Martin-Luther-University Halle-Wittenberg
Institute of Geological Sciences
Mineralogy / Geochemistry
Prof. Dr. Dr. H. Pöllmann
Von-Seckendorff-Platz 3, 06120 Halle, Germany, Tel.: +49-0345-5526110
FAX:+49-0345-5527180, email: [email protected]
The phase stability of Brownmillerite and Perovskite of the system
CaFeO2.5 - CaMnO2.8
Matthias Zötzl, Tel.: +49-0345-5526124, email: [email protected]
Introduction
Highly reactive calcium alumina cements can be formed by addition of manganese ore to a raw meal composed of bauxite and limestone [1].
Lowered production temperatures and lower costs of raw material are obtained. The new formed phases are manganese solid solutions such as
monocalciumaluminate and brownmillerite which are primary minerals of calcium aluminate cements. The ferrite phase in cements acts as a host and
scavenger for many elements present in cement [2]. The addition of manganese to calcium alumina cements leads to the formation of a high amount of
ferrite phase because it is incorporated in a ferrite phase. Considering the partial substitute the Al-raw material (high cost of bauxite) by manganese, the
3+
field of hydraulic brownmillerite solid solutions in the system CaFeO2.5-CaMnO3-x was investigated. In the CaFeO3-x-CaMnO3-x - system Fe was
3+
replaced by Mn within the crystalline structure of CaFeO2.5. The areas of different phases were determined by X-ray diffraction in a temperature range
of 1300°C.
Mol-%
Fig. 1: Phase relations in the system CaFeO2.5 - CaMnO2.8
Phase relations in thesystem CaFeO2.5 - CaMnO2.8
The phase relations in the system CaFeO2.5-CaMnO2.8 are shown at Figure 1. CaFeO3-x crystallises as CaFeO2.5 as brownmillerite structure-type. The
3+
3+
3+
oxidation state of iron in this compound is Fe only. According to [2] the substitution of Fe by Mn in CaFeO2.5 is concluded by C2F0.83Mn0.17O2.5. Few
3+
quantities of Mn -ions cause a change of the cell parameters in which the magnitude of the b-axis increases and that of a0 and c0 decreases. The unit cell
3
3
3+
3+
volume is decreased from V = 448.08 Å (CaFeO2.5) to 443.50 Å (Ca2Fe0.83Mn0.17O2.5) by incorporation of smaller Mn - (0.62 Å) than Fe - (0.67 Å)
ions. Another CaFe0.5Mn0.5O3-x (as a second phase) could be observed by an addition of above 17 mol % of manganese. An addition of more than 50
mol% manganese leads to an increase of an oxygen defect perovskite-phase. In the field between 17-50 mol% manganese content diminishes and the
brownmillerite structure regressed. Further, only one phase - CaFe0.5Mn0.5O3-x - could be formed by a substitution of 50 mol % iron by manganese. The
compound CaFe0.5Mn0.5O3-x has a cubic crystal structure with lattice parameter a0 = 3.779 ± 0.0004 Å. The stability field of various oxygen defect
3+
3+
perovskites lies within the area of 50-100 mol% substitution of Fe by Mn . CaMnO3-x crystallises as oxygen defect perovskites-phase and its
4+
3+
stoichiometric value was determined by TPR-measurements with CaMn 0.60Mn 0.40O2.8. The compound has a primitive orthorhombic structure with
4+
3+
lattice parameters of a0 = 7.622 (± 0.0037) Å, b0 = 15.006 (± 0.0033) Å and c0 = 7.457 (± 0.0009) Å. The structure of CaMn 0.60Mn 0.40O2.8 has a lattice
consisting of MnO6 octahedra and MnO5 square pyramids contrary to that of brownmillerite-type structure which consists of an alternating layers of
octahedra and tetrahedral. In the analysed system and temperature field, brownmillerite-solid solutions crystallise with the following Ca(Fe1-xMnx)O2.5
3+
3+
3+
4+
4+
3+
≤ x≤ 0.17). The stability of brownmillerite (Fe , Mn ) or perovskites (Fe , Fe , Mn , Mn ) depends on different pO2 caused by a variable
(0.0
(0.0≤
sintering temperature range. The CaFeO2.5 substitution of 50 mol% of iron by manganese leads at a temperature range of 1300°C - 1500°C to the
formation of only one perovskite phase with CaFe0.5Mn0.5O3-x. The solid state reaction at 1150°C had shown a second phase with brownmillerite
structure, contrary to [3] who synthesised CaFe0.5Mn0.5O3-x under similar conditions (1150-1200°C under air) and analysed an orthorhombic
brownmillerite structure with alternating FeO4 tetrahedra and MnO6 octahedra, only. The oxidation control of sintering conditions is crucial to avoid
the crystallisation of hydraulic brownmillerite or inert perovskite.
Oxygen
Fig. 2: Schematic structures of a brownmillerite (left) and
2+ 3+
perovskite (middle). The crystal structure of A B O2.5
2+
3+
3+
brownmillerite (A = Ca ; B = Mn , Fe ), an oxygen-deficient
perovskite type, consists of alternating layers of corner sharing
BO6-octahedra and BO4-tetrahedra. The perovscite structure
2+ 4+
2+
4+
4+
A B O3 (A = Ca ; B = Mn , Fe ) composed of alternating
2+
layers of corner sharing BO6-octahedra. The A -ions located in
the open spaces in-between are not shown. The structure of
oxygen defect perovskites CaMnO2.5 (at the right) is a
arrangement of MnO6 octahedra and MnO5 square pyramids .
The modified Figure is described by [4].
B-Cation
Oxygen-Vacancy
Brownmillerite
(ABO2.5)
References
[1] PÖLLMANN, H. & OBERSTE-PADTBERG, R. (2001) IMO Communications, 139-148
[2] PUERTAS, F. & GLASSER, F. P. (1987) Advances in Cement Research, 1, 31-34
[3] NAKAHARA, Y., KATO, S., SUGAI, M., OHSHIMA, Y. & MAKINO, K. (1997) Materials Letters, 30, 163-167
[4] PÖPPELMEIER, K. R., LEONOWICZ, M. E., LOGO, J. M. (1982) J. Solid State Chem., 44, 89-98
Perovskite
(ABO3)
O2-defect-Perovskite
(CaMnO3-x, x = 0.5)