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Correlation between spin-phonon coupling and the magnetoelectric properties of and were investigated using ceramic samples in this work. The experimental results evidence a stronger spin-phonon coupling in than in , as well as a clear anomaly in the phonon contribution for the dielectric constant. This anomaly is located at the paramagnetic-incommensurate antiferromagnetic phase-transition temperature of , corroborating the existence of a magnetoelectric effect in this system. The absence of the magnetoelectric effect in is due to the existing disorder revealed in this compound by the striking thermal metastability and the large decrease in the activation energy associated with the dielectric relaxation processes. This kind of behavior has no counterpart in . In fact, it has been confirmed that the existence of spin-phonon coupling is not a sufficient condition …