M.T. Caldes

Characterization of perovskite systems derived from Ba2In2O5□: Part I: the oxygen-deficient Ba2In2(1-x)Ti2xO5+x□1-x (0≤x≤1) compounds

Ba2In2(1−x)Ti2xO5+x□1−x (0≤x≤1) compounds have been prepared by solid state reaction. At room temperature (RT), when x increases, the progressive filling of oxygen vacancies, concomitant with the substitution of Ti for In, first induces (for 0<x≤0.075) a disorder in the plane of oxygen vacancies observed in Ba2In2O5□. Then, it suppresses the distortion to orthorhombic symmetry; for 0.075<x≤ 0.15, the symmetry becomes tetragonal and a formation of domains is observed. For 0.15<x<1, all members adopt a disordered cubic perovskite (DCP) structure at RT. Conductivity measurements between 450 and 800 °C show that the change from brownmillerite to tetragonal structure when x increases from 0.075 to 0.1 induces a drastic decrease of the activation energy. The highest oxide-ion conductivity is observed for 0.1≤x≤0.33: ∼0.5×10−2 S cm−1 at 700 °C.

Characterization of perovskite systems derived from Ba2In2O5□: Part II: The proton compounds Ba2In2(1−x)Ti2xO4+2x(OH)y [0≤x≤1; y≤2(1−x)]

Abstract The proton compounds Ba 2 In 2(1− x ) Ti 2 x O 4+2 x (OH) y [0≤ x ≤1; y ≤2(1− x )] were prepared by reacting Ba 2 In 2(1− x ) Ti 2 x O 5+ x □ 1− x (0≤ x ≤1) phases with water vapor at ∼200 °C. For 0≤ x ≤0.20, the filling of oxygen vacancies is almost complete. For larger x values, it decreases significantly down to ∼30% only for x =0.7. The crystal structure of the end member Ba 2 In 2 O 4 (OH) 2 ( x =0) was reinvestigated by a combination of techniques including 1 H and 2 D NMR and electron, X-ray and neutron diffraction. The actual cell is eight times larger than that previously published. The structure analysis confirms that the sheet consisting of parallel chains of In(2)O 4 tetrahedra and parallel rows of oxygen vacancies in the parent structure of Ba 2 In 2 O 5 □ has been converted into a In(2)O 6 octahedral perovskite-like sheet. It demonstrates that the protons are bonded only to the O atoms around this In(2) site. When x increases, the change in reduced perovskite cell volume, concomitant with the water uptake, remains small for x x values. The proton conductivity was measured between room temperature and 180 °C. The highest conductivity at 180 °C, σ 180 ≈10 −6 S cm −1 , is observed for x ∼0.3.

Evidence for a monoclinic distortion in the ferroelectric Aurivillius phase Bi3LaTi3O12

Abstract An alternative chemical approach was used to prepare the ferroelectric Aurivillius phase Bi 3 LaTi 3 O 12 (BLT1.00) at low temperature (450°C) leading to well-crystallized compounds at 750°C. The existence of 90°-oriented twin domains, characteristic of a ferroelectric state, was revealed in selected area electron diffraction and microdiffraction studies. Though the 3D reconstruction of the reciprocal lattice enables to propose the non-centrosymmetrical orthorhombic space group (SG) B 2 cm , a further convergent-beam electron diffraction study was undertaken in order to ascertain the symmetry. The monoclinic symmetry was clearly identified. BLT1.00 crystallizes in the polar SG B 1 c 1 (no. 7, non-standard setting of P 1 c 1) instead of I 4/ mmm , as previously reported.

A new 'Chimie Douce' approach to the synthesis of Sr1-xLa1+xAl1-xMgxO4 with K2NiF4 structure type

Abstract Synthesis of Sr 1− x La 1+ x Al 1− x Mg x O 4 solid solution is achieved by a new soft chemistry route called the nitrate polyacrylamide gel (NPG) process. A comparison between the ceramic method and the NPG route is presented. This method shows that well-crystallized single-phase product can be obtained either at 1200 °C for 12 h or 900 °C for 120 h instead of 1400 °C for 108 h by the ceramic method. BET measurement has shown an enhancement of the surface area up to 7.5 m 2 g −1 . The structural analysis by XRPD or electron microscopy does not reveal any deviation from the ideal K 2 NiF 4 -type structure.