I~2La4TisOI8 crystallizes in the trigoml system ~ group R3 ) with the ~-cell parmaeters: a = 5.584 (1) A; c = 41.I76 (8)/~; Z= 3. The: structure has been solved from single crystal X-my diffr~ data to a final RI = 0.0285. Ba and La atcmm are twelve-fol~ coordinated and Ti atoms, six-fold eoordinamd. The structure can be described as consisting of identical perovskite-like Mocks, five tother-Sharing Ti06 ~ thi~ separated by layers of vacant ~a. ~ distoffion of the cation and ardon sublattices has been analysed and a ~ order has been evidenced, INTRODUCTI~ Corwf, lex oxides with cation-deficient perovskite-related stmerare and general fomL-da A,l~lO3n (rt.~3) have been synthesized in rramerous systems with AfCa, Sr, Ba, La and B=M÷II(Mg, Zn,~,Ni, Co);M+IH(B, Fe,. Ti} ; M +IV (Ti, Ru) ; M+V(Nb, Ta) ; M+VI(W, Re) [1-2I]. Most of them form a ~ogous series of trigona[ structures wh~ can be described as deriving from the perovskite structure by the periodic introduction of intrinsic stacking fanlts inthe c~c close packing of AO3 mixed layers. The voluminous A cations are twelve-fold coordinated. Wig.ln