Incommensurate phases in barium sodium niobate

Abstract

The x-ray-diffraction spectra of Ba2NaNb50&5 have been investigated by means of i precession camera between room temperature and 500'C, across the orthorhombic-tetragon il transition near Tt =300'C. Long exposures have revealed reflections, onsetting with the orthorhombic phase, which are incommensurate with respect to the periodicity of the basic crystal lattice.Referred to the pseudotetragonal lattice vectors, the modulation vector is k = (1+5)(a + b )/ 4+e /2 where 5 increases, on heating, from a value of 0.01 at room temperature, up to 0.115 at T~. Its variations exhibit an upward jump it Tlat=2'75'C from 0.035 to 0.085. These results show for the first time, the existence of two incommensurate phases in birium sodium niob ite.These incommensurate phases have the same modulation direction. They both h &ve &n iver &ge orthorhombic symmetry and are therefore ferroelastic on symmetry grounds. Above T~, precursor effects are disclosed in the form of strong diffuse sc ittering rods which condense it Tĩ nto the satellite reflections. Birefringence and DTA (differential thermal inalysis) me isurements are consistent with the x-ray results.

Incommensurate modulation structure in ferroelectric Ca0.28Ba0.72Nb2O6 single crystals of tungsten bronze structure

“…The occupational disorder has been identified as the cause of the structural modulation and related to the incommensurable character found in the TTB compounds. [7][8][9][10][11] It may be expected that the unusual dielectric properties of CBN and its phase-transition behavior are associated with disordered states of the incommensurate ͑ICS͒ tetragonal phase. Recently we observed normal-torelaxor ferroelectrics transition around 252°C and macrodomain-to-microdomain switching in CBN-28 crystals.…”

Study by high resolution electron microscopy and electron diffraction of the La0.03Sr0.255Ba0.7Nb2−yTiyO6−y/2 ferroelectric ceramic

“…8,11,13 ED patterns have revealed a superlattice through the ͓110͔ zone exhibiting structural variations respect to the TTB cell, promoted by the collective shearing of the oxygen octahedra, provoking a structural variation along c ͑supercell͒. The succession of reflections does not show equal separation in the superlattice plane, which gives the incommensurable character in the system LSBNT.…”

“…A fourth vector is needed, the modulation wave vector kϭ͓(1ϩ␦)/4͔a*ϩ͓(1ϩ␦)/4͔b*ϩ͓c*/2͔, that takes into account the incommensuration parameter ␦. 11 The extra spots are actually situated at positions ͓h,k,l͔͑1/4ϩ␦, 1/4ϩ␦, 1 2 ͒ and ͓h,k,l͔͑3/4ϩ3␦, 3/4ϩ3␦, 1 2 ͒.…”

“…This occupational disorder has been identified as the cause of the structural modulation and related with the incommensurable character found in the TTBs compounds. [8][9][10][11][12] Recently, the modulated phase and incommensurate structure of the modified Sr 1Ϫx Ba x Nb 2 O 6 ,Sr 1Ϫx Ba x Nb 2 O 6 compound and other TTB structures have been studied by means of HRTEM in combination with electron diffraction, 8 -11,13 since this combined experimental technique has the dual advantage of extracting the real and reciprocal space information.…”

Growth and Characterization of Centimeter‐Sized Ba2LaFeNb4O15 Crystals from High‐Temperature Solution under a Controlled Atmosphere

“…[47] Simon et al [48] showed that SBN crystals exhibit anisotropic dielectric behavior. They behave as relaxors along the [001] direction and as ferroelectrics along the [100] direction; such features are possibly related to the existence of structural modulations.…”