First-principles study of structural and vibrational properties of SrZrO3

in Physical Review. B, Condensed Matter and Materials Physics (2012), 85

Using first-principles calculations, we investigate the electronic, structural, and vibrational properties of SrZrO3. We start from the high-symmetry cubic perovskite phase, for which the phonon ... [more ▼]

Using first-principles calculations, we investigate the electronic, structural, and vibrational properties of SrZrO3. We start from the high-symmetry cubic perovskite phase, for which the phonon dispersion curves are reported.We point out the coexistence of structural antiferrodistortive instabilities at the R and M zone-boundary points and a ferroelectric instability at the zone center.We showthat the strong antiferrodistortive motions suppress ferroelectricity and are responsible for the orthorhombic ground state as in CaTiO3. The structural properties <br />of possible intermediate phases and of the orthorhombic Pnma ground state are reported. For the latter, an assignment of IR and Raman zone-center phonon modes is proposed. The main features of the ferroelectric instability are also discussed, and we show that a ferroelectric ground state can even be induced in SrZrO3 by strain engineering. [less ▲]

Linear magnetoelectric effect by orbital magnetism

in Physical Review Letters (2012), 109(19),

We use symmetry analysis and first-principles calculations to show that the linear magnetoelectric effect can originate from the response of orbital magnetic moments to the polar distortions induced by an ... [more ▼]

We use symmetry analysis and first-principles calculations to show that the linear magnetoelectric effect can originate from the response of orbital magnetic moments to the polar distortions induced by an applied electric field. Using LiFePO 4 as a model compound we show that spin-orbit coupling partially lifts the quenching of the 3d orbitals and causes small orbital magnetic moments (μ (L)≈0.3μ B) parallel to the spins of the Fe2 + ions. An applied electric field E modifies the size of these orbital magnetic moments inducing a net magnetization linear in E. © 2012 American Physical Society. [less ▲]

Unexpectedly Large Electronic Contribution to Linear Magnetoelectricity

in Physical Review Letters (2011), 106(10),

We show that the electronic part of the linear magnetoelectric response, usually omitted in first-principles studies, can be comparable in magnitude to that mediated by polar lattice distortions, even in ... [more ▼]

We show that the electronic part of the linear magnetoelectric response, usually omitted in first-principles studies, can be comparable in magnitude to that mediated by polar lattice distortions, even in strong magnetoelectrics. Using a self-consistent response to a Zeeman field for noncollinear spins, we show how polarization emerges in magnetoelectrics through both electronic and lattice contributions-analogous to the high- and low-frequency responses of dielectrics. The approach we use is computationally simple, and can be used to study linear and nonlinear responses to magnetic fields. [less ▲]

J dependence in the LSDA plus U treatment of noncollinear magnets

in Physical Review. B, Condensed Matter and Materials Physics (2010), 82(22),

We re-examine the commonly used density-functional theory plus Hubbard U (DFT+U) method for the case of noncollinear magnets. While many studies neglect to explicitly include the exchange-correction ... [more ▼]

We re-examine the commonly used density-functional theory plus Hubbard U (DFT+U) method for the case of noncollinear magnets. While many studies neglect to explicitly include the exchange-correction parameter J, or consider its exact value to be unimportant, here we show that in the case of noncollinear magnetism calculations the J parameter can strongly affect the magnetic ground state. We illustrate the strong J dependence of magnetic canting and magnetocrystalline anisotropy by calculating trends in the magnetic lithium orthophosphate family LiMPO(4) (M=Fe and Ni) and difluorite family MF(2) (M=Mn, Fe, Co, and Ni). Our results can be readily understood by expanding the usual DFT+U equations within the spinor scheme, in which the J parameter acts directly on the off-diagonal components which determine the spin canting. [less ▲]

First-principles study of competing ferroelectric and antiferroelectric instabilities in BaTiO3/BaO superlattices

in Physical Review. B : Condensed Matter (2010), 82(4), 045426

We report a first-principles study of (BaTiO3) m/ (BaO) n superlattices for a wide range of periodicities m/n. We show that such a system develops a polar zone-center instability for sufficiently large m ... [more ▼]

We report a first-principles study of (BaTiO3) m/ (BaO) n superlattices for a wide range of periodicities m/n. We show that such a system develops a polar zone-center instability for sufficiently large m/n ratio, which can be understood, at least qualitatively, from a simple electrostatic model and should lead to a ferroelectric ground state. However, the analysis of the phonon-dispersion curves also points out the appearance of stronger antiferroelectric instabilities at the zone boundaries around m=4, before the critical ratio for ferroelectricity is reached and which still dominate beyond it. The dominant character of the antiferroelectric instability is explained from the depolarizing field which hardens the ferroelectric mode. This analysis allows us to predict that, (BaTiO3) m/ (BaO) n superlattices should present an antiferroelectric ground state for m larger than 4, which should smoothly evolve to a multidomain structure for increasing m values and only become ferroelectric for large m. [less ▲]

Correlation between optical constants and crystal chemical parameters of ZrW2O8

in Journal of Solid State Chemistry (2009), 182(10), 2762-2768

The refractive indices of ZrW2O8, measured at wavelengths of 435.8-643.8 nm, were used to calculate n(D) at lambda = 589.3 nm and n(infinity) at lambda = infinity from a one-term Sellmeier equation ... [more ▼]

The refractive indices of ZrW2O8, measured at wavelengths of 435.8-643.8 nm, were used to calculate n(D) at lambda = 589.3 nm and n(infinity) at lambda = infinity from a one-term Sellmeier equation. Refractive indices, n(D) and dispersion values, A, are, respectively, 1.8794 and 114 x 10(-16) m(2). The high dispersion, relative to other molybdates, tungstates and Zr-containing compounds, is attributed to the low value of E-o = 7.7 eV and mean cation coordination number. Total electronic polarizabilities, alpha(total), were calculated from n(infinity) and the Lorenz-Lorentz equation. The unusually large difference between the observed polarizability of 20.087 angstrom(3) and the calculated total polarizability alpha(T) of 17.59 angstrom(3) (Delta = +12.4%) is attributed to (1) a large M-O-W angle, (2) a high degree of W 5d-O(terminal) 2p and Zr nd-O 2p hybridization, and (3) unusually high oxygen displacement factors, B(O), normalized to B(W). (C) 2009 Elsevier Inc. All rights reserved. [less ▲]

New insight into the concept of ferroelectric correlation volume

in Journal Of Computational And Theoretical Nanoscience (2008), 5(4), 517-520

Using an effective Hamiltonian derived from first-principles calculations, we discuss the concept of "ferroelectric correlation volume" and show that it cannot be uniquely defined: it is not the size but ... [more ▼]

Using an effective Hamiltonian derived from first-principles calculations, we discuss the concept of "ferroelectric correlation volume" and show that it cannot be uniquely defined: it is not the size but the anisotropic shape of the polar region that is crucial for stabilizing a ferroelectric distortion into a ferroelectric materials. To provide basic understanding of this result, the stability of a polar region is analyzed in term of intra-chain and inter-chain interactions. [less ▲]

First-principles study of barium titanate under hydrostatic pressure

in Physical Review B (2006), 74(18),

The behavior of the ferroelectric instability of cubic barium titanate under increasing hydrostatic pressure is investigated from first principles. Beyond a rapid suppression and then reappearance of this ... [more ▼]

The behavior of the ferroelectric instability of cubic barium titanate under increasing hydrostatic pressure is investigated from first principles. Beyond a rapid suppression and then reappearance of this instability at about 140 GPa, we highlight a sudden change in the dynamical behavior of BaTiO3 at a pressure of approximate to 20 GPa. We show that the ferroelectric instability at high pressure has a totally different character than at atmospheric pressure. From the analysis of the interatomic force constants, we demonstrate that the destabilizing role of the short-range forces at high pressure must be related to the change of the soft mode eigenvector rather than to an original electronic effect. [less ▲]

Direct evidence for ferroelectric polar distortion in ultrathin lead titanate perovskite films

in Physical Review b (2006), 73(9),

X-ray photoelectron diffraction is used to directly probe the intracell polar atomic distortion and tetragonality associated with ferroelectricity in ultrathin epitaxial PbTiO3 films. Our measurements ... [more ▼]

X-ray photoelectron diffraction is used to directly probe the intracell polar atomic distortion and tetragonality associated with ferroelectricity in ultrathin epitaxial PbTiO3 films. Our measurements, combined with ab initio calculations, unambiguously demonstrate noncentrosymmetry in films a few unit cells thick, imply that films as thin as three unit cells still preserve a ferroelectric polar distortion, and also show that there is no thick paraelectric dead layer at the surface. [less ▲]