Magnetic properties and neutron diffraction study of two manganese sulfosalts: monoclinic MnSb₂S₄ and benavidesite (MnPb₄Sb₆S₁₄)

Abstract

Mn²⁺Sb₂S₄, a monoclinic dimorph of clerite, and benavidesite (Mn²⁺Pb₄Sb₆S₁₄) show well-individualized single chains of manganese atoms in octahedral coordination. Their magnetic structures are presented and compared with those of iron derivatives, berthierite (Fe²⁺Sb₂S₄) and jamesonite (Fe²⁺Pb₄Sb₆S₁₄). Within chains, interactions are antiferromagnetic. Like berthierite, MnSb₂S₄ shows a spiral magnetic structure with an incommensurate 1D propagation vector [0, 0.369, 0], unchanged with temperature. In berthierite, the interactions between identical chains are antiferromagnetic, whereas in MnSb₂S₄ interactions between chains are ferromagnetic along c-axis. Below 6 K, jamesonite and benavidesite have commensurate magnetic structures with the same propagation vector [0.5, 0, 0]: jamesonite is a canted ferromagnet and iron magnetic moments are mainly oriented along the a-axis, whereas for benavidesite, no angle of canting is detected, and manganese magnetic moments are oriented along b-axis. Below 30 K, for both compounds, one-dimensional magnetic ordering or correlations are visible in the neutron diagrams and persist down to 1.4 K.

Introduction

Iron complex minerals of the sulfosalt type, showing well-individualized single chains of iron atoms in octahedral coordination, have been studied previously for their magnetic properties. Magnetic structures of berthierite (Fe²⁺Sb₂S₄) (Winterberger et al. 1990) and jamesonite (Fe²⁺Pb₄Sb₆S₁₄) (Léone et al. 2004) have been thus determined. In this paper, we compare these results with the magnetic structures obtained for manganese derivative compounds, monoclinic Mn²⁺Sb₂S₄, a dimorph of clerite (orthorhombic, isotypic with berthierite) and benavidesite, Mn²⁺Pb₄Sb₆S₁₄ (isotypic with jamesonite).

MnSb₂S₄ has been synthesized by solid-state reaction of MnS and Sb₂S₃ following Pfitzner et al. (2000). It crystallizes in the monoclinic symmetry, space group C2/m, with unit cell parameters a = 12.743(3), b = 3.799(1), c = 15.106(3) Å and β = 113.91(3), Z = 4. It is not isotypic with berthierite (FeSb₂S₄), which crystallizes in the orthorhombic symmetry, space group Pnma, with unit cell parameters a = 11.412(2), b = 3.763(1), c = 14.161(3) Å, Z = 4, but it contains the same type of chains of edge-sharing octahedra [MnS₆] parallel to b-axis, 6.65 Å apart (Fig. 1).

Fig. 1

Crystal structure of monoclinic MnSb₂S₄ (left), chains of manganese atoms in octahedral coordination (right)

Benavidesite (MnPb₄Sb₆S₁₄) has been synthesized by solid-state reaction of MnS, PbS and Sb₂S₃ (Léone et al. 2003). It crystallizes in the monoclinic symmetry, space group P2 ₁ /c, with unit cell parameters a = 4.0216(8), b = 19.178(4), c = 15.837(3) Å and β = 91.89(3), Z = 2. It is isotopic with jamesonite (FePb₄Sb₆S₁₄) (Fig. 2). Mn octahedra form single chains parallel to a-axis, 12.4 Å apart.

Fig. 2

Crystal structure of jamesonite (benavidesite), Fe(Mn)Pb₄Sb₆S₁₄ (left); chains of iron (manganese) atoms in octahedral coordination (right)

Experimental

Magnetic measurements were carried out on a Quantum Design SQUID magnetometer using powder. The data were corrected for the sample holder contribution and the core diamagnetism using the Pascal’s constants.

Calorimetric measurements were performed on a homemade specific heat setup using semi-adiabatic method. Data recorded on sintered pellets were corrected for the sample holder and grease contributions.

The neutron diffraction experiments were performed at the Laboratoire Léon Brillouin (CEA-CNRS) in Saclay (France) using the G4.1 diffractometer (λ = 2.4266 Å). Height diagrams were collected in the 2θ range 7–86.9° between 1.5 and 160 K (1.5, 5, 10, 15, 20, 25, 40, 160 K) for MnSb₂S₄; the data were collected during 3 h at 1.5 K and during 1.5 h at the other temperatures. Sixteen diagrams were collected in the 2θ range 7–86.9° between 1.6 and 45 K (1.6, 3, 4, 5, 6, 7, 10, 13, 16, 18, 21, 24, 27, 30, 34, 45 K) for benavidesite; the data were collected during 5 h at 1.6 K and during 2.5 h at the other temperatures. The samples were set in a cylindrical vanadium can and held in a liquid helium cryostat. Nuclear and magnetic structures were refined using the Rietveld-type (Rietveld 1969) Fullprof (Rodriguez-Carvajal 1990) program. The nuclear scattering lengths and iron magnetic form factor were those included in this program.

Results

Magnetic measurements and specific heat of monoclinic MnSb₂S₄

Figure 3 shows the thermal variation of both susceptibility and reciprocal susceptibility of powder under a magnetic field of 1 kOe. At high temperature (above 150 K), the reciprocal susceptibility agrees with a Curie–Weiss law. The effective magnetic moment μ_eff = 5.9(1) μ_B is close to the expected value for high spin Mn²⁺ (5.8–6.0) (Carlin 1986). The Curie paramagnetic temperature θ = −63(1) K indicates prevalent antiferromagnetic interactions. The susceptibility curve shows a maximum at 25(1) K, corresponding to an antiferromagnetic transition. These results confirm those of Kurowski (2003) (θ = −66(1)°, T _N = 26.5(5) K) except for the effective magnetic moment given by this author, which seems too high (μ_eff = 6.86(5) μ_B).

Fig. 3

Temperature dependence of the susceptibility of monoclinic MnSb₂S₄ (H = 1 kOe)

Figure 4 represents the evolution of the susceptibility and of the specific heat versus temperature. The specific heat curve shows a long-range magnetic ordering transition at 23 K corresponding to the antiferromagnetic transition observed on the susceptibility curve, and an additional small peak at 25 K.

Fig. 4

Observation of the magnetic transition by susceptibility and specific heat measurements of MnSb₂S₄

The difference between the zero field cooled and the field-cooled data for a field applied of 100 Oe, below 50 K, is illustrated in Fig. 5. The magnetization curves versus applied field recorded at different temperatures are linear above 25 K, showing that the transition temperature has been reached.

Fig. 5

Susceptibility of MnSb₂S₄ (H = 100 Oe): field cooled (FC) and zero field cooled (ZFC) (left), magnetization versus applied magnetic field (right)

Magnetic structure of monoclinic MnSb₂S₄

Figure 6 shows neutron diffraction patterns collected at 1.5, 25 and 160 K. The neutron diffraction pattern recorded at 160 K is strictly due to the nuclear scattering. The sample contains some Sb₂S₃ and MnS as main impurities. The pattern recorded at 25 K evidences the (111) magnetic peak of MnS. Additional peaks of magnetic origin appear below 25 K. Magnetic peaks observed at 1.5 K correspond to a 3D long-range magnetic ordering of manganese magnetic moments. They can be indexed from the nuclear cell parameters using an incommensurate 1D propagation wave-vector along b-axis, equal to k = [0, 0.369(1), 0]. Two modulation models were tested, sinusoidal and helicoidal. The best refinement is obtained for the helicoidal model (R _mag = 0.093). Figure 7 shows observed, calculated and difference neutron diffraction patterns. Figure 8 represents the magnetic structure at 1.5 K. Manganese magnetic moments are oriented in the (a, c) plane. Along the chains of edge-sharing octahedra [MnS₆] (along b-axis), there is an angle of 133° between adjacent magnetic moments; along the a-axis, the angle is 66.85° and it is zero along c-axis. The magnetic moment reaches 4.43(6) μ_B at 1.5 K. The evolution of the magnetic moment values versus temperature is represented in Fig. 9. It confirms the magnetic transition at 25 K. It is remarkable that the propagation vector is unchanged, to the precision of measurements, from 1.5 to 20 K. In his thesis, Kurowski (2003) chose a sinusoidal model for the magnetic structure of MnSb₂S₄ (k _y = 0.371(1); M = 4.6(1) μ_B) but later, he and co-authors (Matar et al. 2005; Kurowski et al. unpublished) indicated that the structure is helicoidal, without any detail.

Fig. 6

Part of neutron diffraction patterns of monoclinic MnSb₂S₄ at 1.5, 25 and 160 K; at 1.5 and 25 K, the peak marked with an asterisk is the (111) magnetic peak of MnS

Fig. 7

Observed (cross), calculated (solid line) and difference (solid line at the bottom) neutron diffraction patterns at 1.5 K of MnSb₂S₄. The first series (up) of Bragg reflexion markers corresponds to the nuclear structure, the second series to the magnetic structure, the third series to Sb₂S₃ and the fourth (down) to MnS

Fig. 8

MnSb₂S₄ magnetic structure: drawing in perspective (left), in projection along the b-axis (right)

Fig. 9

Evolution of the magnetic moment versus temperature for monoclinic MnSb₂S₄

Magnetic measurements and specific heat of benavidesite (MnPb₄Sb₆S₁₄)

Previously (Léone et al. 2003), we showed that benavidesite (MnPb₄Sb₆S₁₄) has an effective magnetic moment μ_eff = 5.7(1) μ_B and a Curie paramagnetic temperature θ = −40 K, characteristic of high spin Mn²⁺ with antiferromagnetic interactions (Fig. 10). The evolution of the specific heat versus temperature (Fig. 11) and the zero field cooled (ZFC), field cooled (FC) susceptibility curves for an applied field of 100 Oe (Fig. 12) show a long-range magnetic ordering transition at about 6 K. Between 6 and 45 K, ZFC and FC susceptibility curves do not coincide: this difference may be attributed to 1D magnetic ordering, or to correlations along the manganese octahedra which form single chains in the structure. The magnetization curves versus applied field recorded are linear at 5 K.

Fig. 10

Temperature dependence of the susceptibility of benavidesite (MnPb₄Sb₆S₁₄) (H = 1 kOe)

Fig. 11

Observation of the magnetic transition by specific heat measurements of benavidesite

Fig. 12

Susceptibility of benavidesite (H = 100 Oe): field cooled (FC) and zero field cooled (ZFC) (left), magnetization versus applied magnetic field (right)

Magnetic structure of benavidesite (MnPb₄Sb₆S₁₄)

Figure 13 shows neutron diffraction patterns collected at 1.6, 6 and 45 K. The neutron diffraction pattern recorded at 45 K is strictly due to the nuclear scattering. The sample contains one Mn-free impurity, robinsonite (Pb₄Sb₆S₁₃). A series of extra peaks is present at 1.6 K corresponding to the three-dimensional long-range magnetic ordering of the manganese magnetic moments. These magnetic peaks can be indexed in doubling the a parameter of the crystal cell. These extra peaks have disappeared at 6 K. Like jamesonite (FePb₄Sb₆S₁₄) (Léone et al. 2004), at around 2θ = 18°, the background of all neutron diagrams exhibits a step which disappears at around 30 K. In order to determine the magnetic structure, to limit the number of parameters, we used the results of the group theory calculation. Contrary to jamesonite, the best fit (Fig. 14) is obtained for the “Γ ₄ ” irreductible representation and not for “Γ ₂ ”. Despite the results of the group theory calculation define the moment vector by three parameters, the magnetic moments are strictly oriented along b-axis (Fig. 15) and reach 3.2(1) μ_B at 1.6 K. The magnetic reliability factor (R _mag = 0.159) is a little bit high (R _mag = 0.126 for jamesonite): two possible explanations are the weak intensity of the magnetic peaks compared to nuclear one, as well as the small amount of sample.

Fig. 13

Part of neutron diffraction patterns of benavidesite (MnPb₄Sb₆S₁₄) at 1.6, 6 and 45 K. Relatively to the diagram at 45 K, those at 6 and 1.6 K have been shifted up for clarity. For each temperature, a line is drawn to show the evolution of the background

Fig. 14

Observed (cross), calculated (solid line) and difference (solid line at the bottom) neutron diffraction patterns at 1.6 K of benavidesite. The first series of Bragg reflexion markers corresponds to the nuclear structure, the second series to the magnetic structure, the third series to robinsonite (Pb₄Sb₆S₁₃)

Fig. 15

Benavidesite (MnPb₄Sb₆S₁₄) magnetic structure

Discussion

Comparison between berthierite (FeSb₂S₄) and monoclinic MnSb₂S₄

Within single chains of Fe or Mn octahedra, according to Goodenough-Kanamori rules (Goodenough 1963), superexchange interactions at 90° are antiferromagnetic. In berthierite (FeSb₂S₄) (Winterberger et al. 1990), the distances between neighbouring chains are smaller (6.19 Å) than those in MnSb₂S₄ (6.65 Å), and the interactions are stronger, leading to a 3D long-range magnetic ordering at higher temperature, 50 K instead of 25 K. Both compounds show spiral magnetic structures with a similar incommensurate 1D propagation vector (k _y = 0.394 for FeSb₂S₄ and 0.369 for MnSb₂S₄), unchanged with temperature, and no short range magnetic order is detectable on neutron diagrams just above T _N. In berthierite, the interactions between identical chains (6.19 Å apart) are antiferromagnetic, whereas in MnSb₂S₄, interactions between chains along c-axis (Fig.8) (7.55 Å apart) are ferromagnetic. The maximum magnetic moment values, 4.1 μ_B for iron II and 4.4 μ_B for manganese II, are close to expected ones, respectively 4 and 5 μ_B.

Comparison between jamesonite (FePb₄Sb₆S₁₄) and benavidesite (MnPb₄Sb₆S₁₄)

The ZFC-FC susceptibility measurements, the evolution of the specific heat and the powder neutron diffraction patterns of benavidesite (MnPb₄Sb₆S₁₄) show, like for jamesonite (FePb₄Sb₆S₁₄) (Léone et al. 2004), a 3D long-range magnetic ordering at 6 K. Jamesonite is a canted ferromagnet and iron magnetic moments are mainly oriented along a-axis. On the contrary, no angle of canting is detected for benavidesite, and manganese magnetic moments are oriented along b-axis.

For both compounds, we observed an increase on the background in the neutron diffraction pattern at about 2θ = 17°, below 30 K. This “step” can be most probably attributed to 1D magnetic ordering, or correlations along the iron or manganese octahedra which form single chains in the structure: the distance between iron or manganese atoms along these chains is equal to the a parameter (4.02 Å), so Q = π/a = 0.78 Å⁻¹ and it corresponds to 2θ = 17.5 [=2 arcsin(Qλ/4π)]. Below 6 K, a few chains remain uncorrelated giving rise to this 1D magnetic signal. Probably, it explains the smaller maximum magnetic moment value, 3.3 μ_B for iron II and 3.2 μ_B for manganese II, than expected, respectively 4 μ_B and 5 μ_B. Morimoto et al. (2007), by Mössbauer spectroscopy of a synthetic jamesonite sample, did not detect any development of this short-range magnetic order. They explain the Mössbauer spectra by the development of a spin-glass state and a 3D magnetic order.

Comparison between (berthierite, monoclinic MnSb₂S₄) and (jamesonite, benavidesite)

All these compounds present crystal structures with 1D organization in which iron or manganese octahedra form single chains. In jamesonite and benavidesite, the distances between neighbouring chains are larger (12.3 Å) than in berthierite and monoclinic MnSb₂S₄, so the interactions are weaker, leading to a 3D long-range ordering at lower temperature (6 K), a commensurate magnetic structure, and a 1D magnetic correlations below 30 K. For berthierite and MnSb₂S₄, the interactions between chains lead to a spiral magnetic structure and no short-range order is detectable.