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Calorimetric study of the thermodynamic properties of Mn 5 O 8
Author(s) -
Zhang Pinghui,
Liu Jue,
Page Katharine,
Navrotsky Alexandra
Publication year - 2019
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/jace.15951
Subject(s) - bixbyite , pyrolusite , manganese , cryptomelane , endothermic process , enthalpy , chemistry , phase diagram , gibbs free energy , metastability , materials science , inorganic chemistry , oxide , phase (matter) , thermodynamics , manganese oxide , adsorption , physics , organic chemistry
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.