Bismuth tri-iodide (BiI3) is an intermediate band gap semiconductor with potential for room temperature gamma-ray detection applications. Remarkably, very different band gap characteristics and values of BiI3 have been reported in literature, which may be attributed to its complicated layered structure with strongly bound BiI6 octahedra held together by weak van der Waals interactions. Here, to resolve this discrepancy, the band gap of BiI3 was characterized through optical and computational methods and differences among previously reported values are discussed. Unpolarized transmittance and reflectance spectra in the visible to near ultraviolet (UV-Vis) range at room temperature yielded an indirect band gap of 1.67 ± 0.09 eV, while spectroscopic ellipsometry detected a direct band gap at 1.96 ± 0.05 eV and higher energy critical point features. The discrepancy between the UV-Vis and ellipsometry results originates from the low optical absorption coefficients (α ∼ 102 cm−1) of BiI3 that renders reflection-based ellipsometry insensitive to the indirect gap for this material. Further, electronic-structure calculations of the band structure by density functional theory methods are also consistent with the presence of an indirect band gap of 1.55 eV in BiI3. Based on this, an indirect band gap with a value of 1.67 ± 0.09 eV is considered to best represent the band gap structure and value for single crystal BiI3.

No abstract

Coprecipitation synthesis methods followed by microwave sintering techniques were utilized to obtain dense phase pure Bi 2 Ti 2 O 7 polycrystalline ceramic pellets. No evidence of secondary phases was found in the powder or pellets. This maiden achievement allowed for primary thermophysical, crystallographic, and dielectric characterization of this ceramic compound. Density functional theory was used to model the structure of the pyrochlore, from which the theoretical X-ray diffraction pattern was obtained to determine the purity of the experimental compound. Discrepancies among reports in literature regarding the structure, stability, and supposed ferroelectricity of this material are discussed and clarified. A modification to the phase diagram of the Bi 2 O 3 ÀTiO 2 system is proposed based on the results of the present investigation. In addition, and contrary to prior reports, the dielectric characterization of Bi 2 Ti 2 O 7 reveals a linear dielectric with high permittivity values at room temperature (115 at 500 kHz), and more remarkably, a temperature and frequency dependent dielectric relaxation.

No abstract

No abstract

Molecular dynamics simulations are used to characterize ferroelectricity on the (001) surfaces of PbTiO3 (PT), one of the most widely studied ferroelectric materials. Two different empirical interatomic shell model potentials are used. Both PbO and TiO2 surface terminations in PT under open circuit electrical boundary conditions are characterized. The results are found to be in good agreement with the results of density functional theory calculations. The atomic relaxations, interlayer spacings and surface rumplings of each of the four possible surface terminations are analyzed. The deviation of the polarization from the bulk value is observed to be larger when the polarization points out of the surface than when it points into the surface. Analysis of the surface energies for free-standing films shows that polarization parallel to the surface is energetically more favorable than the polarization normal to the surfaces.

In direct contrast to recent reports, density functional theory predicts that the most stable structure of Bi2Ti2O7 pyrochlore is a cubic Fd3¯m space group by accounting for atomic displacements. The displaced Bi occupies the 96g(x,x,z) Wyckoff position with six equivalent sites, which create multiple local minima. Using nudged elastic band method, the transition states of Bi cation hopping between equivalent minima were investigated and an energy barrier between 0.11 and 0.21 eV was determined. Energy barriers associated with the motion of Bi between equivalent sites within the 96g Wyckoff position suggest the presence of dielectric relaxation in Bi2Ti2O7.

Based on effective medium theory, the Bruggeman equation and Maxwell‐Wagner (M‐W) equation were applied to extract the permittivity of SrTiO3 and BaTiO3 powders in slurries with various solids loadings. It was found that the margin of error for the calculated permittivity broadened with decreasing solids loading and increasing powder permittivity. However, the margin of error for permittivity calculated from the Bruggeman equation is consistently smaller than that from the M‐W equation. This smaller margin of error resulted in the applicability of the Bruggeman equation to extract the permittivity for all of the slurries in this work, in contrast to the M‐W equation which is restricted to dilute SrTiO3 slurries. Therefore, for slurries of powders with high permittivity (e.g., BaTiO3) or slurries with high solid loading the Bruggeman equation is necessary instead of the M‐W equation.