From T to T′-La2CuO4 via Oxygen Vacancy Ordered La2CuO3.5

T-La2CuO4 can be transformed into T?-La2CuO4 via a two-step topotactic reaction mechanism. First T-La2CuO4 is reduced by CaH2 at 200 °C to La2CuO3.5 which can be subsequently reoxidized to T?-La2CuO4 at 300 °C in air. La2CuO3.5 does not adopt the Sr2CuO3 type ?S-phase? structure, as taken for granted for almost 20 years, but constitutes an oxygen deficient T?-framework, with copper in 4-fold planar and 2-fold linear dumbbell coordination. Upon heating the T-La2CuO4 phase is reobtained from T?-La2CuO4 above 650 °C, confirming the ?T-phase? to be the thermodynamically stable modification at least at higher temperature.