Strongly frustrated magnetism of the metallic pyrochlore oxide Pr2Ir2O7 has been revealed by single crystal study. While Pr 4f moments have an antiferromagnetic RKKY interaction energy scale of /T*/ = 20 K mediated by Ir 5d-conduction electrons, no magnetic long-range order is found except for partial spin freezing at 120 mK. Instead, the Kondo effect, including a lnT dependence in the resistivity, emerges and leads to a partial screening of the moments below /T*/. Our results indicate that the underscreened moments show spin-liquid behavior below a renormalized correlation scale of 1.7 K.

Single crystals of YbAlB 4 were grown in excess Al flux. Plate-and needle-shaped crystals were found. The plates are found to be -YbAlB 4 , which crystallizes with the ThMoB 4 structure type in space group Cmmm (No. 65), Z ) 4, with lattice parameters of a ) 7.3080(4), b ) 9.3150(5), and c ) 3.4980(2) Å. The needle-shaped crystals were identified as the first form of YbAlB 4 which crystallizes with the YCrB 4 structure type in space group Pbam (No. 55), Z ) 4, with lattice parameters of a ) 5.9220(2), b ) 11.4730(3), and c ) 3.5060(5) Å. While both compounds have heavy fermion ground states with Isinglike magnetic anisotropy, the electronic specific heat coefficients (γ) differ. The -phase has a γ value near 300 mJ mol, more than twice that of the R-phase, γ ) 130 mJ mol. A comparison of the structures and physical properties of both polymorphs is presented.