Search results for "PEROVSKITE"

Quantum chemical modelling of electron polarons and excitons in ABO3perovskites

2000

Quantum chemical calculations using the intermediate neglect of the differential overlap (INDO) method, combined with the large unit cell periodic model argue for an existence of the self-trapped electrons in KNbO3 and KTaO3 perovskite crystals. An electron in the ground state occupies predominantly t2g orbital of a Nb4+ ion. Its orbital degeneracy is lifted by a combination of the breathing and Jahn-Teller modes where four nearest equatorial O atoms are displaced outwards and two oxygens shift inwards along the z axis. Triplet exciton is shown to be in a good approximation of a pair of nearest Jahn-Teller electron and hole polarons (a bipolaron) which is very likely responsible for the `gr…

Electronic Properties of Mixed-Valence Manganates:  The Role of Mn Substitutional Defects

2002

Single-phase perovskites in the solid solution series La0.7+yA0.3-yMn1-xMxO3 (with 0.00 ≤ x ≤ 0.10; A = Sr2+, M = Cu2+, Zn2+, Sc3+, Cr3+, Co3+, and Ga3+; A = Ba2+, M = Cu2+, Zn2+, and Sc3+) have been prepared via the acetic acid solutions freeze-drying method. This soft procedure makes possible strict stoichiometric control, and the synthetic variables allow one to maintain a constant proportion of Mn4+ (ca. 32%) in the 47 compounds prepared. In this way, the concentration of cationic vacancies at A and B sites is practically negligible in all cases. X-ray powder diffraction patterns corresponding to the 47 compounds have been completely indexed with rhombohedral perovskite cells. The cryst…

Photoluminescence quantum yield exceeding 80% in low dimensional perovskite thin-films via passivation control

2017

Quasi-2D perovskites with the BA : MA molar ratio equal to 3 : 3 show a remarkable PLQY exceeding 80%, thanks to the use of an electron donor as the passivating agent. These films have been applied in LEDs that exhibit high brightness exceeding 1000 cd m−2 and current efficiencies >3 cd A−1.

EXAFS study of ceria–lanthana-based TWC promoters prepared by sol–gel routes

2003

Extended X-ray absorption fine structure (EXAFS) experiments at the Ce K- and La K-edges were performed on ceria–lanthana–alumina three-way catalysts promoters prepared by sol–gel routes, in order to investigate the effect of lanthanum doping on the ceria structure. The formation of Ce1−xLaxO2−x/2 solid solution, already observed by X-ray diffraction, was confirmed by EXAFS analysis, while no experimental evidence of a Ce–Al interaction was found. In presence of cerium and aluminum, lanthanum is involved in the formation of solid solution with CeO2 and of La–Al compounds. When the La:Al molar ratio is sufficiently high, the growth of a tridimensionally ordered LaAlO3 perovskite compound is …

Working Principles of Perovskite Solar Cells

2018

Thermodynamic stability of stoichiometric LaFeO 3 and BiFeO 3 : a hybrid DFT study

2017

BiFeO3 perovskite attracts great attention due to its multiferroic properties and potential use as a parent material for Bi1−xSrxFeO3−δ and Bi1−xSrxFe1−yCoyO3−δ solid solutions in intermediate temperature cathodes of oxide fuel cells. Another iron-based LaFeO3 perovskite is the end member for well-known solid solutions (La1−xSrxFe1−yCoyO3−δ) used for oxide fuel cells and other electrochemical devices. In this study an ab initio hybrid functional approach was used for the study of the thermodynamic stability of both LaFeO3 and BiFeO3 with respect to decompositions to binary oxides and to elements, as a function of temperature and oxygen pressure. The localized (LCAO) basis sets describing th…

BacSr1−cTiO3 perovskite solid solutions: Thermodynamics from ab initio electronic structure calculations

2005

We suggest theoretical prediction for Ba"cSr"1"-"cTiO"3 perovskite solid solutions (BST) combining ab initio DFT/B3PW calculations and alloy thermodynamics. This approach is based on calculations of a series of ordered super-structures in Ba-Sr simple cubic sublattice immersed in the rest TiO"3 matrix. Although these structures are unstable with respect to the decomposition, the results of total energy calculations allow us to extract the necessary energy parameters and to calculate the phase diagram for the solid solutions (alloys). A novel approach applied to the BST system enables to predict that at T>400 K Ba and Sr atom distribution is random. But below this temperature at small c Ba a…

Hansen theory applied to the identification of nonhazardous solvents for hybrid perovskite thin-films processing

2018

Abstract Metal-halide perovskites have become the most studied material for efficient next-generation solar cells, in part because of the possibility of depositing high quality semiconducting perovskites by simple solution-based methods. However, the majority of solvent systems implemented in literature for deposition of lead halide perovskites are hazardous to handle. Investigation of alternatives perovskite processing methods are hence key to safely upscale the perovskite photovoltaic manufacturing. In this manuscript we use the Hansen theory to find suitable nonhazardous solvents to solubilize two lead salts, PbBr2 and PbI2, used to fabricate the corresponding methylammonium (MA) lead ha…

Anion ordering and defect structure in Ruddlesden-Popper strontium niobium oxynitrides

2004

The crystal structure of the n = 1 member of the Ruddlesden-Popper family (SrO)(SrNbO(2)N)(n) was refined by the Rietveld method using neutron powder diffraction data. This complex crystallizes in the I4/mmm space group with cell parameters a = 4.0506(2) and c = 12.5936(9) angstroms. The refined composition was Sr(2)NbO(3.28)N(0.72), which corresponds to a formal oxidation state for Nb of +4.72, meaning 72% Nb(V) and 28% Nb(IV). The nitrogen atoms order in the equatorial sites of the niobium octahedra according to Pauling's second crystal rule as the more charged anion occupies the site showing the larger bond strength sums. Pauling's second crystal rule is shown to be able to predict the d…

Semi-empirical simulations of surface relaxation for perovskite titanates

2000

The (100) and (110) surface relaxations are calculated for SrTiO 3 and BaTiO 3 perovskite thin films. By means of a semi-empirical shell model, the positions of atoms in 16 near-surface layers placed atop a slab of rigid ions are calculated. Surface rumpling and surface-induced dipole moments are calculated for all possible surface terminations. Our results for the (100) surface structure are in good agreement with ab-initio plane-wave pseudopotential calculations and LEED experiments. The surface energy for the Ba-, Sr-, TiO-terminated (110) surfaces is found to be much larger than that for the (100) surface. In contrast, the surface energy for the asymmetric O termination, where outermost…