CDW-induced negative thermal expansion in two-dimensional conductor η-Mo4O11

Two-dimensional oxide η-Mo4O11 exhibits almost isotropic thermal expansion in the normal phase, while anisotropic negative thermal expansion (NTE) in the charge density wave (CDW) phase below Tc1=105 K, where a remarkable anomalous expansion occurs along the a-axis. The incommensurate nesting vector along the b-axis is independent of temperature, q1=(0.000(1) 0.2335(1) 0.0000(5)). We propose that the CDW-induced NTE occurs as a result of the structural relaxation of the MoO6 octahedra layers along the stacking direction with the aid of the open spaces around the MoO4 tetrahedra.