Anomalous thermal expansion in the square-net compounds RE ₄ T Ge₈ (RE = Yb, Gd; T = Cr-Ni, Ag)

The family of materials RE₄TGe₈ (RE = Yb, Gd; T = transition metal) exhibits directional zero thermal expansion (ZTE) via a process that is associated with the linking of planar square nets in the third dimension. The Ge square nets in these compounds exhibit commensurate long-range modulations similar to those observed in charge-density-wave compounds. The ZTE is manifested in the plane of the square nets from 10 to 300 K with negligible volume expansion below ∼160 K. The specific atomic arrangement in RE ₄TGe₈ enables a Poisson-like mechanism that allows the structure to contract along one direction as it expands only slightly in the perpendicular direction.