Y2O3:Eu and the Mössbauer isomer shift coefficient of Eu compounds from ab-initio simulations

We report on a full potential density functional theory characterization of Y2O3 upon Eu doping on the two inequivalent crystallographic sites 24d and 8b. We analyze local structural relaxation,electronic properties and the relative stability of the two sites. The simulations are used to extract the contact charge density at the Eu nucleus. Then we construct the experimental isomer shift versus contact charge density calibration curve, by considering an ample set of Eu compounds : EuF3, EuO,EuF2, EuS, EuSe, EuTe, EuPd3 and the Eu metal. The, expected, linear dependence has a slope of α= 0.054 mm/s/Å3, which corresponds to nuclear expansion parameter ∆R/R= 6.0·10−5.αallows to obtain an unbiased and accurate estimation of the isomer shift for any Eu compound. We test this approach on two mixed-valence compounds Eu3S4 and Eu2SiN3, and use it to predict theY2O3:Eu isomer shift with the result +1.04 mm/s at the 24d site and +1.00 mm/s at the 8b site.

Magnetism in Iron-Zirconium Systems

The discrete variational method is used to solve the Khon-Sham equation in the spin-polarized local density approximation for Fe-Zr and Fe-Zr-H clusters, representing iron-zirconium and hydrogenated iron-zirconium alloys. The local magnetic moment and hyperfine field at the Fe site were found to decrease, whereas the contact charge density and occupation number were found to increase with Zr contents. The Zr site in clusters with high Fe contents acquires a negative magnetic moment. When H is trapped at an interstitial site next-neighboring an Fe atom, the magnetic moment and hyperfine field are enhanced, while the contact charge density is reduced. ‘The opposite occurs when H occupies a neighboring interstitial site to Fe. For Zr atoms the local magnetic moment is found to become less negative with H at the neighboring position. We conclude from this calculation that H is trapped in Fe-Zr systems at positions which are nearest to Zr and next-nearest to Fe atoms.