New Phases of YBaCuGeO Superconductors Identified from X-ray Diffraction and Infra-red Absorption Measurements

Authors: AHMED ABO-ARAIS, MOHAMAD AHMAD TAHER DAWOUD

Abstract: X-ray powder diffraction patterns and infra-red absorption spectra have been evaluated and analysed for the Y_1 Ba_2 Cu_3 O_{7-\delta} - Ge_x compound samples prepared by the solid state reaction with x values ranging from 0.0 to 1.13. All samples show bulk superconductivity above liquid nitrogen temperature using the levitation test (Meissner effect). Samples with Ge content up to x = 0.2 have offset T_c between 83K and 92K while the sample with x = 1.13 shows semiconducting behavior above 100K. As a result of the solid state interaction between YBCO and Ge, new phases are observed and determined, mainly three phases are concluded from X-ray powder diffraction analysis: (i) Ba2GeO4 (ii) Y2BaCuO5 (iii) BaCO3. The unit cell parameters a, b and c of the orthorhombic superconducting phase are calculated for all the prepared samples. The anisotropy factor is evaluated and related to the new structural phases in YBCO-Ge composite system. The I-R absorption spectra for the samples with orthorhombic symmetry have been determined. The phonon modes between ~ 400 cm^{-1} and 630 cm^{-1} are attributed to the Cu - O octahedron and pyramid vibrations for the CuO_2 -planes and CuO-chains, while the peaks in the range from ~ 700 cm^{-1} to ~ 860 cm^{-1} may be due to defects such as the new phase Ba2GeO4 and the green phase Y2BaCuO5. The obtained results are discussed according to the superconductor - semi-conductor composite model and with the phonon-mediated charge transfer between CuO_2 -planes and CuO- chains through apex oxygen (BaO).

Keywords: Superconductors, X-ray, Infra-red, YBCO, Ge.

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