Growth of single crystals of CdSe with oxygen, introduced by stoichiometric control to suppress the formation of native Se and Cd vacancies, generates oxygen centers replacing Cd (O Cd) rather than Se (O Se) as expected. This antisite substitution is unambiguously singled out by the host isotope fine structure of the nearest neighbor (NN) Se atoms in the localized vibrational modes (LVMs) of O Cd. When the stoichiometry control favors the formation of Cd vacancies, three infrared signatures gamma1, gamma2 and gamma3 appear ascribable to the LVMs of O Se in association with a Cd vacancy in the NN position as (O Se-V Cd) centers. Polarization measurements establish the monoclinic Cs symmetry for these centers. As a function of temperature, they display a remarkable two-step symmetry transformation, Cs-->C3v-->Td, due to the dynamic switching of the O Se-V Cd dangling bond.
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