Two Magnetic Orderings and a Spin–Flop Transition in Mixed Valence Compound Mn₃O(SeO₃)₃

Abstract

A mixed-valence manganese selenite, Mn₃O(SeO₃)₃, was successfully synthesized using a conventional hydrothermal method. The three-dimensional framework of this compound is composed of an MnO₆ octahedra and an SeO₃ trigonal pyramid. The magnetic topological arrangement of manganese ions shows a three-dimensional framework formed by the intersection of octa-kagomé spin sublattices and staircase-kagomé spin sublattices. Susceptibility, magnetization and heat capacity measurements confirm that Mn₃O(SeO₃)₃ exhibits two successive long-range antiferromagnetic orderings with T_N1~4.5 K and T_N2~45 K and a field-induced spin–flop transition at a critical field of 4.5 T at low temperature.

1. Introduction

Mixed valence transition metal (TM) oxides with three-dimensional electronic configurations are of great significance in the fields of materials chemistry, electrochemical energy and condensed matter physics due to their diverse crystal structures and electronic configurations [1,2]. From the ancient application of Fe₃O₄ in the compass to today’s copper-based high temperature superconducting materials, mixed valence TM oxides exhibit exciting and unusual chemical and physical behaviors, including high-temperature superconductors [3], colossal magnetoresistance [4], ion deintercalation [5], metal-insulator transition [6], electrocatalysis/photocatalysis [7,8], etc. More specifically, copper oxides with bidimensional characters, together with the mixed valency of Cu⁺/Cu²⁺ or Cu²⁺/Cu³⁺, are responsible for superconducting properties [9,10]. The ferromagnetic (FM) material La_0.67Sr_0.33MnO₃ exhibits metallic conductivity due to the Zener double exchange mechanism between Mn³⁺ and Mn⁴⁺ ions, but BaFe₁₂O₁₉ (also an FM material) is insulative due to the limitation of the ratio of Fe²⁺ and Fe³⁺ ions [11]. Compound K₂Cr₈O₁₆ (hollandite), with a rare Cr³⁺/Cr⁴⁺ mixed valence state, exhibits a metal-insulator transition in a FM state [12]. The transition metal valence state of cathode material LiMO₂ (M = Mn, Co, Ni) will switch back and forth between M²⁺ and M⁴⁺ during charging and discharging processes in Li-ion batteries [13,14]. X. Yu et al. reported the experimental observation of skyrmionic bubbles with various topological lattices in colossal magnetoresistive manganite La_1−xSr_xMnO₃ [15]. In order to discover new materials with unusual physical/chemical properties, it is necessary to explore new mixed-valence transition metal compounds. The compound Mn₃O(SeO₃)₃ (Mn^IIMn^III₂O(SeO₃)₃) was first reported by Wildner [16]. Structure analysis confirmed that this compound shows a channel structure with a three-dimensional magnetic topological framework formed by the intersection of octa-kagomé spin sublattices and staircase-kagomé spin sublattices; however, there are few studies regarding its magnetic properties. In this paper, we report the discovery of a mixed valence manganese selenate Mn₃O(SeO₃)₃. Magnetic measurements indicate that this compound possesses two successive antiferromagnetic (AFM) transitions at low-temperature. Moreover, a spin–flop transition is observed at 2 K with an applied magnetic field of ~4.5 T.

2. Experimental Section

2.1. Synthesis of Mn₃O(SeO₃)₃

Single crystals of Mn₃O(SeO₃)₃ were obtained using a conventional hydrothermal method. A mixture of 2 mmol Mn(NO₃)₂·xH₂O (3 N, 0.3943 g), 2 mmol LiI (2 N, 0.2704 g), 1 mmol SeO₂ (4 N, 0.1110 g) and 5 mL deionized water was sealed in an autoclave equipped with a Teflon liner (28 mL). The autoclave was gradually heated to 230 °C at a rate of 1 °C/min, held for 4 days and then naturally cooled to room temperature. The product contained the desired black noodle-like crystals with a 90% yield. The crystals’ sizes and morphologies were characterized using a stereomicroscope and field emission scanning electron microscopy (FE-SEM, SU8100, Hitachi, Tokyo, Japan). Figure S1 shows images of the crystals under the stereomicroscope and FE-SEM. It can be observed that the crystal size is approximately 0.3 × 0.08 × 0.05 mm. The product’s impurities were manually removed under a microscope. Powdered samples were prepared for physical measurement by crushing small single crystals; purity was confirmed by powder X-ray diffraction (XRD) analysis (Figure 1). Moreover, the reagent LiI acted as a mineralizer, as the quality of crystals was unsatisfactory without it.

2.2. Methods

XRD patterns were collected using a Bruker D8 diffractometer with Cu-K_α radiation (λ~1.5418 Å) at room temperature. Rieltveld refinement was performed using GSAS-EXPGUI software [17]. Refined crystal structures were analyzed using VESTA software [18]. Furthermore, element analysis was observed using FE-SEM with an X-ray energy-dispersive spectrometer (EDS). EDS analysis confirmed the molar ratio of Mn/Se as 3.1/2.0, which is in good agreement with the X-ray structure analysis. Thermogravimetric analysis (TGA) of Mn₃O(SeO₃)₃ was collected on NETZSCH STA 449C instruments with an Al₂O₃ crucible from 50 to 900 °C at a rate of 10 °C/min under N₂ atmosphere.

Magnetic measurements of a powdered sample of Mn₃O(SeO₃)₃ were performed using a PPMS (Quantum Design, San Diego, CA, USA). The powdered sample (20.6 mg) was placed in a plastic capsule, which was suspended in a copper tube slot. Magnetic susceptibility was measured at 0.1 T from 2 to 300 K. Magnetization was measured at different temperatures at applied field from 0 to 9 T. Heat capacity was measured with the same PPMS system at zero field and determined using a relaxation method on a 5.6 mg sample.

3. Results and Discussion

The structure of compound Mn₃O(SeO₃)₃ was first reported by Wildner [16]. Mn₃O(SeO₃)₃ crystallizes in the monoclinic system with the space group C2/m. As shown in Figure S2, both Mn and Se atoms have three crystallographic sites. The oxidation state is +2 for Mn1 and +3 for Mn2/Mn3. All manganese atoms are coordinated by six oxygen atoms forming MnO₆ distorted octahedra; Mn–O bond lengths range from 2.100(1) to 2.361(8) Å for Mn1²⁺O₆ octahedra and from 1.854(2) to 2.310(6) Å for Mn2³⁺O₆ and Mn3³⁺O₆, respectively. In other words, the degree of distortion for Mn1²⁺O₆ octahedra is smaller than that of Mn2³⁺O₆ and Mn3³⁺O₆. This is due to the Mn³⁺ (t_2g³e_g¹) octahedron with a remarkable Jahn–Teller effect, which may induce a larger structure distortion than Mn²⁺ (t_2g³e_g²). All selenium atoms are in trigonal pyramid geometry with a stereoactive lone pair of 4 s² in Se⁴⁺ ions; the Se-O bond lengths are approximately 1.70 Å. It should be noted that Se1/Se2/Se3 atoms are surrounded by 4/5/6 manganese atoms with a Se-O-Mn route, respectively. These 4/5/6 manganese atoms contain two Mn²⁺ atoms and 2/3/4 Mn³⁺ atoms, respectively.

As shown in Figure 2, Mn₃O(SeO₃)₃ shows a tunnel structure along the b-axis, in which the framework is constituted by MnO₆ octahedra and SeO₃ trigonal pyramids. Mn1O₆ octahedra share their edges (O5–O6) to form uniform [-Mn1-] chains along the b-axis. Mn³⁺O₆ octahedra are interconnected via edge-sharing oxygen atoms, forming a two-dimensional [-Mn³⁺-] layered structure parallel to (001). The detailed linkage mode between manganese ions is shown in Figure 3. Two Mn2O₆ octahedra connect to each other by edge-sharing oxygen atoms (O4–O4) to form a [Mn₂O₁₀] dimer along the a-axis. The Mn2-O4-Mn2 angle is 101.34(9)°. Mn3O₆ octahedra are interconnected by corner-sharing oxygen atoms (O7) to form uniform [-Mn3-] chains along the b-axis. One Mn2O₆ octahedron and two Mn3O₆ octahedra are connected in an isosceles triangle configuration. The neighbored [-Mn³⁺-] layers are separated by [-Mn1-] chains and SeO₃ trigonal pyramids. Furthermore, we noted that Mn1O₆ octahedra are interconnected with Mn2O₆ octahedra via conner-sharing O5 atoms, but Mn1O₆ and Mn3O₆ octahedra are connected by SeO₃ groups in the manner of Mn1-O-Se-O-Mn3. After removing the nonmagnetic SeO₃²⁻ groups, the topological arrangement of magnetic Mn ions is a three-dimensional framework (Figure 2b). Mn³⁺ ions form a two-dimensional octa-kagomé lattice parallel to (001) (Figure 2c). The adjacent octa-kagomé layers are connected by [-Mn1-] chains. It is significant that there is a staircase-kagomé lattice composed of Mn²⁺ and Mn³⁺ parallel to (100) in the magnetic topological framework (Figure 2d). The shortest Mn–Mn distance in both [-Mn1-] and [-Mn3-] chains is 3.332(9) Å. However, the detailed connection mode of MnO₆ octahedra in [-Mn1-] chains are edge-sharing, whereas in [-Mn3-] chains it is corner-sharing. The Mn-O-Mn angles in [-Mn1-] and [-Mn3-] chains are 89.77(1)°/102.90(8)° and 127.61(5)°, respectively. The Mn2--Mn2 and Mn2–Mn3 distances in the octa-kagomé lattice are 3.150(0) Å and 3.069(6) Å, respectively, whereas the Mn1–Mn2 distance is 3.902(1) Å.

Figure 4a shows the temperature dependence of magnetic susceptibility χ(T) of Mn₃O(SeO₃)₃ measured at 0.1 T. Magnetic susceptibility increases with decreasing temperature; two peaks can be observed at T_N1~4.5 K and T_N2~45 K., showing AFM transitions. At high temperature (80–300 K) inverse susceptibility χ⁻¹(T) follows the Curie–Weiss law with a Weiss temperature of θ = −8.89 K and a Curie constant of C = 11.03 emu·mol⁻¹·K. The effective magnetic moment is calculated to be μ_eff = 5.42(3) μ_B, obtained by μ²_eff = 8C/n, where n = 3. This value of μ_eff is slightly smaller than the spin-only value of 5.91(6) μ_B for Mn²⁺ (3d⁵, high spin) and larger than the spin-only value of 4.89(9) μ_B for Mn³⁺ (3d⁴, high spin). As Mn ions are mixed-valent, the theoretical magnetic moment of the titled compound is μ_theo = 5.25(9) μ_B obtained by the equation μ²_eff = [μ²_eff (Mn²⁺) + 2μ²_eff (Mn³⁺)]/3. The value of μ_eff is quite close to that of μ_theo, confirming that Mn ions in the structure are mixed valence. The negative value of θ suggests the presence of dominative AFM interactions between neighboring Mn ions. Figure S3 shows the χT-T curve, in which the value of χT decreases with decreasing temperature, which is characteristic of typical AFM interactions. As shown in Figure 4b, the heat capacity data of Mn₃O(SeO₃)₃ show a λ-type peak at T~45 K and a corner-type transition at 4.5 K, providing concrete evidence for the two long-range magnetic orderings observed in the magnetic susceptibility curves.

To further investigate the magnetic properties of the system, magnetization (M) as a function of applied field (H) was observed at 30 K and 2 K. As shown in Figure 5, at 30 K, magnetization increased linearly with increasing field, and did not saturate at 9 T. Furthermore, no hysteresis or remanent magnetization was observed. These features of the M–H curve suggest that the magnetic anomaly at T~45 K is the onset of an AFM ordering. At 2 K, the magnetization (M) shows a linear increase in magnetization at low field, indicative of a characteristic AFM ground state. A clear change in slope in the magnetization is observed at approximately 4.5 T, indicating field-induced magnetic transition. Furthermore, no hysteresis can be observed on the M–H curve.

It is well-known that the magnetic properties of solid magnets are strongly related to their structural features. The three-dimensional manganese topological framework of Mn₃O(SeO₃)₃ is formed by the intersection of octa-kagomé lattices and staircase-kagomé lattices. Firstly, we know that the kagomé-like lattices containing equilateral or isosceles triangle sublattices may exhibit strong frustrated magnetic properties [19,20,21]. We note the value of frustration factor, f =|θ|/T_N~0.20, with Weiss temperature θ = −8.89 K and Neel temperature T_N2~45 K, ruling out spin frustration in the system. It is well known that primary magnetic interactions originate from the superexchange of Mn-O-Mn. A detailed description of superexchange interactions is shown in Figure 2c,d and itemized in

Table 1. Geometric parameters of dominant magnetic superexchanges in Mn₃O(SeO₃)₃ *.

J1	d(Mn-O) (Å)	d(Mn-Mn) (Å)	Mn-O-Mn Angle (°)	Magnetism
J1	Mn1-2.130(7)-O6-2.130(7)-Mn1 Mn1-2.361(4)-O5-2.361(4)-Mn1	3.332(9)	Mn1-O6-Mn1…102.90(8)AFM-w Mn1-O5-Mn1…89.77(1)FM-S	FM
J2	Mn3-1.857(1)-O7-1.857(1)-Mn3	3.332(9)	Mn3-O7-Mn3…127.61(5)AFM-S	AFM
J3	Mn1-2.361(4)-O5-2.278(1)-Mn2	3.902(1)	Mn1-O5-Mn2…114.49(4)AFM-S	AFM
J4	Mn2-2.049(5)-O3-2.310(4)-Mn3 Mn2-1.854(2)-O7-1.857(1)-Mn3	3.069(6)	Mn2-O3-Mn3…89.29(7)FM-S Mn2-O7-Mn3…111.59(8)AFM-S	?
J5	Mn2-1.896(5)-O4-2.169(5)-Mn2 Mn2-2.169(5)-O4-1.896(5)-Mn2	3.150(0)	Mn2-O4-Mn2…101.34(9)AFM-w × 2	AFM

* The S or W behind AFM or FM (in the Mn-O-Mn Angle (°) column) refers to the magnitude of the J value; S refers to strong and W refers to weak.

; there are five main magnetic exchange interactions, numbered J₁–J₅, within the three-dimensional spin-lattice according to the Goodenough–Kanamori–Anderson rules (GKA rules) [22]. There are two isosceles triangular topological configurations composed by J₁–J₄ on the staircase-kagomé spin sublattice. According to the GKA rules, J₂ and J₃ are AFM interactions. J₁ and J₄ both have two Mn-O-Mn superexchange interactions, as the corresponding MnO₆ octahedra share their edges. In general, the spin exchange parameter, J, can be written as J = J_FM + J_AFM, where J_FM indicates ferromagnetic exchange and J_AFM indicates antiferromagnetic exchange (J_FM > 0 and J_AFM < 0) [23]. So, J₁ = J_1FM(O5) + J_1AFM(O6), and the AFM interaction via O6 in J₁ is negligible; this means that J₁ ≈ J_1FM(O5) > 0. Using the same analytical method, the spin exchange parameter, J₄, is ambiguous, as the magnitude of J_4AFM(O7) is difficult to calculate. J₅ should be a weak AFM interaction. This analysis indicates that AFM interactions are dominant in the system, which is consistent with the negative value of θ. It is significant that the neighboring octa-kagomé sublattices are separated by [-Mn1-] chains and that the neighboring staircase-kagomé sublattices are connected by [Mn₂O₁₀] dimers. It is safely said that these two long-range magnetic orders are driven by interlayer magnetic coupling. It is noteworthy that [-Mn1-] chains are composed of Mn²⁺ ions. If Mn²⁺ ions are replaced with nonmagnetic ions with a radius similar to Mn²⁺, such as Mg²⁺ or Zn²⁺ [24], an octa-kagomé lattice composed of Mn³⁺ ions might form. The expected compounds may exhibit spin-liquid or other quantum physical properties [25,26]. Exploratory synthesis of related compounds is underway.

As shown in Figure 6, Mn₃O(SeO₃)₃ undergoes a slow weight gain of approximately 0.20% from 100 to 200 °C and a slow weight loss of approximately 0.40% from 300 to 400 °C. As the weight of the sample is only approximately 7 mg, slight weight gain/loss may be caused by instrument error. As the temperature rises, two successive steps of weight loss of approximately 57% occur from 450 to 630 °C, corresponding to the calculated 59% loss of 3SeO₂. The 2% difference may also be caused by instrument error. Based on this analysis, the final residues of Mn₃O(SeO₃)₃ should be Mn₃O₄; however, this was difficult to characterize, as the residues melted in the Al₂O₃ crucible after being heated to 900 °C. We re-selected the sample and sintered it in a smooth quartz crucible at 800 °C in a nitrogen atmosphere for 10 min. We then scraped off the sintered product and performed powder X-ray diffraction analysis. As shown in Figure S4, the residue was confirmed as Mn₃O₄ (PDF #80-0382). This result is consistent with the decomposition characteristics of most manganese-based compounds.

4. Conclusions

A mixed-valence compound, Mn₃O(SeO₃)₃, was successfully synthesized using a conventional hydrothermal method. The reagent LiI acted as a mineralizer. Mn₃O(SeO₃)₃ was shown to have a channel structure with a three-dimensional magnetic topological framework formed by the intersection of octa-kagomé spin sublattices and staircase-kagomé spin sublattices. Magnetic and specific heat data confirmed that Mn₃O(SeO₃)₃ exhibits two successive long-range AFM orderings with T_N1~4.5 K and T_N2~45 K, and a field-induced spin–flop at a 4.5 T critical field at low temperature. Moreover, magnetic measurements confirmed that the ratio of Mn²⁺/Mn³⁺ ions in this compound is 1:2, which is consistent with the structural analysis. The exploratory synthesis of Mg²⁺ or Zn²⁺ replaced compounds is in progress.