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 and 75°C have been investigated in (K0.95Li0.05)(Nb1–x Ta x )O3 (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°C, 3.0 kV/cm, and 30.3°C, 2.0 kV/cm, respectively. The tricritical point at which the first-order phase transition changes to the second-order phase transition has been estimated at 10.4℃ 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.