Phase Equilibrium Diagrams of the Binary Systems LaMnO 3 -SrMnO 3 and LaMnO 3 -CaMnO 3
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ABSTRACT The quasi binary systems LaMnO 3 - SrMnO3 and LaMnO 3 - CaMnO 3 have been studied. Both systems show a miscibility gap below about 1400'C in air. This phenomenon causes the decomposition of single phase (LaSr)MnO 3_xand (LaCa)MnO 3.x solid solution with intermediate La:Sr or La:Ca ratios into La rich SrMnO 3_•or CaMnO 3.,, and Sr or Ca rich LaMnO3_x at lower temperatures. At 1400 'C a structure transformation of (LaSr)MnO3 from orthorhombic to rombohedral has not been observed and the structure of La0 .7Sr0.3MnO, has been determined to be orthorhombic with a = 0.54927 ± 0.0009 nm, b = 0.54582 ± 0.0009 nm, and c = 0.76772 ± 0.0034 nm. INTRODUCTION The binary systems La20 3-Mn20 3 [1], SrO-Mn 20 3 [2] and, CaO-Mn 20 3 [3] contain the perovskite structured phase ABO 3 with A = Sr, Ca or La and B = Mn. For LM with high a Mn4 content, e.g. Sr or Ca doped LM, a rombohedral structure has been reported for temperatures of 1000-1200 °C [1,4,5]. Whereas, at a lower Mn4÷ content (low Sr or Ca content) LM crystallizes in an orthorhombic structure [1,5]. [4,5] observed the phase transition of Sr doped LM from the orthorhombic to the rhombohedral modification at 1200 'C in air at about xs, = 0.175. [5] observed that at lower Mn 4 content (xs, below 0.175) orthorhombic and rhombohedral LM coexist at 1000 'C in air. SrMnO 3_, (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 combined with an increase of the Mn4 content of the phase [6]. The crystal structure of CaMnO 3 (CM) is orthorhombic at high and low Mn 4 contents, but the axis parameter significantly varies with the Mn'4 content [7,8]. It is known that LM and SM as well as LM and CM form complete solid solution series at temperatures of about 1400 'C in air. The extensions of the solid solution series at lower temperatures have not been studied, yet. EXPERIMENTAL Samples with the compositions La 0.5Sr 0.5MnO 3_• and La0osCa0.5MnO3 _,were prepared using La 20 3, SrCO 3, CaCO 3 and, MnO 2 (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
327 Mat. Res. Soc. Symp. Proc. Vol. 602 0 2000 Materials Research Society
(batch 1). A second batch of samples with the composition La05Sr 05MnO 3.x and Lao5Ca0.5 MnO 3_,,respectively, were sintered at 1400 'C in air and subsequently annealed at 1350, 1300, 1200, 1000, 900 and 800 'C in air for up to 500 h and furnace cooled (batch 2). In addition, samples with the composition La 1.,SrxMnO 3.yand La,_xCa•MnO 3.ywith 0 < x < 1 were prepared in air at 1200 as well as 1400 'C and at 1300 as well as 1400 'C, respectively. Phase identifications were performed using scanning electron microscopy (SEM) with EDX (Zeiss DSM 982 Gemini) and x-ray diffraction (Siemens
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