EXAFS study of lead-free relaxor ferroelectric BaTi(1-x)Zr(x)O3 at the Zr K-edge

Extended X-ray absorption fine structure (EXAFS) experiments at the Zr K-edge were carried out on perovskite relaxor ferroelectrics BaTi(1-x)Zr(x)O3 (BTZ) (x = 0.25, 0.30, 0.35), and on BaZrO3 for comparison. Structural information up to 4.5 A around the Zr atoms is obtained, revealing that the local structure differs notably from the average Pm-3m cubic structure deduced from X-ray diffraction. In particular, our results show that the distance between Zr atoms and their first oxygen neighbors is independent of the Zr substitution rate x and equal to that measured in BaZrO3, while the X-ray cubic cell parameter increases linearly with x. Furthermore, we show that the Zr atoms tend to segregate in Zr-rich regions. We propose that the relaxor behavior in BTZ is linked to random elastic fields generated by this particular chemical arrangement, rather than to random electric fields as is the case in most relaxors.Comment: 13 pages, 12 figures, 4 tables. Submitted to Phys. Rev.

Investigation of the Jahn-Teller Transition in TiF3 using Density Functional Theory

We use first principles density functional theory to calculate electronic and magnetic properties of TiF3 using the full potential linearized augmented plane wave method. The LDA approximation predicts a fully saturated ferromagnetic metal and finds degenerate energy minima for high and low symmetry structures. The experimentally observed Jahn-Teller phase transition at Tc=370K can not be driven by the electron-phonon interaction alone, which is usually described accurately by LDA. Electron correlations beyond LDA are essential to lift the degeneracy of the singly occupied Ti t2g orbital. Although the on-site Coulomb correlations are important, the direction of the t2g-level splitting is determined by the dipole-dipole interactions. The LDA+U functional predicts an aniferromagnetic insulator with an orbitally ordered ground state. The input parameters U=8.1 eV and J=0.9 eV for the Ti 3d orbital were found by varying the total charge on the TiF$_6^{2-}$ ion using the molecular NRLMOL code. We estimate the Heisenberg exchange constant for spin-1/2 on a cubic lattice to be approximately 24 K. The symmetry lowering energy in LDA+U is about 900 K per TiF3 formula unit.Comment: 7 pages, 9 figures, to appear in Phys. Rev.

New lead-free perovskite relaxor ceramics derived from BaTiO₃ and containing scandium

New lead-free compositions of AMO3 perovskite type were synthesized in the BaTi1-x(Nb1/2Sc1/2)xO3(0 ≤ x ≤ 1) system, with regards to the comparable electronic configurations of the various 6-coordinated cations. Special solid state reaction was used. The effect of the replacement of titanium by scandium and niobium in the B-site on structural and dielectric properties was investigated. For this, we have used X-ray diffraction and dielectric characterizations. Dielectric measurements performed on ceramics with 0 ≤ x ≤ 0.25 composition revealed that the small change in composition from BaTiO3 has a strong influence on the dielectric properties: fast decreasing of the ferroelectric transition temperature for low values of x and evolution from classical ferroelectric for 0 ≤ x ≤ 0.075 to relaxor behavior for 0.075 < x ≤ 0.25

The Inorganie Fluoride and Oxyfluoride Ferroelectrics

The ferroelectric fluorides belong to six families of type (NH$_4$)2BeF$_4$, BaMnF$_4$, SrAlF$_5$, Na$_2$MgAlF$_7$, K$_3$Fe$_5$F$_{15}$ or Pb$_5$Cr$_3$F$_{19}$. The oxyfluorides can be separated in two groups: i) the true oxyfluorides with a relatively high F/O value (K$_3$MoO$_3$F$_3$, Na$_5$W$_3$O$_9$F$_5$, Pb$_5$W$_3$O$_9$F$_{10}$, Bi$_2$TiO$_4$F$_2$, ...); ii) the compositions derived from ferroelectric oxides (perovskite, tetragonal tungsten bronze, pyrochlore, LiTaO$_3$, ...) characterized by a continuous F-O substitution. The phase transitions and the physical properties are discussed. The origin of the spontaneous polarization is described structurally. The variations of the Curie temperature with the type of substitution are related to chemical bonding. The potential interest of these materials for applications is briefly given.Les ferroélectriques fluorés appartiennent à six familles de type (NH$_4)_2$BeF$_4$, BaMnF$_4$, SrAlF$_5$, Na$_2$MgAlF$_7$, K$_3$Fe$_5$F$_{15}$ ou Pb$_5$Cr$_3$F$_{19}$. Les oxyfluorures peuvent être séparés en deux groupes : i) les oxyfluorures “vrais” comportant un rapport F/O relativement élevé (K$_3$MoO$_3$F$_3$, Na$_5$W$_3$O$_9$F$_5$, Pb$_5$W$_3$O$_9$F$_{10}$, Bi$_2$TiO$_4$F$_2$, ...), ii) les compositions dérivées des oxydes ferroélectriques (perovskite, bronze quadratique de tungstène, pyrochlore, LiTaO$_3$, ...) et caractérisées par une substitution F-O continue. Les transitions de phases et les propriétés physiques sont analysées. L'origine de la polarisation est précisée sur le plan structural. Les variations de la température de Curie avec le type de substitution sont reliées à des considérations de liaison chimique. L'intérêt potentiel de ces matériaux pour les applications est brièvement abordé