Crystal, magnetic and electronic structures and properties of new BaMnPnF (Pn = As, Sb, Bi)

  • A Corrigendum to this article was published on 04 December 2013

Abstract

New BaMnPnF (Pn = As, Sb, Bi) are synthesized by stoichiometric reaction of elements with BaF2. They crystallize in the P4/nmm space group, with the ZrCuSiAs-type structure, as indicated by X-ray crystallography. Electrical resistivity results indicate that Pn = As, Sb and Bi are semiconductors with band gaps of 0.73 eV, 0.48 eV and 0.003 eV (extrinsic value), respectively. Powder neutron diffraction reveals a G-type antiferromagnetic order below TN = 338(1) K for Pn = As and below TNs = 272(1) K for Pn = Sb. Magnetic susceptibility increases with temperature above 100 K for all the materials. Density functional calculations find semiconducting antiferromagnetic compounds with strong in-plane and weaker out-of-plane exchange coupling that may result in non-Curie Weiss behavior above TN. The ordered magnetic moments are 3.65(5) μB/Mn for Pn = As and 3.66(3) μB/Mn for Pn = Sb at 4 K, as refined from neutron diffraction experiments.

Introduction

The recent discovery of high temperature superconductivity in F-doped LaFeAsO1 (1111 family) initiated an extensive research into analogous materials. This research led to the discoveries of superconductivity in doped BaFe2As22,3 (122 family), LiFeAs4 (111 family) and FeSe5 (11 family), among others. All these families feature two dimensional (2D) structures with FeAs or FeSe layers, which contain edge-shared FeAs4 or FeSe4 tetrahedra. Fe atoms are formally divalent; hole-, electron-, or isovalent-doping inside or outside of layers can result in superconductivity.

Among the Fe-based superconductors (FeSCs), the highest superconducting transition temperatures (TCs) are reported for the 1111 family6,7. In the search for non-Fe-based oxypnictides, varieties of physical properties are found such as diamagnetism in LaZnAsO8, itinerant ferromagnetism in LaCoAsO9,10, semiconducting antiferromagnetism in LnMnPnO11 (Ln = La-Sm; Pn = P, As) and superconductivity in LaNiAsO12 (TC ~ 3 K). Interestingly, although such oxyarsenides have been studied in detail, reports on non-oxide 1111 fluoroarsenides with ZrCuSiAs-type structure, i.e. BFeAsF (B = alkaline-earth metal), are relatively scarce.

For the 1111 fluopnictides BFeAsF, rare-earth substitution at the alkaline-earth metal (B) site is found to give the highest TC of 56 K for Sr0.5Sm0.5FeAsF and Ca0.4Nd0.6FeAsF13,14. The only known non-Fe transition-metal-based 1111 fluoropnictides are BaMnPF15, ACuChF and AAgChF (A = Sr, Ba, Eu; Ch = S, Se, Te)16,17. However, it should be noted that Cu and Ag in ACuChF and AAgChF are nominally monovalent and therefore, BaMnPF is the only known 1111 fluoropnictide containing divalent non-Fe transition metal. Other known 1111 fluoropnictides, include those of group 12 metals, namely SrZnPF, BaZnPF, BaZnSbF and BaCdSbF15,18. According to literature on the properties of 1111 fluoropnictides, SrCuChF19,20, BaCuChF21 and EuCuChF20 (Ch = S, Se and Te) are p-type semiconductors with Seebeck coefficients ranging from +10 to +620 μV/K. BaZnPF15 and BaCdSbF18 are also semiconductors with band gaps of Eg = 0.5 eV and 0.25 eV, respectively. Moreover, BaMnPF is a semiconducting antiferromagnet with a temperature independent magnetic susceptibility up to 300 K15.

Considering the fact that doped BFeAsF fluoropnictides are among the highest TC (56 K) FeSCs, there is an incentive to explore for superconductivity in similar 1111 structures and even non-Fe-based analogs. This is the report of the synthesis of new 1111 ZrCuSiAs-type BaMnPnF (Pn = As, Sb, Bi) (Figure 1) and a comprehensive study of their crystal, magnetic and electronic structures. We report nuclear structures from powder and single crystal X-ray diffraction and magnetic structures from neutron powder diffraction. In addition, we present thermodynamic and transport properties from temperature dependent electrical transport data, temperature- and field-dependent magnetization data. Moreover, we report density functional theory (DFT) electronic structure calculations.

Figure 1
figure1

Tetragonal ZrCuSiAs-type structure of BaMnPnF (Pn = As, Sb, Bi).

The unit cell contains transition metal pnictide layers of [MnPn] that are made of MnPn4 tetrahedra.

Results

Synthesis, crystal chemistry and stability

Three new compounds of BaMnAsF, BaMnSbF and BaMnBiF are synthesized using the solid-state sintering method. The compounds crystallize in the primitive ZrCuSiAs-type tetragonal P4/nmm (No. 139, Pearson symbol tP8) and are isotypic to the lighter BaMnPF (Figure 1)15.

EDS results confirm the presence of small crystallites with BaMnSbF and BaMnBiF compositions after the first heating step, which allows for collection of single-crystal x-ray diffraction data (Table 1). There are four crystallographically unique atoms in the asymmetric unit cell, all located in special positions (Tables 2, 34, S1). BaMnPnF structure, similar to LaFeAsO, can be viewed with cationic [BaF]+ and anionic [MnPn] layers alternating along the c-axis. These layers are built upon edge-sharing FBa4 and MnPn4 tetrahedra, respectively. Following the discovery of superconductivity in the F-doped LaFeAsO, the ZrCuSiAs-type structure and its relationship with other structures have been extensively studied11,22,23.

Table 1 For BaMnPnF (Pn = Sb, Bi), selected single crystal X-ray diffraction data and structure refinement parameters
Table 2 For BaMnPnF (Pn = Sb, Bi), atomic coordinates and equivalent isotropic (Ueqa) displacement parameters, at 173 K, from single crystal X-ray diffraction refinements
Table 3 For BaMnPnF (Pn = Sb, Bi), selected bond distances (Å) and angles (°), at 173 K, from single crystal X-ray diffraction
Table 4 Refined structural parameters for BaMnAsF and BaMnSbF at 4 K from neutron powder diffraction, in P4/nmm

From single crystal X-ray data (Table 3), Mn-Sb and Mn-Bi bond distances in the [MnPn] layers are dMn-Sb = 2.7767(5) Å and dMn-Bi = 2.8479(11) Å, respectively; the Mn-Mn distances are dMn-Mn = 3.1595(4) Å in BaMnSbF and dMn-Mn = 3.196(1) Å in BaMnBiF. The tetrahedral angles are 110.65(1)° and 107.14(3)° in MnSb4 and 111.73(3)° and 105.04(5)° in MnBi4 tetrahedra. The interlayer distances shown by the distance between barium and pnictogen atoms in adjacent layers are dBa-Sb = 3.6489(6) Å in BaMnSbF and dBa-Bi = 3.671(1) Å in BaMnBiF. For comparison, MnAs4 tetrahedra in BaMnAsF are characterized by the distances dMn-As = 2.60460(4) Å and dMn-Mn = 3.0221(1) Å and bond angles 109.0789(8)° and 110.259(2)°, from the room temperature PXRD data (see Table S1). The bond distances and angles in BaMnPnF are very close to those reported for the related BaMn2Pn2 ternary phases24,25,26, which also contain anionic [MnPn] layers.

PXRD patterns along with Rietveld refinements results for BaMnAsF, BaMnSbF and BaMnBiF are illustrated in Figures 2 to 4. There are no Bragg peaks in the 2θ range of 5–20° for Pn = As and Sb and so the low angle regions are not shown for them. The broad background below 2θ ~ 25° in Pn = Sb and Bi (Figure 34) are caused by the polycarbonate cover. X-ray diffraction refinement detects a small amount of BaF2 crystalline impurity (less than 1% by mass) in Pn = As and Bi products, whereas no impurity peaks are observed in the Pn = Sb compound.

Figure 2
figure2

For BaMnAsF, (a) Rietveld refinement (red line) of powder X-ray diffraction data (black dots), shown along with Bragg positions for 1111 tetragonal I4/mmm phase (pink tick marks) and BaF2 impurity (turquoise tick marks); (b) temperature-dependence of refined lattice parameters (dash lines are guide to the eye).

Figure 3
figure3

For BaMnSbF, (a) Rietveld refinement (red line) of powder X-ray diffraction data (black dots), shown along with Bragg positions for 1111 tetragonal I4/mmm phase (pink tick marks) with no impurities; (b) temperature-dependence of refined lattice parameters (dash lines are guide to the eye).

The green data points in (b) are a- and c- lattice parameters from single crystal X-ray diffraction refinements.

Figure 4
figure4

For BaMnBiF, (a) Rietveld refinement (red line) of powder X-ray diffraction data (black dots), shown along with Bragg positions for 1111 tetragonal I4/mmm phase (pink tick marks) and BaF2 impurity (turquoise tick marks).

All BaMnPnF samples have MnO impurities if ground in air during sample sintering stages. Even in the final product form, ground-in-air BaMnSbF shows signs of oxidation in the PXRD data, while BaMnBiF burns in air. The extreme air sensitivity of ground BaMnBiF limits its full characterization through neutron diffraction experiments. Notwithstanding these facts, the heating of pellets under ambient conditions up to 130°C and subsequent PXRD measurements demonstrate that BaMnPnF are air-stable in pellet form.

The refined room temperature lattice parameters from the PXRD data are a = 4.2739(1) Å and c = 9.5875(2) Å for BaMnAsF (Rp = 8.54%, wRp = 11.52%), a = 4.4791(1) Å and c = 9.8297(2) Å for BaMnSbF (Rp = 4.59%, wRp = 6.03%) and a = 4.5384(1) Å and c = 9.8929(2) Å for BaMnBiF (Rp = 4.05%, wRp = 5.40%). The refinement of PXRD data on BaMnAsF and BaMnSbF show (Figures 2b, 3b) that there are no structural anomalies upon cooling and the unit cell volumes contract uniformly. A survey of the Inorganic Crystal Structure Database (ICSD)16 and the Pearson's Handbook17 reveals new features about the BaMnPnF family. First, they are the only reported compounds in each of the corresponding Ba-Mn-Pn-F (Pn = As, Sb, Bi) phase diagrams so far. Second, BaMnBiF can also be considered to be the first bismuthide with the ZrCuSiAs-type structure, considering that Bi in BiCuSeO27 serves as a cation. Third, together with BaMnPF, the new compounds are the first phases with Mn and F to adopt the ZrCuSiAs structure type. Fourth, the cell volumes of BaMnSbF and BaMnBiF (see Table 1) are the largest within this structure type along with that of BaCdSbF18. As expected from the large unit cell volumes and lattice parameters of BaMnSbF and BaMnBiF, the bond distances are also longer than those reported for other compounds that adopt this structure type8,11.

The thermal behaviors of BaMnAsF and BaMnSbF are studied through TGA/DTA (Figure S1). Both BaMnAsF and BaMnSbF are stable up to 1300 K (~1030°C), after which they decompose. Additionally, pellets of BaMnAsF and BaMnSbF are separately vacuum sealed inside silica tubes and heated to 1500 K with periodic visual monitoring. This is done because the DTA on BaMnAsF (Figure S1) shows two peaks upon heating and it is not immediately clear if the first peak corresponds to the melting of the compound. No visual change occurs to the pellets up to ~1430 K, after which molten liquid is clearly visible in both samples. These molten pieces contain MnAs and MnSb binaries according to the EDS results.

Physical properties

For BaMnPnF, temperature dependence of electrical resistivity results are plotted in Figure 5. The compounds show semiconducting behavior, with room temperature resistivity values of ρ300K(BaMnAsF) = 3.6 × 105 Ω cm, ρ300K(BaMnSbF) = 2.4 × 104 Ω cm and ρ300K(BaMnBiF) = 0.135 Ω cm. Similarly, BaMnPF is also reported as a semiconductor15. The semiconducting behavior supports the charge-balanced nature of the compounds according to [Ba2+F][Mn2+Pn3−]. dρ/dT derivatives of the resistivity data are featureless in the measured range. Calculated band gaps from the Arrhenius fit (lnρ = lnρ0 + Eg/2kBT) are Eg(BaMnAsF) = 0.73 eV, Eg(BaMnSbF) = 0.48 eV and Eg(BaMnBiF) = 0.003 eV (see Figure 5). These band gap values follow the expected periodic trend based on electronegativities of pnictogen elements28. It is interesting that for BaMnBiF, both the temperature dependence of electrical resistivity and the calculated narrow band gap are quite different when compared to the other Pn members. For BaMnBiF, ρ(T) decreases upon cooling down to ~85 K, then changes slope and sharply increases below 80 K. From the trend in the band gaps in this family, it can be speculated that BaMnBiF is an extrinsic semiconductor29.

Figure 5
figure5

Temperature dependence of electrical resistivity, ρ(T), for polycrystalline pellets of (a) BaMnAsF and BaMnSbF and (b) BaMnBiF.

Insets illustrate lines (in red) corresponding to Arrhenius fit for the temperature region.

The calculated band gaps for BaMnAsF and BaMnSbF are two orders of magnitude larger than those reported for the narrow gap semiconductors of BaMn2Pn2, with band gaps of 6–54 meV24,25,26. Such a difference may be attributed to the fact that BaMnPnF contain an additional insulating [BaF]+ layer in the structure, which reduces the band dispersion in the c-axis direction. Further comparisons can be made with the isostructural quaternary phases based on manganese. Most of LnMnPnO (Ln = rare-earth metal) compositions have been reported to be semiconductors with varying band gaps11, with exceptions such as metallic PrMnSbO30.

Temperature- and field-dependent magnetization results for BaMnAsF are plotted in Figure 6a. ZFC and FC χ(T) data overlap for BaMnAsF and are measured up to ~800 K. There is an upward tail in χ(T) below ~15 K, which is probably due to paramagnetic impurities, above which magnetic susceptibility increases slowly with rising temperature. There is another change in the slope of χ(T) at ~550 K, above which χ(T) starts to plateau. M(H) plots both at 5 K and 100 K are linear. χ(T) and M(H) data, coupled with resistivity data, suggest that BaMnAsF is likely a local moment antiferromagnet.

Figure 6
figure6

Temperature dependence of magnetic susceptibility, χ(T), for (a) BaMnAsF, (b) BaMnSbF and (c) BaMnBiF.

For BaMnAsF, inset shows a linear field-dependence of magnetization, M(H), at 5 K and 100 K. Note that (b) and (c) are derived using magnetization data under 20 kOe and 30 kOe assuming χ ≈ ΔMH, in order to eliminate ferromagnetic impurities (see text).

Field dependence of magnetization (not shown) for BaMnSbF and BaMnBiF suggest presence of ferromagnetic impurities. From a linear fit to the M(H) data at 5 K, saturation magnetization values of Msat(BaMnSbF) = 0.039 μB/mol Mn and Msat(BaMnBiF) = 0.006 μB/mol Mn are obtained. Assuming that the sources of the ferromagnetic impurities are MnSb or MnBi binaries16,17, the Msat values correspond to 1.2% (molar) MnSb, or 0.13% MnBi impurity concentrations. Consequently, in order to find intrinsic behavior for BaMnSbF and BaMnBiF compounds, magnetic susceptibility measurements were performed under two applied fields (20 kOe and 30 kOe), then subtracted from each other according to χ ≈ ΔMH; the resulting χ(T) plots are shown in Figure 6b, c. Although there are low temperature upturns below ~100 K, χ(T), similar to BaMnAsF, increase slowly with rising temperature.

The long range antiferromagnetic order found from neutron diffraction results (see next section) at TN = 338(1) K for BaMnAsF and TN = 272(1) K for BaMnSbF do not manifest clearly in the magnetic susceptibility data (Figure 6). Magnetic susceptibility data without a feature at TN is not unique to BaMnAsF and BaMnSbF; for example, χ(T) for LaMnPO31 (TN = 375(5) K) is featureless up to 800 K; this was attributed to a very strong exchange coupling in the compound. Cases of increasing χ(T) above TN have also been reported for BaMn2As232, BaMn2Bi226, BaFe2As23 and LaFeAsO33.

Neutron powder diffraction

Results of the neutron powder diffraction (NPD) experiments are summarized in Table 4 and Figures 7 and 8 and S2 to S4. Rietveld refinement of the NPD data at 4 K gives lattice parameters of a = 4.2642(1) Å and c = 9.5586(3) Å for BaMnAsF (RBragg = 3.54%, RF = 2.4%) and a = 4.4636(1) Å and c = 9.7885(4) Å for BaMnSbF (RBragg = 4.85%, RF = 3.2%). Magnetic peaks can be indexed by a wave vector k = (0 0 1/2), giving a magnetic structure (Figure 8a) with antiferromagnetic (AF) coupling between nearest-neighbor Mn ions in the [MnPn] layers. The adjacent [MnPn] layers are also antiferromagnetically coupled, leading to a G-type AF order and a magnetic unit cell twice the size of the chemical unit cell. The NPD data at 4 K yield ordered moment values of 3.65(5) μB/Mn for BaMnAsF and 3.66(3) μB/Mn for BaMnSbF, which are lower than the value of 5.00 μB/Mn expected for the S = 5/2 (high spin) Mn2+ ions, assuming μ = gB with g = 2.

Figure 7
figure7

(a) Rietveld plots of the neutron powder diffraction data for BaMnAsF collected at 4 K using 1.54 Å and 2.41 Å wavelengths.The two most prominent magnetic reflections, indexed as (1 0 1/2) and (1 0 3/2), are indicated by arrows. (b) Trends in lattice parameters for BaMnAsF as obtained from the NPD data. Dashed lines are intended as a guide to the eye.

Figure 8
figure8

(a) The magnetic structure for BaMnAsF and BaMnSbF.Temperature dependence of (1 0 0.5) magnetic peak for (b) BaMnAsF and (c) BaMnSbF. Insets of (b) and (c) show evolution of (1 0 1/2) and (1 0 3/2) magnetic peaks with lowering temperature in low Q region.

Although lower than expected, the ordered moment values of 3.65(5) μB/Mn for BaMnAsF and 3.66(3) μB/Mn for BaMnSbF suggest local moment antiferromagnetism in these compounds. In comparison, the ordered moment values are 0.2–1.0 μB/Fe in the delocalized spin-density-wave (SDW) antiferromagnets of AEFe2As234 and LnFeAsO22. Reduced moments and G-type ordering have also been reported for BaMn2P2 (4.2(1) μB/Mn)35 and BaMn2As2 (3.88(4) μB/Mn)34. Among the quaternary ZrCuSiAs-type compounds of Mn, the ordered moments are 3.28(5) μB/Mn for LaMnPO (AF in the [MnP] layer and ferromagnetic, F, between the layers)31, 3.34(2) μB/Mn for LaMnAsO (intralayer AF and interlayer F)36 and ~3 μB/Mn for PrMnSbO (AF C-type)30. For these compounds, the reduced ordered moments have been attributed to a strong hybridization between pnictogen p and Mn d orbitals10,37.

Figure 8b and 8c show temperature dependence of the (1 0 1/2) magnetic peak. AF ordering temperatures of TN (BaMnAsF) = 338(1) K and TN (BaMnSbF) = 272(1) K are obtained from a power law fit I(T) M2(T) (1-T/TN) of the order parameter, with β(BaMnAsF) = 0.36(3) and β (BaMnSbF) = 0.31(1), suggesting 3D Heisenberg class29. It is interesting that temperature dependence of the refined lattice parameters of BaMnAsF (Figure 7b) show a nonlinear trend around room temperature; this may be indicative of a magnetoelastic coupling in BaMnAsF.

Electronic structure calculations

All three compounds are found to be strongly magnetic with substantial Mn moments, in spite of the strong covalency between Mn and pnictogens. We perform calculations for both ferromagnetic (F) and in-plane nearest-neighbor antiferromagnetic (AF) spin configurations for the materials and consider additional magnetic configurations for BaMnAsF. In all cases, the AF configuration was lower in energy than the F configuration. The energy differences between the two configurations are 0.48 eV/f.u for BaMnAsF, 0.37 eV/f.u for BaMnSbF and 0.33 eV/f.u for BaMnBiF. These energies are extremely high and indicate high magnetic ordering temperatures. For comparison, the prototypical ferromagnet, bcc Fe (TC = 1043 K), has a F – AF energy difference of ~0.4 eV in density functional calculations38.

The qualitative reasons for these high energies may be seen in the electronic structures. The calculated densities of states for BaMnAsF for the in-plane AF order are shown in Figure 9, with the corresponding band structures in Figure 10 (the plots for BaMnSbF and BaMnBiF are provided in Figures S5S6). The fluorine and barium atoms are fully ionic and the resulting ionic [BaF]+ layers form insulating barriers between the [MnPn] layers in the crystal structure as is seen in the relatively weak dispersion of the occupied bands along the c-axis (Γ-Z) in Figure 10. As may be seen, the band structures with this magnetic order are semiconducting. The calculated gaps are 0.70 eV, 0.56 eV and 0.42 eV for BaMnAsF, BaMnSbF and BaMnBiF, respectively. The values for BaMnAsF and BaMnSbF are close to those found from transport data, which is the expected behavior of a material where the gap is between different transition metal d-manifolds and where Mott-Hubbard type Coulomb correlations are not large39. This is in contrast to simple semiconductors where DFT gaps are underestimates and especially Mott-Hubbard insulators where DFT calculations give either no gaps or gaps much smaller than experiment. The inferred relative weakness of Mott-Hubbard correlation effects in these materials is reminiscent of the FeSCs, although it should be noted that this does not mean that the FeSCs are uncorrelated40,41.

Figure 9
figure9

Electronic densities of states and projection for BaMnAsF in the nearest neighbor in plane antiferromagnetic state.

The densities of states are per formula unit, both spins and the energy zero is at the valence band maximum.

Figure 10
figure10

Band structure for BaMnAsF in the nearest neighbor in plane antiferromagnetic state.

The energy zero is set to the valence band maximum.

We find very strong covalency between the spin polarized Mn d orbitals and the pnictogen p orbitals in all three compounds. This is seen in the electronic densities of states for the nearest neighbor antiferromagnetic state (Figure 9). As may be seen from the projections the hybridization is strongly spin dependent, providing an explanation for the high energy scale associated with magnetic order. This is further seen by comparing the calculated electronic structures for the F and AF order. Figure 11 compares the Mn d projection of the spin resolved density of states (DOS) for BaMnAsF with these two orders. There is a strong reconstruction of the electronic structure when going to the less favorable F order. In fact, this reconstruction is so strong that the semiconducting gap is closed and because of this the magnetization is reduced from the nominal value of 5 μB/Mn to lower values of 4.0 μB, 4.3 μB and 4.4 μB for BaMnAsF, BaMnSbF and BaMnBiF, respectively (calculated based on the total spin magnetization in the cell, not the moments in LAPW spheres). As discussed e.g. by Goodenough42, cases where the covalency and electronic structure are strongly affected by magnetic order are cases where high exchange couplings can be expected.

Figure 11
figure11

Comparison of Mn-d projected electronic DOS for BaMnAsF, with nearest neighbor in-plane antiferromagnetic ordering (AF) and ferromagnetic ordering (F).

The energy zero is the valence band maximum for the AF case and the Fermi energy for the metallic F case.

In general local moment magnetism has two ingredients: (1) moment formation and (2) the interactions between the moments that lead to order. While divalent Mn can occur in different spin states depending on the strength of the hybridization with ligands, here we find a near high spin case for all the magnetic orderings considered with similar spin moments in the Mn LAPW spheres for the F and AF cases (as well as the other cases for BaMnAsF). However, the energy associated with moment formation is not so much smaller than the ordering energy. For BaMnAsF in particular, non-spin-polarized calculations (no moments) yield an energy 1.27 eV/f.u. higher than the nearest neighbor AF order, i.e. less than three times higher than the F order (0.48 eV/f.u.).

Consistent with this, we find very weak magnetic interactions in the c-axis direction. For BaMnAsF, we calculated the energy for doubled cells along c-axis for the F and in-plane AF cases with and without alternation of spins along the c-axis. The energy differences were below 1 meV per formula unit implying that this is a very highly two dimensional magnetic system.

In the case of BaMnAsF, we also considered other AF in-plane configurations. These were for the order observed in the Fe-pnictides consisting of chains of like spin Mn atoms (the so-called SDW order) and the double stripe pattern found in FeTe43 (see Ref. 43 for a depiction of these orders). On a per formula unit basis, we find that the SDW type ordering and the double stripe ordering are 0.12 eV and 0.21 eV higher than the nearest neighbor AF order, respectively. Therefore, in agreement with the NPD data, we conclude that the nearest neighbor in-plane order is the probable ground state.

As mentioned, our calculations included spin orbit. In this case the energy depends on the spin orientation through the magnetocrystalline anisotropy. While this energy is small compared to the ordering energy, it is relevant to the magnetic behavior. For BaMnAsF, we did calculations with the moments oriented in the a-axis direction as well along the c-axis direction. For the other cases discussed, the calculations are done with the moments along the c-axis. With the experimental crystal structure the uniaxial c-axis direction is favored by 1.0 meV/f.u. While this is a small energy, if correct, the result implies that there will not be a strong Kosterlitz-Thouless type reduction in the ordering temperature due to the near 2D character of the material. To summarize, the calculations find that these materials are local moment antiferromagnetic semiconductors with moderate band gaps and strong spin-dependent hybridization between the Mn d states and the pnictogen p states. The materials are rather two dimensional both electronically and magnetically. The strong hybridization leads to reconstructions of the band structure with changes in magnetic order. This in turn underlies very high magnetic energy scales consistent with high TN that are confirmed in neutron diffraction.

Discussion

Three new 1111 fluoropnictides with the ZrCuSiAs-type structure, namely BaMnAsF, BaMnSbF and BaMnBiF, are synthesized by reacting elements with BaF2. The initial reactions of these components give small plate crystallites that are suitable for structure refinements using single crystal X-ray diffraction. Subsequent two-step annealing procedure results in approximately single-phase products (<2% impurity content from PXRD and magnetization results). For BaMnPnF, the unit cell volumes and bond distances are much larger compared to the isostructural Mn-based oxypnictides.

Temperature dependence of electrical resistivity suggest that BaMnPnF are semiconductors with band gaps of Eg(BaMnAsF) = 0.73 eV, Eg(BaMnSbF) = 0.48 eV and Eg(BaMnBiF) = 0.003 eV. These values are comparable to the theoretical gaps of 0.70 eV for BaMnAsF and 0.56 eV for BaMnSbF, but not 0.42 eV for BaMnBiF. The large discrepancy in the band gap for BaMnBiF derived from the resistivity data is likely due to the influence of doping. Based on electronic structure calculations, BaMnPnF are strongly magnetic with a preferred G-type antiferromagnetic ground state. Neutron powder diffraction (NPD) results give evidence for the G-type antiferromagnetic order below TN = 338(1) K for BaMnAsF and TN = 272(1) K for BaMnSbF. However, temperature dependence of magnetization χ(T) data on these polycrystalline pellets show no anomalies at TN and χ(T) increase with increasing temperature above TN. Similar featureless χ(T) data and non-Curie-Weiss behavior was recently reported for LaMnPO31 and increasing χ(T) above ordering temperatures are observed for BaMn2As232, BaMn2Bi226, BaFe2As23 and LaFeAsO33. Such behavior may be indicative of strong exchange coupling among Mn moments in BaMnPnF. Another important feature of BaMnPnF phases is a strong hybridization between Mn and Pn states, which is responsible for reduced ordered moment values of 3.65(5) μB/Mn for BaMnAsF and 3.66(3) μB/Mn for BaMnSbF, as determined from the NPD data at 4 K. The observed semiconducting antiferromagnetic behavior of BaMnPnF (Pn = As, Sb and Bi) is similar to the reported behavior in Mn-based oxypnictides LnMnPnO (Ln = La-Sm; Pn = P, As)11.

In conclusion, further studies of transition-metal-based ZrCuSiAs-type compounds in general and BaMnPnF in particular, are warranted. Although electronic structures of BaMnPnF are not similar to that of the superconducting Fe-based 1111 compounds, recent reports show that 1111 phases may demonstrate interesting variation of electrical and magnetic properties with doping and under applied pressure; for example, antiferromagnetic insulator LaMnAsO turns ferromagnetic metal when doped with hydrogen44. Another Mn-based compound, LaMnPO, transforms from an AF insulator to an AF correlated metal under pressure31. In addition, there is continued interest in 1111 phases due to the recent discovery of a high thermoelectric efficiency in BiCuSeO, which is further enhanced by introduction of Cu defects27.

Methods

Synthesis

Dendritic Ba, Mn and As pieces and Sb and Bi granules with purities greater than 99.9% are used as received from Alfa Aesar. Ultrapure BaF2 powder, from Ventron Alfa Inorganics, is dried at 200°C for 3 h before using. Reactants in the stoichiometric ratio of Ba:BaF2:Mn:Pn = 1:1:2:2 are weighed inside a helium-filled glovebox and put into alumina crucibles. The alumina crucibles are then transferred into silica tubes and sealed under vacuum. The reaction mixtures are heated to 1000°C (30°C h−1; dwell 6 h), then to 900°C (10°C h−1; dwell 6 h) and subsequently to 300°C (30°C h−1) after which the furnaces are switched off. This initial sintering step produced multi-phase reaction products, along with μm-size crystallites of 1111 phase in Pn = Sb and Bi, which are extracted for structural determinations using single crystal X-ray diffraction. The products are reground and pelletized inside the glovebox. The pellets are then placed inside alumina crucibles, enclosed in silica tubes, vacuum sealed, then annealed for a second time at 900°C (dwell 60 h). The third sintering step is the repeat of the latter annealing procedure. This only results in marginal improvement of phase purity as judged by slightly lower BaF2 impurity levels compared to the second sintering step.

Characterization

X-ray diffraction

For BaMnPnF (Pn = As, Sb, Bi), powder X-ray diffraction (PXRD) data are collected on a PANalytical X′Pert PRO MPD X-ray Diffractometer using monochromated Cu-Kα1 radiation. Scans are performed in 5–65° (2θ) range, with a step size of 1/60° and 20–100 seconds/step counting time. Low temperature data collections are carried out using an Oxford Phenix closed cycle cryostat. Due to the air sensitivity of finely ground powders of Pn = Sb and Bi, the powders are loaded in a protective holder with a polycarbonate cover, inside the glovebox. Rietveld refinements are performed using GSAS45,46 software package. PXRD results are summarized in Figures 2 to 4 and Table S1 (see Supporting Information).

For Pn = Sb and Bi, single crystal x-ray diffraction data are collected on a Bruker SMART APEX CCD-based diffractometer, which employs Mo Kα radiation (λ = 0.71073 Å). Crystals are selected under a microscope and cooled to 173(2) K under a cold nitrogen stream. The data collection, data integration and refinement are carried out using the SHELXTL software package47. SADABS is used for semi-empirical absorption correction based on equivalents. The structures are solved by direct methods and refined by full matrix least-squares methods on F2. All sites are refined with anisotropic atomic displacement parameters and full occupancies. Details of the crystallographic data and structure refinement parameters are given in Table 1. Positional and equivalent isotropic displacement parameters, along with refined interatomic distances and angles are provided in Tables 2 and 3, respectively. Further details of the crystal structure studies are summarized in the form of CIF (Crystallographic Information File) file (see Supporting Information).

Elemental analysis

Energy-dispersive X-ray spectroscopy (EDS) measurements are carried out using a Hitachi-TM3000 scanning electron microscope equipped with a Bruker Quantax 70 EDS system. Data acquisition is carried out with an accelerating voltage of 15 kV in 2–3 min scanning time. EDS results confirm elemental compositions of 1:1:1:1 for BaMnAsF, BaMnSbF and BaMnBiF.

Physical property measurements

For BaMnPnF (Pn = As, Sb, Bi), DC magnetization measurements are carried out using a Quantum Design Magnetic Property Measurement System. Temperature dependence of magnetization experiments are performed on polycrystalline pellets under applied fields of 10 kOe, 20 kOe and 30 kOe. Both zero field cooled (ZFC) and field cooled (FC) data are collected. Magnetization measurements as a function of field are carried out at 5 K, 100 K and 300 K. Four-probe electrical resistivity measurements are carried out on a Quantum Design Physical Property Measurement System.

Thermal analysis

Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) on BaMnPnF (Pn = As, Sb) are performed using a Pyris Diamond TG/DTA from Perkin Elmer, under a stream of ultra-high purity argon gas. The measurements are carried out on 20–25 mg pellet pieces in the temperature interval of 323–1573 K (20 K min−1).

Neutron powder diffraction

Neutron powder diffraction experiments on BaMnPnF (Pn = As and Sb) are carried out using the HB-2A neutron powder diffractometer at the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL). Measurements are performed using two different wavelengths of λ = 1.536 Å and 2.410 Å provided by the (115) and (113) reflections of a vertically focusing Ge monochromator; this allows for optimization of the instrument resolution function for specific Q ranges. The data are collected by scanning the detector array consisting of 443 He tubes in two segments to cover the total 2θ range of 6–150° in steps of 0.05°; overlapping detectors for the given step average the counting efficiency of each detector48. For the measurements, 4 g powder samples are confined inside vanadium containers. For below room temperature measurements, samples are loaded inside a JANIS top-loading closed-cycle refrigerator, while for high temperatures experiments up to 800°C, samples are loaded in an ILL vacuum furnace equipped with Nb heating elements. Rietveld refinements are performed using the FULLPROF program49. Spin configurations compatible with the crystal symmetry are generated by group-theory analysis using the program SARAh50. In Pn = As compound, impurity phases of BaF2 (1.5% weight fraction) and Mn2As (2.5%) are noted; in Pn = Sb compound, BaF2 (2.5% weight fraction) and Mn1−xO (2.4%) impurities are found.

Electronic structure calculations

First principles calculations are performed using the experimental crystal structure information data. The calculations are done within DFT using the generalized gradient approximation (GGA) of Perdew, Burke and Ernzerhof51 and the general potential linearized augmented planewave (LAPW) method52 as implemented in the WIEN2k code53. Well-converged basis sets and Brillouin zone samplings are employed, along with LAPW sphere radii of 2.4 Bohr for Ba, Mn, As, Sb and Bi and 1.9 Bohr for F. Local orbitals54 are added to the basis to include semi-core states and spin-orbit is included in the calculations.

Change history

  • 04 December 2013

    A correction has been published and is appended to both the HTML and PDF versions of this paper. The error has not been fixed in the paper.

  • 04 December 2013

    New BaMnPnF (Pn = As, Sb, Bi) are synthesized by stoichiometric reaction of elements with BaF2. They crystallize in the P4/nmm space group, with the ZrCuSiAs-type structure, as indicated by X-ray crystallography. Electrical resistivity results indicate that Pn = As, Sb, and Bi are semiconductors with band gaps of 0.73 eV, 0.48 eV and 0.003 eV (extrinsic value), respectively. Powder neutron diffraction reveals a G-type antiferromagnetic order below TN = 338(1) K for Pn = As, and below TN = 272(1) K for Pn = Sb. Magnetic susceptibility increases with temperature above 100 K for all the materials. Density functional calculations find semiconducting antiferromagnetic compounds with strong in-plane and weaker out-of-plane exchange coupling that may result in non-Curie Weiss behavior above TN. The ordered magnetic moments are 3.65(5) μB/Mn for Pn = As, and 3.66(3) μB/Mn for Pn = Sb at 4 K, as refined from neutron diffraction experiments.

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Acknowledgements

This work was supported by the Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. Work at the High Flux Isotope Reactor, Oak Ridge National Laboratory, was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy. We thank R. Custelcean for his help with the single crystal X-ray diffraction measurements.

Author information

B.S. conceived the experiments, prepared the samples, carried out PXRD and SXRD experiments, electrical resistivity and magnetic susceptibility measurements and wrote the paper. D.J.S. performed electronic structure calculations. V.O.G. carried out NPD experiments. A.S.S. initiated the project, assisted with the magnetic susceptibility measurements, supervised the experiments, analyzed the data and wrote the paper. All authors reviewed the manuscript.

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Saparov, B., Singh, D., Garlea, V. et al. Crystal, magnetic and electronic structures and properties of new BaMnPnF (Pn = As, Sb, Bi). Sci Rep 3, 2154 (2013). https://doi.org/10.1038/srep02154

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