Synthesis of the new high pressure Vernier phase Sr0.74CuO2

The strontium oxide–copper oxide binary phase diagram has been reinvestigated under oxygen pressures of up to 600 bar, in the temperature range 600–850 °C. A new, high-pressure phase, Sr0.74CuO2, has been isolated and characterised. The oxygen stoichiometry, confirmed by thermogravimetry, gives an average formal copper valence of ca. 2.5. X-Ray and electron diffraction analyses suggest that Sr0.74CuO2 has a Vernier structure incommensurate in b. The subcells are body-centred orthorhombic: aI= 6.8182(12)Å, bI= 3.7113(7)Å, cI= 11.0203(17)Å, and face-centred orthorhombic: aII= 6.8182(12)Å, bII= 2.7290(3)Å, cII= 11.0203(17)Å. The subcells combine into a coincidence (super)cell with b= 92.79(1)Å, which is 25bI and 34bII. The structure of one of the subcells appears to be related to that of Ca0.82CuO2 and NaCuO2. Sr0.74CuO2 is non-superconducting, at least above 4 K.