T*-type substituted neodymium cuprates Nd_1.2Sr_0.8-xY(Ho, Er, Yb)_xCuO_4-δ: Crystal chemistry and electrical conductivity

The four T*-type solid solutions Nd_1.2Sr_0.8-xRE_xCuO_4-δ (RE = Y, Ho, Er, Yb) were synthesized and studied in terms of homogeneity range, structural study (neutron powder diffraction) and electrical conductivity. The homogeneity ranges have a small extent around the value x = 0.4. The main crystal chemical properties are as follows: a full ordering of Sr²⁺ and Y³⁺ (Ho, Er, Yb) in the Rocksalt (RS) and Calcium Fluoride (CF) slabs, respectively, Nd³⁺ distributed on both slabs; an overall oxygen deficiency δ in the range 0.15-0.18; oxygen vacancies distributed in the CF slabs (12%) and the equatorial `CuO<sub>2</sub>' planes; interstitial oxygen in the RS slabs (# Nd<sub>0.6</sub>Sr<sub>0.4</sub>O<sub>1.04</sub>). Electrical resistivity measurements (RT - 4 K) of samples annealed under p<sub>O(2)</sub> up to 1800 bars point to a `dirty metal' behavior up to 25 K (Y, Ho) and 70 K (Er), followed by a semiconductive regime at lower temperature. The big difficulty to get a superconductive transition in these phases and more generally in the T*-type cuprates is strongly dependent on the nature and the amount of small lanthanoidic cation in the CF slabs.