X-ray investigations of phase transition in Bi 2 TeO 5 single crystals

Authors [1] consider that bismuth tellurite Bi2TeO5 (BTO) possesses ferroelectric properties having undergone the second-type phase transition (PT) at the temperature about 780 ◦C. BTO has Abm2 orthorhombic lattice a=11.602 A, b=16.461 A, c=5.523 A according to [2]. The BTO single crystal belongs to the continuous solid solution group of Bi1−xTexO(3+x)/2 [3]. It is presumed that subcell distorted of fluorite type with the side about 5.5 A 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 Bi2TeO5 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 and the powder of the Bi2TeO5 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 Bi2O3 and TeO2 components 47 and 53 mole % respectively. The sample X-ray patterns were recorded with the DRON 2.0 difractometer (CoKα-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=11.616 A, b=16.451 A and c=5.524 A at room temperature and systematic extinctions for oriented plates taking into account non-centrosymmetry are stowed to the Abm2 space group.