X-RAY INVESTIGATIONS OF PHASE TRANSITION IN Bi_2TeO_5 SINGLE CRYSTALS

Abstract

This work is devoted to researching a phase transition in bismuth tellurite single crystals using a high-temperature X-ray analysis method. The cell parameter step-wise changes at 805• C are revealed. Starting from the data obtained we can say that the first-type transition takes place in bismuth tellurite. Key words: phase transition, bismuth tellurite, X-ray, diffraction maxima PACS: 61.50.KAuthors [1] consider that bismuth tellurite Bi 2 TeO 5 (BTO) possesses ferroelectric properties having undergone the second-type phase transition (PT) at the temperature about 780• C. BTO has Abm2 orthorhombic lattice a=11.602Å, b=16.461Å, c=5.523Å according to [2]. The BTO single crystal belongs to the continuous solid solution group of Bi 1−x Te x O (3+x)/2 [3]. It is presumed that subcell distorted of fluorite type with the side about 5.5Å is a basis of this solid solution structure. The unit-cell is formed by a multiplicity along cell axes 2x3x1 respectively. The temperature studies of changing cell parameters and symmetry have not been carried out earlier. Therefore the aim of this work is X-ray research of Bi 2 TeO 5 single crystals in a wide temperature range including the area of phase transition supposed.The (100), (010) and (001) single crystal plates oriented and polished with the size of 10x15x2 mm 3 and the powder of the Bi 2 TeO 5 single crystals grown were used. The single crystal boules of bismuth tellurite were prepared as in the work [4] from the melt by the Czochralski technique at the starting ratio of the Bi 2 O 3 and TeO 2 components 47 and 53 mole % respectively. The sample X-ray patterns were recorded with the DRON 2.0 difractometer (Co Kα -radiation filtered). For the hightemperature measurements the UVD-2000 accessory was used. In table 1 the positions and the relative intensities of X-ray maxima from the BTO single crystal powder are presented at room temperature. The difractograms of BTO single crystal cuts show a=11.616Å, b=16.451Å and c=5.524Å at room temperature and systematic extinctions for oriented plates taking into account non-centrosymmetry are stowed to the Abm2 space group.

High Porosity in Nanostructured n-Type Bi2Te3 Obtaining Ultralow Lattice Thermal Conductivity

“…However, after the SPS process, diffraction peaks of as-sintered pellet can only be indexed as the rhombohedral structured Bi 2 Te 3 . The disappearance of Bi 2 TeO 5 during the sintering process should be attributed to the sublimation of Bi 2 TeO 5 under SPS heat treatments and large currents . The XPS was further utilized to verify the sublimation of Bi 2 TeO 5 during the SPS process.…”

“…The disappearance of Bi 2 TeO 5 during the sintering process should be attributed to the sublimation of Bi 2 TeO 5 under SPS heat treatments and large currents. 27 The XPS was further utilized to verify the sublimation of Bi 2 TeO 5 during the SPS process. The full XPS spectra of both Bi 2 Te 3 nanopowder and nanostructured Bi 2 Te 3 pellet (shown in Figure S2) indicate the presence of Bi 4f and Te 3d energy states, without any energy states detected for other elements, except C 1s and O 1s.…”

“…Here, we prepared nanostructured n -type Bi 2 Te 3 with porous structure via a facile solvothermal method and subsequently sintered them into pellets through the sparking plasma sintering (SPS), as schematically shown in Figure . During the sintering process, the impurity Bi 2 TeO 5 was sublimed under the heating environment and large plasma currents . Consequently, nanostructured Bi 2 Te 3 with porous structure was fabricated.…”

“…During the sintering process, the impurity Bi 2 TeO 5 was sublimed under the heating environment and large plasma currents. 27 Consequently, nanostructured Bi 2 Te 3 with porous structure was fabricated. With the synergistic effects of both pores and nanostructure, strong phonon scatterings were induced, leading to ultralow κ l < 0.1 W m −1 K −1 at 420 K. Consequently, a promising zT of 0.97 was achieved at 420 K, which is among the highest values reported for pure n-type Bi 2 Te 3 .…”