Cation-Deficient Perovskite-Related (Ba,La)nTin-DELTAO3n (n>4DELTA) Microphases in the La4Ti3O12–BaTiO3 System: An HRTEM Approach

The large nonstoichiometric domain, observed in the La4Ti3O12-rich part of the La4Ti3O12–BaTiO3 system, has been shown to correspond to a continuous series of cation-deficient perovskite-related (Ba,La)nTin-O3n (n4) microphases. The crystal structures of these microphases have been analyzed by electron diffraction and high-resolution electron microscopy. They can be described as coherent intergrowths of P-perovskite-like blocks La4Ti3O12 and Q-perovskite-like blocks BaLa4Ti4O15, respectively, constitutive of the simple basic terms n=4 and n=5 of the previously identified (Ba,La)nTin-1O3n series. The corresponding BaQLa4(P+Q)Ti3P+4QO12P+15Q microphases can thus be denoted by 4P5Q in a compact form. In fact, only the 4P51 intergrowth sequences have been observed. Eight of these are clearly shown by direct imaging. No intergrowth terms were observed for compositions ranging from BaLa4Ti4O15 (n=5) to Ba2La4Ti5O18 (n=6). The ability of the system to develop coherent intergrowths has been discussed in the light of information provided by an accurate analysis of crystal structures of the basic members of the series: La4Ti3O12 (n=4), BaLa4Ti4O15 (n=5), and Ba2La4Ti5O18 (n=6).