  The giant and negative dielectric tunability of Pb(Fe1/2Nb1/2)1-xTixO3 single
crystals is reported. A low field of 120 V/cm can induce a great reduction of
the capacitance, and the tunability is larger than 80% in low frequency range
(<1 MHz) at room temperature. This giant tunability is ascribed to the
interfacial polarization at the interface of electrode/sample. A negative
dielectric tunability detected only in the tetragonal sample can be also
attributed to the interfacial polarization. The origin of the giant and
negative tunabilities is discussed with the multipolarization-mechanism model
and equivalent circuit model, respectively.
