Technetium incorporation in scheelite: insights from first-principles

Atomistic investigations of crystalline scheelite, CaWO4, and Tc-99-bearing scheelite, CaWO4:Tc, have been carried out using density functional theory. The lattice constants, bulk modulus, and volume compression data of CaWO4 have been calculated and compared with experimental data, with a focus on predictive understanding of Tc-99 incorporation in CaWO4. Defect formation energies have been computed for several possible interstitial (I) and substitutional (S) sites of Tc-99 in CaWO4. Both I(Oh) and S(W) sites were found to be energetically favourable for Tc doping. X-ray diffraction (XRD) spectra for each Tc-99 defect type have been simulated to help interpret the complex experimental XRD patterns. This work on CaWO4: Tc provides insights into materials generated during nuclear weapons testing and useful spectral signatures for nuclear forensics.