BaMn₉^II(VO₄)₆(OH)₂: a homospin ferrimagnet with a broken spinel-lattice of B-sites

Abstract

A new vanadate of BaMn₉^II(VO₄)₆(OH)₂ was synthesized by a conventional hydrothermal method. BaMn₉^II(VO₄)₆(OH)₂ crystallizes in the cubic space group Pa3̄, which exhibits an edge-sharing MnO₆ octahedral network structure with cavities occupied by Ba(VO₄)₆¹⁶⁻ groups. The lattice built by Mn²⁺ ions shows the reverse triangular dipyramid Mn₇ in the system, which can be considered as a broken spin lattice of B-sites in spinels. The title compound shows a ferrimagnetic behavior with T_C = 18 K.

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Introduction

Transition-metal based compounds with a spinel structure have been studied intensively in the fields of condensed matter physics and crystal engineering, due to the experimental possibility to investigate the interplay of spin, charge, and orbital degrees of freedom of magnetic ions based on a topologically frustrated lattice of B-sites.¹ The spinel compounds can be generally written as A^IIB^III₂X₄ (A = divalent transition-metal ions or Mg²⁺; B = trivalent transition-metal ions, Al³⁺ or Ga³⁺; X = O, S or Se), which usually crystallize in the cubic system with a space group Fd3̄m, showing the X anions arranged in a cubic closely-packed lattice parallel to (111). In a typical spinel structure, A^II ions often occupy 1/8 of the tetrahedral site while B^III ions occupy 1/2 of the octahedral site in the three-dimensional framework. One of the most remarkable structural features is that the A-site ions form a diamond lattice, while the B-site ions form a network of corner-sharing regular tetrahedra, known as the pyrochlore lattice. Such unique structural features can lead to strong spin fluctuations originating from geometrical frustration (GF) in general. The early investigation of strong GF in the B-site ions can be indeed traced back to the work of Anderson in 1956.² The representative examples of a strong GF for the B-sites of spinels are CdCr₂O₄ ³ and ZnCr₂O₄,⁴ where the rare 1/2 magnetization plateau appears under an applied magnetic field. However, normal cubic spinels FeSc₂S₄ ⁵ and MnSc₂S₄ ⁶ with only the A-site occupied by magnetic ions are also found to exhibit strong GF effects, both in the spin and in the orbital sector. Besides, unusual magnetic behaviours of GF, transition-metal based compounds with a spinel structure are also found to exhibit various exotic physical phenomena. These include the Verwey transition in magnetite,⁷ colossal magnetoresistance in Cu doped FeCr₂S₄,⁸ multiferroic behavior and colossal magnetocapacitive effect in CdCr₂S₄ and HgCr₂S₄,⁹ the heavy fermion behavior in LiV₂O₄,¹⁰ spin–orbital liquid in FeSc₂S₄,¹¹ spin dimerization in CuIr₂S₄ ¹² and MgTi₂O₄,¹³ and the spin-Peierls-like transitions in three-dimensional solids.¹⁴ Hence, the discovery and synthesis of new spinel compounds have given many exciting topics in physics and chemistry, which have attracted great scientific interest.

In this paper, we report on the first synthesis of a vanadate BaMn₉^II(VO₄)₆(OH)₂ with an edge-sharing octahedral network, which is isomorphic to the mineral of nabiasite.¹⁵ The topological geometry of magnetic ions can be viewed as the destroyed spinel-lattice of B-sites due to the absence of L_3̄. Our experimental results combined with magnetic susceptibility and magnetization show that BaMn₉^II(VO₄)₆(OH)₂ exhibits a ferrimagnetic transition at ∼18 K, while a 1/3 plateau is observed in the magnetization curve.

Experiment section

Syntheses of BaMn₉^II(VO₄)₆(OH)₂

Synthesis of BaMn₉^II(VO₄)₆(OH)₂ was proceeded by the reaction of a mixture of 1 mmol Ba(CH₃COO)₂·H₂O (3 N, 0.2734 g), 5 mmol Mn(CH₃COO)₂·4H₂O (3 N, 1.2254 g), and 0.6077 mmol V₂O₅ (3 N, 0.1105 g) in 10 mL H₂O in an autoclave equipped with a Teflon liner (28 mL) through a typical hydrothermal reaction. The autoclave was put into a furnace which was then heated at 230 °C for 3 days under autogenous pressure, followed by slowly cooling to room temperature at a rate of 3 °C h⁻¹. Black crystals with an octahedron-shape were separated by ultrasonic cleaning for several hours and then washed with distilled water. Elemental analysis (Fig. S1†) of single crystals was done through the energy dispersive spectrometry (EDS), confirming that no other elements can be detected except for Ba, Mn, and V. Further an average molar ratio of Ba/Mn/V was found to be 1.03(1) : 8.95(2) : 6.00(1), agreeing with the result determined from single-crystal X-ray structural studies. Powdered samples for magnetic measurements were prepared by crushing single crystals of BaMn₉^II(VO₄)₆(OH)₂. The quality of the powdered samples was confirmed by powder XRD studies (Fig. S2†).

X-ray crystallography

The black octahedron-shaped crystals of BaMn₉^II(VO₄)₆(OH)₂ (∼0.2 mm × 0.15 mm × 0.07 mm) were selected and mounted on glassy fibers for single crystal X-ray diffraction (XRD) measurements. Data collections were performed on a Rigaku Mercury CCD diffractometer equipped with a graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 293 K. The data sets were corrected for Lorentz and polarization factors as well as for absorption by the Multi-scan method.¹⁶ The structure was solved by direct methods and refined by full-matrix least-squares fitting on F² by SHELX-97.¹⁷ All non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms were located at calculated positions and refined with isotropic thermal parameters. The final refined structural parameters were checked by the PLATON program.¹⁸ Crystallographic data and structural refinements are summarized in Table 1. The final refined atomic positions and structural parameters are shown in the ESI (Tables S1–3†).

Table 1 Crystal data and structure refinements for BaMn₉(VO₄)₆(OH)₂

Formula	BaMn9(VO4)6(OH)2
a R 1 = ∑||Fo| − |Fc||/∑|Fo|, wR2 = {∑w[(Fo)^2 − (Fc)^2]^2/∑w[(Fo)^2]^2}^1/2.
Fw	1355.43
Space group	Pa3̄
a/Å	12.8373(2)
T/K	Room temp
λ/Å	0.71073
V/Å^3	2115.54(6)
Z	4
D calcd/g cm^−3	4.249
μ (Mo-Kα)/mm^−1	9.619
GOF on F^2	1.132
R 1, wR2 [I > 2σ (I)]^a	0.0273, 0.0758
R 1, wR2 (all data)	0.0273, 0.0759

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Magnetic measurements

Magnetic measurements were performed using a commercial Quantum Design Physical Property Measurement System (PPMS). Powdered samples of BaMn₉^II(VO₄)₆(OH)₂ (29.360 mg) were placed in a gel capsule sample holder which was suspended in a plastic drinking straw. Magnetic susceptibility was measured at 0.1 T from 300 to 2 K, and magnetization was measured at 2 K in an applied field from −8 to 8 T (field scan of 0.1 T per step). Moreover, low-temperature magnetic susceptibility of BaMn₉^II(VO₄)₆(OH)₂ was also measured with field-cooling (FC) and zero-field-cooling (ZFC) regimes from 2 to 30 K under 50 Oe.

Results and discussion

Structural description

BaMn₉^II(VO₄)₆(OH)₂ crystallizes in the cubic space group Pa3̄ with a = 12.837(3) Å and Z = 4. The asymmetric unit contains one Ba, one V, and three Mn atoms. As shown in Fig. 1, V⁵⁺ ions are tetrahedrally coordinated forming distorted VO₄ tetrahedra with V–O bond lengths ranging from 1.675(3) to 1.770(2) Å, while Mn²⁺ ions are octahedrally coordinated in oxygen-ligand geometry, forming MnO₆ octahedra. Mn²⁺ ions have three independent crystallographic sites (Mn1, Mn2, and Mn3) with Wyckoff positions of 8c, 4a, and 24d, respectively. Both Mn(1) and Mn(2) ions form nearly regular octahedra with Mn–O bond lengths ranging from 2.142(7) Å to 2.171(1) Å while Mn(3) ions form distorted octahedra connected with a OH group, leading to an unusual Mn–O bond length of 2.447(1) Å.

Fig. 1 View of the coordination environments of (a) Mn1, (b) Mn2, (c) Mn3, and (d) V sites.

As shown in Fig. 2a, the three dimensional architecture of BaMn₉^II(VO₄)₆(OH)₂ can be described as an edge-sharing MnO₆ octahedral network with cavities occupied by Ba(VO₄)₆¹⁶⁻ groups. It is noted that Ba atoms are coordinated by twelve atoms with Ba–O bond lengths of 2.860(7) Å or 3.166(1) Å and sharing an edge with six VO₄ groups, forming Ba(VO₄)₆¹⁶⁻ groups (Fig. 2b). All oxygen atoms are arranged in a cubic closely packed lattice parallel to (111), which is similar to the X anions in AB₂X₄ spinels. Bond valence calculations indicate that Mn, Ba and V atoms are in their expected oxidation states of +2, +2 and +5. The calculated total bond valences for three Mn atoms are 2.05, 1.94, and 1.93, while Ba and V atoms are found to be 2.06 and 4.96, respectively. In addition, O3 atoms in this compound are calculated to be 0.947, confirming that the O3 atoms should be in OH groups for charge balancing the formula.

Fig. 2 (a) The crystal structure of BaMn^II₉(VO₄)₆(OH)₂ showing (b) Ba(VO₄)₆¹⁶⁻groups surrounded by the nearest {Mn₂₂O₈₄(OH)₆}_∞ building units.

There is a unique building unit {Mn₂₂O₈₄(OH)₆}_∞ in the network (Fig. 3a), which is constructed by one Mn(1)O₆, three Mn(2)O₆, and eighteen Mn(3)O₅(OH) octahedra with an edge-sharing manner. It is noted that Mn(2)O₆ and six Mn(3)O₅(OH) octahedra form a reverse triangular dipyramid centred by a Mn(2)²⁺ ion, while Mn(1)O₆ octahedra are surrounded by three adjacent reverse triangular dipyramids. It is also noted that such a building unit {Mn₂₂O₈₄(OH)₆}_∞ with an edge-sharing manner (Fig. 3a) can also be regarded as the destroyed spinel lattice with a lack of MnO₆ octahedra among reverse triangular dipyramids.

Fig. 3 The linkage of an edge-sharing building unit for (a) {Mn₂₂O₈₄(OH)₆} units, (b) the plane Mn₄ groups with L₃ symmetry, and (c) Mn₇ groups centered by Mn(2) with L₋₃ symmetry in BaMn^II₉(VO₄)₆(OH)₂.

As shown in Fig. 3b, Mn(1) ions connect to three Mn(3) ions via edge-sharing oxygen atoms with the Mn(1)–O–Mn(3) angle of ∼89° and the Mn(1)⋯Mn(3) separation of ∼3.05 Å. This connection with a L₃ symmetry is quite similar to that of a regular honeycomb lattice as seen in BaNi₂(VO₄)₂.¹⁹ As shown in Fig. 3c, Mn(2) ions connect to six Mn(3) ions via edge-sharing O(4) atoms, showing a regular Mn(2)O₆ octahedron in the centre of reverse triangular dipyramids with the crystallographic symmetry of L_3̄. The linkages between Mn(3) ions have two different paths of Mn₃(μ₃-O(4)) and Mn₃(μ₃-O(3)H), where the distances of Mn(3)⋯Mn(3) are ∼3.22 and ∼3.34 Å, respectively. This indicates that the reverse triangular dipyramids are slightly distorted, which are different from the regular reverse triangular dipyramids in spinel compounds. However, such distorted reverse triangular dipyramids are quite similar to those in Cu₂Cl(OH)₃ with Cu₇(μ₃-OH).²⁰

Removing nonmagnetic O atoms and OH groups from the building units {Mn₂₂O₈₄(OH)₆}_∞, the 3D topological spin network of magnetic Mn²⁺ ions is shown in Fig. 4a. Compared with spinel compounds (Fig. 4b), similar and different topologies are clearly seen. The reverse triangular dipyramids are quite similar to those of spinels with a same symmetry operation of L_3̄. However, such reverse triangular dipyramids of spinels also connect to each other via corner-sharing, while those of BaMn₉^II(VO₄)₆(OH)₂ are separated by Mn(1) sites, showing the disappearance of L_3̄. If Mn(1) sites can be substituted by a reverse triangular-dipyramidal Mn₇ unit, the titled compound may exhibit a spinel structure of the B site. Hence BaMn₉^II(VO₄)₆(OH)₂ may be considered to have a broken spin lattice of B-sites in spinels.

Fig. 4 Spin-lattice unit built by reverse triangular dipyramids in (a) BaMn^II₉(VO₄)₆(OH)₂ and (b) spinel compounds.

Magnetic properties

Fig. 5 shows temperature dependence of the magnetic susceptibility measured at 0.1 T. Magnetic susceptibility increases with decreasing temperature, while a rapid upturn is seen at ∼18 K, indicating the inset of a ferromagnetic correlation. A typical Curie–Weiss behavior is observed above 180 K, giving the Curie constant C = 38.08(7) emu K mol⁻¹ and Weiss temperature θ = −172.3(1) K. The effective magnetic moment of Mn²⁺ ions in the system is calculated to be 5.817(6)μ_B, which is close to the theoretical value of 5.916μ_B for Mn²⁺ ions (S = 5/2, g = 2) obtained by μ_eff² = gS(S + 1). As shown in Fig. 6, the value of χT is observed to be 24.72 emu mol⁻¹ K at 300 K and decreases gradually with decreasing temperature, while a rapid increase is observed at ∼20 K and reaches its maximum with 435.9 emu mol⁻¹ K, confirming the appearance of a ferromagnetic component in the system. Such ferromagnetic correlation can also be identified by the irreversible behavior of magnetic susceptibility measured with zero-field cooling and field cooling regimes below ∼18 K (the inset of Fig. 6). However, the negative Weiss temperature shows that the dominative interactions between magnetic Mn²⁺ ions are antiferromagnetic in nature. These results of negative Weiss temperature and ferromagnetic correlation suggest that magnetic transition at ∼18 K is of the ferrimagnetic type.

Fig. 5 The magnetic susceptibility and the corresponding reciprocal one of BaMn^II₉(VO₄)₆(OH)₂.

Fig. 6 The plot of χT versus temperature (T). The inset shows the magnetic susceptibilities measured with zero-field cooling (ZFC) and field cooling (FC) regimes.

Fig. 7 shows the isothermal magnetization as a function of an applied field at 2 K. The magnetization increases rapidly with the increasing field and tends to saturate at the field up to 8 T. The saturated magnetization is assumed to be ∼1.6μ_B, which is close to 1/3 saturated moment of M_S = 5μ_B for a high spin Mn²⁺ ion. This shows that a 1/3 plateau can be observed with the increasing field, supporting in return a ferrimagnetic ground state in the system. As shown in the inset of Fig. 7, the absence of hysteresis and remnant magnetization near H = 0 suggests that BaMn₉^II(VO₄)₆(OH)₂ may be a soft magnetic material.

Fig. 7 The curve of magnetization versus an applied field at 2 K. The inset shows an enlarged view at a low field.

It is well-known that a typical ferrimagnet usually consists of two heterospin carriers with an antiferromagnetic exchange coupling between the nearest neighbouring magnetic ions. A well-known example is magnetite Fe₃O₄ with a mixed valence of Fe²⁺ and Fe³⁺ ions. Compared to heterospin ferrimagnets, homospin ferrimagnets seem to be quite interesting, in which carriers are arranged in the particular way so that antiferromagnetic interaction cannot cancel the magnetic moments. This may be due to the different spin moments arising from homo-magnetic ions in special geometrical topologies or noncompensation of the magnetic moments. For BaMn₉^II(VO₄)₆(OH)₂, all of the Mn²⁺ ions seem to form a broken spin lattice of B-sites in spinels with a high frustration effect. Thus we suggest that the ferrimagnetic ground state of BaMn₉^II(VO₄)₆(OH)₂ may be due to the spins of Mn²⁺ ions arranged in a geometrically frustrated sub-lattice, which is similar to Cu₂Cl(OH)₃ ²⁰ and SrFe₃(PO₄)₃O.²¹ To judge whether spin frustration occurs in a magnetic system, an empirical measurement has been suggested by defining the value of f = |θ_CW|/T_c, where θ_CW is the Weiss temperature and T_c is the ordering temperature. The value of f > 10 indicates a strong frustration effect in magnetic systems.²² We note the f value of 9.5 for BaMn₉^II(VO₄)₆(OH)₂ on the basis of the ordering temperature (T_c) of ∼18 K and the Weiss constant (θ) of ∼172 K, supporting the frustration effect in the system.

Conclusions

A new vanadate of BaMn₉^II(VO₄)₆(OH)₂ has been synthesized by a hydrothermal method. BaMn₉^II(VO₄)₆(OH)₂ crystallizes in the cubic space group Pa3̄, which exhibits an edge-sharing MnO₆ octahedral network structure with cavities occupied by Ba(VO₄)₆¹⁶⁻ groups. The lattice built by Mn²⁺ ions shows the reverse triangular dipyramid Mn₇ in the system, which can be considered as a broken spin lattice of B-sites in spinels. The titled compound exhibits a ferrimagnetic behavior below 18 K, while a 1/3 plateau can be observed in the magnetization curve. The nature of such ferrimagnetic properties for a homospin system is suggested to originate from the spins arranged in a geometrically frustrated lattice.

Acknowledgements

This work was financially supported by the National Basic Research Program of China (no. 2012CB921701).

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Footnote

† Electronic supplementary information (ESI) available: X-ray crystallographic files in CIF format, displacement parameters (Table S1), important bond lengths and angles (Table S2), energy-dispersive X-ray spectroscopy (Fig. S1), simulated and experimental XRD patterns (Fig. S2). ICSD 427745. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt02930e

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