The goal of this contribution is

The goal of this contribution is to fill the existing gap in the structural studies. In the first stage, we have focused our attention on examining the features of the temperature evolution of the structure of the high-temperature phase of solid (1–x)Pb(Fe2/3W1/3O3)–(x)PbTiO3 solutions in the MPB region. Unlike previous studies, the composition–temperature phase diagram was scanned along the temperature axis (the vertical dashed lines in Fig. 1).
Samples and measurement procedure The PFW and PFW–PT samples were fabricated by E.A. Dyadkina (Skripchenko) at the Voronezh State Technical University by the standard ceramic technology using double sintering [7]. PbFe2/3W1/3O3 and PbTiO3 powders were synthesized in advance at temperatures of 1050 and 920K, respectively, in air for 3 h. The synthesized material was then thoroughly dry-milled and mixed in the required proportions. The resulting powder mixture was granulated. The final sintering stage was carried out at temperatures of 1170 and 1200K for x = 0.2 and 0.3, respectively, also in air for 3 h. Next, the obtained ceramic samples were ground into fine (∼1μm) powders that were poured into cylindrical vanadium containers. The diffraction spectra were measured by a high-resolution E9 neutron diffractometer and a high-intensity E2 diffractometer (Helmholtz Zentrum Berlin (HZB), the BER II reactor). The incident neutron wavelength was 1.79Å in the first case and 1.21Å in the second case. The samples in vanadium containers were placed in a cryogenic oven that allowed performing measurements in the temperature range from 90 to 500K; the temperature was maintained with a stability no worse than ±2K. The resulting diffraction patterns were processed with the standard full profile analysis program FULLPROF [8].
Results and discussion The first stage involved testing whether the samples obtained were actually solid PFW–PT solutions rather than a mechanical mixture of these compounds. Since it is known that the Pb(Fe2/3W1/3)O3 sphingosine 1-phosphate receptor modulator is in the cubic phase (Pmm) and PbTiO3 is in the tetragonal phase (P4mm) in the high-temperature range (300–500K), the diffraction profiles were described in a two-phase approximation within the mechanical mixture model. Data processing has revealed that the obtained diffraction patterns cannot be adequately described by the hypothesis of the mechanical mixture of the initial components, in particular, Rf = 76% and Rf = 16%, where Rf and Rf are the R-factors for the tetragonal and the cubic phases, respectively. At the same time, using the model of the solid solution of stoichiometric composition in the cubic phase with the space group Pmm lead to a significant improvement of the R-factor (Rf = 3.8%). Fig. 2 shows a typical diffraction pattern for the (0.8)PFW–(0.2)PT compound, obtained at a temperature of 345K. The dots represent the experimental results, and the lines the approximation of the experimental data within the model of the solid solution of stoichiometric composition. The line at the bottom of the figure is the residual between the approximation and the experimental data. Primary analysis of the temperature evolution of the diffraction patterns identified two groups of peaks, (hkl), for which at least two indices are different, and (hhh), with fundamentally different temperature behavior: additional shoulders (or wings) emerging and subsequent splitting are observed with a decrease of temperature for the (hkl) peaks, while the width and the shape of the (hhh) peaks remain unchanged. Fig. 3 shows as an example the temperature evolution of the (310) and (222) reflections of the solid solution of the (1−x)PFW–(x)PT compound at х = 0.2. For comparison, the same figure also shows the instrumental profile of the peak intensity, calculated for the temperature T = 345K within the space group Pmm without taking into account the size effects and the elastic stresses. The (310) peak was selected to represent the (hkl) group, as it is located at large values of 2θ and therefore small changes in the structural factor lead to significant distortions of this reflection. The (222) peak was selected for comparison as a peak of the (hhh) type because it is located the closest to (310). All peak intensities were normalized to unity making it possible to analyze the shapes of the peaks.