Localised magnetism in 2D electrides

Abstract

In this work, we investigate intrinsic magnetic properties of monolayer electrides LaBr2 and La2Br5, where excess electrons do not reside at any atomic orbital and act as anions located at interstitial regions. Having demonstrated that conventional first-principles approaches are incapable of treating such non-atomic magnetic orbitals largely underestimating insulating band gaps, we construct effective electronic models in the basis of Wannier functions associated with the anionic states to unveil the microscopic mechanism underlying magnetism in these systems. Being confined at zero-dimensional cavities in the crystal structure, the anionic electrons will be shown to reveal an exotic duality of strong localisation like in d-and f -electron systems and large spatial extension inherent to delocalised atomic orbitals. While the former tends to stabilise a Mott-insulating state with localised magnetic moments, the latter results in direct exchange between neighbouring anionic electrons that dominates over antiferromagnetic superexchange interactions. On the basis of the derived spin models, we argue that any long-range magnetic order is prohibited in LaBr2 by Mermin-Wagner theorem, while intersite anisotropy in La2Br5 stabilises weakly coupled ferromagnetic chains along the monoclinic b axis. Our study shows that electride materials combining peculiar features of both localised and delocalised atomic states constitute a unique class of strongly correlated materials.Introduction. Electrides are a unique class of ionic materials, where the electron density is neither localised at any atomic orbital, nor fully delocalised like in metals. Instead, their electrons occupy interstitial regions formed by cavities in the crystal structure, where they act as anions [1, 2]. Materials with anionic electrons offer versatile functionalities, such as high electrical conductivity [3], ultra-low work functions [4], and non-linear optical responses [5], ranging their applications as electron emitters [6], battery anodes [7], and agent catalysts [8].Their physical properties are in large part determined by topology of the voids confining anionic electrons. In 2003, Matsuishi et al [9] demonstrated the first inorganic material [Ca 24 Al 28 O 64 ] +4 ·4e − stable at room temperature, which shows the high density of anionic electrons trapped in the crystallographic cages, forming a quasizero-dimensional electride with high electronic conductivity due to tunnelling through the cages [10, 11]. Recently, a novel layered electride [Ca 2 N] + ·e − has been reported, where owing to a highly delocalised nature the quasi-two-dimensional anionic electrons confined in the interlayer regions display an extreme electron mobility and long mean scattering time [12, 13]. Later on, strong localisation of anionic electrons has been observed in a layered transition-metal hypocarbide [Y 2 C] 2+ ·2e − with the quasi-two-dimensional anionic electrons in the interlayer spaces [14, 15]. Finally, it was shown that Ca 2 N can be exfoliated into two-dimensional...

Electrides: a review

“…For example, the anionic electrons in the predicted 0D electrides LaBr 2 and La 2 Br 5 have been calculated to be spatially extended, and their overlap at neighbouring cavities gives rise to ferromagnetism via direct exchange. 126,127 LaBr 2 and La 2 Br 5 are also theoretically shown to be Mottinsulators. 127 A Mott-insulator transition takes place if the energetic penalty for double occupancy of electrons (the on-site coloumb interaction) is large enough to drive open an energy gap at the Fermi level.…”

“…126,127 LaBr 2 and La 2 Br 5 are also theoretically shown to be Mottinsulators. 127 A Mott-insulator transition takes place if the energetic penalty for double occupancy of electrons (the on-site coloumb interaction) is large enough to drive open an energy gap at the Fermi level. When this occurs, a material that would be metallic under an elementary band picture can become a semiconductor or insulator.…”

“…For example, the Hubbard-type model was constructed for LaBr 2 and La 2 Br 5 by using the Wannier functions representing the anionic electrons. 127 Overall, the exploration of topological properties in electrides appears barely to have begun to scratch the surface of the rich materials chemistry and physics we see here. Especially in this section, we see a real demand for experimental characterisation to help guide and support the theoretical community in identifying the crucial features of topological materials that are so sensitive to theoretical parameters.…”

Negative Piezoelectric Coefficient in Ferromagnetic 1H-LaBr2 Monolayer

“… 10 It combines peculiar features; for example, its electron density shows neither complete localization at an atomic site nor metal-like delocalization, but rather it occupies the center of the hexagon from which originate localized magnetic moments. 11 , 12 Very recently, it has been predicted that this magnetism can be utilized for valley polarization. 10 However, its piezoelectric properties have not been investigated to date.…”

Floating Interlayer and Surface Electrons in 2D Materials: Graphite, Electrides, and Electrenes

“…The contribution of Y to the magnetization density, estimated from the ionic radius of 0.9 Å for Y 3+ , accounts for only 7.1% of the total magnetization, implying that the ferromagnetism is induced by the anionic nonnuclear electrons. Such Stoner‐type ferromagnetism originating from the spin polarization of anionic electrons has been predicted to occur in YCl, [ 62 ] LaBr 2 , [ 63 ] La 2 Br 5 , [ 63 ] and Ca 5 Ga 2 N 4 [ 64 ] at ambient pressure and in alkali metals under ultrahigh pressures. [ 65 ]…”