Influence of the calcium excess in the structural and spectroscopic properties of the complex perovskite Ba3CaNb2O9

Abstract

The effects of the change in the Ca/Nb ratio in the structure and spectroscopic properties of the solid solution Ba 3 Ca 1+x Nb 2-x O 9-δ with x00-0.25 are investigated. The solid solution undergoes a transition from an hexagonal phase (2:1 order) to a cubic phase (1:1 order) with increasing calcium content as indicated by X-Ray diffraction and Raman. The calcium excess matches with a gradual increase in blue colour due to a reduction at the high temperatures of sintering. Optical absorption experiments showed that there are different absorption bands in the region 0.45-4.5 eV corresponding tentatively to OH vibrations, polarons and monovalent oxygen vacancies. The series properties change gradually with calcium excess with the exception of Ba 3 Ca 1.18 Nb 1.82 O 9-δ that showed a slight change attributable to the presence of protons in the lattice. These changes can be used to quantify protons in complex perovskites.

Ordering effect on the electrical properties of stoichiometric Ba3CaNb2O9-based perovskite ceramics

“…The Thangadurai group has studied the partial replacement of Ba by K on A-site and Ti substitution for Nb in the stoichiometric Ba 3 CaNb 2 O 9 [16,17]; The Fanglin Chen group has reported the effects of Ce and Y doping on the Ca and Nb ions in complex perovskite Ba 3 Ca 1.18 Nb 1.82 O 9-δ proton conductor [9,18]. In all cases, the nonstoichiometry leads to a formation of oxygen vacancies and a modification in the B-site ordering from 1:2 to 1:1 type [19]. In the 1:2 ordered systems, the Ca and Nb cations are alternately distributed in the sequence -Ca-Nb-Nb-Ca-Nb-Nb-along the [111] c direction of the parent cubic cell [20].…”

“…x O 9-δ system. In this material, two processes take place with the substitution of Ca 2+ for Nb 5+ : oxygen vacancy generation and gradual transition from 1:2 trigonal to 1:1 cubic [19,64]. From the defect chemistry point of view, the formation of oxygen vacancies is designated by the next reaction [in Kröger-Vink notation]: Interestingly, the reported values between 0.70 and 0.92 eV suggests that the oxygen migration along NbO 6 occurs in the Ba 3 Ca 1+x Nb 2-x O 9-δ ceramics [66].…”

“…x O 9-δ system. In this material, two processes take place with the substitution of Ca 2+ for Nb 5+ : oxygen vacancy generation and gradual transition from 1:2 trigonal to 1:1 cubic [19,64]. From the defect chemistry point of view, the formation of oxygen vacancies is designated by the next reaction [in Kröger-Vink notation]:…”

High Proton Conductivity in Ba5Er2Al2ZrO13, a Hexagonal Perovskite-Related Oxide with Intrinsically Oxygen-Deficient Layers

“…The hexagonal perovskite-related oxides, which have structures with hexagonal close-packed A O 3 (h) layers or sequences of hexagonal and cubic (c) A O 3 (and/or oxygen-deficient A O 3−δ where δ is the oxygen vacancy content) close-packed layers, exhibit a variety of crystal structures. − High oxide-ion conductivity is reported in the hexagonal perovskite-related oxides, − while the reports on the proton conduction are quite rare. Proton conductivities in Ba 2 (Zn 2/3 Ta 1/3 ) 2 O 5 and Ba 3 Ca 1+ x Nb 2– x O 9−δ with hexagonal symmetry were reported, − but their structures lack hexagonal close-packing and hence are not categorized as hexagonal perovskite-related oxides. Several materials such as Sr 3 R Nb 3– x Ti x O 12−δ ( R = La, Nd), BaTi 1– x Sc x O 3– δ , and Ba 4 M 2 O 9 ( M = Nb, Ta) have been investigated as proton conducting materials, but their proton conductivities (e.g., <10 –5 S cm –1 at 400 °C) are far below those of representative proton conductors. − …”

Probable thermoelectric materials for promising candidate of optoelectronics for Ba‐based complex perovskite compounds

“…Complex perovskites Ba 3 BB′ 2 O 9 are composed of at least two different B cations that can take a variety of valences 1,2 . The difference in the nature of the B cations leads to an ordering in the B site.…”