A study of the unit-cell dimensions and symmetry of certain ferroelectric compounds of niobium and tantalum at room temperature

Abstract

An account is given of the methods of preparation of the recently reported ferroelectric compounds KTaO3, KNbO3, NaNbOs and NaTaO3, and of an investigation of their structure from powder photographs. At room temperature KTaO3 is cubic and the other compounds orthorhombic, the distortion from the simple perovskite structure being similar to that which BaTiO3 shows between-5 and 90 ° C..From an examination of the variation of Curie point with lattice parameter of these compounds, it is suggested that the factor primarily responsible for the ferroelectricity is the degree of homopolar bonding of the tantalum, niobium or titanium ion with the oxygen ion. Introduction In a recent note Matthias (1949) reported that the niobates and tantMates of sodium and potassium, which are pseudo-isomorphous with BaTi08, exhibit ferroelectric properties. These salts gave ~ piezoelectric response and showed dielectric hysteresis loops, with peaks in their dielectric constant versus temperature curve corresponding to changes in crystal symmetry. These changes in symmetry were observed by studying crystals, of 1-2mm. cube edge, with the polarizing microscope. The room-temperature symmetry of the compounds was given as pseudo-tetragonal from a study of the X-ray powder photographs, with an axial ratio of _~ 0.99 for the niobates, and rather closer to unity for the tantalates. The observations described below, which have been previously briefly reported (Vousden, 1951), were undertaken in order to make accurate determinations of the unit-cell dimensions of these compounds.