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Decomposition of NiMn2O4 spinels

Published online by Cambridge University Press:  31 January 2011

G. D. C. Csete de Györgyfalva  and
I. M. Reaney
G. D. C. Csete de Györgyfalva
Affiliation:
Department of Engineering Materials, Sir Robert Hadfield Building, Mappin St. University of Sheffield, Sheffield, S1 3JD, United Kingdom
I. M. Reaney
Affiliation:
Department of Engineering Materials, Sir Robert Hadfield Building, Mappin St. University of Sheffield, Sheffield, S1 3JD, United Kingdom
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Abstract

Thermal analysis and x-ray diffraction have confirmed that single-phase, cubic-spinel-structured NiMn2O4 (2:1 Mn2O3:NiO) begins to decompose into a rocksalt and a second spinel-structured phase above 907 °C. The decomposition product in samples air-quenched from 900 and 1200 °C was therefore investigated using transmission electron microscopy. Samples quenched from 900 °C (below decomposition temperature) did not show the expected single-phase microstructure, but instead grains contained nanoregions of a lenticular fringe contrast. Samples quenched from 1050 to 1200 °C were generally composed of Mn-rich spinel grains in addition to grains containing Mn-rich spinel precipitates (30–50 nm) surrounded by a Ni-rich rocksalt matrix. As temperature increased, the spinel grains and precipitates became clearly tetragonal, exhibiting a ferroelastic domain structure arising from a cooperative Jahn–Teller distortion. A decomposition mechanism based on the degree of inversion is proposed to explain these microstructures. Slow cooling samples from 1250 °C resulted in partial recomposition, leading to a microstructure principally composed of cubic spinel and regions of much smaller spinel structured precipitates (50–120 nm) in a rocksalt structured matrix. The slow-cooled samples showed a larger increase in resistance over time than single-phase samples did when held at 400 °C. X-ray diffraction measurements carried out before and after electrical characterization showed a reduction in the amount of rocksalt structured material present in slow-cooled samples.

Type
Articles
Information
Journal of Materials Research , Volume 18 , Issue 6 , June 2003 , pp. 1301 - 1308
Copyright
Copyright © Materials Research Society 2003

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