A “Star” Antiferromagnet: A Polymeric Iron(III) Acetate That Exhibits Both Spin Frustration and Long‐Range Magnetic Ordering

Abstract

The preparation of new geometrically spin-frustrated magnetic materials [1] that approximate theoretical models [1,2] is a challenge. Although the Mermin-Wagner theorem [3] indicates that long-range magnetic order can exist in two dimensions at zero Kelvin, order can be destroyed either by quantum fluctuations or geometric frustration even at this temperature. Theoretical studies indicate that the ground state of a spin-1/2 Heisenberg antiferromagnet is most likely to be semiclassically ordered.[4] However, the interplay of geometric frustration and quantum fluctuations has been found to give rise to a paramagnetic ground state without semi-classical long-range order in two types of lattice.The first of these lattices is the famous KagomØ lattice (T8) and the second is the so-called "star" lattice (T9; Scheme 1), which may serve as a new example of a quantum paramagnet. [4,5] The triangles are corner-sharing in the KagomØ lattice whereas they are separated by a bridge in the star lattice, which means that their next-nearest-neighbor exchange interactions are different. [4,5] The magnetic J exchange pathways in the KagomØ lattice are all equivalent, whereas the intra-triangular J T pathway in the star lattice is weaker than the inter-triangular J D pathway. In contrast to the rapid development of KagomØ-type antiferromagetic lattices [6, 7] and related, geometrically spin-frustrated lattices, [8] there appears to date to be no report of a compound with a genuine star lattice. [7][8][9] including the desired magnetically frustrated star lattice. This star lattice can be described in vertex notation as 3.12 2 (see Scheme S1 in the Supporting Information), a lattice that is a uniform, three-connected twodimensional net with large voids.[10] Three-connected node subunits that prefer to bond in a planar fashion, such as the basic cationic iron(III) carboxylate cluster [Fe 3 [11] where L may be water, methanol, or pyridine, must be used to avoid three-dimensional connections. These carboxylate clusters are good potential building blocks because they are easily prepared, prefer planar bonding, and the R groups and L ligands can easily be varied. [12] The cationic [Fe 3 + moiety has previously served as a six-or three-connected node (see Scheme S2 in the Supporting Information) to form either three- [12a-e] or zero-dimensional [12f] porous frameworks depending upon the nature of the carboxylate, which may be either fully or partially substituted by dicarboxylates; the L ligands are usually retained as terminal ligands. Although no example is known to date, it should be possible to substitute the L ligands located in the triangular [Fe 3

The Heisenberg antiferromagnet on the square-kagomé lattice

“…These studies are motivated by the recent progress in synthesizing quasi-two-dimensional magnetic materials which exhibit exciting quantum effects [3][4][5][6][7][8][9][10]. Even spin systems on more exotic frustrated lattices such as the star lattice [11,12], the maple-leaf lattice [13,11] and the triangulated kagomé lattice [14] have been synthesized recently [15][16][17].…”

Spin-half Heisenberg antiferromagnet on two archimedian lattices: From the bounce lattice to the maple-leaf lattice and beyond

“…Indeed, most of these lattices are found to be underlying lattice structures of the magnetic ions of various magnetic compounds, such as CaV 39 Recently there has been also synthesized the magnetic compound 40) with maple-leaf lattice structure as well as hybrid cobalt hydroxide materials 41 with maple-leaf like lattice structure which may stimulate increasing interest in this lattice. Moreover, in the natural mineral spangolite (Cu 6 Al(SO 4 )(OH) 12 Cl·3H 2 0) the magnetic copper ions sit on the lattice sites of the maple-leaf lattice.…”

Competing quantum paramagnetic ground states of the Heisenberg antiferromagnet on the star lattice

“…Our motivation for this study is two-fold. First, the recent synthesis of a 'star lattice' organic Iron Acetate quantum magnet 18 raises the possibility that a S = 1/2 variant may possibly be synthesized in the near future and our results should be applicable to such systems. Second, this lattice has a sufficiently different geometry from more commonly studied quantum magnets -it may be viewed either as a variant of the kagome lattice or as a decorated honeycomb lattice -which allows us to explore the effect of this new lattice geometry on possible spin liquid physics and valence bond solid phases in quasi-2D systems.…”