Acta Crystallographica Section A

Foundations of Crystallography

Volume 55, Part 5 (September 1999)

Acta Cryst. (1999). A55, 828-839    [doi:10.1107/S0108767399002068]

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

A. M. Abakumov, R. V. Shpanchenko, O. I. Lebedev, G. Van Tendeloo, S. Amelinckx and E. V. Antipov

Abstract: The structures of -Ba₃RAl₂O_7.5 and -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 and 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 form has an orthorhombic unit cell whereas the 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, = 91.325  (1)°, R_I = 0.023, R_P = 0.053), the structure of the form (space group Cmc2₁) was deduced from ED and HREM data. The important feature of the and 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 phase or randomly resulting in the high-temperature 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.

Online 1 September 1999

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