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

Abstract

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, Sr_0.74CuO₂, 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 Sr_0.74CuO₂ has a Vernier structure incommensurate in b. The subcells are body-centred orthorhombic: a_I= 6.8182(12)Å, b_I= 3.7113(7)Å, c_I= 11.0203(17)Å, and face-centred orthorhombic: a_II= 6.8182(12)Å, b_II= 2.7290(3)Å, c_II= 11.0203(17)Å. The subcells combine into a coincidence (super)cell with b= 92.79(1)Å, which is 25b_I and 34b_II. The structure of one of the subcells appears to be related to that of Ca_0.82CuO₂ and NaCuO₂. Sr_0.74CuO₂ is non-superconducting, at least above 4 K.