Effect of doping and oxygen vacancies on the octahedral tilt transitions in the BaCeO3 perovskite (original) (raw)
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Diffusion and Defect Data Pt.B: Solid State Phenomena, 2011
From neutron diffraction it is known that the BaCeO 3 perovskite undergoes a sequence of phase transformations from high temperature cubic C to rhombohedral R, to orthorhombic O1 (Imma) and to orthorhombic O2 (Pnma). Doping Y 3+ on the Ce 4+ site introduces charge compensating O vacancies (V O ) that may be partially filled with OH complexes with exposition to H 2 O, so making the material an ionic conductor.
Effect of O vacancies on the Young’s modulus of the BaCe[sub 1−x]Y[sub x]O[sub 3−δ] perovskite
Applied Physics Letters, 2009
The effect of oxygen vacancies on the elastic properties of BaCe 0.9 Y 0.1 O 3−␦ is studied by measuring the complex Young's modulus between 80 and 850 K varying the content of O vacancies from 0 to nearly 0.05. The Young's modulus measured at a fixed temperature above 300 K may change by more than 20% but this is due to a shift of the rhombohedral-orthorhombic transition by 250 K and to proton and vacancy hopping. Below 100 K these effects are frozen and the filling of the O vacancies with OH ions increases the Young's modulus by ϳ1.3%.
arXiv (Cornell University), 2023
Several neutron diffraction, Raman spectroscopy, and thermoanalytical experiments conducted over decades have revealed that the perovskite-structured BaCeO 3 goes through a series of temperature-induced structural phase transitions. However, it has been frequently observed that the number of phases and the sequence in which they appear as a function of temperature differ between experiments. Insofar as neutron diffraction experiments are concern, in the temperature range of 4.2 to 1273 K, four structures are crystallographically well characterized with three transitions: orthorhombic Pnma → orthorhombic Imma [563 K] → rhombohedral R-3c [673 K] → cubic Pm-3m [1173 K], which lately have been reciprocally realized in the studies of polarized Raman spectroscopy. In contrast, thermoanalytical methods such as dilatometry showed multiple singularities corresponding to at-least three more structural phase transitions at around 830 K, 900 K, and 1030 K, in addition to those recorded by neutron studies. In account of these conflicting experimental findings, we computed free energy phase diagram for BaCeO 3 polymorphs employing crystal structure data mining in conjunction with first principles electronic structure and phonon lattice dynamics. A total of 34 polymorphs have been predicted, the most stable of which follows the Glazer classification of the perovskite tilt system, and it has been found that a number of these polymorphs are thermodynamically competing with Pnma as the temperature rises. In particular, it has been predicted that the orthorhombic Cmcm and tetragonal P4/mbm phases surpass Pnma at 666 K and 1210 K, respectively. At any temperature, two alternate tetragonal phases (P4 2 /nmc and I4/mcm) are also found to be 20 to 30 meV less favored than the Pnma. While the calculated stability order of the predicted polymorphs is in acceptable agreement with the results of neutron diffraction, the transitions observed in thermoanalytical studies could be ascribed to the development of four novel phases (Cmcm, P4/mbm, P4 2 /nmc, and I4/mcm) at intermediate temperatures. However, we analyze that the rhombohedral R-3c phase predominantly stabilized over a broad temperature field, masking all subsequent phases up until the cubic Pm-3m. Consequently, the novel phases predicted to occur in thermoanalytical studies are only fleetingly metastable. The calculated phonons additionally demonstrate that the high temperature phases are not quenchable down to room temperature. The theoretical results presented reconcile the apparent inconsistencies observed thus far in the experiments.
Inorganic Chemistry, 2011
We report the effect of donor-doped perovskitetype BaCeO 3 on the chemical stability in CO 2 and boiling H 2 O and electrical transport properties in various gas atmospheres that include ambient air, N 2 , H 2 , and wet and dry H 2 . Formation of perovskite-like BaCe 1Àx Nb x O 3(δ and BaCe 0.9Àx Zr x Nb 0.1 O 3(δ (x = 0.1; 0.2) was confirmed using powder X-ray diffraction (XRD) and electron diffraction (ED). The lattice constant was found to decrease with increasing Nb in BaCe 1Àx Nb x O 3(δ , which is consistent with Shannon's ionic radius trend. Like BaCeO 3 , BaCe 1Àx Nb x O 3(δ was found to be chemically unstable in 50% CO 2 at 700°C, while Zr doping for Ce improves the structural stability of BaCe 1Àx Nb x O 3(δ . AC impedance spectroscopy was used to estimate electrical conductivity, and it was found to vary with the atmospheric conditions and showed mixed ionic and electronic conduction in H 2 -containing atmosphere. Arrhenius-like behavior was observed for BaCe 0.9Àx Zr x Nb 0.1 O 3(δ at 400À700°C, while Zr-free BaCe 1Àx Nb x O 3(δ exhibits non-Arrhenius behavior at the same temperature range. Among the perovskite-type oxides investigated in the present work, BaCe 0.8 Zr 0.1 Nb 0.1 O 3(δ showed the highest bulk electrical conductivity of 1.3 Â 10 À3 S cm À1 in wet H 2 at 500°C, which is comparable to CO 2 and H 2 O unstable high-temperature Y-doped BaCeO 3 proton conductors.
The effect of disorder in Ba2YTaO6 on the tetragonal to cubic phase transition
2011
Synchrotron X-ray diffraction and Raman spectroscopy have been used to study the structure of the complex perovskite Ba 2 YTaO 6 , at temperatures down to 100 K. Where the Ta and Y cations exhibit longrange rock-salt like ordering, Ba 2 YTaO 6 displays a continuous phase transition from a high temperature cubic structure, described in space group Fm3 m, to a tetragonal, I4/m, structure near 260 K. This transition is inhibited if extensive disorder and/or vacancies are/is present in the sample.
Local Oxygen-Vacancy Ordering and Twinned Octahedral Tilting Pattern in the Bi0.81Pb0.19FeO2.905 Cubic Perovskite, 2012
The structure of Bi0.81Pb0.19FeO2.905 was investigated on different length scales using a combination of electron diffraction, high-resolution scanning transmission electron microscopy, synchrotron X-ray powder diffraction, and Mössbauer spectroscopy. In the 80−300 K temperature range, the average crystal structure of Bi0.81Pb0.19- FeO2.905 is a cubic Pm3̅m perovskite with a = 3.95368(3) Å at T = 300 K. The (Pb2+, Bi3+) cations and O2− anions are randomly displaced along the ⟨110⟩ cubic directions, indicating the steric activity of the lone pair on the Pb2+ and Bi3+ cations and a tilting distortion of the perovskite framework. The charge imbalance induced by the heterovalent Bi3+→ Pb2+ substitution is compensated by the formation of oxygen vacancies preserving the trivalent state of the Fe cations. On a short scale, oxygen vacancies are located in anion-deficient (FeO1.25) layers that are approximately 6 perovskite unit cells apart and transform every sixth layer of the FeO6 octahedra into a layer with a 1:1 mixture of corner-sharing FeO4 tetrahedra and FeO5 tetragonal pyramids. The anion-deficient layers act as twin planes for the octahedral tilting pattern of adjacent perovskite blocks. They effectively randomize the octahedral tilting and prevent the cooperative distortion of the perovskite framework. The disorder in the anion sublattice impedes cooperative interactions of the local dipoles induced by the off-center displacements of the Pb and Bi cations. Magnetic susceptibility measurements evidence the antiferromagnetic ordering in Bi0.81Pb0.19FeO2.905 at low temperatures.
First-principles study of the cubic perovskitesBiMO3(M=Al, Ga, In, and Sc)
Physical Review B, 2007
We systematically investigated the structure, electronic properties, zone-center phonon modes, and structure instability of four cubic perovskite BiMO 3 compounds, with three of the M ions being IIIB metals ͑Al, Ga, and In͒ and one IIIA transition-metal Sc, using first-principles density-functional calculations. Optimized lattice parameters, bulk moduli, band structures, densities of states, as well as charge density distributions are calculated and compared with the available theoretical data. Our results are in good agreement with those previously reported in the literature. All the BiMO 3 oxides considered in the present work are semiconductors with an indirect band gap between the occupied O 2p and unoccupied Bi 6p states varying between 0.17 and 1.57 eV. Their electronic properties are determined mainly by Bi-O bonding, which, in turn, depends on the M-O bonding. Ferroelectric properties of these oxides come from the 6s 2 lone pair on the A-site Bi ion and is similarly affected by the M ions through their influence on the Bi-O bonding, as suggested by our calculations of density of state, Born effective charge, and soft modes. The existence of soft modes and eight ͓111͔ minima suggests that the phase transition in BiAlO 3 has a mixed displacive and order-disorder character. There is evidence that ferroelectricity is absent in BiGaO 3. Our investigation suggests that the BiMO 3 oxides or their modified versions are promising ferroelectric, piezoelectric, multiferroic, and photocatalytic materials.
Microstructure dynamics in orthorhombic perovskites
Physical Review B, 2010
Anelastic loss mechanisms associated with phase transitions in BaCeO 3 have been investigated at relatively high frequency ϳ1 MHz and low stress by resonant ultrasound spectroscopy ͑RUS͒, and at relatively low frequency ϳ1 Hz and high stress by dynamic mechanical analysis ͑DMA͒. Changes in the elastic moduli and dissipation behavior clearly indicate phase transitions due to octahedral tilting: Pnma ↔ Imma ↔ R3c ↔ Pm3m structures at 551 K, 670 K, and 1168 K, and strain analysis shows that they are tricritical, first-order, and second-order phase transitions, respectively. Structures with intermediate tilt states ͑R3c and Imma structures͒ show substantial anelastic softening and dissipation associated with the mobility of twin walls under applied stress. The Pnma structure shows elastic stiffening which may be due to the simultaneous operation of two discrete order parameters with different symmetries. In contrast with studies of other perovskites, BaCeO 3 shows strong dissipation at both DMA and RUS frequencies in the stability field of the Pnma structure. This is evidence that ferroelastic twin walls might become mobile in Pnma perovskites and suggests that shearing of the octahedra may be a significant factor.
Structural variants in ABO3 type perovskite oxides. On the structure of BaPbO3
2001
The structure of BaPbO 3 has been re®ned from powder neutron and synchrotron X-ray diffraction data at both room temperature and 15 K. At room temperature the structure is orthorhombic, space group Imma a 6:02991; b 8:50941; c 6:60941 A Ê . R p 5.26, R wp 6.48, x 2 2.45%. On cooling the structure transforms to a monoclinic form, space group C2/m, a 8:55481 b 8:48081 c 6:01181 A Ê b 134:8061: R p 5.30, R wp 6.50, x 2 2.64%. q