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2021 : Volume 1, Issue 1

Phase equilibria in the MnO-Mn₂O₃-FeO-Fe₂O₃-Sb₂O₃-Sb₂O₅ System in Air at 1200?
Author(s) : BG Golovkin ¹

¹ Department of Chemistry , Public Institute of Natural and Human Sciences , Russia

Glob J Chem Sci

Article Type : Research Article

DOI : https://doi.org/10.53996/2769-6170.gjcs.1000107

Abstract

Using the methods of X-ray phase and X-ray Densitometric analysis, the phase equilibria between oxides of manganese, iron and antimony have been investigated in an air atmosphere at temperatures up to 1250? in an air mosphere at normal pressure. The phase diagram of the system at 1200? was built MnO-Mn₂O₃-FeO-Fe₂O-Sb₂O₃-Sb₂O₅. A new phase was found Mn_12-2x,sup>2+Fe_2x²⁺Sb³⁺Sb₅⁵⁺O₂₆(0?x?1), with edge compositions FeMn₅Sb₃O₁₃ and Mn₆Sb₃O₃ (a=8.5003?0.0025Å; b=8.0064?0.0025Å; c=11.5779?0.0025Å; Z = 3; ?obs. = 5.7 g/cm³; ?calc = 5.6991 g/cm³). The phase exists in the temperature range 1180-1230^oC and can be obtained by quenching, but always with a large admixture of Mn₂Sb₂O₇ and spinel Mn₁₁²⁺Mn₁₃³⁺Sb₉³⁺O₄₄.The reason for this behavior is that air molecules have different temperatures, as a result of which the phase composition of the reaction mixture cannot be strictly related to one temperature, and different phases can be stable at different temperatures.

Keywords

Antimonates of iron and manganese; X-ray Densitometric method; Molecule temperature; Temperature distribution of molecules

Introduction

Investigation of the phase composition of the MnO-Mn₂O₃-FeO-Fe₂O₃-Sb₂O₃-Sb₂O₅ system is of interest for obtaining sensor and magnetic materials, spin glasses, catalysts, and materials used in memory devices and microwave technology [1–5]. We previously studied the phase composition and magnetic properties of systems containing similar components: Cu₂BSbO₆, where B=Fe₃₊,Mn₃₊,Ga₃₊ with a bixbyite structure [5–7] Mn₂BSbO₆ (B=Fe,V,Cr,Ga,Al) at high pressure [8] systems CaO-CoO-Co₂O₃-MnO-MnO₂ [4] Ca–Mn–Sb–O [9] in an atmosphere of air at normal pressure, and also obtained and studied the properties of Mn₃FeTiSbO₉ and Mn₄FeTi₂SbO₁₂ [1,2].The purpose of this work is to study phase equilibria and construct a phase diagram of a system consisting of oxides of iron, manganese, and antimony. In accordance with the Fe-O phase diagram [10] there are three oxides in the system in air: metastable FeO (wustite), disproportionating when heated to iron and Fe₃O₄ (magnetite) and Fe₂O₃, existing in four different modifications [11–13]. α-Fe₂O₃ (hematite) is a stable modification [14–16]. Above 1457^oC, it is reduced to Fe₃O₄ [10]. There are three oxides in the Sb–O system under air conditions. Antimony oxide Sb₂O₃ is known in two polymorphic modifications: low-temperature orthorhombic (α-Sb₂O₃, valentinite) and high-temperature cubic (α-Sb₂O₃, senarmontite) with a temperature of a reversible enantiotropic phase transition in an inert atmosphere of about 500^oC. In air at this temperature, Sb₂O₃ is oxidized to ?-Sb₂O₄(cervantite), which at 790^oC turns into monoclinic α-Sb₂O₄ (clinoservantite). Pure Sb₂O₅ exists in two polymorphic modifications and can be obtained only under oxygen pressure [5,17,18]. When pure or hydrated Sb₂O₅ is heated in air above 267^oC, Sb₆O₁₃ with a cubic pyrochlore structure is first formed, transforming at 830^oC in to Sb₂O₄, which quickly evaporates at higher temperatures [5,17].In the system FeO-Fe₂O₃-Sb₂O₃-Sb₂O₅ there is antimonate FeSbO₄ (mineral tripuchite) with a rutile structure, which decomposes above 1270^oC to form hematite [19–21] and FeSb₂O₄ (mineral safarzikite), isostructural Pb₃O₄ in vacuum at 700^oC [22]. As shown in [21], the synthesis or finding in nature of the previously described compounds FeSb₂O₆ with the trirutile structure, Fe₂Sb₂O₇ with the pyrochlore structure, and Fe₂Sb₂O₆ was erroneous, and no one has yet succeeded in obtaining such compounds. We have also carried out unsuccessful attempts to obtain these compounds from stoichiometric mixtures under a layer of a finely dispersed amorphous substance or a silicate glass melt [23]. In the first case, the main product of the reaction was FeSbO₄, and when using the melt, a mixture of unknown phases was formed. Of manganese oxides in air up to 877^oC, α-Mn₂O₃ (kurnakite) is stable, in the range 877–1160^oC α-Mn₃O₄ (hausmanite with a tetragonal structure) is stable, at 952–1160^oC a metastable modification with an orthorhombic lattice can exist, and in the range 1160-1567^oC there is a cubic spinel α-Mn₃O₄. Above these temperatures, MnO is formed (mp=1842^oC) [3]. In accordance with the Fe–Mn–O phase diagram in air [14,15], the system contains Fe_xMn_2-x)O₃ solid solutions based on the α-Mn₂O₃ (bixbyite) structure, with the corundum (hematite) structure, Fe_xMn_3-xO₄ with a hausmanite structure and cubic spinel with a MgAl₂O₄ structure. In the system MnO-Mn₂O₃-Sb₂O₃-Sb₂O₅ under atmospheric conditions, Mn²⁺Sb₂O₆ is formed, which exists in four different modifications [17] (decomposes at 1100^oC [24] and Mn₂²⁺Sb₂O₇. At temperatures above 900^oC, Mn₂Sb₂O₇ crystallizes in the structure of trigonal 3T-weberite [25,26], and at temperatures below 500^oC it is fixed in the form of cubic pyrochlore [27]. The Mn_1-xSb_1+xO₄ (0 ≤ x ≤ 0,33) phase with the rutile structure, decomposing at 800^oC [17], was synthesized by the sol-gel method. Also known is the mineral manganostibite Mn₈Sb₂O₁₅ with the structure of orthorhombic hausmanite [17,24]. Under conditions of oxygen deficiency, tetragonal Mn²⁺Sb₂O₄, iso-structural Pb₃O₄ [24,28,29] and Mn₃²⁺Sb₂O₆ [29] are formed. Of the ternary compounds, only the mineral melanostibit Mn₂FeSbO₆ with an limonite structure is known. As shown in [16,30,31], this compound can be obtained only under conditions of comprehensive compression under a pressure of 3–5 GPa and a temperature of 900^oC. At a pressure of 6 GPa and the same temperature, this compound transforms into a perovskite structure.

Experiment

To prepare the samples, we used Sb₂O₃ (pure grade), Fe₂O₃ and MnO₂ (analytical grade). The samples were synthesized by annealing the corresponding stoichiometry mixtures in alundum crucibles in air at normal atmospheric pressure with intermediate (at least three times) grinding with a gradual increase in temperature from 800^oC to 1250^oC, followed by annealing for 8–100 hours and quenching in air at room temperature. The change in the phase composition of the samples was monitored by the XRD method using a DRON-2 diffract meter in CuKα - radiation. The oxygen content in the edge compositions of the Mn_12-2x²⁺Fe_2x²⁺Sb³⁺Sb₅⁵⁺O₂₆ (0?x?1) phase was determined by the X-ray– Densitometric method [32].

Phase equilibria of the MnO-Mn₂O₃-FeO-Fe₂O₃-Sb₂O₃-Sb₂O₅ system investigated in air atmosphere at normal pressure in the temperature range 800-1250^oC. At temperatures below 1160^oC, no new compounds were found in the system. FeSbO₄ Antimonates is in equilibrium with manganese antimonates Mn²⁺Sb₂O₆, Mn₂²⁺Sb₂O₇, hematite and a solid solution with a hausmanite structure based on Mn₃O₄, which is still in equilibrium with Mn₂²⁺Sb₂O₇ and solid solution Fe_2-xSb_xO₃ (0 ≤ x ≥ 0,04) with the structure of hematite. Antimony oxide at these temperatures exists in the form ?-Sb₂O₄ and is in equilibrium with iron and manganese antimonates. However, with prolonged heating, it volatilizes and ceases to be detected. Phase equilibria in the temperature range 1180-1230^oC are shown by the phase diagram [Figure 1].

Figure 1: Phase diagram of the system MnO-Mn₂O₃-FeO-Fe₂O₃-Sb₂O₃-Sb₂O₅ in air atmosphere at 1200^oC.

Under these conditions, a new (black and low-conductive electric current) phase of variable composition Mn_12-2x²⁺Fe_2xSb₃₊Sb₅⁵⁺O₂₆ (0 ≤ x ≥ 1), in equilibrium with Mn₂²⁺Sb₂O₇ and a phase with a cubic spinel structure (hausmanite transforms into a spinel structure at 1160^oC). On the basis of the structure of hematite, there are solid solutions Fe_(1-0,05x)Mn_{0,05xO_1.5}Fe_(1-0,02x)Sb_0,02xO_1,5(0 ≤ x ≥ 1). Hematite containing antimony Fe_1,96Sb_0.04O₃ is insoluble even in aqua regia. Otherwise, the phase equilibrium diagram coincides with that at lower temperatures. A new phase of any composition Mn_12-2x²⁺Fe_2x²⁺Sb³⁺Sb₅⁵O₂₆ (0 ≤ x ≥ 1) can be obtained only by quenching, but always, with a certain amount of impurity pyroantimonate Mn₂²⁺Sb₂O₇ and spinel Mn₈Sb₃O₂₂ of the presumptive composition Mn₁₁²Mn₁₃³⁺Sb₉³O₄₄. The boundary compositions of the new phase are the compounds Mn₆Sb₃O₁₃ and FeMn₅Sb₃O₁₃²⁺.

The ratio of manganese to antimony of the new phase was determined on the basis of quantitative X-ray phase analysis of the phase content in the corresponding samples.The X-ray characteristics of the new phase are presented in [Table 1]. The phase crystallizes in anorthorhombic system with parameters a = 8.5003 ± 0.0025Å; b = 8.0064 ± 0.0025Å; c = 11.5779 ± 0.0025Å.

d_calc,Å	hkl	Mn₆Sb₃O₁₃ new phase are the compounds		FeMn₅Sb₃O₁₃
d_calc,Å	hkl	I/I₀,%	d_obs,Å	I/I₀,%	d_obs,Å
8.5003	100	0	8.5	16	8.5
6.8519	101	–	–	19	6.9
5.7895	002	–	–	20	5.8
4.2503	200	23	4.25	38	4.25
4.0032	020	16	4.003	–	–
3.5709	211	53	3.573	40	3.573
3.1497	212	100	3.151	100	3.151
2.914	220	50	2.918	51	2.926
2.8334	300	60	2.829	29	2.829
2.8244	221	60	2.829	29	2.829
2.6688	030	37	2.672	10	2.67
2.6711	310	37	2.672	10	2.672
2.5462	130	46	2.555	47	2.548
2.5449	302	46	2.554	47	2.548
2.434	132	23	2.436	20	2.436
2.2182	231	16	2.22	–	–
2.1251	400	16	2.13	16	2.13
2.0016	040	36	2.009	42	2.009
2.0115	125	36	2.0086	42	2.0086
1.9949	402	30	1.996	29	1.996
1.8817	106	26	1.8864	27	1.8791
1.8815	142	26	1.8864	27	1.8791
1.7854	414	20	1.786	20	1.7827
1.713	404	33	1.7143	–	–
1.7131	135	33	1.7143	–	–
1.7162	216	33	1.7143	–	–
1.7024	500	–	–	30	1.7006
1.6463	044	35	1.6434	–	–
1.6348	340	33	1.6327	–	–
1.6013	050	23	1.5989	23	1.599
1.5861	051	23	1.5824	22	1.5838
1.5286	027	16	1.5296	16	1.5292
1.4242	018	16	1.4233	16	1.4236

Table 1: X-ray characteristics Mn_12-2x²⁺Fe_2x²⁺Sb³⁺Sb₅⁵⁺O₂₆ (0 ≤ x ≥ 1)

Since it was not possible to obtain a new phase in its pure form, the X-ray Densitometric method was used to determine the oxygen content and the number of formula units in a unit cell [32–34]. The composition Mn₆Sb₃O_13,5-α was taken as a model phase. The true composition of the desired phase turns out to be dependent on the value of α to be found. rexp was determined from the dependence of the density of MnO, Sb₂O₅ and manganese antimonates [18,24–27] on the composition [Figure 2]. The ordinate shows the ratio of the manganese to antimony content in the form of the corresponding antimonates formulas. Therefore, the phase Mn₆Sb₃O₁₃ corresponds to the composition Mn₄Sb₂O₉.

Figure2: Dependence of densityon the composition of manganese antimonites.

As can be seen from this figure, this dependence is close to straight-line. From which it follows that the density of the model phase? should be approximately equal to the experimental value of the sought phase ρ≈ρexp. ≈5.7 g/cm³, obtained from this dependence for the composition, the relative content of manganese in which corresponds to that in the model phase Mn:Sb = 2. Then the number of formula units Z of the model phase can be found from the expression [32-34]:

r =ZM/NV (1)

According to the formula:

Z = [NVr/M] (2)

Where M is the molecular weight of the formula unit of the model phase, V is the unit cell volume, N is Avogadro's number, and square brackets mean that the number in parentheses is taken, rounded to the nearest integer value. The correctness of the choice of the composition of the model phase is confirmed by the fact that the fractional number enclosed in square brackets only slightly differs from the whole number. In our case, the number of formula units? Mn₆Sb₃O_13.5-α, found by formula (2) is Z=[2.9966] ≈ 3, provided that α = 0.5. For other values of α, the number in square brackets turns out to be very different from an integer. After we have been able to estimate the values of α and Z, we can use formula (1) to find the X-ray values of the density of the desired phase. In particular, for the phase Mn₆Sb₃O₁₃ ρrent. = 5.6991 g/cm3, and for the FeMn₅Sb₃O₁₃ ρrent. = 5.6925 g/cm³, phase, which are close to the experimental density of 5.7 g/cm³. Considering that manganese in antimonites is always in a bivalent state, the composition of the resulting new phase can be represented as Mn_12-2x²⁺Fe_2x²Sb³⁺Sb₅⁵⁺O₂₆(0 ≤ x ≥ 1). At a temperature of ~ 1250^oC, this phase no longer exists. Instead, in products of various compositions, only Mn₂Sb₂O₇, reflections are recorded, while in compositions richer in manganese, spinel reflections based on Mn₃O₄ are already manifested. Below 1180^oC, the same mixture of Mn₂²⁺Sb₂O₇ with spinel is again recorded. Apparently, at temperatures above 1230^oC under atmospheric conditions Mn₃²⁺Sb₂O₆ is already formed [29]. The strongest Mn₂Sb₂O₇ reflection coincides with one of the peaks of the new phase, and the positions of the strongest Mn2+3Sb2O6 reflections coincide with the positions of the strongest Mn₂²⁺Sb₂O₇. Reflections this allows us to make assumptions about the existence of regions of homogeneity based on Mn₂²⁺Sb₂O₇.

The discussion of the results

From the phase equilibria studied and presented in the diagram in Figure 1, the main discovery is the detection of a phase of variable composition Mn_12-2x²⁺Fe_2x²⁺Sb₃₊Sb₅⁵⁺O₂₆ (0 ? x ? 1). This phase, however, cannot be obtained in its pure form under atmospheric conditions at any temperature and, practically at any annealing time, both with slow cooling and quenching to subzero temperatures. Permanent impurities, the amount of which, roughly, based on a comparison of the intensities of X-ray reflections, is 20–30 %. The impurities contain pyroantimonate Mn₂Sb₂O₇ and a phase with a spinel structure. Assuming that these products are the result of the decomposition of the new phase, the chemical composition of the spinel can be estimated. Let us write down the decomposition reaction for the edge composition Mn₆Sb₃O₁₃ Mn₆Sb₃O₁₃ = aMn₂Sb₂O₇ + bMn_3-xSb_xO₄ (3) And, the balance equation for chemical elements in both sides of equation (3): 6=2a+3b-bx (4) 3=2a+bx (5) 13=7a+4b (6)Having solved the equations (4–6) and substituting the obtained values of the unknowns into the equation (3), we get: Mn₆Sb₃O₁₃ = 0.6Mn₂Sb₂O₇ + 2.2Mn_2.18 Sb_0.82O₄ (7) The spinel composition Mn_2.18Sb_0.82O₄ can also be represented by the formulas Mn₈Sb₃O₂₂ and Mn₁₁²⁺Mn₁₃³⁺Sb₉³⁺O₄₄. For compositions containing iron, it will be part of the impurity spinel, but since iron can have different oxidation states, depending on the temperature and pressure, it can enter both the cationic and anionic spinel sub lattices. The reason for the non-existence of a new phase at temperatures below 1180^oC may be that the phase exists only in the region of high temperatures ?1180-1230^oC, and when cooled to lower temperatures leads to its decomposition into manganese pyroantimonate and spinel Mn_2.18Sb_0.82O₄. In this case, the phase could be obtained by quenching, i.e. by rapid cooling of the reaction products to subzero temperatures in an almost pure form, containing only minor impurities of decomposition products. However, in practice, the amount of impurities turns out to be very large, up to 30 percent or more, for any composition corresponding to its region of existence. This forces us to look for another reason for the impossibility of obtaining this phase in its pure form by the ceramic method. In [34–37] by extrapolating the concept of temperature for a large number of gas molecules to one molecule, it was shown that the temperature ? of a monoatomic molecule of radius r and mass m, moving with speed v, is

Where l is the free path of the molecule without collisions [37], k is the Boltzmann constant, and E is the kinetic energy of the molecule. In [35], a method was proposed for calculating the temperature distribution of n gas molecules of radius r depending on the gas temperature T. The method consists in calculating the gas temperature, the mean free path of molecules? l?_s, at which the number of molecules n_? with a temperature ?=T is maximum, and then the desired distribution by the formula

These results show that at a given gas temperature, the total mass of molecules contains a large number of molecules whose temperatures are much different from the total gas temperature. It follows from this that ceramic synthesis technologies fundamentally do not allow obtaining highly pure substances. And in those cases when the temperature regions of existence of the corresponding phases are close, then they will all form and exist simultaneously in the form of mixtures that are equilibrium for the given conditions. Obviously, in our case, a new phase of variable composition Mn_12-2x²⁺Fe_2x²⁺Sb³⁺Sb₅⁵O₂₆ (0 ? x ? 1) under atmospheric conditions air at normal pressure and temperatures ?1180-1230^oC coexists with manganese pyroantimonate Mn₂Sb₂O₇ and a phase of the supposed composition Mn_2.18Sb_0.82O₄ with a spinel structure, and in pure form at any ratio and any temperatures of the initial components under these conditions has a zero region existence. The reason for this behavior of the new phase is that the oxygen molecules contained in the air have different temperatures, as a result of which the phase composition of the reaction mixture cannot be strictly related to one temperature, and different phases can be stable at different temperatures. Phase equilibrium thus refers not to one temperature, but to a range of temperatures.

Conclusion

The phase equilibria of the MnO-Mn₂O₃-FeO-Fe₂O₃-Sb₂O₃-Sb₂O₅ system in air and normal pressure up to temperatures of 12500^oC have been investigated.
A phase diagram of this system at 1200? was constructed.
A new phase was discovered Mn_12-2x²⁺Fe_2x²⁺Sb³⁺Sb₅⁵⁺O₂₆ (0?x?1), with edge compositions FeMn₅Sb₃O₁₃ and Mn₆Sb₃O₁₃.
The X-ray characteristics of the new phase were determined.
The region of existence of the new phase in the temperature range of 1180 –1230? was determined.
It was found that the new phase always exists under the synthesis conditions only in a mixture with manganese pyrovadate Mn₂Sb₂O₇ and a phase of the supposed composition Mn_2.18Sb_0.82O₄ with a spinel structure.
It was concluded that the reason for this behavior of the new phase is that the oxygen molecules contained in the air have different temperatures, as a result of which the phase composition of the reaction mixture cannot be strictly related to one temperature, and different phases can be stable at different temperatures. Phase equilibrium thus refers not to one temperature, but to a range of temperatures.

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CORRESPONDENCE & COPYRIGHT

*Corresponding Author: BG Golovkin, Public Institute of Natural and Human Sciences, Ekaterinburg, Russia.

Copyright: © 2021 All copyrights are reserved by BG Golovkin, published by Coalesce Research Group. This This work is licensed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution and reproduction in any medium, provided the original author and source are credited.