Synthesis and Characterization of New Germanate Pyrochlores, A$_2$Ge$_2$O$_7$ (A = Tb, Yb, Er)

The titanate pyrochlores, A$_2$Ti$_2$O$_7$, have yielded some of the most well-studied geometrically frustrated magnetic materials. A new class of pyrochlores with germanium on the B-site is now being investigated. The germanates, which in many cases share ground states with their titanate analogues, are far more highly correlated due to the smaller B-site cation. Two germanate pyrochlores, Ho$_2$Ge$_2$O$_7$ and Dy$_2$Ge$_2$O$_7$, were previously synthesized and characterized as new spin ice compounds [1-3]. We now present the new germanate pyrochlores, A$_2$Ge$_2$O$_7$ with A = Tb, Yb, and Er. Based on the titanates, three distinctly different magnetic ground states can be expected for these materials: Er$_2$Ti$_2$O$_7$ has an ``order-by-disorder'' mechanism, Yb$_2$Ti$_2$O$_7$ is a quantum spin ice and Tb$_2$Ti$_2$O$_7$ is a spin liquid. Preliminary measurements on Tb$_2$Ge$_2$O$_7$ indicate that it too is a spin liquid down to at least 0.35 K. We will present the characterizations of A$_2$Ge$_2$O$_7$ (A = Tb, Yb, Er) and compare them to the titanates.\\[4pt] [1] H. D. Zhou et al., Nature Communications 2, 478 (2011). \\[0pt] [2] H. D. Zhou et al., Phys. Rev. Lett. 108, 207206 (2012).\\[0pt] [3] A. M. Hallas et al., accepted for publication in Phys. Rev. B (2012).