A total of 48 ternary Bi-Mn-Sb samples were prepared and investigated for the construction of the isothermal sections and 17 samples for the liquidus surface projection.
The combined results of XRD and SEM measurements are listed together with experimental information in Table 2 and were used to construct the isothermal sections at 320 and 600 °C; they are illustrated in Fig. 2 and 3. EDX area scans were used to determine the composition of the matrix solidified from the liquid during quenching, especially in the 600 °C section, which has been denoted by “solidified liquid = L” in Table 2. DTA results are collected in Table 3. Boundaries of phase fields confirmed by at least one sample are drawn using thin solid lines.
Isothermal Section at 320 °C
The 320 °C isothermal section, illustrated in Fig. 2, shows the three binary phases α-BiMn, Mn2Sb and MnSb together with the ternary phase ~ MnBi0.9Sb0.1, designated here by τ, but there is no indication whatsoever of a continuous solid solution between α-BiMn and MnSb as it had been found at 250 °C by Dannöhl and Gmöhling. Despite the difference of 70°, it must be concluded that such a continuous solid solution does not exist at any temperature. This is the more probable as the experimental conditions, i.e. annealing at 250 °C for 750 h, would not be sufficient to achieve equilibrium conditions. Consequently, the optimized phase diagram by Uemitsu et al., which is mostly based on the experimental work by Dannöhl and Gmöhling, cannot be correct.
The ternary τ phase, first described by Andresen et al., is located close to the α-BiMn phase field. Powder XRD as well as SEM/EDX measurements confirmed that it is a true ternary compound and not an extended solubility of Sb in α-BiMn. According to the phase equilibria shown in Fig. 2 the solubility of Sb in α-BiMn is less than 1 at.% at 320 °C. Single crystal measurements showed that the ternary phase has a hexagonal structure (see chapter 4.4.). The narrow single-phase field of τ ranges from about 4 to 10.3 at.% Sb and from about 49.2 to 53.4 at.% Mn.
The isothermal section shows six three-phase fields of which five were confirmed by at least one sample. The three-phase field (L + α-BiMn + τ) was also confirmed by one sample but EDX results showed that its composition is very close to the phase boundary; therefore, it is shown by dashed lines. Both binary compounds, Mn2Sb and MnSb, show a rather low solid solubility of less than 4 at.% Bi. In samples containing MnSb in equilibrium with solid Bi1−xSbx, no ternary solid solubility was detected at all.
Isothermal Section at 600 °C
The isothermal section at 600 °C contains no additional solid phase besides the binary phases Mn2Sb and MnSb and the solid solution Bi1−xSbx. Two three-phase fields could be experimentally confirmed.
Concluding from the two binary systems, a continuous liquid phase field is suggested at this temperature, starting between ~ 72 and 100 at.% Bi on the Bi-Mn side, and ending between 13 and 21 at.% Mn on the Mn-Sb side. However, all SEM/EDX measurements of samples containing MnSb and L show a Mn content of less than 2 at.% for the quenched liquid phase as determined from area scans (Table 2). This is, especially on the Sb rich side, significantly lower than expected from the binary systems. It must be assumed that, already during the quenching process, Mn and Sb crystallize as MnSb in addition to the MnSb crystallites growing during the annealing process. The crystallization during quenching leads to a depletion of Mn in the liquid. The same applies to MnSb present in samples located in the single phase field L. An example is shown in Fig. 4(a) (sample composition Bi9.5Mn5Sb85.5) where the MnSb crystallites are directly surrounded by brighter areas containing less Sb. A further indication of MnSb phase formation during quenching is the small size of those crystallites.
The shape of the liquid phase boundary in the Bi-rich corner is deduced from the two samples Bi86Mn8Sb6 and Bi89Mn3Sb8. Corresponding SEM backscattered images are shown in Fig. 4(b) and (c). A detailed micrograph of sample Bi86Mn8Sb6 shows that primary crystals of MnSb are surrounded by Mn2Sb. This indicates an equilibrium of the three phases MnSb, Mn2Sb and L, whereby the sample must be located outside of the liquid boundary. On the other hand, sample Bi89Mn3Sb8 shows long thin needles of MnSb in an L(Bi) matrix (with L(Bi) designating Bi that had crystallized out of the liquid on quenching), which precipitated fast during the quenching process. This indicates that the sample is located in the single phase field L close to the phase boundary. The composition of the liquid was determined through EDX area scans of the solidified matrix in quenched samples. However, the concentration values of Bi and Sb show some scattering. All compositions listed in Table 2 are average values. Due to the discrepancies in the Mn content of the quenched liquid, the phase boundary itself is shown as a dashed line.
In the Sb-rich corner of the ternary phase diagram, a small section consists of L and solid (Sb). The boundaries of this two-phase field are defined on the one hand by the binary diagrams and on the other hand by a sample with the composition Bi4Mn7Sb89. This latter sample shows the three phases MnSb (formed during the quenching process), (Sb) and L; on the other hand, the sample exhibits a melting point of 589 °C, which is 11° lower than the annealing temperature. Thus, it can be assumed that it is situated in the L single-phase field close to the L + (Sb) two-phase field. The sample Bi4Mn7Sb89 shows, similar to sample Bi2Mn25Sb73 (Fig. 4d), a globular eutectic structure composed of L(Sb) and MnSb.
In contrast to the isothermal section at 320 °C the binary MnSb phase field is shifted to higher Mn contents and dissolves more, i.e. up to 6 at.% Bi. Figure 3 represents the best compromise of all available experimental results.
Liquidus Projection, Fields of Primary Crystallization and Ternary Reactions
All observed liquidus values from DTA measurements and results of a study of primary crystallization, based on 17 as-cast samples, were combined with literature data for the binary subsystems to construct a liquidus surface projection of the entire Bi-Mn-Sb system. It is shown in Fig. 5, and the corresponding experimental values are listed in Table 4. Bold solid lines correspond to monovariant lines which are based on experimental results, whereas dashed lines indicate estimated monovariant lines; in both cases arrows point to lower temperatures. The isothermal lines are drawn as thin dotted lines.
Because of using carbonized crucibles, the temperatures of thermal effects evaluated from DTA measurements are subject to an uncertainty, estimated to be up to 10 °C.
Compared with the data by Dannöhl and Gmöhling and Uemitsu et al. the liquidus projection in Fig. 5, including the fields of primary crystallization, looks quite different. On the other hand, the monovariant line originating from the binary eutectic MnSb + Sb is in perfect agreement with the results by Durand-Charre et al. (shown in Fig. 5 by small open circles) and by Dannöhl and Gmöhling.
The liquidus surface is dominated by the two large primary crystallization fields of MnSb and Mn2Sb and the four fields of the different Mn modifications. Because it is not possible to quench the γ- and δ-Mn modifications, their primary crystallization fields are estimated from the binary phase diagrams and drawn with dashed lines. Both the fields of primary crystallization of MnSb and Mn2Sb reach far to the Bi corner, i.e. to around 95 at.% Bi or slightly more.
From the micrographic texture of sample Bi83.7Mn10Sb6.3 (Fig. 4e) it is ambiguous if MnSb or Mn2Sb is the primary crystallization phase. Therefore, the sample is shown, as the only one, directly on the border between the two crystallization fields as a rhombus.
As can be seen from the liquidus projection in Fig. 5, a series of invariant reactions occurs in the a very narrow composition range (estimated between about 94 and 97 at.% Bi), as well as in a very narrow temperature range. Although it is virtually impossible to deduce the correct sequence of reactions from experimental results, it is attempted to provide the most logical Scheil diagram in Fig. 6. A corresponding schematic magnification of the liquidus projection in the Bi-rich corner is presented as an inset in Fig. 5. It shows very narrow fields of primary crystallization for the ternary phase τ as well as for α-BiMn and β-BiMn where the ternary phase τ itself is formed in a ternary peritectic reaction.
Figure 7 provides an isopleth for a constant Sb concentration of 10 at.%. Since the phase fields in the Bi-rich corner cannot be derived unequivocally from the experimental results (DTA and XRD) this part has been left out in the diagram.
The Ternary Phase BixMnSb1−x
The DTA measurements of several bulk phases, containing the τ phase, indicated that it is stable up to the ternary peritectic reaction at about 500 °C (see Tables 3 and 5).
In contrast to Andresen et al. or Göbel et al. who found an orthorhombic structure, space group P2221, a hexagonal structure (space group P63/mmc) was found in the current study for this ternary phase.
Bi0.8MnSb0.2 crystallizes with its own structure type that can be derived from the NiAs structure. The asymmetric unit of the structure contains 5 atomic positions which are listed together with their Wyckoff sites and site symmetries in Table 6. However, one of these positions (Sb, 6h) is essentially vacant as the refined occupation factor is only 1.5%. This position which is only poorly occupied, seems to be a partial delocalization of Sb from the Bi position. Compared with the basic NiAs-type—realized e.g. in MnSb and BiMn—the cell parameters a, b = 8.5714(4) Å are approximately doubled whereas the parameter c = 5.7113(3) Å is slightly shorter.
In Bi0.8MnSb0.2 the ratio of Mn-atoms to main group element atoms is 1:1, similar to the NiAs-type. However, the distribution of atoms in the unit cell differs significantly from the NiAs- and the related filled Ni2In-type structure, as discussed below.
The NiAs and Ni2In structures can be described based on the different stacking sequences of the 36 triangular nets (NiAs: Ni
; Ni2In: Ni
) where the subscript representing the z value and superscript the different point positions relative to the cell origin. In the case of Bi0.8MnSb0.2, the 36 net for the main group elements at z = 1/4 and z = 3/4 is formed by the mixed position M2 (2c) together with Bi1 (6h). The respective 36 net for the transition metal at z = 0 and z = 1/2 is incomplete, as it contains only the fully occupied Mn1 position, but the corresponding 2a position is not occupied resulting in a vacancy on every 4th position of the 36 net. The Mn2 (2d) position, on the other hand, is part of an incomplete 36 net at z = 1/4 and z = 3/4 with only one quarter of the net positions being occupied.
Thus, the crystal structure of Bi0.8MnSb0.2 is related to NiAs as well as to Ni2In. The main group elements Bi and Sb occupy exactly the same net positions at z = 1/4 and z = 3/4 as in NiAs (As
) and in Ni2In (In
). The Mn1 atoms occupy three quarter of the Ni
net (present in NiAs as well as in Ni2In) and the Mn2 corresponds to one quarter of the additional Ni-net in Ni2In (Ni
As mentioned previously the crystal structure of the phase was described in the space group P2221 by Andresen et al. and Göbel et al. (Fig. 8a and b). A transformation of the orthorhombic cell according to [2 0 0/-1 1 0/0 0 1] from a = 4.304(5) Å, b = 7.449(4) Å, c = 5.742(4) Å, V = 181.4 Å3 results in a cell metric of a = 8.608(10) Å, b = 8.603(4) Å, c = 5.742(4) Å, α = β = 90°, γ = 120.02(9)°, V = 368.2 Å3. This is close to the hexagonal cell a = 8.5714(4), c = 5.7113(3) Å, V = 363.39(3) Å3 of the present investigation determined by single crystal XRD. Not considering any partial occupation of distinct atom positions, the crystal structure of Bi0.8MnSb0.2 reported by Göbel et al., is structurally related to. A vacant position in the crystal structure described by Andresen et al. is occupied by Mn atoms. A transformation of the orthorhombic cell parameters a = 4.27, b = 7.40, c = 5.70 Å by [2 0 0/− 1 1 0/0 0 1] results in a pseudo-hexagonal cell metric of a = 8.54, b = 8.54, c = 5.70 Å, α = β = 90°, γ = 119.88°.
Topological relations between Bi0.9MnSb0.1 respectively Bi0.8MnSb0.2 and the compound presented here are not evident. A comparison of the three different crystal structures is shown in Figs. 8(a) to (c).
According to the current study, a violation of the hexagonal cell symmetry of the ternary phase BixMnSb1−x is not likely as the anisotropies of the displacement parameters of all atoms are not conspicuous. An attempt to release the restriction of the cell parameters a and b results in insignificant differences only. Merging of symmetry equivalent reflections gives no hint for the requirements of a lowering of the Laue symmetry. One could argue that the ratio of the occupation factors for the mixed occupied position M2 (Bi2:Sb2 ~ 1:2) might indicate a reduction of the P63/mmc symmetry. However, these atoms occupy the position 2c with site symmetry −6m2; order might be achieved only by an enlarged unit cell. A careful investigation of the reciprocal lattice obtained from the single-crystal experiments gave no evidence for any super structure reflections.
The composition measurements by SEM (BSE image in Fig. 4f) resulting in Bi41.3Mn50.7Sb8, are in good agreement with the calculated value Bi41.7Mn49.7Sb8.6 (Bi0.8MnSb0.2) from single crystal model. Sb and Bi occupy the Wyckoff position 2c jointly.
Structural parameters, coordinates and displacement parameters as well as interatomic distances are given in Tables 6, 7, and 8. The refined atomic coordinates and structural parameters obtained by powder XRD are given in Table 9; Fig. 9 displays a 2θ segment of the powder XRD pattern between 19° and 69°. The crystal structure itself is shown in Fig. 8(c).
Further structural information was deposited with Fachinformationszentrum KarlsruheFootnote 1 and can be obtained on quoting the depository number CCDC 1881822.