Synthesis and Physico-chemical Study of Few Mn-based Ternary Chalcogenides of General Formula MII2AIVQ4

Abstract

Chalcogen (Group 16 of Periodic Table) and chalcogenide compounds are known to humankind from the early days of history. These compounds possess diversified structural features along with several potential applications in various fields like energy, data storage, medicine etc. Oxygen containing compounds (oxides) are the more attractive choice in the chalcogen group as these are relatively easy to synthesize and tuning of the physical properties can be achieved without much problem. On the other hand, compounds with other elements in the chalcogen family (S, Se and Te) are troublesome as their synthesis is not easy and their intentional tuning of the properties is also difficult to do. Further, the metal sulfides, selenides and tellurides are comparatively less studied and understood as compared to the oxides.
Magnetic chalcogenide compounds have a great potential in the field of spintronic and related applications. The 3d-transition metal based ternary chalcogenides, specifically Mn, Fe and Cr based chalcogenides are of significant interest from the view point of magnetism as these transition metals contain more number of unpaired electrons. Particularly, compounds with the general formula MII2AIVQ4 (M = Transition metal; A = Si, Ge, and Sn; Q = S, Se, and Te) are found to exhibit rejuvenated research interest because of their distinct structural and complex magnetic properties. These compounds largely crystallize in olivine structure type. However, Mn2SnS4 is the only compound in the MII2AIVQ4 (M = 3d transition metal; A = Si, Ge, and Sn; Q = S, Se and Te) series known so far with ordered defect rock salt-type structure. Very few reports are available so far in literature on its crystal structure and magnetic properties. In this thesis, the compound, Mn2SnS4 as well as its doped variants are synthesized via sealed tube solid state high temperature reaction method and detailed study of the structure and properties (mainly magnetic) have been done.
Pure phase of Mn2SnS4 was successfully synthesized after several trials and optimized reaction temperature profile from high purity elements. DFT based theoretical calculations on this compound revealed that the valence d-electrons of the magnetic Mn atoms are completely spin-polarized and total Mn–S bond energy contribution (though with low bond energy per Mn–S) is considerably higher than Sn–S bond energy contribution per unit cell. XPS study proved that the oxidation states of the elements in the compound, Mn2SnS4, are +2, +4 and −2 for Mn, Sn and S respectively. Magnetic measurements which were performed in a broad temperature range (3 K to 300 K) revealed the existence of two transitions: i) An ordered antiferromagnetic transition around 152 K (which is previously known) and ii) A weak ferromagnetic transition around 53 K (previously unknown). A high value of magnetization at low temperature and bifurcation of ZFC and FC measurement results were also obtained in the magnetic study. AC susceptibility measurement on Mn2SnS4 also confirms two magnetic transitions and none of the transitions possess any spin glass dynamics. The magnetic material is showing a very high value of frustration parameter, f (around 4), indicating the presence of a frustration in the system. Temperature variable neutron diffraction study was conducted at 3 K, 50 K, 100 K, 200 K and 300 K to understand the microscopic magnetic structure of Mn2SnS4. The magnetic structure determined at 3 K and 100 K revealed that there is a “canted” antiferromagnetic arrangement of spins at low temperature that diminishes at higher temperature (which also explains the higher value of magnetization at low temperature). The specific heat study was only successful to capture the second magnetic transition (i.e., around 152 K).
In order to observe the effect of substitution on the structure and magnetic properties, an aliovalent p-block element was partially substituted at the Sn-site to induce a mixed valence state at the Mn-site. Several trial reactions with different p-block elements were attempted, out of which antimony (Sb) doping was successful. The room temperature neutron diffraction measurement revealed that the dopant Sb occupies the Mn-site instead of Sn-site and expels equivalent amount of Mn to Sn-site resulting disorder at both the sites. However, the oxidation state investigation by XPS, revealed that neither Mn nor Sn changed their oxidation state due to aliovalent substitution, rather the dopant, antimony, exists in mixed valent state with equal amount Sb3+ and Sb5+. The magnetic measurements showed that upon doping Sb in Mn2SnS4, the second transition around 152 K (in parent compound) remained unaffected while the first transition temperature increases with successive substitution. The substituted compounds retained their antiferromagnetic character as that of the parent compound, Mn2SnS4, however, the substitution with a non magnetic ion eased the frustration within the system resulting in the decrease in the frustration parameter value. More interesting information was being found after the thermoremanent magnetization study i.e., substitution induced a low temperature stable spin dynamic state which was not present in the parent compound. This change in magnetic property may be the result of antimony induced Mn- and Sn-site mixing that creats more disorder at the Mn-site.
Substitution of d-block element, such as Cr, Fe at Mn-site in Mn2SnS4 were carried out to understand the effect of different d-electrons containing species on the magnetic properties. Site occupancy of the elements were obtained from the room temperature neutron diffraction study of one typical composition from each substitution series. It was found that chromium is fully occupied at the Mn-site while iron distributes itself to both Mn- (mostly) and Sn-site (partly) generating different degree of disorder upon substitution. The XPS analysis point out to the fact that both the dopants (Cr and Fe) exist in trivalent state (Fe3+ and Cr3+) while some amount of tin reduces from +4 to +2 state (Sn4+ and Sn2+) to maintain the charge neutrality. The evaluation of magnetic properties of Fe- and Cr- doped compounds exhibited contrasting results with respect to the effect on the transition temperatures as well as type of magnetic properties. In case of Cr-doping, both the transition temperatures were observed (as that of the parent compound) with small change in 1st transition temperature (53 K in undoped compound to 37 K in the doped compounds) and no change in the 2nd transition temperature (around 152 K). However, in Fe-doped compounds, a striking phenomenon was observed, as the first transition temperature (around 53 K for pure compound) completely disappeared while the second transition temperature (152 K in the pure compound) increased with higher amount of Fe substitution (around 174 K for 9% of Fe). The frustration parameter value is showing a lower value (than pure compound) in both chromium and iron doping, indicating a release of frustration. AC susceptibility measurements also support the change in the magnetic transition temperatures in both the doping series. M-H isotherm study show that Fe-substituted compounds retained antiferromagnetism while Cr-susbstituted compounds show ferrimagnetism due to incomplete cancellation of spin moment.
Further to understand the effect of substitution of f-block element on the magnetic properties, Ho-doped (Mn-site) Mn2SnS4 compounds were synthesized. The PXRD analysis and refinement proved a pure phase formation in Mn2−xHoxSnS4 till x = 0.2. The XPS analysis revealed that holmium exists in trivalent state i.e., Ho3+ and to maintain the charge neutrality, part of Sn4+ get reduced to Sn2+. The magnetic measurement in these compounds exhibited similar results as that of iron doping. The 1st transition at 53 K for undoped compound vanished upon doping while the 2nd transition temperature (at 152 K for pure Mn2SnS4) increased to a slightly higher temperature i.e., around 160 K for 10% of Ho substitution. In this case also, the frustration parameter shows a lower value than that of the parent Mn2SnS4 indicating a lowering of the frustration present in the system. However, all the doped compounds remained antiferromagnetic in nature (as found from the analysis of inverse susceptibility data, negative value of Weiss constant, θW), similar to the parent compound Mn2SnS4. These magnetic results indicate that the magnetic d-f exchange interaction is complicated in nature.