Communication pubs.acs.org/cm
KMn3O2(Ge2O7): An S = 2 Magnetic Insulator Featuring Pillared Kagome Lattice Matthew S. Williams, J. Palmer West, and Shiou-Jyh Hwu* Department of Chemistry, Clemson University, Clemson, South Carolina 29634-0973, United States S Supporting Information *
(see Supporting Information). Crystals of 1 and 2 form column-like morphology and crystallize in an ideal space group, P6 3/mmc (No. 194). However, the kagome lattice is conceivably distorted from the ideal one due to, as discussed later, the disorder about the Ge2O7 unit. Structurally, it is closely related to volborthite (C2/m), Cu3V2O7(OH)2·2H2O,9 regarding pillared kagome of edge-shared CuO6 octahedra; see Figures S3 and S4 (Supporting Information). The title compound features a kagome lattice in [Mn3IIIO8]∞ layers (Figure 1, left) that are interconnected through sharing the
A structurally perfect, geometrically frustrated magnetic (GFM) lattice has long been sought since it presents an ideal construct for the model study relevant to the unusual physics of strongly correlated spin systems such as resonating-valence-bond state proposed by Anderson in high Tc superconductivity.1 With GFM systems, a spin cannot simultaneously minimize magnetic interactions with neighboring spins, but instead, fluctuates between many different spin configurations. Theoretically, this highly degenerate ground state cannot be illustrated by a single minimized solution to the Hamiltonian equation for the antiferromagnetic interactions between the spins. This quantum spin fluctuation has been thought to suppress long-range magnetic order (LRMO) and is often complemented by exotic ground state phenomena such as spin-glass, spin-ice, and spinliquid behaviors. The major features of fundamental and technological importance2−9 that frustrated systems are examined for are often perturbed by lattice imperfection that lifts the degeneracy of otherwise anticipated infinite ground state.2 Structurally, a perfect GFM crystal for model study must possess trigonal (or hexagonal) symmetry inherent of the space groups such as P63/mmc (No. 194) that the title compound adopts. Kagome antiferromagnets (KAFMs) are a canonical example of GFM systems under extensive study. Kagome, a distinctive Japanese basket weave pattern, is used as a specialized term for a two-dimensional (2D) lattice made of corner shared triangles of magnetic ions. Over the past decade, the search for ideal S = 1/2 KAFM compounds for model studies3 has mostly been done via hydrothermal methods through which, for instance, a rarely synthesized S = 1/2 KAFM herbertsmithite (ZnCu3(OH)6Cl2)4 exhibiting structurally perfect kagome has received considerable attention with novel theoretical developments.5 In many other kagome systems, the essential infinite degeneracy is found to be perturbed due to lattice imperfection such as in some herbertsmithites,6 the jarosite family,7 (based on the KFe3(OH)6(SO4)2 parent), and volborthite.8 Intrinsic to the low-temperature (700 °C), fluxgrowth methods. AMn3O2(Ge2O7), where A = K (1), and for comparative studies, K0.35(1)Rb0.65(1) (2) as well as solid solution (Figures S1 and S2, Supporting Information), were synthesized © 2014 American Chemical Society
Figure 1. (left) Projected view showing a single [Mn3O8]∞ kagome sheet where Mn3 units, connected by μ3-oxo oxygen, O(1), are highlighted by pink (corner-sharing) triangles. (right) Perspective view of 1 looking along the b axis showing the [Mn3O8]∞ layers are pillared by Ge2O7 pyrogermanate units (in polyhedra), see text.
terminal oxygen atoms of the Ge2O74− units (Figure 1, right). It is evident that the A-site cations can be replaced partially or possibly in full via cation substitution. This is promising for future correlation studies concerning chemical modification via (aliovalent) cation substitution to allow charge, lattice, orbital, and spin degrees of freedom to be examined.10 In this communication, we report the structure and initial magnetic studies of this new class of kagome germanates. The latter shows that 1 has established LRMO that is partially suppressed at higher magnetic fields and, as shown in 2, can be interrupted via random K+/Rb+ cation substitution. The kagome lattice in 1 exhibits hexagonal nets made of Mn3 equilateral triangles, as opposed to pseudo hexagonal with Cu3 isosceles in volborthite (Figure S4, Supporting Information). As listed in Table 1, the Mn3 triangular units adopt a unique and comparable size where dMn−Mn = 2.9399(4) Å for 1 and 2.9420(4) Å for 2. As expected, there is only one distinct Mn3+ Received: October 1, 2013 Revised: January 30, 2014 Published: February 3, 2014 1502
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size of A-site cations. The Ge2O7 pyrogermanate unit found in the title compound forms a bent geometry as a result of disordered bridging atoms, O(2) (Figure S6, Supporting Information). This nonlinear ∠Ge−O(2)−Ge configuration, see Table 1, allows structural flexibility for incorporating different sized A-site cations. It should be noted that the orientation of the GeO4 tetrahedra is intuitively turned in order to preserve the tetrahedral ∠Ge−O−Ge angles, thus the distortion of the kagome net via the shared oxygen atoms, μ2− O(3). On the average, otherwise, the interlayer Mn···Mn distances (≡ c/2) are increased upon substituting K+ with larger Rb+ cations, 6.860(2) Å for 1 to 6.898(2) Å for 2. Yet, the Mn− O distances and intralayer Mn···Mn distances remain noticeably the same. Nevertheless, an increased interlayer distance (c/ 2 = 6.926 Å) is abstracted from the PXRD pattern of the “x = 0.75” attempt. This increase could correspond to a more stretched or even a linear Ge−O(2)−Ge, thus a less disordered Ge2O7 unit. Further investigation including crystal growth and magnetic investigations for comparative studies is underway. The preliminary study concerning the temperature-dependent magnetic susceptibility, χ, of 1 and 2, shows multiple features deviating from Curie−Weiss (C−W) behavior; see Figure 2. The inverse magnetic susceptibilities, χ −1, of the two
Table 1. Selected Bond Distances and Angles for AMn3O2(Ge2O7), A = K (1), K0.35(1)Rb0.65(1) (2) 1a MnO6 Mn−O(1) Mn−O(1) Mn−O(3) × 4 GeO4 Ge−O(2) × 3 Ge−O(3) × 3 Mn−Mn (intra) Mn−Mn (inter) ∠Mn−O(1)−Mn × 3 ∠Mn−O(3)−Mn Ge−Ge ∠Ge−O(2)−Ge
2b
1.886(2) Å 1.886(2) Åa 2.094(3) Åa,b,c
1.887(3) Å 1.887(3) Åa 2.090(5) Åb.c,d,e
1.764(4) Åd,e 1.7465(4) Åd,f 2.9399(4) Åc,g.h.i 6.860(2) Å 102.5(1)°g,h 89.2(1)°i 3.409(1) Å 150.22(4)°e
1.765(4) Åf,g 1.750(5) Åg 2.9420(4) Åb,c,h,i 6.898(2) Å 102.4(1)b,c 89.5(1)b,c 3.439(1) Å 153.97(4)°f,g
a Symmetry codes. 1: (a) −x + 1, −y + 1, −z; (b) x − y, x, −z; (c) −x + y + 1, −x + 1, z; (d) −y, x − y, z; (e) −x + y, −x, −z + 1/2; (f) −x + y, −x, z; (g) −x + y, −x + 1, z; (h) −y + 1, x − y + 1, z; (i) −y + 1, x − y, z. bSymmetry codes. 2: (a) −x + 2, −y + 1, −z + 1; (b) −y + 1, x − y, z; (c) −x + y + 1, −x + 1, z; (d) x − y + 1, x, −z + 1; (e) y + 1, −x + y + 1, −z + 1; (f) x − 1, y, −z + 1/2; (g) x − 1, y, z; (h) −x + y + 2, −x + 2, z; (i) −y + 2, x − y, z.
site where the MnO6 octahedral unit reveals a Jahn−Teller (JT) compression along the axial bonds of the μ3-oxo oxygen O(1). The average bond distances, dMn−O of 2.025 Å for 1 and 2.022 Å for 2, are only slightly larger than 1.995 Å, the sum of the Shannon crystal radii of a high spin Mn3+ (CN = 6) and O2− ions.11 The [Mn3IIIO8]∞ kagome layer is made of fused MnO6 octahedra adopting one of the two possible lattice types discovered thus far. Through sharing edges of octahedra, trans along the a-axis and cis along [110], the octahedral layer possesses two intralayer superexchange pathways, Mn−μ3,O(1)−Mn and Mn−μ2,O(3)−Mn, as opposed to one (μ2,Olinkage) by corner sharing such as those found in jarosite (Figure S5, Supporting Information). On the opposite side of the Mn3 triangular faces from the μ3-oxo oxygen, there are Asite cations (Figure S4, Supporting Information). As shown in Figure 1, right, these A-site cations reside in the channels along [010] of the extended framework made of [Mn3IIIO8]∞ layers and Ge2O7 spacers. Oxyanions are commonly known as a nonmagnetic spacer12 employed to establish, in this case, a magnetically quantized 2D kagome lattice. The oxyanions studied thus far in kagome systems include SO42− in jarosites,6 ZnO610− in herbertsmithite,3 and VO43− in β-vesignieite, BaCu3V2O8(OH)2.13 These oxyanions intuitively reduce interlayer superexchange that would otherwise induce perturbations that lift the essential spin degeneracy for a purely frustrated kagome lattice. The Ge2O76− pyrogermanate anion is known for its propensity to adopt a linear configuration as a pillar. In 1 and 2, like V2O74− pyrovanadate in volborthite,8 the Ge2O7 units “space” the [Mn3O8]∞ layers presumably responsible for magnetic confinement. It should be noted that nonmagnetic A-site cations included for charge balance can also serve as a spacer. Their effect to the spin dynamics due to electrostatic charge, however, is unknown. Thus, its critical distance with respect to spin perturbation to kagome lattice is worthy of study. The distance between kagome planes is evidently dictated by the combination of the geometry of the Ge2O7 spacer and the
Figure 2. Molar magnetic susceptibility (χ; solid) and inverse magnetic susceptibility (χ−1; open) vs temperature (T) contrasting 1 (triangles) and 2 (squares) in an applied field of 0.01 T. Top inset shows a zoomed in low-temperature region of χ. Bottom inset shows χ of 1 with varying applied fields of 0.01 and 0.5 T (open circles) at T < 80 K.
compounds at T > 120 K are comparable, and the temperaturedependent behavior follows C−W rather closely. At T < 120 K, the deviations from C−W become more prominent for 1. Fitting the data to C−W, χ−1 = (T − θ)/C, from 120 to 300 K (0.01 T) gave a Curie constant, C, of 47.9(3) cm3 K mol−1, a Weiss constant, θ, of −63(2) K, and an effective magnetic moment, μeff, of 5.5(4) μB for 1. A similar fit for 2 yielded a C of 48.8(4) cm3 K mol−1, a θ of −78(3) K, and a μeff of 5.6(5) μB. The experimentally determined effective moments are comparable with the spin-only value, 4.9 μB, for the free Mn3+ (S = 2) ion. The negative Weiss constants indicate predominant local antiferromagnetic (AFM) interactions as required for magnetic frustration. As to the low temperature region, the broad maximum (or plateau) observed around 85 K in χ, especially evident in 1, represents a development of an AFM short-range order as 1503
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disordered Ge2O76− unit. This material is available free of charge via the Internet at http://pubs.acs.org.
observed in other kagome antiferromagnets such as volborthite. 1 shows a prominent ferromagnetic (FM) feature as suggested in the χT and the ZFC/FC plots at an onset of ∼46 K; see Figures 3 and S7 (Supporting Information), but only a small
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Science Foundation through grant DMR-0077321, and the purchase of the single crystal X-ray diffractometer was made possible through ESR9108772, CHE-9207230, and 9808165. The authors are indebted to Dr. Don VanDerveer for assistance in crystallographic studies and Dr. Wendy Queen for assistance with magnetic studies and fruitful discussions.
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Figure 3. Molar magnetic susceptibility and temperature product (χT) vs temperature (T) comparing 1 (triangles) and 2 (squares). Data was collected with an applied magnetic field of 0.01 T. The solid horizontal line corresponds to 5.0(1) μB comparable with 4.9 μB, the expected, free ion, magnetic moment of Mn3+. Inset shows χT of 1 with two different fields.
kink at around 20 K in 2. This difference could be due to random substitution of K+/Rb+ which in turn introduces a possible spin-glass state in KAFMs. This FM feature in 1 is suppressed upon applying a larger magnetic field (i.e., 0.5 T) likely due to the 2D nature. However, the canted AFM may exist at low temperatures as a result of finite DzyaloshinskyMoriya (DM) antisymmetric exchanges in the kagome net of magnetic atoms (Mn).14 The θc and Tc are low and so is the |θC|/Tc ratio (1.4 for 1), possibly also due to the convoluted magnetic interactions through multiple exchange pathways. In addition, a short-range order associated with A-site cations could exist, and inelastic neutron scattering is in order. In summary, we have reported an ideal class of kagome solids (P63/mmc, No. 194) synthesized by high-temperature, fluxgrowth methods. Compounds studied thus far include KMn3O2(Ge2O7) manganese(III) oxo-germanate and some K+/Rb+ ion-substituted derivatives. Single crystal X-ray structure studies suggest the kagome magnetic planes are likely distorted due to the disorder of the bridging oxygen in Ge2O7 pillar units. Preliminary magnetic studies indicate an existence of weak ferromagnetism, which is likely related to the canted antiferromagnetic order. The exact nature of foregoing KAFMs with respect to the magnetic ground state is unclear, and we are awaiting the crystal growth of ion-substituted derivatives for comparative studies. Nevertheless, this family of compounds opens up exciting possibilities in model studies by tweaking factors that govern the spin state of magnetic frustration via controlled chemical modifications which are oftentimes difficult to achieve through hydrothermal methods.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental methods; crystallographic data; PXRD patterns of solid solution; SEM image and EDX analysis of a single crystal of 2; structures of AMn3O2(Ge2O7), volborthite, jarosite, and 1504
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