CRYSTALLOGRAPHIC EVOLUTION, T'-]T-ASTERISK-]T, IN PR2-XSRXCUO4-DELTA

In our comprehensive crystallographic investigation of the quasi-binary Pr2CuO4-Sr2CuO3 system, we observe the structural evolution T' --> T* --> T --> Sr2CuO3 phase with increasing Sr substitution in Pr2-xSrxCuO4-delta, that is distinctly different from the behavior in Sr-doped La2CuO4 or Nd2CuO4. From the results of structural refinement using neutron powder diffraction and valence study from PrL3 X-ray absorption, we find that the stabilization of the T* phase in Pr1.6Sr0.4CuO4 is associated with the preferential occupation of large Sr ions in the T-type half unit cell and the presence of Pr ions in the T'-type half unit cell having an average valence larger than 3+. Furthermore, the presence of large in-plane oxygen deficiency is related to the stabilization of the T structure in Pr1.0Sr1.0CuO3.6. Formal Cu valence in the T* or T compound, deduced from an oxygen stoichiometry study using thermogravimetric analysis and neutron structural refinement, is larger than 2+; however, neither superconductivity nor metallic behavior is observed.