Cd implantation in α-MoO3: An atomic scale study

Lamellar α-MoO3 crystals were implanted with low fluence of radioactive Cd111m ions at ISOLDE-CERN. Subsequently, we have probed the interaction of the Cd impurity in the lattice with native point defects, such as oxygen vacancies, as a function of annealing temperature using the time differential perturbed angular correlations nanoscopic technique. The experimental data were complemented and interpreted by modeling different Cd-defect configurations in α-MoO3 with first-principles density functional theory (DFT). The agreement between experiments and DFT simulations shows that only the interstitial Cd (CdI) prevails in the van der Waals gap, by inducing a polaron effect. Upon raising the annealing temperature, CdI is able to trap hole charge carriers resultant from the oxygen vacancies VO. Oxygen vacancies were found to form most commonly at two-fold coordinated (O2) atoms. According to comparison DFT results with the experimental electric field gradient values (Vzz and η) and the calculated formation energies for different defect complexes, the configuration of CdI with two (O2) vacancies (VO2), located at different planes, is found to be more favorable and stable than the other defect configurations. The electron-polaron formation around the Cd impurity at an interstitial site is enhanced by inducing (O2) vacancies with the creation of hole polaron states.