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

Abstract

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.