High-Pressure Study of X-Ray Diffuse Scattering in Ferroelectric Perovskites

Abstract

We present a high-pressure x-ray diffuse scattering study of the ABO3 ferroelectric perovskites BaTiO3 and KNbO3. The well-known diffuse lines are observed in all the phases studied. In KNbO3, we show that the lines are present up to 21.8 GPa, with constant width and a slightly decreasing intensity. At variance, the intensity of the diffuse lines observed in the cubic phase of BaTiO3 linearly decreases to zero at ∼ 11 GPa. These results are discussed with respect to x-ray absorption measurements, which leads to the conclusion that the diffuse lines are only observed when the B atom is off the center of the oxygen tetrahedron. The role of such disorder on the ferroelectric instability of perovskites is discussed. Because of the simplicity of their structure and their actual or potential applications, perovskites ABO 3 and their derivatives are amongst the most studied compounds in solid state science. Perovskites stabilize a large variety of states such as superconductivity, ferroelectricity (FE) and multiferroism, but despite decades of studies, the mechanisms of these transitions are still a matter of debate [1]. This is the case of the oxygen perovskites such as SrTiO 3 and KTaO 3 , which only exhibit incipient FE, while PbTiO 3 , BaTiO 3 and KNbO 3 stabilize this instability in well defined series of FE transitions.Upon heating at ambient pressure, barium titanate and potassium niobate undergo the same famous sequence of rhomboedric-orthorhombic-tetragonal-cubic (R-O-T-C) first-order phase transitions [2,3]. During this sequence, the polarization changes direction from the [111], to the [110] and [100] pseudo-cubic directions, before vanishing above the Curie temperature T C in the cubic phase. The mechanism of this sequence of phase transition has been often discussed from two limiting cases: the soft mode -or displacive -theory, in which a transverse optical (TO) mode frequency gradually soften when approaching the phase transition [4], and an order-disorder scenario [5], in which some atoms lying on symmetryequivalent sites, order on preferential ones. In fact, current theories favor an intermediate situation, in which soft phonons and disorder play a role [6,7].Pressure has proven to be an essential parameter to study phase transitions in perovskites [8]. Because applying pressure results in a symmetrization of chemical bonds, drastic effects on order-disorder phase transitions are expected. Indeed, the same sequence of phase transitions is observed as a function of pressure in KNbO 3 and BaTiO 3 [9]. At 300 K, FE is lost at 10 GPa (2 GPa) and is no more observed at low temperature above ∼ 26 GPa (∼ 6.5 GPa) in KNbO 3 (BaTiO 3 ). The existence of the O-T transition under pressure has been questioned in KNbO 3 [10], while in other studies its transition pressure was found at 6 GPa [11] or 8.5 GPa [9]. In this letter, we will show that high pressure x-ray diffuse scattering measurements give a unique opportunity to study the transitions and the FE instability as a function of the nature and the position of...

Elastic softening and central peaks in BaTiO3 single crystals above the cubic-tetragonal phase-transition temperature

“…The microscopic nature of these polar precursors has recently been revealed by x-ray absorption fine structure 10 and nuclear magnetic resonance measurements 11 to be composed of dynamic off-center displacements of Ti ions along eight equivalent ͗111͘ directions, consistent with the original eight-site model suggested for the order-disorder component in BaTiO 3 . 4 However, although the disorder at the B-site ions is expected to enhance the ferroelectric instability, 12 exact roles and microscopic natures of displacive and order-disorder components during the phase transition are still controversial. In addition, the relation of two-mode behaviors, recently revealed by a first-principles-based calculation and far-infrared reflectivity measurements, 13 to these two phase-transition components remains far from complete understanding.…”

Transitions de phase structurales

“…D'abord parce que les progrès en matière de techniques expérimentales permettent occasionnellement de « revisiter » des problèmes non résolus [61][62][63][64][65]. Et ensuite parce que les « nouveaux » matériaux s'avèrent aussi riches en instabilités structurales de toutes sortes, que les autres et il est toujours souhaitable, même si cela ne constitue pas un objet de recherche en soi, de savoir identifier les mécanismes qui donnent lieu à ces changements de structure, ne serait-ce que pour mieux comprendre en quoi ils peuvent affecter les propriétés « utiles » des dits matériaux.…”

Characterization of Pb-Free KNbO3- and (Na,Bi)TiO3-Based Piezoelectric Single-Crystals Using X-ray and Neutron Diffraction

“…Using single-crystal diffuse scattering measurements with laboratory X-rays, the 1-dimensional correlations among B-atom displacements in BaTiO 3 and KNbO 3 were earlier estimated to be between 50 Å and 100 Å [54,55]. More recent measurements with high-energy X-rays provide estimates of the order of 2 nm [56,57], which is closer to that predicted from theoretical calculations [58]. In addition to 1-dimensional correlations, 2-dimensional correlations in the form of nanoscale polar clusters among B-atom off-center displacements are also possible [59,60].…”

“…Most remarkably, in situ diffraction with synchrotron X-rays have shown that the nanoscale correlations for atomic displacements in both BaTiO 3 and KNbO 3 are highly susceptible to applied pressure and electric fields. The susceptibility of B-site atomic displacements on applied pressure was studied by Ravy et al from in situ high-energy X-ray diffraction experiments [56]. They showed that the intensity of the diffuse scattering lines, as described above, are strong functions of the magnitude of applied pressures.…”