Structural, magnetic, and heat-capacity studies on Zn- and Ga-substituted ${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

The compound ${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ is not superconducting and exhibits antiferromagnetic ordering of the Pr moments with Néel temperature ${\mathit{T}}_{\mathit{N}}$ of ∼17 K. We have carried out magnetic, heat-capacity, and neutron-diffraction structural studies on Zn- and Ga-substituted ${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. The Zn and Ga dopants presumably substitute at the plane and the chain Cu sites, respectively. Our measurements reveal that ${\mathit{T}}_{\mathit{N}}$ is reduced in both cases; the reduction being faster for Ga substitution than for Zn substitution. For 10 at. % Zn substitution ${\mathit{T}}_{\mathit{N}}$ is ∼15–16 K, while for the same substitution level of Ga it is ∼12–14 K. It is rather unusual that the plane-site substitution has relatively less effect on ${\mathit{T}}_{\mathit{N}}$ than the chain-site substitution. From neutron-diffraction studies the cause of such contrasting behavior may be understood in terms of changed planar Cu-${\mathrm{O}}_2$ and Pr-${\mathrm{O}}_2$ bond lengths. The electronic specific-heat coefficient γ of the substituted samples is found to be large as in the parent ${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ compound.