Enhanced Born charge and proximity to ferroelectricity in thallium halides

Abstract

Electronic-structure and lattice-dynamics calculations on thallium halides show that the Born effective charges in these compounds are more than twice larger than the nominal ionic charges. This is a result of cross-band-gap hybridization between Tl p and halogen-p states. The large Born charges cause giant splitting between longitudinal and transverse-optic phonon modes, bringing the lattice close to ferroelectric instability. Our calculations indeed show that cubic TlBr develops ferroelectric instabilities upon lattice expansion starting at 2%. It is remarkable that the apparently ionic thallium halides with a simple cubic CsCl structure and large differences in electronegativity between cations and anions can be very close to ferroelectricity. This can lead to effective screening of defects and impurities that would otherwise be strong carrier traps and may therefore contribute to the relatively good carrier transport properties in TlBr radiation detectors.

Dynamic Stereochemical Activity of the Sn2+ Lone Pair in Perovskite CsSnBr3

“…[73][74][75][76] Systems with these electronic features exhibit a strong electron-phonon coupling (and atomic displacement-polarization coupling), with implications for charge transport and defect tolerance. 14 Comparison with isostructural CsPbBr 3 and CsCaBr 3 allows us to examine the influence of the s 2 lone pair electrons on the electronic structure and lattice dynamics: Pb 2+ is predicted to have a lesser tendency for stereochemical activity due to greater relativistic The band structure of CsPbBr 3 is similar to that of CsSnBr 3 , but the competing effects of longer metal-Br bond length, reduced ionicity of metal-Br interaction, and stronger spinorbit coupling result in a wider bandgap and larger carrier effective masses at this level of theory. The large electronegativity difference between Ca and Br causes CsCaBr 3 to be an insulator, with rather localized electronic states.…”

Electronic structure and photovoltaic application of BiI3

“…22 Large static dielectric constants and Born charges were suggested to indicate effective screening of defects and impurities which would otherwise trap charge carriers. 31 Therefore we propose that the relatively high Z * of BiI 3 reported here and previously 21,22 may be correlated with favorable transport properties.…”

Transparent conducting properties of SrSnO3 and ZnSnO3

“…While ferroelectricity as strictly defined is incompatible with metallic conduction, we note that semiconductors with high dielectric constants are generally expected to have high mobility due to screening of impurities and other defects. 36,53 This plus the high plasma FIG. 5.…”

“…ZnSnO 3 is ferroelectric, and therefore it and modifications of it may be expected to have high mobility when doped due to the high dielectric constants. 36 In general, scattering in doped non-magnetic oxide films can come from point defects, extended defects (dislocations, grain boundaries, etc. ), and intrinsic electron-electron and electron-phonon scattering.…”