Dielectric and Optical Studies of the Low-Temperature Phase in NaNbO3: Effect of Manganese Dopant

Pure and Mn-doped NaNbO3 crystals have been investigated dielectrically between 380 and 20 K. Below room temperature, no phase transition is observed in pure sodium niobate. With increasing Mn content, a structural phase transition is induced around 140 K. The dielectric constant shows a maximum for the sample with 0.7 wt.%, and shows large thermal hysteresis. In the low-temperature region, ferroelectric activities could not be detected in either pure or lightly doped NaNbO3. The phase diagram of T c vs the concentration of Mn is obtained. Domain motion associated with this phase transition is observed around the transition temperature. This evidence proved that the low-temperature phase is induced by light doping with Mn ions. The uniaxial stress decreases T c; dT c/dσ=-80 K/kbar on heating and -600 K/kbar on cooling.