Average and local atomic-scale structure in BaZr_xTi_1−xO₃(x= 0.10, 0.20, 0.40) ceramics by high-energy x-ray diffraction and Raman spectroscopy

Abstract

High-resolution x-ray diffraction (XRD), Raman spectroscopy and total scattering XRD coupled to atomic pair distribution function (PDF) analysis studies of the atomic-scale structure of archetypal BaZrxTi(1-x)O3 (x = 0.10, 0.20, 0.40) ceramics are presented over a wide temperature range (100-450 K). For x = 0.1 and 0.2 the results reveal, well above the Curie temperature, the presence of Ti-rich polar clusters which are precursors of a long-range ferroelectric order observed below TC. Polar nanoregions (PNRs) and relaxor behaviour are observed over the whole temperature range for x = 0.4. Irrespective of ceramic composition, the polar clusters are due to locally correlated off-centre displacement of Zr/Ti cations compatible with local rhombohedral symmetry. Formation of Zr-rich clusters is indicated by Raman spectroscopy for all compositions. Considering the isovalent substitution of Ti with Zr in BaZrxTi1-xO3, the mechanism of formation and growth of the PNRs is not due to charge ordering and random fields, but rather to a reduction of the local strain promoted by the large difference in ion size between Zr(4+) and Ti(4+). As a result, non-polar or weakly polar Zr-rich clusters and polar Ti-rich clusters are randomly distributed in a paraelectric lattice and the long-range ferroelectric order is disrupted with increasing Zr concentration.

Chemical and structural effects on the high-temperature mechanical behavior of (1−x)(Na1/2Bi1/2)TiO3-xBaTiO3 ceramics

“…It is very well known that Raman spectroscopy is sensitive to the short-range, i.e., few unit cells scale, and thus can detect structural features that are overseen by laboratory-scale XRD. 35 As can clearly be seen from Fig. 9, a Raman spectrum has been detected above T C /T M for all compositions, although-in a cubic centrosymmetric phase-no Raman activity is expected.…”

“…This, notwithstanding the room-temperature phase of NBT-75BT, maintains its ferroelectric character. In fact, the mode at 715 cm À1 is prominent at low temperatures and disappears (albeit in a diffuse manner) with increasing T. 35 A full relaxor state, i.e., for the whole temperature range, would have implied here measuring a similar spectrum for all temperatures. 42,46,47 Shvartsman and Lupascu 24 reported relaxor behavior for the x ¼ 0.75 composition in a certain temperature range (185-225 C).…”

“…These two features, together with the peak at $715 cm À1 , are generally regarded as the signature of the ferroelectric phase in BT. 35,42,43 Thus, it is evident from Figure 8 that pure BT is in its tetragonal ferroelectric phase;…”

“…The prominence of the mode at 715 cm À1 , however, suggests room-temperature ferroelectricity for this composition. 35 The dip at 180 cm À1 is also not visible for x ¼ 0.75; this is probably due to the appearance of a strong A-site mode centered at $170 cm À1 . This mode, already appearing in NBT-85BT but with lower intensity, has already been observed in other NBT-based materials, and has been ascribed to clustering on the A-site.…”

“…Moreover, in substituted BT, relaxation of the selection rules due to compositional disorder further broadens the spectral signature. 35 Thus, in this work, we follow a reasoning based on the ion mass and the position of substituent cations within the perovskite unit cell in order to identify the vibrational complex giving rise to a particular Raman peak in the spectrum. This analysis takes into account also the possibility that clusters rich in one substituent exist, and can be included in the concept of "hard-mode spectroscopy."…”