Influence of lattice defects on the high pressure properties of Ni0.66Mn2.34O4 NTC ceramics

The properties and hence the applications of negative temperature coefficient (NTC) oxides, Ni_xMn_3-xO₄, strongly depend on the Ni:Mn ratio, valence states, defects and cation distribution in the spinel structure. Here, the accuracy of determination of nickel and manganese cations distribution in the spinel structure of Ni_0.66Mn_2.34O₄ oxide, has been improved by taking into account the presence of tetrahedral vacancies. Additionally, the structural stability of cubic Ni_0.66Mn_2.34O₄ was investigated as a function of pressure up to 9.5 GPa using synchrotron radiation angle-dispersive X-ray powder diffraction and a diamond anvil cell. The bulk modulus and its first derivative were determined by fitting the Birch-Murnaghan equation of state (EoS) model to the experimental pressure-volume data. Rietveld refinement of the X-ray powder diffraction data reveals that the cubic spinel structure is stable upon compression to 9.5 GPa. The XRD data yielded a bulk modulus of K₀ = 126(7) GPa, with a pressure derivative K′ = 12(2). The obtained data were discussed in terms of defects in the cationic sublattice and compared with the elastic parameters of NiMn₂O₄.