Cd implantation in α−MoO3 : An atomic scale study

Lamellar $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{MoO}}_{3}$ crystals were implanted with low fluence of radioactive $^{111\mathrm{m}}\mathrm{Cd}$ 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 $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{MoO}}_{3}$ with first-principles density functional theory (DFT). The agreement between experiments and DFT simulations shows that only the interstitial Cd (${\mathrm{Cd}}_{I}$) prevails in the van der Waals gap, by inducing a polaron effect. Upon raising the annealing temperature, ${\mathrm{Cd}}_{I}$ is able to trap hole charge carriers resultant from the oxygen vacancies ${V}_{\mathrm{O}}$. 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 (${V}_{zz}$ and $\ensuremath{\eta}$) and the calculated formation energies for different defect complexes, the configuration of ${\mathrm{Cd}}_{I}$ with two (O2) vacancies (${V}_{\mathrm{O}2}$), 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.