PRASEODYMIUM VALENCY FROM CRYSTAL-STRUCTURE IN PR-BA-CU-O AND (Y-PR)-BA-CU-O SINGLE-CRYSTALS

The substitution of Pr for Y leads to materials with a general formula (Y1-vPrv) (Ba2-x′Prx′) (Cu3-yXy)O6+x′/2-y+z and with a structural transition around x = v + x′ ≈ 0.5. For x ≤ 0.5 the crystals are orthorhombic, Y-Ba-Cu-O type, but with a lowered distortion. For v ≥ 0.5 the crystals are tetragonal, La1.5Ba1.5Cu3O7±z type, with the characteristic tri-twinning of this phase. The Pr valency, in the range 3-3.2+ depending on preparation conditions, is determined from interatomic distances. Orthorhombic crystals of PrBa2Cu3-yXyOz prepared at high temperatures exhibit a high amount of defects, y ≈ 0.25 on the Cu(1) site and are semiconductors with a T- 1/4 activation law attributed to the praseodymium valence fluctuation.