-principles study of ferroelasticity in CaCl 2 and As 2 O 5

Ferroelectrics, 2000

First-principles calculations are used to characterize ferroelastic phase transitions in calcium ... more First-principles calculations are used to characterize ferroelastic phase transitions in calcium chloride (CaCl2) and arsenic pentoxide (As2O5). We demonstrate that CaCl2, in accord with experiment, behaves as a pseudoproper ferroelastic with an optical instability as ultimate origin of the phase transition. In the case of As205, however, although an elastic constant is essentially unstable, a much more complex situation is

Research paper thumbnail of First-principles study of the structural instabilities in hexagonal barium titanate: Coupling between the soft optical and the acoustic Modes

Ferroelectrics, 2000

Hexagonal BaTiO3 undergoes a structural phase transition to an orthorhombic C2221 phase at To = 2... more Hexagonal BaTiO3 undergoes a structural phase transition to an orthorhombic C2221 phase at To = 222 K. The transition is driven by a soft optical mode with E2u symmetry whose couplings force the appearance of a spontaneous E2g strain (improper ferroelastic character). Staying within the same E2u subspace, the system could in principle settle into a second (not observed) orthorhombic phase (Cmc21). We have carried out a first-principles investigation of these questions, studying the structure of the existing C2221 and the virtual Cmc21 phases, and describing the spontaneous E2g strain in accord with the experimental observations. In addition, we show that the occurrence of C221 instead of Cmc2l cannot be explained by the E2u soft modes themselves and, therefore, must be related to their couplings with secondary order parameters. A more detailed analysis proves that the E2g strains do not account for the experimental preference.

Research paper thumbnail of Displacive vs. order-disorder in structural phase transitions

Ferroelectrics, 2000

The quantitative assessment of the displacive degree of a structural phase transition is discusse... more The quantitative assessment of the displacive degree of a structural phase transition is discussed both from a microscopical and an empirical viewpoint. Using Monte Carlo results for the Φmodel as a qualitative guide, the relation between both perspectives is shown. Based on the general trends observed, an empirical parameter for describing the displacive degree of any structural phase transition is

Research paper thumbnail of Monte Carlo simulation of dielectric response near a phase transition: displacive vs. order—disorder character

Computer Physics Communications, 1999

Research paper thumbnail of Tabla V