The phase transitions and crystal structures of Ba₃RM₂O_7.5 complex oxides (R = rare-earth elements, M = Al, Ga)

The structures of α-Ba₃RAl₂O_7.5 and β-Ba₃RM₂O_7.5 complex oxides (R = rare-earth elements, M = Al, Ga) have been studied by a combination of X-ray diffraction, electron diffraction (ED) and high-resolution electron microscopy (HREM). The α and β forms have cell parameters related to the perovskite subcell: a = 2a_per, b = a_per(2)^1/2, c = 3a_per(2)^1/2, however, the α form has an ortho­rhombic unit cell whereas the β form adopts monoclinic symmetry. The crystal structure of monoclinic Ba₃ErGa₂O_7.5 was refined from X-ray powder data (space group , a = 7.93617 (9), b = 5.96390 (7), c = 18.4416 (2) Å, β = 91.325 (1)°, R_I = 0.023, R_P = 0.053), the structure of the α form (space group Cmc2₁) was deduced from ED and HREM data. The important feature of the α and β structures is the presence of slabs containing strings of vertex-sharing tetrahedral Al₂O₇ pairs. Two almost equivalent oxygen positions within the strings can be occupied either in an ordered manner leading to the low-temperature β phase or randomly resulting in the high-temperature α structure. The critical temperature of this order–disorder phase transition was determined by high-temperature X-ray diffraction and by differential thermal analysis (DTA). In situ ED and HREM observations of the second-order phase transition confirmed the symmetry changes and revealed numerous defects (twins and antiphase boundaries) formed during the phase transformation.