Ab initiolinear response and frozen phonons for the relaxorPbMg1∕3Nb2∕3O

Abstract

We report first principles density functional studies using plane wave basis sets and pseudopotentials and all electron linear augmented plane wave (LAPW) of the relative stability of various ferroelectric and antiferroelectric supercells of PMN for 1:2 chemical ordering along [111] and [001]. We used linear response with density functional perturbation theory (DFPT) which is implemented in the code ABINIT to compute the Born effective charges, electronic dielectric tensors, long wavelength phonon frequencies and LO-TO splittings. The polar response is different for supercells ordered along [111] and [001]. Several polar phonon modes show significant coupling with the macroscopic electric field giving giant LO-TO splittings. For [111] ordering, a polar transverse optic (TO) mode with E symmetry is found to be unstable in the ferroelectric P3m1 structure and the ground state is found to be monoclinic. Multiple phonon instabilities of polar modes and their mode couplings provide the pathway for polarization rotation. The Born effective charges in PMN are highly anisotropic and this anisotropy contributes to the observed huge electromechanical coupling in PMN solid solutions.2

Electron low energy-loss functions of Pb(Mg1/3Nb2/3)O3: Theory and experiment

“…Based on the random site model for the CODs, a 30 atom supercell of PMN was built up with the B-sites ordering along the ͓111͔ direction, which is similar to the structure in Ref. These computed results of the symmetry transition are in agreement with the reports of Choudhury et al 35 and Prosandeev et al 36 for the P3m1 supercell. The supercell consisted of a NaCl-type 1:1 ordering of two sublattices, the first of Nb and the second of composition Mg 2/3 Nb 1/3 with layering of Mg and Nb in ͑111͒ planes on the mixed sublattice.…”

Structural, electronic, and optical properties of ferroelectric KTa1/2Nb1/2O3 solid solutions

“…This choice is suggested in the work of other researchers. [2][3][4][5]18 Geometry optimization was first carried out by using the Broyden-Fletcher-Goldfarb-Shannon ͑BFGS͒ scheme and USP methods to obtain a stable ferroelectric structure. After the optimization, electric, and optical properties were calculated by using the NCP methods.…”

“…More recently, there has been a flurry of activities to theoretically study the effect of the ordered microregions. [1][2][3][4][5][6][7] Potassium tantalite niobate ͑KTa 1−x Nb x O 3 ͒, which is the solid solution of KNbO 3 and KTaO 3 , is important for many applications, including electro-optics, holography, and second harmonic generation. 8,9 KTa 1−x Nb x O 3 crystals are ferroelectric materials that display both displacivelike and orderdisorder-like properties.…”

Finite-temperature properties of the relaxorPbMg1/3Nb2/3O3from atomistic simulations

“…The left inset of panel (d) displays the resulting dipolar configuration at 10 K, when random fields are turned off. temperature close to 0 K [54] and 0.41 C/m 2 at 150 K [55] as well as with first-principles calculations yielding values ranging between 0.40 and 0.65 C/m 2 at 0 K [38,39]-which further emphasizes the capability of our H eff to realistically mimic the complex properties of PMN. Figure 1(b) also indicates that the polarization decreases on heating and then, at about 250 ± 50 K, experiences a sudden jump down leading to its vanishing.…”

“…u i is therefore directly proportional to the local electric dipole moment in cell i. It is technically chosen to be centered on the Pb sites, as consistent with the known fact that lead ions significantly move of center in Pb-based perovskites [28,29,[37][38][39][40]; (2) Nb/Mg-centered dimensionless local displacements v i that are related to the inhomogeneous strain inside each cell [36]; (3) The homogeneous strain tensor, η H [36]. Such effective Hamiltonian also depends on {σ j } variables that characterize the atomic configuration of the PMN solid solution.…”