Calorimetric investigation of NdBa2Cu3Ox single crystals

The specific heat of Nd(Y)Ba2Cu3Ox single crystals without substitution of Nd for Ba has been measured for various oxygen contents. In comparison with YBa2Cu3Ox, the maximum attainable transition temperature of the crystals i s about 4 K higher and shifted from x = 6. 92 to x = 7.0, the highest possi ble oxygen content. In spite of the difference in the T-c(x) dependences, t he jump in the specific heat Delta C/T-c at T-c in both systems increases c ontinuously being maximum at x = 7.0. Bond valence sum calculations based o n neutron diffraction data point to a retarded generation of holes with inc reasing oxygen content in the Nd system and a different hole distribution b etween the chains and the planes in comparison to YBa2Cu3Ox accounting for the different behaviour of both systems.