New data on the polymorphous transformations and T-x phase diagrams of Na_1-xLi_xNbO₃ and Na_1—xK_xNbO₃ solid solutions

Abstract

Na_1-x Li _x NbO₃ (0 x 0.006) and Na_1—x K _x NbO₃ (0 x 0.10) solid solution crystals have been grown from the flux. Dielectric, X-ray and optical studies of the crystals revealed the presence of new phase transitions in the temperature range from—50C to 200C. The results obtained imply the necessity of substantial correction of the known T-x phase diagrams of NaNbO₃-based solid solutions in the compositional range adjacent to pure NaNbO₃

Thus the dielectric, optical, and structural studies and the analysis of the corresponding published data indicate that, in addition to the six well-known phase transitions in NaNbO₃ one more phase transition occurs in the vicinity o150C which is close to a second-order phase transition and is assumed to take place between two antiferroelectric orthorhombic phases. Phase transition in the 150-200 temperature range occurs in NLN and NKN crystals as well. In the antiferroelectric-ferroelectric crossover compositional range one more first-order phase transition is observed at 30-100C both in NLN and NKN crystals. The existence of the phase transitions in NaNbO₃ and NaNbO₃-based solid solutions in the 30-200C temperature range (which falls within the working interval of numerous NaNbO₃-based materials) can produce a considerable effect on the temperature and time stability of various characteristics of these materials. This fact should necessarily be taken into account in designing new NaNbO₃-based materials for various purposes. In particular, it is important to refine the corresponding temperature-composition phase diagrams of NaNbO₃-based solid solutions and the theoretical models which describe the sequence ot phase transitions in NaNbO₃

Key Words:

• Ferroelectrics
• perovskites
• phase transitions
• solid solutions