The (Na1 - x Kx )0.5 Bi0.5 TiO3 perovskite solid solution is investigated using x-ray diffraction (XRD) and Raman spectroscopy in order to follow the structural evolution between the end members Na0.5 Bi0.5 TiO3 (rhombohedral at 300 K) and K0.5 Bi0.5 TiO3 (tetragonal at 300 K). The Raman spectra are analysed with special regard to the hard modes and suggest the existence of nano-sized Bi3+ TiO3 and (Na1 - 2x K2x )+ TiO3 clusters. The complementary use of XRD and Raman spectroscopy suggests, in contrast to previous reported results, that the rhombohedral tetragonal phase transition goes through an intermediate phase, located at 0.5 x 0.80. The structural character of the intermediate phase is discussed in the light of sub- and super-group relations.

https://iopscience.iop.org/article/10.1088/0953-8984/12/14/305/pdf

An x-ray diffraction and Raman spectroscopy investigation of A-site substituted perovskite compounds: the (Na1-xKx)0.5Bi0.5TiO3 (0 x1) solid solution

With the purpose of fabricating ceramics where ferroelectric and magnetic order coexist, ceramics of Pb(Fe1∕2Nb1∕2)O3 have been prepared using the traditional ceramic method following three different routes. The first is a direct via starting from oxide reagents and the other two use different kinds of FeNbO4 precursors with either monoclinic or orthorhombic structures. Crystallographic and surface morphological studies were carried out by the powder x-ray diffraction and scanning microscopy techniques. The presence of Fe2+, detrimental to the ferroelectric and magnetic performance, was evaluated by x-ray photoelectron spectroscopy. The samples showed no structural differences, uniformly distributed grains, a ferro-paraelectric transition temperature at 110°C and a normal diffuse phase transition (nonrelaxor behavior). Differences in the degree of diffuseness, densities and grain size were observed depending on the kind of precursor. Measurements of dc and ac electrical resistivity, dielectric constant an...

Frequency-temperature response of ferroelectromagnetic Pb(Fe1∕2Nb1∕2)O3 ceramics obtained by different precursors. Part I. Structural and thermo-electrical characterization

Ferroelectric BaFe0.5Nb0.5O3 (BFN) ceramic is synthesized by the solid-state reaction technique for the first time. The x-ray diffraction of the sample at room temperature shows a monoclinic phase. Dielectric studies of the sample show a frequency dependence of the temperatures at which the dielectric permittivity (real and imaginary) peaks. The temperature variations of the real and imaginary components of the dielectric permittivity show broad maxima. There is evidence for Vogel-Fulcher-type relaxational freezing. The analysis of the real and imaginary parts of the dielectric permittivity with frequency has been performed assuming a distribution of relaxation times as confirmed by Cole-Cole plots as well as the scaling behaviour of the dielectric loss. All these observations clearly suggest that BFN is a relaxor ferroelectric. The Mossbauer spectrum of the sample at room temperature shows a symmetric doublet, with the iron being trivalent.

https://iopscience.iop.org/article/10.1088/0953-8984/14/2/311/pdf

Structural and dielectric studies of BaFe0.5Nb0.5O3

The magnetic and dielectric properties of ferroelectromagnet (FEM) Pb(Fe1/2Nb1/2)O-3 (PFN) single crystals have been investigated. Anomalies in the dielectric constant as a function of temperature have been observed near its Neel temperature of similar to143 K, which are indicative of the so-called magnetoelectric coupling between the ferroelectric and antiferromagnetic orders in this compound. The results coincide with previous theoretical predications. It is demonstrated that a weak magnetic transition appeared at a temperature a little below the Neel temperature, confirming the earlier prediction from the Monte Carlo simulation in a qualitative sense. The magnetic hysteresis measurement proves that there does exist weak magnetic order in PFN crystal below its Neel temperature. The Mossbauer spectra also support the existence of the magnetic order.

Magnetoelectric coupling in ferroelectromagnet Pb ( Fe 1 ∕ 2 Nb 1 ∕ 2 ) O 3 single crystals

We have carried out a detailed Rietveld analysis of x-ray powder diffraction data of Pb(ZrxTi1−x)O3 (PZT) compositions across the morphotropic phase boundary (MPB) region (x=0.515, 0.520, 0.525, 0.530). It is shown that the structure of PZT is pure tetragonal for x⩽0.515 with space group P4mm. In the MPB region, 0.515<x<0.530, the tetragonal and monoclinic (space group: Cm) phases are found to coexist as a result of a first order phase transition between the low temperature monoclinic and high temperature tetragonal phases. Further, arguments are advanced to show that the hitherto believed rhombohedral structure (FRHT) of PZT for 0.530⩽x⩽0.62 is more likely to be monoclinic.

Vincent Bonny

Papers

Phase transitions in disordered lead iron niobate: X-ray and synchrotron radiation diffraction experiments