Molecule-like cluster magnetism and cationic order in the new hexagonal perovskite Ba₄Sn_1.1Mn_2.9O₁₂

Abstract

The new hexagonal perovskite phase of composition Ba₄Sn_1.1Mn_2.9O₁₂ has been synthesized by solid-state reactions at 1673 K. The crystal structure has been investigated using X-ray and neutron diffraction. The hexagonal perovskite structure has an ordered arrangement of Sn and Mn ions on the corner-sharing octahedral centers and the face-sharing octahedral centers respectively. Short Mn–Mn distances have been evidenced in the face-sharing trimer of MnO₆ octahedra. The magnetic susceptibility shows magnetic cluster behavior, with cluster formation temperature ∼220 K. Antiferromagnetic order has been observed at T_N ∼ 6 K. Ba₄Sn_1.1Mn_2.9O₁₂ is a semiconductor with a transport activation energy of 0.61 eV.

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1 Introduction

Manganese oxides have attracted enormous attention during the last decades mainly due to their technological applications as heterogeneous catalysts,¹ Li batteries,^2,3 SOFC electrode materials,⁴ and magnetic⁵ and also CMR materials.⁶ This has motivated the investigation of the hexagonal perovskite BaMnO_3−δ system, which shows a diversity of crystal structures and magnetic properties.^7–11 A series of BaMnO_3−δ polytypes have been found, for example, 2H → 15R → 8H → 6H → 10H → 4H, as the oxygen vacancy δ increases.^7,10 Additional polymorphs (i.e. 9R-Ba_0.875Sr_0.125MnO₃) are generated upon Sr substitution on the Ba site.¹⁰ Further perovskite polytypes may be produced when Mn is replaced by other cations.

Many Mn-substituted BaM_xMn_1−xO₃ systems have been investigated, for instance, with M = Ca,^12,13 Ti,^14,15 In,^16,17 Sn,¹⁸ Sb,¹⁹ Fe,²⁰ Ru,^21,22 Ir,²³ and rare earth Ln.^24,25 This led to the uncovering of a range of M-cation (partially) ordered 12R perovskites Ba₄Ti₂Mn₂O₁₂,¹⁴ Ba₄InMn₃O_11.5,¹⁶ Ba₄REMn₃O₁₂ (RE = Ce and Pr),²⁴ Ba₄YMn₃O_11.5,²⁶ and Ba₄NbMn₃O₁₂.²⁷ The Ba₄MMn₃O₁₂ 12R perovskite consists of close-packed BaO₃ layers in a (cchh)₃ sequence. The larger-sized substituted M ions are hosted in the corner-sharing octahedral center M1, and the Mn ions occupy the face-sharing octahedral centers M2 and M3, as shown in Fig. 1a. Partial disorder of M and Mn elements occurs on the M2 site when the M ion has a similar size with 6-coordinated Mn⁴⁺ (0.53 Å), i.e. Ti⁴⁺ (0.605 Å, CN = 6).²⁸

Fig. 1 The crystal structures viewed along [110] of (a) 12R Ba₄MMn₃O₁₂ and (b) 10H Ba₅MMn₄O₁₅, showing the stacking sequence of BaO₃ and corner-sharing octahedron center M1 and face-sharing octahedron center M2 and M3. The MnO₆ and MO₆ octahedra are shaded in yellow and green respectively. The barium and oxygen atoms are shown as large cyan and small red spheres.

The BaMnO_3−δ polymorphs show the long-range antiferromagnetic (AFM) order of all the Mn spins, with T_N = 220–270 K.^9,10 However, the magnetic order is suppressed in the 12R perovskites with trimers of face-sharing MnO₆ octahedra on low-content nonmagnetic doping. No magnetic order is observed for Ba₄InMn₃O_11.5 (ref. 16) and Ba₄Ti₂Mn₂O₁₂,¹⁴ whilst Ba₄YMn₃O_11.5 (ref. 26) and Ba₄CeMn₃O₁₂ (ref. 29) undergoes an AFM transition at T_N = 4–6 K. The Ba₄NbMn₃O₁₂ sample experiences a ferrimagnetic transition at 42 K and an additional magnetic abnormality at ∼6 K.²⁷ The latter one should be AFM transition, as suggested by first-principle calculations.³⁰ Furthermore, Ba₄NbMn₃O₁₂ and Ba₄Ti₂Mn₂O₁₂ show cluster magnetism, with linear Mn₃ (and Mn₂) magnetic clusters arranged in triangular plains, leading to magnetic frustrations.^30,31 Therefore, the 12R perovskite materials are potential quantum materials, such as quantum spin liquid.^32,33 The exploration of new 12R perovskite BaM_xMn_1−xO₃ is of great interest.

The BaSn_1−xMn_xO₃ system contains one cation-ordered perovskite 10H Ba₅Sn_1.1Mn_3.9O₁₅.¹⁸ The Ba₅MMn₄O₁₅ 10H polytype has close-packed BaO₃ layers in (cchhh)₂ sequences, and features tetramers of face-sharing MnO₆ octahedra, alternating with MO₆ octahedra (Fig. 1b). Considering the close relationship between the 10H and 12R perovskites, we wonder whether the 12R polymorph BaSn_1−xMn_xO₃ can exit. If the 12R BaSn_1−xMn_xO₃ is stabilized, it would be a good material with magnetic clusters and frustration. Motived by such ideas, we investigate a series of BaSn_1−xMn_xO₃. Herein we report the synthesis, crystal structure, and physical properties of 12R Ba₄Sn_1.1Mn_2.9O₁₂ perovskite.

2 Experimental

2.1 Synthesis

The polycrystalline Ba₄Sn_1+xMn_3−xO₁₂ (−0.1 < x < 0.2) were prepared via high-temperature solid-state reaction, using BaCO₃ (99.5%, Alfa Aesar), SnO₂ (99%, Alfa Aesar), and MnO₂ (99.5%, Alfa Aesar) reagents as raw materials. These starting materials were carefully weighed according to the correct stoichiometries, and mixed thoroughly with alcohol in an agate mortar and pestle, and heated in an alumina crucible at 1273 K for 10 h to decompose the carbonate. Then the material was reground, pressed into pellets (20 tons per cm²), and placed on the platinum foil, and sintered at 1673 K for a total of 24 h with heating and cooling rates of 5 K min⁻¹, leading to dense pellet ceramic samples. At several intervening steps, the material was re-ground and re-pressed, and powder X-ray diffraction (XRD) was performed at each step to track the reaction to completion. The densities of the final ceramic pellets were calculated using their geometric sizes (diameters and thicknesses) and the masses.

2.2 Characterization

The phase purity of the samples was characterized by XRD using a Panalytical X'pert Powder diffractometer with Cu Kα radiation operated at 40 kV and 40 mA. Data were collected over the 2θ range 5–120° for Rietveld analysis carried out using the Topas Academic software.³⁴ Bond valence sums (BVS) were calculated by Brown and Altermatt's method.³⁵

Time-of-flight (TOF) neutron diffraction data were collected from ∼5 g fine powder samples of Ba₄Sn_1.1Mn_2.9O₁₂ using the general-purpose powder diffractometer (GPPD) at the China Spallation Neutron Source (CSNS). Selected area electron diffraction (SAED) patterns were recorded using a JEOL JEM-2100F transmission electron microscopy (TEM) with a point resolution of 1.9 Å and operated at 200 kV.

Magnetic susceptibility data were acquired using a Quantum Design MPMS-3 superconducting quantum interference device (SQUID) magnetometer. DC magnetic susceptibilities were recorded in a 1000 Oe applied field while warming the sample from 2 to 300 K, following both zero-field cooling (ZFC) and field cooling (FC). AC magnetic susceptibilities from 2 K to 10 K were also measured at step mode at different frequencies of 100 Hz, 250 Hz, 500 Hz, and 1000 Hz.

AC impedance spectroscopy (IS) measurements were carried out from room temperature (RT) to 773 K using a Solartron 1260 frequency response analyzer over the 10⁻¹–10⁷ Hz frequency range. Before the IS measurements, the silver paste was coated on the opposite faces of the pellets and fired at 823 K for 30 min to remove the organic component and form electrodes. The impedance data analysis and equivalent circuit fitting were carried out with Zview software.

Inductively coupled plasma optical emission spectroscopy (ICP-OES) was performed on the Ba₄Sn_1.1Mn_2.9O₁₂ sample to determine the cationic contents using a PerkinElmer Optima 8000 inductively coupled plasma optical emission spectrometer. The scanning electron microscopy (SEM) imaging and X-ray energy dispersive spectroscopy (EDS) elemental analysis were performed using a GeminiSEM 300 (ZEISS, Germany) scanning electron microscope equipped with an Ultim Max (Oxford, U.K.) EDS spectrometer.

3 Results and discussions

3.1 Phase formation

The initial Ba₄SnMn₃O₁₂ samples consisted of mixed 12R and 10H perovskite phases (Fig. 2). The Sn doping amount was varied to eliminate the 10H phase, and the XRD patterns of the Ba₄Sn_1+xMn_3−xO₁₂ are shown in Fig. 2. The amount of the 10H phase is reduced at an increase of x from −0.1 to 0.1 and a single-phase 12R product is obtained at the composition of Ba₄Sn_1.1Mn_2.9O₁₂. Further increasing the Sn substitution rate leads to a secondary Sn-rich cubic perovskite phase, i.e. BaSnO₃, as marked with an asterisk. The ICP analysis on the Ba₄Sn_1.1Mn_2.9O₁₂ sample shows the Ba : Sn : Mn ratio of 4.03 : 1.07 : 2.84, which is essentially identical to the initial cationic ratio. This observation confirms the chemical composition of Ba₄Sn_1.1Mn_2.9O₁₂ for the 12R perovskite phase.

Fig. 2 X-ray diffraction patterns of our Ba₄Sn_1+xMn_3−xO₁₂ samples, showing a pure phase 12R perovskite at the x = 0.1 composition. The inset enlarges the XRD plots of the x = 0 sample for clarity. The reflections from 10H Ba₅Sn_1.1Mn_3.9O₁₅ and BaSnO₃ are marked with arrows and asterisks respectively.

The XRD pattern of Ba₄Sn_1.1Mn_2.9O₁₂ is similar to that of Ba₄NbMn₃O₁₂ (ref. 27) and can be indexed using a rhombohedral hexagonal unit cell with lattice parameters a = 5.7289(4) Å, and c = 28.163(3) Å. The selected area electron diffraction (SAED) pattern along [100] zone axis is shown in Fig. 3. No superstructure reflections are seen, and all the observed reflections agree with the indexing results. The systematic absence of the reflections hh̄0l: h – l ≠ 3n and 000l: l ≠ 3n is consistent with the space group R3̄m.

Fig. 3 The SAED patterns collected along [100] direction for Ba₄Sn_1.1Mn_2.9O₁₂. The indexations were performed using the hexagonal cell (a ∼ 5.72 Å and c ∼ 28.2 Å).

3.2 Crystal structure

Due to their weak X-ray scattering ability, the light oxygen atoms are not well located with X-ray diffraction in the presence of heavy elements such as barium. Neutron diffraction (ND) is more sensitive to oxygen because the neutron scattering length of oxygen (5.083 fm) is comparable to that of Ba (5.07 fm).³⁶ Therefore, to find the oxygen position and possible vacancies, neutron diffraction data were collected on the Ba₄Sn_1.1Mn_2.9O₁₂ sample. The Rietveld refinements against the XRD and ND data simultaneously were performed, using the 12R Ba₄NbMn₃O₁₂ structural model (Fig. 1a). In the first step, the occupancies of Sn and Mn at M1, M2, and M3 sites were refined with the total occupancies on each site subject to unity. A negative value of Sn on the M3 site was found, excluding the presence of Sn at the M3 site. In the following refinement, the occupancies of Sn and Mn were refined only at M1 and M2 sites, with the ratio of Sn : Mn constrained by the initial stoichiometry. In the second step, the oxygen occupancies were refined, and no vacancies were found at both O1 and O2 sites within the refinement error. Therefore, the oxygen occupancies at both sites were set as unity. The final refinement gave a good fit (Fig. 4a and b), with overall R_wp and χ² parameters of 4.99% and 1.20 respectively. The refined structural parameters and selected bond distances and angles are listed in Tables 1 and 2, respectively. The Beq parameters for atoms of the same type on the different sites show a small difference within 20%. The large discrepancy of Beqs between the M1 and M2 sites keeps with their different dominated elements, i.e. Sn and Mn respectively.

Fig. 4 Rietveld refinement plots of (a) XRD data and (b) ND data for the Ba₄Sn_1.1Mn_2.9O₁₂ sample, showing observed (red crosses), calculated (green line) and difference curve (grey line). The blue bars mark the reflection positions for Ba₄Sn_1.1Mn_2.9O₁₂ in both plots.

Table 1 Refined structural parameters of Ba₄Sn_1.1Mn_2.9O₁₂^a

Atom	Site	x, y, z	Occupancy	Beq	BVS
a a = 5.72904(3) Å, and c = 28.1637(3) Å space group: R3̄m.
Ba1	6c	2/3, 1/3, 0.04672(5)	1	0.94(7)	2.37
Ba2	6c	0, 0, 0.12840(4)	1	0.88(7)	2.23
(Sn/Mn)M1	3a	0, 0, 0	0.881(3)/0.119(3)	0.83(3)	4.24/2.81
(Mn/Sn)M2	6c	1/3, 2/3, 0.07695(8)	0.891(1)/0.109(1)	1.46(5)	3.84/5.43
MnM3	3b	1/3, 2/3, 1/6	1	1.19(6)	4.10
O1	18h	0.48062(9), 0.51938(9), 0.12380(4)	1	1.07(6)
O2	18h	0.49856(11), 0.50144(11), 0.29189(4)	1	1.20(6)

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Table 2 Selected bond lengths and angles of Ba₄Sn_1.1Mn_2.9O₁₂

Bond length (Å)	Bond length (Å)	Bond angle (degree)
(Sn/Mn)M1–O2 (×6) 2.0334 (10)	MnM3–O1 (×6) 1.8933 (9)	(Sn/Mn)M1–O2–MnM2 176.400 (2)
(Mn/Sn)M2–O1 (×3) 1.966 (3)	MnM2–MnM3 2.523 (5)	MnM2–O1–MnM3 81.626(3)
(Mn/Sn)M2–O2 (×3) 1.918 (3)

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In the crystal structure of 12R Ba₄Sn_1.1Mn_2.9O₁₂, the M1 site is dominated by the Sn atom (occupancy ∼ 88.1%), and the M2 site is dominated by the Mn atom (occupancy ∼ 89.1%). The BVS values of Mn atom at M1 and M2 sites (2.81 and 3.84 respectively) are smaller than 4, whilst that of Sn atom shows larger values than 4 (4.24 at M1 site and 5.43 at M2 site). This discrepancy arises from the mixed Sn and Mn occupancy because the average bond lengths are used in BVS calculations. But both Mn and Sn atoms most likely show the oxidation states of +4 on local M1 and M2 sites, although trace amounts of Mn³⁺ at M1 site and compensating oxygen vacancies (within the refinement errors) cannot be excluded. Similar phenomena have been observed in the 10H Ba₅Sn_1.1Mn_3.9O₁₅ polymorphs.¹⁸

It should be noted about the distortion of outer MnO₆ octahedra in the trimers. The Mn at the M2 site shifts toward the c-BaO₃ layers, forming three shorter Mn_M2–O2 bonds (∼1.91 Å) and three longer Mn_M2–O1 bonds (∼1.96 Å). The inner Mn_M3O₆ octahedron is undistorted and constrained by its local symmetry of D_3d on the M3 site. Due to the distortion, the Mn–Mn distance inside the trimers (∼2.52 Å) is larger than that (2.407 Å) between undistorted MnO₆ octahedrons in 2H BaMnO₃.³⁷ This suggests that octahedron distortion may help to stabilize the formation of MnO₆ octahedral trimers. This short Mn–Mn distance is comparable with that in elemental α-Mn³⁸ and may suggest a degree of d-orbital overlap between neighboring Mn ions within each trimer forming Mn₃ (and minor Mn₂) magnetic clusters.

Thermal stability was investigated by the temperature varied XRD, as shown in Fig. 5a. No phase transition exists in the title compound until 873 K. The lattice parameters increase linearly as the temperature increases, showing anisotropic thermal expansions of α_a = 12.5 × 10⁻⁶ K⁻¹ and α_c = 10.6 × 10⁻⁶ K⁻¹. This leads to an overall linear volume expansion of α_V = 35.9 × 10⁻⁶ K⁻¹ at 300–873 K temperature range (Fig. 5b).

Fig. 5 Temperature-dependent (a) XRD data for Ba₄Sn_1.1Mn_2.9O₁₂ and (b) cell parameters in relative to the RT values obtained from the XRD data (a₀ = 5.72904(3) Å, c₀ = 28.1637(2) Å, V₀ = 800.54(1) ų), with the fits shown as the solid lines.

3.3 Magnetic properties

The temperature-dependent DC magnetic susceptibilities per Mn ions are shown in Fig. 6a. The susceptibilities change little at 150–300 K and start to increase steeply as the temperature goes down. The maximum value of ZFC susceptibility is observed at about 6.0 K, below which ZFC and FC data are divergent. It suggests an antiferromagnetic (AFM) phase transition around T_N = 6 K.

Fig. 6 (a) Temperature-dependent ZFC (filled circles) and FC (empty circles) magnetic susceptibilities for Ba₄Sn_1.1Mn_2.9O₁₂ per mole of Mn(IV), with low-temperature data shown in the inset. The blue and red lines represent the fitted paramagnetic and the cluster contributions, noted as χ_p and χ_cluster respectively. (b) Inverse susceptibility per mole of Mn(IV). The dashed lines show the slopes corresponding to μ_eff = 1.9 μ_B and μ_eff = 4.2(1) μ_B. The inset to (b) shows the schematics of the possible simple hypothetical arrangements for Mn⁴⁺ (S = 3/2) in the Mn₃ and Mn₂ magnetic clusters of Ba₄Sn_1.1Mn_2.9O₁₂.

Surprisingly, the magnetic susceptibilities above 6 K do not obey the Curie–Weiss law, as shown in Fig. 6b. The attempt fit on 150–300 K yields an μ_eff = 4.2(1) μ_B per Mn⁴⁺, reasonably close to the spin-only value of Mn⁴⁺ 3.9 μ_B, but an enormously large θ_CW = −565 K compared with T_N. The Curie–Weiss fit on 25–80 K yields a much smaller μ_eff = 1.9(1) μ_B per Mn⁴⁺ and a small θ_CW = −22 K. This reduced magnetic moment at intermediate temperatures indicates the formation of AFM magnetic clusters. Considering the crystal structure, the magnetic clusters are predominantly the Mn₃ (Fig. 6b inset) inside the trimers of face-sharing MnO₆ octahedra. The minor Mn₂ clusters (Fig. 6b inset) are also present due to diamagnetic Sn⁴⁺ replacing ∼10.9% Mn⁴⁺ on the M2 site. The Mn₃ (and Mn₂) magnetic linear clusters are observed in the 12R Ba₄NbMn₃O₁₂ (ref. 27 and 30) and Ba₄Ti₂Mn₂O₁₂ (ref. 31) perovskite. The susceptibility of these magnetic clusters, χ_cluster, can be estimated using an analog expression of a spin S = ½ dimmer.³⁹

D is related to magnetic moments involved in cluster formation, while θ′ corresponds to the temperature cluster forms. The total experimental susceptibilities χ can be divided into three parts: χ_cluster, a paramagnetic contribution χ_p = C/(T – θ), and the constant term χ_b. The last term χ_b has two origins: the susceptibility of the sample capsule determined as −6.2 × 10⁻⁶ emu (or equally −1.3 × 10⁻⁴ emu mol⁻¹ Mn⁴⁺) with a blank measurement, and the diamagnetic susceptibility of Ba₄Sn_1.1Mn_2.9O₁₂, which can be estimated to be −2.91 × 10⁻⁴ emu f.u.⁻¹ (or equally −1.00 × 10⁻⁴ emu mol⁻¹ Mn⁴⁺) using Pascal's constant.⁴⁰ Therefore the constant term χ_b is fixed as −2.3 × 10⁻⁴ emu mol⁻¹ Mn⁴⁺ in the following least-square fits against the 10–300 K susceptibilities.

Good fits were obtained (Fig. 6a), leading to C = 0.42(1) K emu Oe⁻¹, θ = −17.1(1) K, D = 0.99(1) K emu Oe⁻¹, and θ′ = 218.7(1) K. The negative Weiss constant indicates antiferromagnetic interactions between the Mn spins. The θ = −17.1(1) K is comparable with the T_N as expected. The cluster formation temperature θ′ depends on the magnetic exchange interaction inside the MnO₆ trimers and is reasonably close to the exchange constant (J_a/k_B = 200 K) in the dimer and trimer of MnO₆ octahedra.³⁰ The overall magnetic moment 3.05(1) μ_B mol⁻¹ Mn⁴⁺, as calculated using μ_eff = (8C + 6D)^1/2, is lower than its spin-only value (3.87 μ_B). This small difference can be acceptable given the presence of both Mn₂ and Mn₃ clusters and the roughness of the cluster model. If we ignore the contribution of minor Mn₂ clusters, then the effective magnetic moments μ_eff per Mn₃ linear cluster 3.17(1) μ_B, as calculated from (24C)^1/2, is essentially identical with the calculated value of 3.05 μ_B mol⁻¹ Mn⁴⁺. This observation seems coincidental and comes from the AFM nature of the Mn₃ linear clusters. Similarly, the Ba₄NbMn₃O₁₂ has one Mn³⁺ ion and two Mn⁴⁺ ions in each trimer of MnO₆ octahedra, thus the μ_eff per Mn₃ cluster is equal to the μ_eff of Mn³⁺.^27,30 The minor Mn₂ clusters are expected to have a ground state of μ_eff = 0 μ_B. The residual Mn spins of dominant Mn₃ clusters order at temperatures below T_N = 6 K via antiferromagnetic Mn⁴⁺–O–(Sn⁴⁺/Mn⁴⁺)_M1–O–Mn⁴⁺ exchange interactions. Similar AFM order below 10 K has been observed for the 12R Ba₄MMn₃O_12−δ (M = Y, Nb, and Ce) with Mn₃ linear clusters.^25–27

In addition, the AC magnetic susceptibility χ′ shows a frequency-independent maximum at about 6 K, as shown in Fig. 7a. This observation rules out a spin glass transition at T_N = 6 K and suggests the long-range AFM transition instead. The AFM order is most likely canted, which leads to a weak ferromagnetic component below T_N, although further neutron diffraction study would be useful to characterize this ground state more fully. This is evidenced by small magnetic hysteresis and saturation in the 2 K M–H curve (Fig. 7b), showing magnetic anisotropy with coercive fields of 50 mT and a saturated moment M_s = 0.09(1) μ_B. These ferromagnetic components disappear above T_N, as indicated by the linear M–H curve at 20 K and 100 K. The weak ferromagnetism below T_N may come from trace amounts of Mn³⁺ at M1 site. The hexagonal average lattice symmetry does not allow long-range orbital order, but when Mn³⁺ ions are present at the M1 sites, local orbital order with respect to the Mn⁴⁺ ions at the six neighboring M2 sites is likely in such a highly connected manganite network. This gives rise to ferromagnetic Mn_M1–O–Mn_M2 interactions, as found in long-range orbitally ordered La_0.5Ca_0.5MnO₃.⁴¹

Fig. 7 (a) Temperature-dependent AC magnetic susceptibilities χ′ at frequencies of 100–1000 Hz, (b) Isothermal M–H curves at 2, 20, and 100 K for Ba₄Sn_1.1Mn_2.9O₁₂ sample under the applied field of −5 T to 5 T. The inset to (b) expands the M–H curves at low field region.

3.4 Conductivity and morphology

To explore the transport properties of Ba₄Sn_1.1Mn_2.9O₁₂, AC impedance data was collected on the dense pellet, which has 93% of the theoretical density. Fig. 8a shows typical complex impedance plots at 573 K measured at different atmospheres, which are essentially identical and consist of one asymmetric semicircle arc. The capacitances for this arc vary within 10⁻¹² to 10⁻¹¹ F cm⁻¹, which could be ascribed to mixed bulk and grain boundary responses. No apparent electrode responses were observed in the impedance data. Fig. 8b shows the total conductivities of the Ba₄Sn_1.1Mn_2.9O₁₂ pellets, which are almost independent of atmospheres. This keeps with very low content of oxygen vacancies in Ba₄Sn_1.1Mn_2.9O₁₂. The conductivities vary from 10⁻¹¹ to 10⁻⁶ S cm⁻¹ in the temperature range 473–750 K. The Arrhenius plots show constant activation energy (E_a) of 0.61(1) eV for the Ba₄Sn_1.1Mn_2.9O₁₂.

Fig. 8 (a) The 573 K complex impedance plots for Ba₄Sn_1.1Mn_2.9O₁₂ pellets measured at the air, argon, and oxygen atmosphere. The numbers denote the logarithms of selected frequencies marked by filled symbols. (b) Temperature-dependent total conductivity of Ba₄Sn_1.1Mn_2.9O₁₂ pellets in air, oxygen and argon, showing constant activation energy of 0.61(1) eV regardless of the atmosphere.

The SEM images in backscattering mode (Fig. 9) show the cross-section morphology of Ba₄Sn_1.1Mn_2.9O₁₂ ceramic, which has homogeneous elements distribution and well-defined grain boundaries. The grain sizes are mainly distributed in the range of 2–10 μm range (Fig. 9a), without significantly anisotropic grain growth, keeping with its good density. The Ba, Sn, Mn elements are uniformly distributed in the cross-section of the pellet, and the Ba : Sn : Mn ratio of 3.8 : 1.1 : 2.5 keeps with their stoichiometric ratio (Fig. 9b–d).

Fig. 9 Microstructure of the Ba₄Sn_1.1Mn_2.9O₁₂ ceramic. (a) SEM image in backscattering mode showing homogeneous elements distribution and well-defined grain boundaries, and EDS mapping images of (b) Ba, (c) Sn, and (d) Mn.

4 Conclusions

The new 12R perovskite Ba₄Sn_1.1Mn_2.9O₁₂ has been synthesized by high-temperature solid-state reactions. The material has an ordered arrangement of Sn and Mn cations, and trimers of face-sharing MnO₆ octahedra arranged in triangular plains. Within trimers of MnO₆ octahedra, the outer MnM₂O₆ octahedra are distorted but the inner MnM₃O₆ octahedra are undistorted. The magnetism of Ba₄Sn_1.1Mn_2.9O₁₂ perovskite can be described in the picture of molecular-like Mn₃ and minor Mn₂ linear clusters. These magnetic cluster forms at ∼220 K due to the strong AFM interaction inside the clusters, and has the effective moment of one Mn⁴⁺ spin per Mn₃ cluster and zero spin per Mn₂ cluster. Weaker inter-cluster magnetic interaction ordered these magnetic clusters antiferromagnetically at lower temperature T_N ∼ 6 K. The Ba₄Sn_1.1Mn_2.9O₁₂ sample has a semiconducting behavior with a transport activation energy of 0.61 eV. Ba₄Sn_1.1Mn_2.9O₁₂ is another rare example of a periodical lattice that shows molecular-like cluster magnetism due to the cationic order beyond orbital molecules.⁴²

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The National Natural Science Foundation of China (No. 51662013, 22161014, 21850410458), Guangxi Natural Science Foundation (No. 2020GXNSFAA297220, 2019GXNSFGA245006, AD19245097), and the Foundation of Guilin University of Technology (No. GUTQDJJ2018115) are acknowledged for their financial support. We acknowledge Prof. He Lunhua at China Spallat Neutron Source for the support in neutron data collection in this work.

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Footnote

† CCDC 2117124. For crystallographic data in CIF or other electronic format see DOI: 10.1039/d1ra07841k

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