Alkaline-earth Zirconates Research Articles

Variable Photoluminescence Intensity Ratio with the Excitation Wavelength in Eu3+-Doped Perovskite-Type Alkaline Earth Zirconates─Possibility of a Unique Visualization of Ultraviolet Light.

Photoluminescence (PL) spectra arising from 4f-4f dipole transitions of Eu3+ doped at both the A and B sites of perovskite-type alkaline earth zirconates obtained at various excitation wavelengths were evaluated. Changes in the excitation wavelength caused obvious differences in the PL intensity ratio of the induced electric dipole (ED) 5D0 → 7F2 transition to the magnetic dipole (MD) 5D0 → 7F1 transition for Eu3+-doped SrZrO3 and CaZrO3, in which only the B site had a center of symmetry. Two charge transfer (CT) bands associated with the excitation of Eu3+ were observed in the spectra of these oxide samples, which arose from the difference in the electronic structure of the Eu-O coordination at the A and B sites. We conclude that simultaneous Eu3+ substitution at sites with and without a center of symmetry, which have different charge transfer energies, induced the observed novel PL changes with a changing excitation wavelength. Our results may enable the development of a new type of ultraviolet light detector.

Optoelectronic pressure dependent study of alkaline earth based zirconates AZrO3 (A = Ca, Ba, Sr) using ab-initio calculations

The physical properties of alkaline-earth zirconates AZrO3 (A = Ca, Ba, Sr) are revealed using density functional theory (DFT) based FP-LAPW+lo approach. The present study investigates the structural, optoelectronic, and thermoelectric features, which are elucidated using GGA-PBEsol functional. The changing A cations from Ba to Sr to Ca result in increasing lattice constant comparable with experimental data and reducing bulk modulus. The CaZrO3 exhibits comparatively higher stiffness or hardness than that of the SrZrO3 and BaZrO3. The applied pressure improves mechanical stability by increasing ductility. Moreover, electronic structures are computed under varying pressures 0–30 GPa. All three compounds show indirect bandgap (Γ–M) up to 20 GPa, and the transition to direct bandgap (Γ–Γ) is illustrated at 30 GPa. Consequently, the significance of optoelectronic applications is revealed. The pressure-dependent various optical parameters are also explored and validation of Penn’s model, transparency, and maximum reflectivity at specific energy ranges expose their possible commercial candidature.

Calcium zirconate photocatalyst and silver cocatalyst for reduction of carbon dioxide with water

Calcium and other alkaline-earth zirconate photocatalysts were prepared for photocatalytic reduction of CO2 with water in an aqueous solution. The CaZrO3 sample showed high selectivity for CO2 reduction although the BaZrO3 sample predominantly promoted water splitting. The CaZrO3 photocatalyst and other zirconate photocatalysts were compared with the corresponding alkaline-earth titanate photocatalysts to discuss their photocatalytic properties. Further, it was also found that some kinds of Ag species on the CaZrO3 sample decreased the photocatalytic activities while metallic Ag nanoparticles gave a positive effect on the photocatalytic activity for CO2 reduction.

Reactions of alkaline earth zirconate refractories with titanium alloys

The severe reactivity of titanium alloys with ceramics is a major challenge for their processing. Up to now refractories to melt and cast titanium alloys are selected on the basis of a low Gibbs energy of formation. This kind of selection assumes that an oxide should be stable if its Gibbs energy of formation is lower than the one of any titanium (sub)oxide. The present contribution reviews that these models trying to explain the stability of ceramic materials in contact with titanium alloys are often misleading. By contrast, a dissolution and evaporation based reaction model is more appropriate to describe the reaction of high temperature ceramics with titanium alloys. These explanations were exemplified by research findings of high temperature reactions of titanium alloys with calcium oxide and yttrium oxide. Based on the discussion on calcium and yttrium oxide, the reactions of alkaline earth zirconates such as calcium and barium zirconate with titanium alloys were discussed. The reaction of alkaline earth zirconates is also highly dependent on the titanium alloy composition. It was also demonstrated that not only thermodynamics but also kinetics should be considered to evaluate refractories for titanium processing.

Optimizing Proton Conductivity in Zirconates through Defect Engineering

Alkaline-earth zirconates (CaZrO3, SrZrO3, and BaZrO3) are under active investigation as solid-state electrolytes in hydrogen fuel cells. Their performance as proton conductors depends critically o...

A combined conductivity and DFT study of protons in PbZrO 3 and alkaline earth zirconate perovskites

The electrical properties of nominally undoped and 4 mol% Y-doped PbZrO 3 have been investigated by AC conductivity measurements and impedance spectroscopy under various p H2O and p O2 at high temperatures. The results indicate that the defect structures are dominated by acceptors (Y dopant and/or Pb vacancies formed during synthesis). In dry atmosphere and at high temperatures, the acceptors are compensated by oxygen vacancies. These are hydrated and replaced by protonic defects (hydroxide ions on oxide ion sites) at higher p H2O and lower temperatures. In oxidizing atmospheres, a minority concentration of electron holes dominates the conductivity. At lower temperatures and in wet atmosphere, a significant protonic conductivity contribution is also observed. Based on p O2 and p H2O isotherms, a model for incorporation of protonic defects has been applied, and the standard enthalpy of hydration of oxygen vacancies in undoped PbZrO 3 has been determined (− 1.07 ± 0.13 eV). The measured total conductivities are influenced by high grain boundary resistance. Hence, the enthalpy is at the present stage assigned to the polycrystalline ceramic material as such. Rough estimates of bulk proton mobility in Y-doped PbZrO 3 yield u H+ 0 = 17 cm 2K/Vs and Δ H m,H + = 0.93 eV. A complementary DFT study of the hydration thermodynamics of PbZrO 3 and the alkaline earth zirconate perovskites AZrO 3 ( A = Ca, Sr, Ba) is also reported. The experimental and theoretical hydration enthalpies are compared with those of other ABO 3 perovskites. Correlations between the hydration thermodynamics and other properties of the materials are discussed.

O–H stretching vibrations of proton conducting alkaline-earth zirconates

Infrared diffuse reflection spectra of O–H( ν OH) and O–D( ν OD) stretching vibration regions are used to characterize proton dissolution sites in high temperature proton conductors (HTPCs) of CaZrO 3, SrZrO 3 and BaZrO 3 doped with trivalent Ga 3+, In 3+ and Y 3+ ions. The number of observed ν OH band was three, five or six and two, respectively, for the CaZrO 3-, SrZrO 3- and BaZrO 3-based samples. It was inferred that the two lowest bands below 2000 cm −1 observed for SrZrO 3-based samples were the ν OH bands for hydrogen bonding of O…H. Therefore, the number of proton dissolution sites was concluded to be three for the CaZrO 3-based samples, three or four for the SrZrO 3-based samples and two for the BaZrO 3-based samples. The ν OH bands were observed at low wave numbers in the order of Sr(Zr,M III)O 3− δ <Ca(Zr,M III)O 3− δ <Ba(Zr,M III)O 3− δ , i.e., hydrogen bonding strength of the dissolved protons is found to be in the order of Ba(Zr,M III)O 3− δ <Ca(Zr,M III)O 3− δ <Sr(Zr,M III)O 3− δ . The average distances between two oxygen atoms, r O–O, in the original alkaline-earth zirconates are reported as r O–O(SrZrO 3)< r O–O(CaZrO 3)< r O–O(BaZrO 3). Strong hydrogen bonding was observed for the alkaline earth zirconate with short r O–O. It was concluded that the chemical environment of protons dissolved was primarily affected by crystal structure of the original zirconates.

Proton conducting alkaline earth zirconates and titanates for high drain electrochemical applications

The mobility and stability of protonic defects in acceptor-doped perovskite-type oxides (ABO 3) in the system SrTiO 3–SrZrO 3–BaZrO 3–BaTiO 3 have been examined experimentally and by computational simulations. These materials have the potential to combine high proton conductivity and thermodynamic stability. While any structural and chemical perturbation originating from the B-site occupation (poor chemical matching of the acceptor-dopant or Zr/Ti-mixing) leads to a significant reduction of the mobility of protonic defects, Sr/Ba-mixing on the A-site appears to be less critical. The stability of protonic defects is found to essentially scale with the basicity of the lattice oxygen, which is influenced by both A- and B-site occupations. The highest proton conductivities are observed for acceptor-doped BaZrO 3. Despite its significantly higher ionic radius compared to Zr 4+, Y 3+ is found to be optimal as an acceptor dopant for BaZrO 3. Mulliken population analysis shows that Y does not change the oxide's basicity (i.e. it chemically matches on the Zr-site of BaZrO 3). The highest proton conductivities have been observed for high Y-dopant concentrations (15–20 mol%). For temperatures below about 700°C, the observed proton conductivities clearly exceed the oxide ion conductivities of the best oxide ion conductors. The high conductivity and thermodynamic stability make these materials interesting alternatives for oxide ion conductors such as Y-stabilized zirconia, which are currently used as separator material for high drain electrochemical applications, such as solid oxide fuel cells.

Potentiometric determination of the gibbs energies of formation of SrZrO3 and BaZrO3

The Gibbs free energies of formation of strontium and barium zirconates have been determined in the temperature range 960 to 1210 K using electrochemical cells incorporating the respective alkaline-earth fluoride single crystals as solid electrolytes. Pure strontium and barium monoxides were used in the reference electrodes. During measurements on barium zirconate, the oxygen partial pressure in the gas phase over the electrodes was maintained at a low value of 18.7 Pa to minimize the solubility of barium peroxide in the monoxide phase. Strontium zirconate was found to undergo a phase transition from orthorhombic perovskite to) with space group Cmcm; D-2h(17) to tetragonal perovskite (t) having the space group 14/mcm; D-4h(18) at 1123 (+/- 10) K. Barium zirconate does not appear to undergo a phase transition in the temperature range of measurement. It has the cubic perovskite (c) structure. The standard free energies of formation of the zirconates from their component binary oxides AO (A = Sr, Ba) with rock salt (rs) and ZrO2 with monoclinic (m) structures can be expressed by the following relations:SrO (rs) + ZrO2 (m) --> SrZrO3 (o) Delta G degrees = -74,880 - 14.2T (+/-200) J mol(-1) SrO (rs) + ZrO2 (m) --> SrZrO3 (t) Delta G degrees = -73,645 - 15.3T (+/-200) J mol(-1) BaO (rs) + ZrO2 (m) --> BaZrO4 (c) Delta G degrees = -127,760 - 1.79T (+/-250) J mol(-1) The results of this study are in reasonable agreement with calorimetric measurements reported in the literature. Systematic trends in the stability of alkaline-earth zirconates having the stoichiometry AZrO(3) are discussed.

Dielectric Properties of Lead Alkaline-Earth Zirconate at Microwave Frequencies

Dielectric properties of alkaline-earth-substituted PbZrO3 ceramics up to microwave frequencies were investigated. Although there were no preferable characteristics in the Ba- or Sr-substituted system, the ceramics in a (PbCa)ZrO3 system showed not only high dielectric constants in the range of 100 to 118, but also high Q values of 1000 to 1150 and small temperature coefficients of resonant frequency of 0 to 30 ppm/ °C. The crystal structure of the Ca-substituted ceramics was also investigated by X-ray powder diffraction. The structure changed from PbZrO3-type orthorhombic to CaZrO3-type orthorhombic through cubic and rhombohedral with increasing Ca substitution. Coaxial dielectric resonators made of the selected above material had Q values in the range of 430 to 450 at the 900 MHz band.

High-refractory cements based on alkaline-earth zirconates

High-refractory cements based on alkaline-earth zirconates