CC BY
0MAGNETIC STATE OF COBALT IN LAYERED CHALCOGENIDES Co7(Se,Te)8
AND Fe4Co3Se8
Piskunov Yu.V.1, Ogloblichev V.V.1, Sadykov A.F.1, Akramov D.F.2, Smol'nikov A.G.1, Gerashchenko A.P.1, Selezneva N.V.2, Baranov N.V.1'2
1 Mikheev Institute of Metal Physics, Ekaterinburg, Russia, piskunov@imp.uran.ru 2 Ural Federal University, Ekaterinburg, Russia
The layered chalcogenides studied in this work belongs to the cation-deficient layered compounds M7X8, where M are transition metal atoms, and X are divalent of Group VI anions S, Se, Te. These compounds are characterized by vacancies in metal layers, as well as the formation of various superstructures as a result of the ordering of vacancies and M atoms in the layers [1]. Depending on the type of 3d metal M, both the structural and magnetic properties of the above chalcogenides can change. Recently the chalcogenides containing cobalt have been considered as promising catalysts, particularly for the water electrolysis [2].
In this work the structural and magnetic properties of the Co7Se8, Co7Te8 and Fe4Co3Se8 compounds have been studied using X-ray diffraction, measurements of the magnetic susceptibility, and NMR spectroscopy on 59Co nuclei. The isotropic magnetic shift and electric field gradient tensors at the location of 59Co nuclei have been determined from NMR spectra of these nuclei. The hyperfine fields in Co ions have been estimated from the temperature dependences of the shift and susceptibility in Co7Te8 and Fe4Co3Se8.
It has been found that the ordering of vacancies and Co atoms in cation layers is absent in the Co7Te8 compound, and its crystal structure is more planar and is characterized by a significantly smaller ratio c/a (c and a are the lattice parameters) compared to Co7Se8. It was shown that both Co7Se8 and Co7Te8 compounds remain a Pauli paramagnets down to the lowest temperatures. Since an increase in the interatomic distances from Co7Se8 to Co7Te8 occurs predominantly in the plane, it does not lead to a stronger localization of electrons or to the appearance of magnetic moments on Co atoms, as could be expected.
It was found that the temperature dependence of the magnetic susceptibility in Fe4Co3Se8 does not show any anomalies in the magnetically ordered region that would indicate a spin-reorientation transition. We suggest that in Fe4Co3Se8 the magnetic moments of Fe are oriented perpendicular to the plane of the layers. From the T-dependence of the susceptibility in the paramagnetic state of Fe4Co3Se8 the sample-averaged value of the effective magnetic moment jeff = 4.5 jb was determined, which, in terms of Fe atoms only, is jeff = 5.90(5) jb. The last value is very close to jeff = 5.80 jb, obtained in [1] for the initial composition of Fe7Se8.
We found out that cobalt ions in Fe4Co3Se8 as well as in completely substituted Co7Se8 do not have their own magnetic moments. However, due to the overlap of the 3d shells of cobalt and iron ions, part of the magnetic moments of Fe can be transferred to the Co ion, leading to the appearance of an induced effective magnetic moment on cobalt . Thus, this study has shown that cobalt ions having no own magnetic
moments in Fe7-xCoxSe8 chalcogenides act mainly as a diluent of the magnetic subsystem of iron.
This work was supported by the Russian Science Foundation (project no. 22-1200220).
References
[1] M. Sato, T. Kamimura, T. Iwata // J. Appl. Phys. - 1995. -V. 57. - P. 3244.
[2] J. Wang, W. Cui, Q. Liu, et al. // Adv. Mater. - 2016. - V. 28. - P. 215.
