The discovery of new materials with appealing functional properties is challenging and such properties are often observed in complex oxides containing transition metals in intermediate or unusual oxidation states which must be stabilised under special synthetic conditions. High-pressure synthesis conditions ("hard") is a great tool to lock instabilities, such as unusual oxidation states or coordination environments, into denser phases that are metastable when recovered to ambient conditions,

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... s leading to unusual properties. Manipulating and tuning the structures and physical behaviour of complex oxides by modifying their anion lattices has gain interest in recent years. This has been promoted by the use of binary metal hydrides as low temperature reducing agents ("soft" chemistry), thus allowing the access to unprecedented structures and unusual oxidation states, for instance as in SrFeO 2 . 1 Hence, it is attractive to explore combined "hard-soft" routes to novel materials by partially relieving the instability of a high-pressure precursor through post-synthesis modification. 2 We have demonstrated this "hard-soft" approach in the high pressure perovskites SrCrO 3 and CaCrO 3 . 15R-SrCrO 2.8 forms a long-period Cr 3+ /Cr 4+ charge-density wave which gives rise to a spin-density wave-type modulation of the magnetic moments with a long, doubled c-axis, periodicity (2c ≈ 69 Å). 2 CaCrO 3 reduction promotes oxygen vacancy ordering along the [001] c direction and generates two new CaCrO 3-δ phases (δ=0.3333 and 0.5). Both phases show alternating tetrahedral (T) and octahedral (O) layers in an OOT and OTOT sequence. 3 * aalopez@staffmail.ed.ac.uk