Extended X-Ray Absorption Fine Structure study of Potassium Niobate
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EXTENDED X-RAY ABSORPTION FINE STRUCTURE STUDY OF POTASSIUM NIOBATE K. H. KIM*, W. T. ELAM, AND E. F. SKELTON Naval Research Laboratory, Washington, D. C., * NRC/NRL Cooperative Research Associate
20375
ABSTRACT Extended x-ray absorption fine structure (EXAFS) of potassium niobate (KNb0 3 ) was measured at the K edge of Nb, at various temperatures to include all phases of the material. The first shell data were analyzed to determine the distances between the niobium atom and its oxygen neighbors. The single 1 shell amplitude almost vanishes around k-7.5 Aat all temperatures, suggesting two distances with a separation of about 0.21 A. Nonlinear fitting was used to determine the structure more carefully and the data at all temperatures can be fit reasonably well with two Nb-O ditances. This is in disagreement with the displacive model which implies only one Nb-O distance in the cubic phase. This result suggests that the niobium atom is displaced along the (111) direction relative to the oxygen atom in all phases. Thus, the transitions are not displacive, but have a strong order-disorder character.
INTRODUCTION The structure of perovskite ferroectric materials in general and that of KNbO 3 in particular has been of much interest [1]. For many years, KNbO 3 was thought to be a displacive ferroelectric and its atomic positions in the unit cell have been determined by neutron diffraction [2]. But more recent studies began to question this description and suggested that the transition fits an order-disorder model [3,4,5] or a cross-over between the two models [6,7]. The determination of atomic positions within a unit cell with x-ray or neutron diffraction method depends heavily on a particluar model. To understand the structure of KNbO 3 better, we studied the material using the EXAFS technique. EXAFS does not depend on any long range order and it can determine the local environment around a particular kind of atom. Hence, EXAFS can answer the question of whether there is only one Nb-O distance in the cubic phase much more directly than x-ray or neutron diffraction method can. The number of Nb-O distances involved in other phases can be determined more directly, also. EXAFS has been used with success by 0. Hanskepetitpierre et al. to study the structure of KTN [8].
EXPERIMENT KNbO 3 was purchased from CERAC company and the lattice parameter was verified with x-ray diffraction. The results agreed with published values. For the low temperature measurements, the usual method of transmission EXAFS sample preparation was used [9]. Fine powder of potassium niobate was sprinkled to a sheet of adhesive tape. After excess powder was removed, the tape was cut into small pieces and several sheets were stacked together to get a proper thickness. However, the same sample can not be used for high temperature measurements as the tape cannot withstand the high temperature. For high temperature measurements, fine powder of potassium niobate was mixed with Sauereisen high temperature cement, which is in powder form. After they are mixe
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