Peculiar Bi-ion dynamics in Na1/2Bi1/2TiO3 from terahertz and microwave dielectric spectroscopy

Dynamics of the main dielectric anomaly in Na1/2Bi1/2TiO3 (NBT) was studied by time-domain THz and microwave spectroscopy, using also previously published data and their new overall fits. Above the dielectric maximum temperature Tm ~ 600 K, the response consists of coupled sub-THz oscillator and a relaxation mode, assigned to strongly anharmonic Bi-ion vibrations and hopping, whose slowing down explains the paraelectric-like permittivity increase to Tm. Below Tm, the main relaxation continues slowing down and additional relaxation, assigned to quasi-Debye losses, appears in the 10^11 Hz range. The oscillator hardens on cooling and takes over the whole oscillator strength. The permittivity decrease below Tm is caused by the reduced strength of the relaxations due to dominance of the rhombohedral phase within the coexistence region with the tetragonal phase. The anharmonic dynamics of Bi is supported by previous structural studies. NBT represents a hybrid between standard and relaxor ferroelectric behaviour

A neutron diffuse scattering study of PbZrO₃ and Zr-rich PbZr_1-xTi_xO₃

A combined neutron diffuse scattering study and model analysis of the antiferroelectric crystal PbZrO3is described. Following on from earlier X-ray diffuse scattering studies, supporting evidence for disordering of oxygen octahedral tilts and Pb displacements is shown in the high-temperature cubic phase. Excess diffuse scattering intensity is found at theMandRpoints in the Brillouin zone. A shell-model molecular dynamics simulation closely reproduces the neutron diffuse scattering pattern. Both in-phase and antiphase tilts are found in the structural model, with in-phase tilts predominating. The transition between disordered and ordered structure is discussed and compared with that seen in Zr-rich PbZr1−xTixO3.</jats:p

Diffuse scattering and local structure modeling in ferroelectrics

The excellent properties of modern ferroelectrics are the result of nanoscale local structure phenomena. Diffuse scattering, sensitive to the atomic correlations, is one of the few techniques that is able to provide detailed information on the local polar order. A review of diffuse scattering studies in ferroelectrics is given with special emphasis on those works where computer simulation techniques are used to analyse the experimental data. The examples comprise some well known materials, such as NaNO2 or BaTiO3, but also modern ferroelectrics: lead-based relaxors, bismuth pyrochlores and BaTiO3 related solid solutions

Monte Carlo and molecular dynamics simulation of disorder in the Ag + fast ion conductors Pearceite and Polybasite

The pearceite-polybasite group of minerals (i.e., pearceite, antimonpearceite, arsenpolybasite, and polybasite) of general stoichiometry [M 6I T 2III S7][Ag 9I CuIS4], with M = Ag+, Cu+ and T = As3+, Sb3+, occur relatively commonly in nature. All have recently been shown to exhibit Ag + fast ion conduction at rather low temperatures (only slightly above or below room temperature[1]). The average crystal structure determination of these materials shows the positions of the Ag+ ions to be smeared out or delocalized within sheets in an ordered framework structure comprised of the other ions. At the same time, strong and highly structured diffuse scattering has been observed, which contains diffuse peaks that are incommensurate with the diffraction peaks of the framework structure. In order to try to understand the origins of the fast ion conduction properties of these materials, we have used computer simulation of a model system (Monte Carlo and molecular dynamics (MD)) to analyze this observed diffuse scattering

Polarized cluster dynamics at the paraelectric to ferroelectric phase transition in BaTiO 3

The mechanism of the prototype ferroelectric phase transition in BaTiO 3 is a matter of intense debate and to a large extent still wrapped in mystery. Precursor phenomena in the form of polarized clusters in paraelectric BaTiO3 are by now believed to represent a key step into the ferroelectric phenomenon. The determination of a slower dynamics of cluster polarization flipping along with a faster order-disorder Ti hopping mode among 111 off-center sites suggests coexistence, instead of mutual exclusion, of displacive and order-disorder types, initially proposed as distinct models. However, no clear picture of the transition state has been proposed so far, which is able to provide insight into the coexistence of the paraelectric and ferroelectric phenomena. Here, by means of a dedicated molecular dynamics approach, we provide a detailed atomistic picture of intermediate regions along the transition. Therein, different time and length scales coexist as they characterize different portions of the same material. From an imbalance of dynamically and more statically polarized clusters in this highly inhomogeneous intermediate, a symmetry breaking step naturally results. Further, we find that ferroelectric nanodomains may host antiferroelectric defects, which appear as an intrinsic feature of the growing BaTiO3 ferroelectric material

Polar nanoregions and diffuse scattering in the relaxor ferroelectric PbMg 1/3 Nb 2 3 O 3

We present a fully atomistic model of polar nanoregions (PNRs) in the relaxor ferroelectric PbMg 1/3Nb 2/3O 3. Our molecular dynamics calculations reproduce both the characteristic form of the neutron diffuse scattering distribution and its temperature dependence. A shell model was used with a modified version of a published interatomic potential that was based on ab initio calculations. The parameters of this potential were optimized for the present work to provide a better description of the O atom interactions, as these are particularly important for neutron scattering. At high temperatures the Pb ions are displaced from their mean site positions in a direction that has an isotropic distribution, but at low temperatures the distribution condenses into eight localized sites displaced from the average position along each of the eight possible 111 directions. At intermediate temperatures (300 K) the distribution is cuboidal with some preference for 111 displacements but with all possible displacement directions present. Longitudinal correlations between the displacements of Pb-Mg/Nb and Pb-O increase monotonically in magnitude as the temperature changes from 700 K to 10 K with the sign of the Pb-O correlation being negative. At low temperatures this increase in correlation results in polar nanoregions that are clearly visible in plots of the local structure, although the exact form of these domains is more difficult to visualize. We show that the form of these PNRs can be revealed by an examination of conditional displacement distributions at low temperatures. Therein a strongly anisotropic cooperative displacement behavior is found. Remnants of this correlation pattern persist at much higher temperatures, but progressively a relatively smaller proportion of the Pb ions appears to be taking part and there is a substantially larger random component. It seems likely that the onset of the characteristic structured diffuse scattering at around 400 K coincides with the appearance of this cooperative displacement behavior

Assessing local structure in PbZn 1/3 Nb 2/3 O 3 using diffuse scattering and reverse Monte Carlo refinement

We use an extensive X-ray diffuse scattering dataset collected from the relaxor ferroelectric PbZn1/3Nb2/3O3 to study the feasibility of refining a nanoscale structure with the reverse Monte Carlo method. Six integer and non-integer reciprocal sections are used with a total number of nearly 105 symmetry-independent data points. Very good agreement between observed and calculated diffuse scattering patterns is achieved with rather subtle diffuse intensity modulations being satisfactorily reproduced. The correlations within the refined local structure are related to the possible physical mechanisms behind them. We discuss the ambiguity of the obtained results and feasible constraining schemes