M.V. Lalic

Electronic structure and optical absorption of the Bi4Ge3O12 and the Bi4Si3O12 scintillators in ultraviolet region: an ab initio study

Ab initio calculations based on density-functional theory have been employed to study structural and electronic properties of Bi4Ge3O12 (BGO) and Bi4Si3O12 (BSO), as well as their optical characteristics in ultraviolet region, up to 40 eV. The electronic structure around the band gap is found to be similar in both compounds, dominated by the O p- and the Bi s-states (valence band top) and the Bi p-states (conduction band bottom). The gap is found to be indirect in both BGO and BSO. The optical spectra are analyzed, compared, and interpreted in terms of calculated band structures. It is shown that the absorption process involves significant energy flow from the O ions to the Bi ions. This fact stresses importance of the first neighborhood of the Bi (six O’s forming an octahedron), which is more distorted in the BSO than in the BGO. The latter difference is mainly responsible for the different absorption characteristics of the BGO and BSO.

An ab-initio study of electronic and optical properties of corundum Al2O3 doped with Sc, Y, Zr, and Nb

Ab-initio calculations based on density functional theory have been employed to study the structural, electronic, and optical properties of yttrium (Y), scandium (Sc), zirconium (Zr), and niobium (Nb) doped α-Al2O3 with corundum structure. Exchange and correlation effects between electrons have been treated by generalized gradient approximation within the Perdew-Burk-Ezrenhof parameterization and by recently developed Tran-Blaha modified Becke-Johnson approach. Most attention in the work has been paid to the impurity d states, whose energy splitting has been analyzed in terms of the crystal field theory and whose influence on the gap size and the offset of the bands around it has been carefully evaluated. The influence of these states on modification of the optical absorption edge and the static dielectric constant of the doped systems has been also studied. It is concluded that only the Y doped α-Al2O3 (1) preserves the size of the band gap of the pure alumina, (2) does not change significantly the band ...

Structural, electronic, optical, and magneto-optical properties of Bi12MO20 (M = Ti, Ge, Si) sillenite crystals from first principles calculations

The structural, electronic, optical, and magneto-optical properties of the three Bi12MO20 sillenites (BMO; M = Ti, Ge, and Si) have been investigated on the basis of the first-principles calculations performed by the full potential augmented plane wave method. The BMO’s linear optical responses are found to be very similar, but their optical rotatory powers and Faraday ellipticities exhibit notable differences in both visible and ultraviolet parts of the spectra. These differences originate from the subtle differences within the BMO’s electronic structures, such as different band-gaps and different offsets of the valence band tops. The latter are found to be caused not by the influence of the M ion electronic states, but by particular behavior of the M–O and the Bi–O chemical bonds.

Influence of Cd impurity on the electronic properties of CuAlO2 delafossite: first-principles calculations

We report on first-principles band-structure calculations of the semiconducting CuAlO2 delafossite compound in the pure form and also with Cd impurity occupying either a Cu or Al position. The computational tool was a full-potential linear augmented plane-wave method, with the generalized gradient approximation accounting for the exchange and correlation effects. The changes caused by the presence of Cd are studied by the analysis of the electronic structure and the electric field gradient (EFG) in both Cd-doped and pure CuAlO2 systems. Good agreement between the calculated and measured EFGs at Cd substituting for Cu or Al atoms in CuAlO2 indicates that the calculations were able to correctly describe the ground state of the system containing the impurity. It is shown that a specific hybridization scheme, involving Cu (and Cd) s and dz2 orbitals and neighbouring O pz orbitals, takes place at the Cu sites in CuAlO2 as proposed earlier. The results of the calculations indicate that the Cd-doped system changes its electrical properties when Cd replaces Cu atoms (producing an n-type semiconductor), but not when it substitutes for Al atoms.

Semiconductor properties of Cu-based delafossites revealed by an electric field gradient study

In this paper we offer an interpretation of the previously observed trend of the electric field gradient (EFG) values measured in a group of semiconducting delafossites CuBO2 (B = Al, Fe, Cr, Nd) at Cd impurities which substitute either Cu or B atoms. Our theoretical study indicates that this EFG trend reveals one of the most subtle details in the electronic spectrum of the compounds, namely whether the impurity states are formed within or out of the band gap. When Cd substitutes the Cu, it exhibits a larger EFG value in CuAlO2 and CuFeO2 than in CuCrO2 and CuNdO2. This occurs because the Cd states form in the first two compounds a shallow band within the gap, but in the second two compounds they do not. When Cd substitutes the B atom it exhibits almost the same EFG in all delafossites. In this case, Cd states are not formed within the gap in any of the compounds. The same interpretation can be applied to the whole family of CuBO2 delafossites, whatever the B atom is. To arrive at these conclusions we analysed and calculated various systems (Cd-doped CuAlO2 and CuCrO2 compounds, and molecular clusters) using the full-potential linear augmented plane wave method.

Electronic structure and electric field gradient calculations for intermetallic compounds

Using the first-principles full-potential linear muffin-tin orbital method in the atomic sphere approximation, we have calculated the electronic structure of the intermetallic compound , and evaluated the electric field gradients (EFGs) at all of the three inequivalent positions (two Hf and one Fe) in its lattice. The main results extracted from the experimental investigations, concerning the different magnitudes and origins of the EFGs at inequivalent Hf sites, are correctly reproduced. The possible mechanisms of formation of the EFGs at these sites are analysed and discussed.

The low-temperature magnetism of cerium atoms in CeMn2Si2 and CeMn2Ge2 compounds

The low-temperature magnetic properties of the Ce atoms in the intermetallic compounds CeMn2Ge2 and CeMn2Si2 were studied. Previous neutron scattering measurements did not detect an ordered moment at Ce atoms in either compound despite the fact that they are surrounded by the Mn moments ordered ferromagnetically in the CeMn2Ge2 and antiferromagnetically in the CeMn2Si2 .C ontrasting with this result, a recen tm easurement performed with the tim ed ifferential perturbed angular correlation (TDPAC) technique showed th ep resence of a pronounced magnetic hyperfine field (MHF) at Ce sites in the CeMn2Ge2 compound and no MHF in CeMn2Si2 .T he absence of the Ce magnetic moment and MHF in the silicide can be understood in terms of too weak a Ce–Ce magnetic interaction while in the germanide the TDPAC result suggests that some magnetic ordering of Ce atoms may occur. Aiming to understand the effects which result in the quenching of the Ce 4f moment in both cases, we performed first-principle sb and-structure calculations for both systems, using the full potential linear augmented plane wave method. It is show nt hat the magnetism of the Ce sublattice has fundamentally different nature in CeMn2Si2 and CeMn2Ge2 .W hile the Ce atoms are intrinsically nonmagnetic in the silicide, having a zero magnetic moment with both spin and orbital contributions identically zero, they display magnetic properties in the CeMn2Ge2 since their very small total moment is composed of finite spin and orbital components which almost cancel each other accidentally.

Full-potential linear-muffin-tin orbital atomic-sphere approximation calculations of the electric field gradient in HCP metals

Abstract In this paper we suggest a new scheme for obtaining electronic contribution to the electric field gradient (EFG) at the nucleus. The EFG for all HCP metals from Be to Cd is obtained from band structure calculations using the recently developed scheme of full-potential (FP) linear-muffin-tin-orbital (LMTO) formalism in the atomic-sphere approximation (ASA). This method is computationally very effective. A comparison with the theoretically most accurate full-potential linear augmented plane wave calculations and experimental values shows very good agreement. According to the investigations presented in this paper, FP–LMTO–ASA method is probably the best choice presently for the calculation of the EFG in more complicated compounds.

Structural, electronic, and optical aspects of Cr doping of the Bi4Ge3O12: an ab initio study

Ab initio calculations based on density functional theory have been employed to study energetic, structural, electronic, and optical properties of Cr doped Bi4Ge3O12 (BGO). Two possible accommodations of Cr impurity have been taken into account: at the Ge (Cr4+) and the Bi (Cr3+) substitution site. For each accommodation the local structure around the Cr has been determined, and in the Cr3+ case the Cr off-site displacement was analyzed. The Cr d-states are positioned at the bottom of the conduction band and within the gap in form of two deeplike (Cr4+) and shallowlike (Cr3+) bands, exhibiting magnetic moments of +1.58 μB and −2.44 μB, respectively. The Cr dominated part of absorption spectrum is calculated and analyzed in terms of Cr band arrangement. Comparison with the experimental BGO:Cr absorption spectrum suggests that it consists of both Cr3+ and Cr4+ contributions, indicating the Cr simultaneous presence at both substitution sites.

Study of Hyperfine Fields in CeIn3 by Electronic Structure Calculations

Electronic band structure calculations for the CeIn3 compound utilizing the full potential linearized augmented plane waves method were performed with the aim to compute the hyperfine fields acting on Ce and In atoms. The latter are found to be in reasonable agreement with the values measured at low temperatures. The 4f orbital contribution dominates the magnetic hyperfine field at Ce ions while the contact field is negligible due to an almost complete cancellation of valence and core contributions. A non-zero magnetic hyperfine field appears at In sites due to the spin-polarization of the 5p sub-shells through the hybridization with the extended 6s, 6p and 5d Ce states which, in turn, are spin-polarized by the Ce 4f states. No net magnetic moment at In is observed since the sum of its 5p sub-shell spins is zero. The 5p shell of In is responsible for the presence of an electric field gradient at In nuclei.