Field-induced effects and ferroelectric critical endpoint in (K0.95Li0.05)(Nb1-x Ta x )O3 single crystals

Dielectric permittivities under dc biasing fields in the temperature range between -169 degrees C and 75 degrees C have been investigated in (K0.95Li0.05)(Nb1-xTax)O-3 (KLNT-x, x = 71% and 74%). Field-induced ferroelectric phase transitions in KLNT-x have been clarified in the dc field applied along the [001](c) direction (in the cubic coordinate). The ferroelectric critical endpoints in KLNT-71% and KLNT-74% have been determined to be 43.5 degrees C, 3.0 kV cm(-1), and 30.3 degrees C, 2.0 kV cm(-1), respectively. The tricritical point at which the first-order phase transition changes to the second-order phase transition has been estimated at 10.4 degrees C and x = 77.4% in the temperature-concentration phase diagram. The field-induced phase transitions and dielectric tunabilities have been discussed on the basis of the Landau-type free energy density.