Calorimetric study of the thermodynamic properties of Mn 5 O 8

Pinghui Zhang, Jue Liu, Katharine Page, Alexandra Navrotsky

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Abstract

Manganese oxides occur widely in nature and have technical applications in various areas. This study quantitatively evaluates the thermodynamic properties of Mn 5 O 8 , a binary manganese oxide that has a layered structure and contains coexisting divalent and tetravalent manganese. Three samples of the Mn 5 O 8 phase with slightly different manganese average oxidation states (Mn AOS) were synthesized using a wet chemical method and annealing. Synchrotron X-ray analysis revealed that the samples contain a small amount of a secondary MnO 2 phase that cannot be identified using laboratory X-ray diffraction. High-temperature oxide melt solution calorimetry in molten sodium molybdate at 700°C showed that all three samples are slightly higher in enthalpy than an isochemical mixture of bixbyite (Mn 2 O 3 ) and pyrolusite (MnO 2 ), probably rendering them metastable in free energy with respect to isochemical mixtures of bixbyite and pyrolusite. However, the energetic metastability (endothermic enthalpy) of Mn 5 O 8 is very small (<6 kJ/mol) and does not depend significantly on the Mn AOS. Thus, although Mn 5 O 8 probably does not appear on the equilibrium Mn-O phase diagram, its small metastability allows its synthesis by a variety of low temperature reactions.

Original languageEnglish
Pages (from-to)1394-1401
Number of pages8
JournalJournal of the American Ceramic Society
Volume102
Issue number3
DOIs
StatePublished - Mar 2019

Funding

We thank the U. S. Department of Energy, Office of Basic Energy Sciences, for financial support of this research (Grant DE‐FG02‐97ER14749) which enabled the synthesis, characterization, and calorimetry at UC Davis as part of the PhD work of PZ. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE‐AC02‐ 06CH11357. Analysis of synchrotron X‐ray data was supported through the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Early Career Research Program, Award No. KC040602, under Contract No. DE‐AC05‐00OR22725. We are grateful to Dr. Nancy Birkner for her help with synthesis and characterization at the start of this project. We thank the U. S. Department of Energy, Office of Basic Energy Sciences, for financial support of this research (Grant DE-FG02-97ER14749) which enabled the synthesis, characterization, and calorimetry at UC Davis as part of the PhD work of PZ. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Analysis of synchrotron X-ray data was supported through the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Early Career Research Program, Award No. KC040602, under Contract No. DE-AC05-00OR22725. We are grateful to Dr. Nancy Birkner for her help with synthesis and characterization at the start of this project.

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