Origin of the absence of a charge density wave in the kagome metal TiV 6 Sn 6 : A rattling chain model–predicted A M 6 X 6 -type charge density wave compound

The rattling chain model has provided a useful phenomenological guideline for identifying AM6X6-type kagome charge density wave (CDW) materials. In this work, we performed a comprehensive experimental and theoretical investigation of TiV6Sn6, a kagome CDW material predicted by the rattling chain model. However, resistivity, magnetization, and specific heat measurements reveal no evidence of a CDW transition in TiV6Sn6. Furthermore, unlike its rare-earth-based sister compound RV6Sn6 (R = Sc, Y, Sm, and Gd-Lu), phonon calculations show that TiV6Sn6 exhibits no imaginary phonon modes under either compressive or tensile strain. The band structure and crystal orbital Hamilton population (COHP) curves of TiV6Sn6 exhibit recognizable differences from those of RV6Sn6. Further analysis attributes these differences to the higher electronegativity of Ti relative to the rare-earth elements. The enhanced electronegativity promotes mixed ionic-covalent bonding, where the covalent character suppresses the rattling motion of Ti, ultimately precluding the formation of a CDW. Our findings suggest that the electronegativity of the filler atom, in addition to its ionic radius, should be incorporated as a supplementary factor into the rattling chain model.