From Ferroelectric to Quantum Paraelectric: KTa 1-x Nb x O 3 (KTN), a Model System
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From Ferroelectric to Quantum Paraelectric: KTa1-xNbxO3 (KTN), a Model System George A. Samara Sandia National Laboratories Albuquerque, NM 87185-1421 ABSTRACT The ABO3 perovskite oxides constitute an important family of technologically important ferroelectrics whose relatively simple chemical and crystallographic structures have contributed significantly to our understanding of ferroelectricity. They readily undergo structural phase transitions involving both polar and non-polar distortions from the ideal cubic lattice. This paper focuses on the mixed perovskite system KTa1-xNbxO3, or KTN, which has turned out to be a model system. While the end members KTaO3 and KNbO3 might be expected to be similar, in reality they exhibit very different properties. Their mixed crystals, which can be grown over the whole composition range, exhibit a rich set of phenomena whose study has added greatly to our current understanding of the phase transitions and dielectric properties of these materials. Included among these phenomena are soft mode response, ferroelectric (FE)-to-relaxor (R) crossover, quantum mechanical suppression of the transition, the appearance of a quantum paraelectric state and relaxational effects associated with dipolar impurities. Each of these phenomena is discussed briefly and illustrated. Some emphasis is on the unique role of pressure in elucidating the physics involved. INTRODUCTION Ever since the discovery of ferroelectricity in BaTiO3 (about sixty years ago) the ABO3 perovskite oxides have been an important family of ferroelectrics whose relatively simple chemical and crystallographic structures have contributed significantly to our understanding of ferroelectricity. They are among the most technologically important ferroelectrics finding a broad range of applications such as transducers, actuators, positioners, and memory devices, based on both their piezoelectric and ferroelectric properties. They readily undergo structural phase transitions involving both polar and non-polar distortions from the ideal cubic lattice (Fig. 1).1,2 In the present paper we focus on the mixed perovskite system KTa1-xNbxO3, or KTN, as a model system. While the end members KTaO3 and KNbO3 might be expected to be similar, in reality they exhibit very different properties. Their mixed crystals, which can be grown over the whole composition range, exhibit a rich set of phenomena whose study has added greatly to our current understanding of the phase transitions and dielectric properties of these materials. Included among these phenomena are soft mode response, pressure-induced ferroelectric (FE)to-relaxor (R) crossover, classical-to-quantum crossover, and effects introduced by lattice defects. The paper touches briefly on these phenomena. As there is an intimate connection between ferroelectricity and lattice strain, a unique aspect of our approach is an emphasis on the use of pressure to study ferroelectric properties and get at the essential physics. The results presented will illustrate the power of this variable.
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