Manifestation of dissimilar types of magnetism in iron- and chromium-substituted Mn2SnS4.

Mn2SnS4 belongs to the MII2AIVQ4 (M = transition metal; A = Si, Ge and Sn; Q = S, Se and Te) class of compounds that crystallizes in the orthorhombic space group Cmmm and shows complex magnetic properties. Here we report the synthesis and magnetic properties of Fe- and Cr-substituted Mn2SnS4 quaternary chalcogenides. All these compounds have been synthesized using a high-temperature solid-state route. Room temperature neutron diffraction studies on the specific compositions of chromium- and iron-substituted compounds were performed to obtain the site occupancy of different elements in the unit cell. The neutron diffraction analysis by employing the Rietveld refinement shows that for the Fe-substituted compound, most of the Fe goes to the Mn site with a small amount at the Sn site, while in the Cr-substituted sample, all the Cr occupy the Mn site. However, the Sn site almost remains intact in the case of the Fe-substituted compound, while it is significantly disordered for the Cr-substituted sample as a fraction of Mn occupies the Sn site and an equivalent amount of Sn occupies the Mn site. XPS study shows that both Cr and Fe exist in the +3 oxidation state, while Mn exists in the +2 state and Sn exists in a mixture of +2 and +4 oxidation states. Magnetic property study of these substituted compounds shows different types of magnetism, which is attributed to the variation of d-electrons of the substituent atom. The chromium-doped compounds show ferrimagnetic character along with two transitions: one transition at ∼37 K and another at ∼152 K. However, in Fe-substituted Mn2SnS4 samples, the low-temperature transition disappears and an increase in the high-temperature antiferromagnetic ordering temperature i.e. from 152 K (Mn2SnS4) to 174 K (Mn1.82Fe0.18SnS4) is observed. The increase in the antiferromagnetic ordering temperature in Mn2-xFexSnS4 may be attributed to the increase in the covalence of Mn/Fe-S-Mn/Fe bonds (shorter) with iron substitution.

Nominal Effect of Mg Intercalation on the Superconducting Properties of 2H-NbSe2.

2H-NbSe2 is a phonon-mediated, Fermi-surface topology-dependent multiband superconductor with an incommensurate charge-density wave (CDW) that coexists at a local level with superconductivity. Usually, the intercalation in 2H-NbSe2 enriches the CDW, enhances the c-axis lattice parameter, and distorts the Fermi surface, which result in a decrease in the superconducting transition temperature (Tc). The rate of decrease of Tc depends on the electronic structure, size, valence, magnetic nature, and electronegativity of the intercalating species. Herein, we report an unusual effect of Mg intercalation on the superconductivity of 2H-MgxNbSe2 (x = 0.0, 0.02, 0.06, 0.08, 0.10, and 0.12) synthesized by a high-temperature solid-state reaction method. Unlike other s- and p-block elements/species as intercalants (Rb, Sn, Ga, and Al) that have a sharp detrimental effect on the Tc of 2H-NbSe2 within 1-5% of intercalation, Mg is found to be an exception. Upon Mg intercalation up to x = 0.06, no remarkable changes in Tc as compared to the parent 2H-NbSe2 (Tc ∼ 6.7 K) are observed, and further intercalation results in a small decrease in Tc (for x = 0.12, Tc = 6.2 K). From heat-capacity measurements, it is inferred that superconducting Mg-intercalated 2H-NbSe2 exhibits strong electron-phonon coupling. Electronic structure calculations on two s-block element intercalated compounds of formula M0.125NbSe2 (M = Mg, Rb) show that Rb s-, p-, and d-states completely overlap with the Nb d states, while the Mg s states lie in a low-energy region as compared to Nb d states, indicating a weak interaction between the intercalant and the Nb sublattice in Mg0.125NbSe2 as compared to Rb0.125NbSe2. These results suggest that the electronic states of the Nb network in 2H-NbSe2 are least altered with Mg intercalation, which could be one of the reasons for the minimal effect on the Tc with intercalation.