Phase diagram studies in the quasi binary systems
LaMnO3–SrMnO3 and LaMnO3–CaMnO3
Peter Majewski, Lars Epple, Michael Rozumek, Heike Schluckwerder, and Fritz Aldinger
Max-Planck-Institut fu¨r Metallforschung, Pulvermetallurgisches Laboratorium, Heisenbergstraße 5,
70569 Stuttgart, Germany
(Received 20 August 1999; accepted 10 February 2000)
The quasi binary systems LaMnO3–SrMnO3 and LaMnO3–CaMnO3 were studied. Both
systems show a miscibility gap at intermediate La:Sr and La:Ca ratios below about
1400 °C in air. This phenomenon causes the decomposition of single-phase
(La,Sr)MnO3−x and (La,Ca)MnO3−x solid solution into La-rich SrMnO3−x + Sr-rich
LaMnO3−x and La-rich CaMnO3−x + Ca-rich LaMnO3−x at lower temperatures,
respectively. At 1400 °C in the system LaMnO3–SrMnO3, a structure transformation of
(La,Sr)MnO3 from orthorhombic to rhombohedral with increasing Sr content was not
observed, and the structure of La0.7Sr0.3MnO3 was determined to be orthorhombic with
a ס
0.54927 ± 0.0009 nm, b ס
0.54582 ± 0.0009 nm, and c ס
0.76772 ± 0.0034 nm.
I. INTRODUCTION
The crystal structure of CaMnO3−x (CM) is ortho-
rhombic at high and low Mn4+ contents, but the cell
parameters significantly vary with the Mn4+ content.12,13
LM and SM as well as LM and CM form complete
solid-solution series at temperatures of about 1400 °C in
air. The extent of solid solution at lower temperatures has
not been studied. However, knowledge of the solid solu-
bility at the operating temperature of solid oxide fuel
cells and the synthesis temperature of Sr- or Ca-doped
LM films (800–1000 °C) is of great interest for the reli-
ability of LM electrodes during operation of fuel cells
and for the preparation of single-phase thin films of LM
materials.
Sr- or Ca-doped LaMnO3−x (LM) is of great techno-
logical interest for magnetic information storage systems
due to its significant magnetoresistive properties.1 In ad-
dition, high-temperature oxygen and electron conductiv-
ity as well as catalytic properties qualifies the compound
to be a cathode material of solid oxide fuel cells (SOFC).2
The maximum values of electron conductivity and mag-
netoresistivity have been observed for compositions with
intermediate contents of alkaline earth elements.2,3
The binary systems La2O3–Mn2O3,4 SrO–Mn2O3,5
6
and CaO–Mn2O3 contain the perovskite-structured
phase ABO3 with A ס
Sr, Ca, or La and B ס
Mn. The
crystal structure of LM is a function of the Mn3+/Mn4+
ratio. For LM with a high Mn4+ content, e.g., Sr- or
Ca-doped LM, a rhombohedral structure has been re-
ported for temperatures of 1000–1200 °C,3,4,7 whereas,
at a lower Mn4+ content (low Sr or Ca content), LM
crystallizes in an orthorhombic structure.4,7 Urushibara
and co-workers3 observed the phase transition of Sr-
doped LM from the orthorhombic to the rhombohedral
modification at 1200 °C in air at xSr ס
0.175.
In addition, recent studies of the magnetic phase dia-
gram of Ca-doped LM suggest that ferromagnetic LM
and antiferromagnetic CM co-exist at intermediate Ca
contents.14 Knowledge of the thermodynamic phase dia-
gram LM–CM is eminent to clarify this phenomenon.
In this work the extents of the solid solutions
(La,Sr)MnO3−x and (La,Ca)MnO3−x are studied using
microprobe analysis and x-ray diffraction (XRD) meas-
urements.
Mitchell and co-workers7 observed that at lower Mn4+
content (xSr below 0.175) orthorhombic and rhombohe-
dral LM coexist at 1000 °C in air.
II. EXPERIMENTAL
Samples with the compositions La0.5Sr0.5MnO3−x and
La0.5Ca0.5MnO3−x were prepared by using La2O3,
SrCO3, CaCO3, and MnO2 (purity > 99%), respectively.
The powder mixtures were calcined at 1200 °C in air for
12 h, ground, and cold isostatically pressed. The samples
were than sintered at 800, 900, 1000, 1200, 1300, 1350,
and 1400 °C in air for 48 h with intermediate regrinding
as well as pressing and finally furnace cooled to room
temperature (batch 1). A second batch of samples with
For Ca-doped LM, a cubic8 (at 950 °C) as well as
orthorhombic structure was reported.9,10 SrMnO3−x (SM)
exhibits a phase transformation from orthorhombic to
orthorhombic distorted hexagonal at about 1400 °C and
to hexagonal at 1035 °C due to uptake of oxygen with
decreasing temperature combined with an increase of the
Mn4+ content of the phase.11 At 1200 °C in air the Mn4+
content is about 90%.11
J. Mater. Res., Vol. 15, No. 5, May 2000
© 2000 Materials Research Society
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