Bandgap behavior and singularity of the domain-induced light scattering through the pressure-induced ferroelectric transition in relaxor ferroelectric A(x)Ba(1-x)Nb(2)O(6) (A: Sr,Ca)

[EN] In this letter, we have investigated the electronic structure of A(x)Ba(1-x)Nb(2)O(6) relaxor ferroelectrics on the basis of optical absorption spectroscopy in unpoled single crystals with A = Sr and Ca under high pressure. The direct character of the fundamental transition could be established by fitting Urbach's rule to the photon energy dependence of the absorption edge yielding bandgaps of 3.44(1) eV and 3.57(1) eV for A = Sr and Ca, respectively. The light scattering by ferroelectric domains in the pre-edge spectral range has been studied as a function of composition and pressure. After confirming with x-ray diffraction the occurrence of the previously observed ferroelectric to paraelelectric phase transition at 4 GPa, the light scattering produced by micro-and nano-ferroelectric domains at 3.3 eV in Ca0.28Ba0.72Nb2O6 has been probed. The direct bandgap remains virtually constant under compression with a drop of only 0.01 eV around the phase transition. Interestingly, we have also found that light scattering by the polar nanoregions in the paraelectric phase is comparable to the dispersion due to ferroelectric microdomains in the ferroelectric state. Finally, we have obtained that the bulk modulus of the ferroelectric phase of Ca0.28Ba0.72Nb2O6 is B-0 = 222(9) GPa. Published by AIP Publishing.J.R.-F. acknowledges the Spanish MINECO for the Juan de la Cierva (IJCI-2014-20513) Program and Dr. Bayarjargal from the Goethe-Universitat Frankfurt for providing the CBN28 samples. This work was supported by Spanish MINECO under Grant No. MAT2016-75586-C4-1-P/2-P. The high pressure x-ray diffraction experiments were performed at MSPD beamline at ALBA Synchrotron (Project 2016021588) with the collaboration of ALBA staff.Ruiz-Fuertes, J.; Gomis, O.; Segura, A.; Bettinelli, M.; Burianek, M.; Muehlberg, M. (2018). Bandgap behavior and singularity of the domain-induced light scattering through the pressure-induced ferroelectric transition in relaxor ferroelectric A(x)Ba(1-x)Nb(2)O(6) (A: Sr,Ca). Applied Physics Letters. 112(4). https://doi.org/10.1063/1.5012111S112

Thermally induced structural changes in incommensurate calcium barium niobate Ca0.28Ba0.72Nb2O6 (CBN28)

The incommensurately modulated crystal structure of relaxor ferroelectric CBN28 was refined at elevated temperatures up to 330 degrees C in the 3+2 dimensional superspace group P4bm(alpha alpha 1/2, -alpha alpha 1/2). The structural modulations mainly consisting of cooperative tilting of NbO6 octahedra and an occupational modulation of the large cation site Me2 persist beyond the diffuse ferroelectric transition with slightly reduced amplitudes. A change of symmetry was not observed. Both symmetrically non-equivalent NbO6 octahedra are distorted by off center shifts of the Nb atoms in the same direction along the tetragonal c-axis. The displacements of the Nb atoms are gradually reduced with increasing temperatures until one of the Nb atoms crosses the center of its coordination polyhedron near the transition temperature T-M adopting an uncompensated anti-ferroelectric configuration. This change is accompanied by enhanced thermal motions of the Nb atoms along the c-axis. Structural distortions and electric polarization do not completely vanish at TM but may persist in fluctuating polar nanodomains. (C) 2012 Elsevier Inc. All rights reserved

Relaxor Behavior of Pure and Cerium Doped CaxBa1-xNb2O6

Here we report the relaxor behavior of pure and cerium doped Czochralski grown lead free relaxor ferroelectric single crystals CaxBa1-xNb2O6 (CBN-x) (0.18 <= x <= 0.35) using temperature dependent elastic behavior. We observed that the dynamic relaxor behavior strongly varies with the variation of Ca content as well as with doping. Evidence is found for a more pronounced relaxor behavior with increasing Ca content and doping. Characteristic temperature T* (temperature at which static behavior of the polar nanoregions begins to appear) found to be unaffected with Ca content variation as well as doping

Characterization of (Bi0.5Na0.5)(1-x)BaxTiO3 grown by the TSSG method

Bi0.5Na0.5TiO3 (BNT) and (Bi0.5Na0.5)(1-x)BaxTiO3 (BNBT; 0.02 <= x(Ba)(melt) <= 0.12) single crystals were grown by the Top Seeded Solution Growth (TSSG) method under conditions near to the thermodynamic equilibrium. The driving force for the crystallization process is only given by the controlled evaporation of the volatile components Bi2O3 and Na2O. The Ba-segregation coefficient in the BNBT-system was determined to be approximate to 0.56 on the basis of EMPA measurements on single crystals grown from melts with a different Ba-content. Congruent melting of BNT is clearly indicated by DSC measurements at 1291 degrees C. BNBT behaves in a similar way whereas melting temperatures are lowered with increasing x(Ba), c(p)(T) calculations and heating/cooling polarizing microscopy was performed for the investigation of the complex domain state behavior during the phase transitions in BNT. (C) 2014 Elsevier B.V. All rights reserved

Determination of the H₂O content in minerals, especially zeolites, from their refractive indices based on mean electronic polarizabilities of cations

It is shown here that the H2O content of hydrous minerals can be determined from their mean refractive indices with high accuracy. This is especially important when only small single crystals are available. Such small crystals are generally not suitable for thermal analyses or for other reliable methods of measuring the amount of H2O. In order to determine the contribution of the H2O molecules to the optical properties, the total electronic polarizability is calculated from the anhydrous part of the chemical composition using the additivity rule for individual electronic polarizabilities of cations and anions. This anhydrous contribution is then compared with the total observed electronic polarizability calculated from the mean refractive index of the hydrous compound using the Anderson–Eggleton relationship. The difference between the two values represents the contribution of H2O. The amount can be derived by solving the equation αcalc=∑iniαicat+∑jαjo×10-NjVm1.2×nj+nW1.2+nw×αW for the number nw of H2O molecules per formula unit (pfu), with the electronic polarizabilities αcat for cations, the values N and αo describing the anion polarizabilities, the number n of cations and anions, and the molar volume Vm, using a value of αW=1.62&thinsp;Å3 for the electronic polarizability of H2O. The equation is solved numerically, yielding the number nw of H2O molecules per formula unit. The results are compared with the observed H2O content evaluating 157 zeolite-type compounds and 770 non-zeolitic hydrous compounds, showing good agreement. This agreement is expressed by a factor relating the calculated to the observed numbers being close to 1 for the majority of compounds. Zeolites with occluded anionic or neutral species (SO3, SO4, CO2, or CO3) show unusually high deviations between the calculated and observed amount of H2O, indicating that the polarizabilities of these species should be treated differently in zeolites and zeolite-type compounds.</p

Polar nanoregions of Ca0.28Ba0.72Nb2O6 probed by second harmonic generation and Raman spectroscopy at high pressure

The pressure-induced ferroelectric phase transition of calcium barium niobate Ca0.28Ba0.72Nb2O6 is studied by second harmonic generation (SHG) and Raman spectroscopy. We observe the ferroelectric to paraelectric phase transition at P-c = 4 GPa and evidence for two additional local changes at 8.5 and 11.5 GPa that are interpreted as the characteristic intermediate P* and the Burns pressure P-B of a relaxor phase. The pressure dependence of the SHG intensity has a Landau critical exponent of gamma = 1.3 between 4 and 8.5 GPa, indicating the presence of static polar nanoregions. At higher pressures, from 8.5 to 11.5 GPa, these nanoregions become dynamic, with gamma = 2.4, and disappear at 11.5 GPa. The polar nanoregions appear when only one of the two non-equivalent octahedral positions of Nb is centrosymmetric. A Landau exponent of gamma = 1 above 11.5 GPa together with a change in the compression mechanism indicates that the system completes a phase transformation to a centrosymmetric space group and the polar nanoregions vanish. (C) 2014 AIP Publishing LLC

High-pressure phase transition of Bi2Fe4O9Bi_2Fe_4O_9

The high-pressure behaviour of Bi2Fe4O9 was analysed by in situ powder and single-crystal x-ray diffraction and Raman spectroscopy. Pressures up to 34.3(8) GPa were generated using the diamond anvil cell technique. A reversible phase transition is observed at approximately 6.89(6) GPa and the high-pressure structure is stable up to 26.3(1) GPa. At higher pressures the onset of amorphization is observed. The crystal structures were refined from single-crystal data at ambient pressure and pressures of 4.49(2), 6.46(2), 7.26(2) and 9.4(1) GPa. The high-pressure structure is isotypic to the high-pressure structure of Bi2Ga4O9. The lower phase transition pressure of Bi2Fe4O9 with respect to that of Bi2Ga4O9 (16 GPa) confirms the previously proposed strong influence of cation substitution on the high-pressure stability and the misfit of Ga3+ and Fe3+ in tetrahedral coordination at high pressure. A fit of a second-order Birch–Murnaghan equation of state to the p–V data results in K0 = 74(3) GPa for the low-pressure phase and K0 = 79(2) GPa for the high-pressure phase. The mode Grüneisen parameters were obtained from Raman-spectroscopic measurements