Thermochemical stability of BaFe 12 O 19 and BaFe 2 O 4 and phase relations in the Ba-Fe-O ternary system
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Turgut M. Giir Center for Materials Research, Stanford University, Stanford, California 94305
Robert Sinclair Department of Materials Science and Engineering, Stanford University, Stanford, California 94305
Stephen S. Rosenblum and Hidetaka Hayashi Applied Electronics Center, Kobe Steel USA, Palo Alto, California 94304 (Received 14 April 1993; accepted 17 February 1994)
The stability of BaFe]2Oi9 and BaFe 2 O 4 was studied by the oxygen coulometric titration technique between 700 °C and 1000 °C using a solid-state electrochemical cell. This temperature range is technologically important for the deposition of BaFe 2 Oi 9 magnetic thin films. The thermodynamic information obtained from the titration measurements was corroborated with structural identification of phases prepared under electrochemically controlled conditions. Accordingly, a section of the Ba—Fe—O ternary phase diagram around the BaFei2Oi9 composition was constructed in this temperature range. The standard Gibbs free energy change for the decomposition of BaFe 12 Oi 9 into BaFe 2 O 4 , Fe 3 O 4 , and O 2 is given by the expression AG°[J/mol] = 7.23 X 105 - 480J. In the oxygen pressure-temperature domain, the thermodynamic stability limits of BaFe 12 Oi 9 and BaFe 2 O 4 are given by the expressions ln[PO2(atm)] = 69.37 - 1.04 X 105 T 1 and ln[P 02 (atm)] = 27.68 - 7.12 X 104 T 1 , respectively. The stability limits determined here help define the process conditions for the successful synthesis of these phases.
I. INTRODUCTION Ferrites with the hexagonal crystal structure are ferrimagnets that have technological importance for many applications such as read/write heads for high speed digital recording, transformer cores, antennas, and microwave circuits. A member of this family, barium ferrite (BaFei 2 Oi 9 ), has long been used in magnetic recording.1 This compound, denoted as BaM after Smit and Wijn,2 has the magnetoplumbite structure3 where the hexagonal unit cell consists of 10 oxygen layers. The oxygen ions form a hexagonal or cubic closed packed lattice along the [001] direction. Five oxygen layers make up one unit formula such that the unit cell contains two BaFe 12 Oi 9 formulas rotated 180° with respect to each other around the c-axis. The lattice constants2 of the unit cell are 23.20 A in the c-direction and 5.88 A in the a-direction. One O 2 " ion in every five oxygen layers is substitutionally replaced by a Ba2+ ion which has a similar ionic radius. Phase relations in the B a - F e - 0 ternary system have been the subject of numerous studies,4"12 many of which were carried out in air, and at elevated temperatures in excess of 1000 °C. One of the earlier studies was by Goto and Takada4 who reported three stable phases J. Mater. Res., Vol. 9, No. 6, Jun 1994 http://journals.cambridge.org
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in this system, namely, Ba 2 Fe 2 O 5 (2BaO • Fe 2 O 3 = B 2 F), BaFe 2 O 4 (BaO • Fe 2 O 3 = BF), and the hexagonal BaFe12Oi9 (BaO • 6Fe 2 O 3 = BaM). The latter (BaFei 2 O19) is a line compound with negligible solubility in either
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