  The rich phase diagrams of magnetically frustrated pyrochlores have
maintained a high level of interest over the past 20 years. To experimentally
explore these phase diagrams requires a means of tuning the relevant
interactions. One approach to achieve this is chemical pressure, that is,
varying the size of the non-magnetic cation. Here, we report on a new family of
lead-based pyrochlores A$_2$Pb$_2$O$_7$ (A = Pr, Nd, Gd), which we have
characterized with magnetic susceptibility and specific heat. Lead is the
largest known possible B-site cation for the pyrochlore lattice. Thus, these
materials significantly expand the phase space of the frustrated pyrochlores.
Pr$_2$Pb$_2$O$_7$ has an absence of long-range magnetic order down to 400 mK
and a spin ice-like heat capacity anomaly at 1.2 K. Thus, Pr$_2$Pb$_2$O$_7$ is
a candidate for a quantum spin ice state, despite weaker exchange.
Nd$_2$Pb$_2$O$_7$ transitions to a magnetically ordered state at 0.41 K. The
Weiss temperature for Nd$_2$Pb$_2$O$_7$ is $\theta_{\text{CW}}$ = $-$0.06 K,
indicating close competition between ferromagnetic and antiferromagnetic
interactions. Gd$_2$Pb$_2$O$_7$ is a Heisenberg antiferromagnet that
transitions to long-range magnetic order at 0.81 K, in spite of significant
site mixing. Below its ordering transition, we find a $T^{3/2}$ heat capacity
dependence in Gd$_2$Pb$_2$O$_7$, confirmation of a ground state that is
distinct from other gadolinium pyrochlores. These lead-based pyrochlores
provide insight into the effects of weakened exchange on highly frustrated
lattices and represent further realizations of several exotic magnetic ground
states which can test theoretical models.
