Disorder in the chalcogen layer in 1T−TaS2−xSex

The charge density wave (CDW) system $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2\ensuremath{-}x}{\mathrm{Se}}_{x}$ was investigated using neutron scattering to determine the nature of the in-plane and out-of-plane disorder. From the local structure analysis, it was deduced that the in-plane star of David motifs are distorted with Se doping just like it was previously observed in $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$. This is due to random Ta displacements away from the ideal trigonal symmetry regardless of whether the CDW state is commensurate, nearly commensurate, metallic, or insulating. In the chalcogen layer, the atoms are also distorted with displacements in the perpendicular direction to the planes. The chalcogen distortions are most pronounced in TaSSe that superconducts below 3.6 K in comparison to the distortions observed in ${\mathrm{TaS}}_{2}$ and ${\mathrm{TaSe}}_{2}$. The implications of the distortions on superconductivity and the effects on the electron pocket proposed to lead to the superconducting state are not understood at present. Unlike in $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ in which both 13c and 3c layer stacking orders were previously reported, in the nearly commensurate metallic state of $1T$-TaSSe and the commensurate CDW metallic state of ${\mathrm{TaSe}}_{2}$, no clear evidence for a stacking order is observed down to 2 K.