Structure and electrical properties in the K1/2Bi1/2TiO3-K1/2Bi1/2ZrO3 solid solution (KBT-KBZ)

International audienceThe present work is devoted to the study of the Zr 4þ /Ti 4 substitution in the K1/2Bi1/2Ti(1-x)ZrxO3 solid solution (x ¼ 0.0 to x ¼ 1.0), based upon the K1/2Bi1/2TiO3 (KBT) ferroelectric compound. The tetragonal distortion of KBT is suppressed by this substitution and leads to the cubic compound K1/2Bi1/2ZrO3 (KBZ). These results agree with the values of the ionic radii of the Zr 4þ and Ti 4 ions (rTi 4þ ¼ 0.605 A˚ and rZr 4þ ¼ 0.72 A˚ ). Close to KBT (x 0.05), the symmetry remains tetragonal. For higher values of x, the ''a'' lattice parameter (cubic indexing) follows Vegard's law, thus confirming the formation of a solid solution. The transformation from tetragonal to cubic proceeds via an intermediate pseudocubic symmetry (0.1 x < 0.5), for which the X-ray diffraction peaks present small broadening and asymmetry. For x 0.5 and up to the KBZ compound (a 4.158 A˚ ), the samples are cubic but some extra peaks are also observed, indicating the occurrence of a secondary phase. The microstructure shows fine-grained ceramic samples for the first range, while for the KBZ-rich range the grains are micrometer-sized and associated to very small grains of the secondary phase. Piezoelectricity is observed for the tetragonal and pseudocubic range, the substitution quickly reducing the piezoelectric properties. The measurement of the dielectric properties revealed close to KBT a dielectric anomaly probably associated to the tetragonal-cubic phase transition. For thepseudocubic and cubic range, a broad dielectric anomaly is observed around 300 8C, corresponding to a relaxor behavior

Formation and transformation of five different phases in the CaSO4-H2O system: Crystal structure of the subhydrate beta-CaSO4 center dot 0.5H(2)O and soluble anhydrite CaSO4

At least five crystalline-phases can be found in the CaSO4-H2O system, which are gypsum CaSO4 center dot 2H(2)O, the subhydrates alpha- and beta-CaSO4 center dot 0.5H(2)O, and the soluble and insoluble anhydrite CaSO4. The formation of these five phases in the CaSO4-H2O system and their transformations were investigated by in situ time-resolved synchrotron radiation powder X-ray diffraction (SR-PXD) in this study. Furthermore, revised structural models for beta-CaSO4 center dot 0.5H(2)O and soluble anhydrite CaSO4 are presented. The hydration of alpha-CaSO4 center dot 0.5H(2)O was studied at 25 degrees C and showed that the reaction with H2O started immediately after mixing the two reactants and that the formation of CaSO4 center dot 2H(2)O was coupled to the depletion of alpha-CaSO4 center dot 0.5H(2)O. The thermal decomposition of CaSO4 center dot 2H(2)O was investigated in the temperature range of 25-500 degrees C and showed the fon-nation of alpha-CaSO4 center dot 0.5H(2)O followed by the formation of soluble anhydrite AIII-CaSO4, which was gradually converted to insoluble anhydrite AII-CaSO4. The thermal decomposition of alpha-CaSO4 center dot 0.5D(2)O was investigated in the temperature range of 25-500 degrees C and showed successive phase transformations to beta-CaSO4 center dot 0.5D(2)O, soluble anhydrite AIII-CaSO4, and insoluble anhydrite AII-CaS04. The two polymorphs of anhydrite coexist in the investigated temperature range of 200-500 degrees C. The hydrothermal decomposition of CaSO4 center dot 2H(2)O was investigated in the temperature range of 25-200 degrees C using a 1 M HNO3 or a 1 M LiCl solution, and in both experiments, CaSO4 center dot 2H(2)O was converted to alpha-CaSO4 center dot 0.5H(2)O and further to insoluble anhydrite AII-CaSO4. A structural model for beta-CaSO4 center dot 0.5H(2)O is proposed on the basis of SR-PXD data and a trigonal unit cell (in hexagonal setting) a = 6.93145(3), c = 12.736 17(4) angstrom, Z = 6, and space group P3(1). A structural model for soluble anhydrite AIII-CaSO4 is also proposed on the basis of powder neutron diffraction data, and a hexagonal unit cell parameters are a = 6.9687(1), c = 6.3004(1) angstrom, Z = 3, and space group P6(2)22

Thermally induced phase transitions of barium oxalates

The thermal decomposition of BaC2O4 center dot 3.5H(2)O and BaC2O4 center dot 0.5H(2)O was investigated using in situ synchrotron X-ray and neutron powder diffraction. The decomposition routes for the barium oxalate hydrates were observed to depend on the applied heating rate. Thermal decomposition of BaC2O4 center dot 0.5H(2)O showed transformation to alpha-BaC2O4 and to beta-BaC2O4 prior to the formation of BaCO3. The decomposition of BaC2O4 center dot 3.5H(2)O showed formation of BaC2O4 center dot 0.5H(2)O at 58 degrees C and the hemi hydrate transforms to alpha-BaC2O4 at 187 degrees C using a relatively fast heating rate of 6.25 degrees C/min. The phase transitions were more complicated using lower heating rate, which also reveal formation of beta-BaC2O4 coexisting with alpha-BaC2O4 along with an unidentified compound. Heating alpha- and beta-BaC2O4 to higher temperatures (T > 400 degrees C) produced BaCO3. A sample of alpha-BaC2O4 was prepared in situ by thermal decomposition of BaC2O4 center dot 3.5H(2)O on a powder neutron diffractometer. The neutron diffraction data has broad diffraction peaks due to small crystallite sizes and overlapping Bragg reflections. [A structural model for alpha-BaC2O4 was derived from the neutron pattern, triclinic, space group P-1, a = 5.127(7), b = 8.905(12), c = 9.068(12) angstrom, alpha = 82.74(1), beta = 99.46(2), gamma = 100.10(1)degrees measured at T= 300 degrees C. The average Ba-O distances are 2.84(3) angstrom and 2.66(3) angstrom for Ba 1 and Ba2 respectively, C-O atom distances in the oxalate ions were found in the range 1.25(3)-1.26(4) angstrom, and C-C distances were 1.60(1)-1.61(1) angstrom]. (C) 2011 Elsevier Masson SAS. All rights reserved