Anomalous lattice contraction and emerging topological phases in Bi-substituted Sm₂Ir₂O₇

We demonstrate that substituting Bi for Sm in the pyrochlore Sm₂Ir₂O₇ induces an anomalous lattice contraction, with Δ a ~ -0.012~~observed at 10% Bi substitution, where 'a' denotes the lattice constant. Beyond 10% Bi substitution, the lattice expands according to Vegard's law. Within this anomalous substitution range, the resistivity shows a 1/T behavior up to 2% Bi-substitution, while near 10% substitution a -lnT dependence is observed. These resistivity behaviors suggest the possibility of a Weyl phase up to 2% Bi substitution, which transforms to a semimetallic quadratic band touching (QBT) topological phase near 10%. For the intermediate composition (Sm0.95Bi0.05)₂Ir₂O₇, the resistivity scales as 1/T1/4, possibly due to its proximity to a proposed quantum critical point at the Weyl-QBT phase boundary [Phys. Rev. X 4, 041027 (2014)]. The samples were characterized using synchrotron powder X-ray diffraction, X-ray near-edge fine structure (XANES), and Extended X-ray absorption fine structure (EXAFS) probes. Additionally, magnetic susceptibility and heat capacity measurements were conducted to provide further support.