Cubic-tetragonal-orthorhombic-rhombohedral ferroelectric transitions in perovskite potassium niobate: neutron powder profile refinement of the structures

Abstract

The orthorhombic and rhombohedral structures of ferroelectric potassium niobate have been determined by neutron diffraction measurements on powdered crystals, using the profile refinement technique to analyse the data. In the orthorhombic phase at 22 "C the atomic displacements along [Oll] from the cubic perovskite positions are 6(K) = -0.006 + SA, 6(Nb) = 0,070A, S ( 0 , ) = -0.138 + 6 A and 6(0,,) 6(0,,,) = -0,125 + 4A. In the rhombohedral phase at 230K, the displacements are along [lll]; 6(K) = -0,013 i 32A, 6(Nb) = 0.078A and 6(0,) = 6(0,,) = S(O,,,) = -0,140 + 4A. As a check on the profile technique, the tetragonal structure was redetermined, and good agreement obtained with the previous single-crystal work, with displacements along [OOl] of S(K) = -0,014 + 26A, 6(Nb) = 0.063A, S ( 0 , ) = -0.117 + 4 A and 6(0,,) = 6(0,,,) = 0099 + 3A. All of these transitions result from the condensation at q = 0 of soft modes in which almost rigid oxygen octahedra vibrate against the potassium and niobium atoms. Our results account for the observed changes in the spontaneous polarization of potassium niobate, and demonstrate the power of the profile refinement technique for the investigation of structural phase transitions.

High-temperature phases of SrZrO3

“…In an earlier paper (Ahtee, Ahtee, Glazer & Hewat, 1977) the structure of SrZrO 3 at room temperature was reported, solved by the neutron powder profile refinement technique (Rietveld, 1969;Hewat, 1973). In this paper we report the structures of the two hightemperature phases at 760 and 900°C and also consider the possible soft modes associated with the corresponding transitions.…”

“…1) extend to sin 0/2 = 0.71 ,/~-~, beyond the limit for Cu Ka X-radiation. In addition, experience with powder refinements on other materials such as K(Ta,Nb)O 3 (Hewat, 1972(Hewat, , 1973Abrahams & Bernstein, 1974) and NHaH2PO 4 , does seem to show a consistency in the orientation of the thermal ellipsoids; if they are very anisotropic they probably indicate a soft vibrational mode which may condense to produce a phase transition at lower temperatures. We shall therefore attempt to understand our temperature factors in terms of the softmode picture postulated above, especially where the ellipsoids are very anisotropic.…”

First-principles and semiempirical calculations forFcenters inKNbO3

“…The atomic positions in all these phases have been determined experimentally. 24 Under these phase transitions the local symmetry of the O vacancy also changes, which can, in principle, affect the optical properties of the F centers. This problem has never been addressed.…”

A neutron powder investigation of the structural changes in K2SnCl6

“…Since single-crystal methods suffer from experimental difficulties and the occurrence of domains (Boysen et al, 1976) neutron powder diffraction has been chosen for the determination. The profile analysis technique as introduced by Rietveld (1967) and modified for anisotropic thermal parameters by Hewat (1973) was used to distinguish between possible structural models and to refine the parameters. Following Jeffrey (1972) the three different phases of KESnCI 6 will be named I (T < Tc2), II (Tc2 < T < T~) and III (T > 7",.…”