Search results for "polaron"

Quantum chemical modelling of polarons and perovskite solid solutions

2001

Abstract Following our previous study [J. Phys.: Condens. Matter 10 (1998) 6271] of a single Nb impurity and Nb clusters in KTaO 3 , we present results of the calculations for a series of perovskite KNb x Ta 1− x O 3 (KTN) solid solutions ( x =0, 0.125, 0.25, 0.75, 1). The quantum chemical method of the intermediate neglect of the differential overlap (INDO) combined with the large unit cell (LUC) periodic model is used. According to the INDO calculations, Nb impurity becomes off-center in KTaO 3 already at the lowest studied concentrations ( x =0.125), in a good agreement with XAFS measurements. We compare our results with previous ab initio FP-LMTO calculations. Quantum chemical calculati…

Computer modelling of point defects in ABO3 perovskites and MgO

2004

We present results for basic intrinsic defects: F-type electron centers (O vacancy which trapped one or two electrons) and hole polarons bound to Mg or K vacancy in ionic MgO and partly covalent KNbO3 perovskite, respectively. We demonstrate that a considerable covalency of the perovskite chemical bonding makes the F-type centers therein much more similar to defects in partly-covalent quartz-type oxides rather than the conventional F centers in alkali halides and ionic MgO. Both one-site (atomic) and two-site (molecular) polarons are expected to coexist in KNbO3 characterized by close absorption energies. Our calculations confirm existence of the self-trapped electron polarons in KNbO3, KTa…

Erratum to: Sub-gap defect density characterization of molybdenum oxide: An annealing study for solar cell applications

2022

Role of Shallow Electron Traps in the Fast Transient Optical Phenomena of Alkali Halide Crystals

2002

We present additional evidences that the same shallow electron traps-atomic alkali impurity centres [M + ] c 0 e - are responsible for both classes (A and B) of the transient IR-absorption bands: (A) bands with maximum at 0.27-0.36 eV in NaCl, KCl, KBr, KI and RbCl (due to shallow electron traps or bound polarons) and (B) bands with maximum at 0.15-0.36 eV in NaI, NaBr, NaCl:I, KCl:I, KBr:I, RbCl:I and RbBr:I (due to on-centre STE or on-centre STE localised at iodine dimer). Both classes of the IR bands have the same location, similar shape (both exactly coincide for KCl:I and KCl at 10 or 80 K), half-width, vibration structure. It is established that the same Mollwo-Ivey plot curves E o =a…

Characterization of the defect density states in MoOx for c-Si solar cell applications

2021

Thin layers of MoOx have been deposited by thermal evaporation followed by post-deposition annealing. The density of states distributions of the MoOx films were extracted deconvoluting the absorption spectra, measured by a photothermal deflection spectroscopy setup, including the small polaron contribution. Results revealed a sub-band defect distribution centered 1.1 eV below the conduction band; the amplitude of this distribution was found to increase with post-deposition annealing temperature and film thickness.

Sub-gap defect density characterization of molybdenum oxide: An annealing study for solar cell applications

2020

AbstractThe application of molybdenum oxide in the photovoltaic field is gaining traction as this material can be deployed in doping-free heterojunction solar cells in the role of hole selective contact. For modeling-based optimization of such contact, knowledge of the molybdenum oxide defect density of states (DOS) is crucial. In this paper, we report a method to extract the defect density through nondestructive optical measures, including the contribution given by small polaron optical transitions. The presence of defects related to oxygen-vacancy and of polaron is supported by the results of our opto-electrical characterizations along with the evaluation of previous observations. As part…

Small radius electron and hole polarons in PbX2 (X = F, Cl, Br) crystals: a computational study

2021

First-principles hybrid density functional theory (DFT) calculations were performed for small radius polarons – self-trapped electrons (STELs) and holes (STHs) in PbX2 (X = F, Cl, Br) crystals, widely used as parent materials for inorganic halide perovskites (CsPbX3) and scintillators. The atomic and electronic structures, spin and charge distributions and formation energies for both types of polarons were predicted for orthorhombic PbF2 and STELs for cubic PbF2. The STH structure was identified in a controversial case of PbCl2. We also confirmed and analyzed in detail experimentally suggested configurations for other cases. It is shown how, due to a delicate balance of ionic and covalent c…

High-temperature transport properties of La0.67Ca0.33MnO3 films

1999

Abstract The giant negative magnetoresistance in manganites has been investigated from the Curie temperature T c up to 600 K (2.6 T c ) in magnetic fields up to 8 T. Nonadiabatic small polaron hopping can successfully describe the temperature dependence of the resistivity. The magnetic field influence on the activation energy is explained by the interaction of unclustered ions with small spin clusters of four ions.

Dielectric relaxation of space charges and polarons in ferroelectric perovskites

2001

Abstract In all ferroelectric perovskites, intentionally introduced or “unwanted” point defects do play a role in the dielectric spectra and in the conductivity. Above room temperature, space charge relaxation at the electrodes interfaces are observed. This can be of interest in the context of the nowadays applications of ferroelectric thin films. At liquid helium temperatures much more localised dielectric relaxation occurs. Special emphasis will be brought on SrTiO3 which has received renewed interest at the beginning of the nineties and for which a wealth of reliable experimental data are available. Considering that a gradual freezing of polarized objects is occurring at low temperatures…

First principles calculations on CeO2 doped with Tb3+ ions

2019

This research was funded by the Latvian Council of Science (under the grant project lzp-2018/1-0147). Authors thank W. Chueh, J. Serra, R. Merkle, A. Popov for fruitful discussions.