Magnetic state of layered cobalt chalcogenides Co$_7$Se$_8$ and Co$_7$Te$_8$

The structural and magnetic properties of the Co$_7$Te$_8$ layered compound have been studied for the first time using X-ray diffraction, measurements of the magnetic susceptibility, and nuclear magnetic resonance spectroscopy on $^{59}$Co nuclei. The nuclear magnetic resonance study of Co$_7$Se$_8$ selenide with the same structural type (NiAs) as Co$_7$Se$_8$ has also been performed for the first time. In contrast to Co$_7$Se$_8$, the ordering of vacancies and Co atoms in cation layers is absent in the Co$_7$Te$_8$ compound, and its crystal structure is more planar and is characterized by a significantly smaller ratio $c_0/a_0$ compared to Co$_7$Se$_8$ ($a_0$ and $c_0$ are the basic lattice parameters of NiAs). The components of the magnetic shift and electric field gradient tensors at the location of Co nuclei have been determined. A significant local charge and spin inhomogeneity of the co-mpounds has been revealed. The hyperfine coupling constant in Co ions has been estimated from the te-mperature dependences of the shift and susceptibility in Co$_7$Te$_8$. An anisotropic increase in the interatomic distances does not increase the localization of $3d$ electrons and does not lead to the appearance of magnetic moments on Co atoms in Co$_7$Te$_8$. This compound remains a Pauli paramagnet down to the lowest temperatures.