SYNTHESIS OF THE NEW HIGH-PRESSURE VERNIER PHASE SR0.74CUO2

The strontium oxide-copper oxide binary phase diagram has been reinves tigated under oxygen pressures of up to 600 bar, in the temperature ra nge 600-850 degrees C. A new, high-pressure phase, Sr0.74CuO2, has bee n isolated and characterised. The oxygen stoichiometry, confirmed by t hermogravimetry, gives an average formal copper valence of ca. 2.5. X- Ray and electron diffraction analyses suggest that Sr0.74CuO2 has a Ve rnier structure incommensurate in b. The subcells are body-centred ort horhombic: a(I) = 6.8182(12) Angstrom b(I) =3.7113(7) Angstrom, c(I) = 11.0203(17) Angstrom, and face-centred orthorhombic: a(II)=6.8182(12) Angstrom, b(II)=2.7290(3)Angstrom, c(II)=11.0203(17)Angstrom. The subc ells combine into a coincidence (super)cell with b= 92.79(1) Angstrom, which is 25b(I) and 34b(II). The structure of one of the subcells app ears to be related to that of C0.82CuO2 and NaCuO2. Sr0.74CuO2 is non- superconducting, at least above 4 K.