Electrochemical Control of Magnetism on the Breathing Kagome Network of Li_xScMo₃O₈

Abstract

Controlling properties within a given functional inorganic material structure type is often accomplished through tuning the electronic occupation, which is in turn dictated by the elemental composition determined at the time of material preparation. We employ electrochemical control of the lithium content, with associated electronic occupancy control, to vary the magnetic properties of a material where a kagome-derived network of Mo3 triangles carry the spin. In this case, Li is electrochemically inserted into LiScMo3O8, a layered compound containing a breathing Mo kagome network. Up to two additional Li can be inserted into LiScMo3O8, transforming it into Li3ScMo3O8. Li2ScMo3O8 prepared by electrochemical lithiation is compared to the quantum spin liquid candidate compound Li2ScMo3O8 prepared through high-temperature solid-state methods, which has a slightly different structural stacking sequence but a similar kagome-derived network. Magnetic measurements are supported by first-principles calculations, showing that electrons remain localized on the Mo clusters throughout the doping series. As x is varied in Li x ScMo3O8, the measurements and calculations reveal the evolution from a diamagnetic band insulator at x = 1 to a geometrically frustrated magnet at x = 2, back to a diamagnetic insulator at x = 3. These results indicate a likelihood of strong coupling between the degree of Li disorder and charge/magnetic ordering over the Mo3 clusters.

Redox Mechanisms, Structural Changes, and Electrochemistry of the Wadsley–Roth LixTiNb2O7 Electrode Material

“…The formation of metal–metal dimers to accommodate the charge donated by Li falls into a class of molecular-orbital like redox mechanisms that is increasingly being explored as an alternative to cation-centric redox mechanisms of conventional battery intercalation compounds. ,− Other compounds exhibiting molecular-orbital redox mechanisms include Na 2 Mn 3 O 7 and Li x ScMo 3 O 8 . , In Na 2 Mn 3 O 7 , redox has been predicted to occur on antibonding states distributed over an extended ring of π-bonded Mn and oxygen orbitals surrounding a cation vacancy . In Li x ScMo 3 O 8 , charge donated by Li is accommodated on molecular orbital-like states derived from Mo metal trimer clusters formed by the hybridization of t 2g orbitals. , …”

Redox Mechanisms upon the Lithiation of Wadsley–Roth Phases

“…From our examination of redox mechanisms in two Nb 2 O 5 polymorphs, we have found that metal–metal bond formation, responsible for significant strain in TiNb 2 O 7 , also occurs in Nb 2 O 5 , suggesting a universality of the metal–metal bond redox mechanism in Wadsley–Roth phases. The dimer redox mechanism belongs to a class of redox processes that occur on extended molecular orbitals. − , …”

“…An alternative is to identify alloying elements that do not favor metal–metal dimer formation and also segregate to the edge-sharing octahedra at the shear boundaries. Wadsley–Roth phases containing Nb, V, and Mo, which are known to readily form metal–metal bonds, − should then be alloyed with elements that have a lower oxidation state. Alloying elements having a low maximum oxidation state will tend to segregate to the edge-sharing octahedra along the crystallographic shear planes and thereby relegate the metal-dimer formers such as Nb, V, and Mo to corner-sharing octahedra where they are unable to form metal–metal bonds.…”

“…The dimer redox mechanism belongs to a class of redox processes that occur on extended molecular orbitals. [48][49][50][51]78 The formation of metal−metal dimers has structural consequences, as it results in a shortening of the distance between edge-sharing cations. This in turn affects the lattice parameters and leads to shape changes of the crystal with changes in the Li concentration.…”

“…The bonding state, which has a lower energy than the unhybridized atom-centric t 2g orbitals, can host two electrons and thereby serve as a redox center to accommodate the electrons donated by Li. The metal-dimer redox mechanism requires edge-sharing octahedra containing early transition metals such as V, Nb and Mo, which are known to form metal–metal bonds in oxides. − It is an unconventional redox mechanism that occurs on molecular-orbital-like states distributed over multiple ions within the solid. − …”