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Small electric-field-induced frequency shifts have been observed for resonant modes associated with three defect systems. For NaI: Clˉ, the shifts have been used to measure the quartic anharmonic terms of the interionic potential of the impurity ion. For KBr: Li+, the quartic anharmonic terms are found to be very small, and an harmonic potential which includes a central barrier with the barrier height less than the zero-point energy of the oscillator is required to explain the experimental results. For NaCl: Cu+, only an "on-center" resonant-mode configuration is consistent with the experimental results.