Synthesis and structural study of hexagonal perovskites in the Ba5Ta4O15-MZrO3 (M = Ba, Sr) system

New ration-deficient perovskite-like oxides Ba5MTa4ZrO18 and Ba5MTa4Zr2O21 (M = Ba, Sr) were synthesized and studied by X-ray diffraction, electron di ffraction, and high-resolution electron microscopy (HREM). The Ba5SrTa4ZrO1 8 (space group R (3) over bar m, (hcccch), a = 5.80162(8) Angstrom, c = 42. 5403(8) Angstrom, R-1 = 0.039, R-p = 0.082) and Ba5Sr2Ta4ZrO21 (space group R (3) over bar m, (hccccch)(3), 5.7998(1) Angstrom, c 49.595(1) Angstrom, R-1 =0.033,R-p = 0.080)crystal structures were refined from X-rag powder di ffraction data, The results of structure refinements were confirmed by HREM observations. The formation of the Ba5Sr4Ta4Zr3O24 compound was revealed a nd the structure of this compound was deduced from HREM (space group P (3) over bar m1, hcccccch, a approximate to 5.8 Angstrom, c approximate to 18.9 Angstrom). All these compounds belong to the A(n)B(n-1)O(3n) homologous se ries of cation-deficient hexagonal perovskite-like oxides with n = 6, 7, an d 8 and may be formally described as intergrowth structures with alternatin g Ba5Ta4O15-type and Ba(Sr)ZrO3-type structural blocks along the c axis. (C )1998 Academic Press.