Hopping motions in cobalt manganese spinel oxides with high cobalt concentration (Co x Mn 3−x O 4 , 2.3 ≤ x ≤ 2.7) are investigated in order to clarify the origin of unusual electrical behaviors as negative temperature coefficient (NTC) thermistors. Based on the resistance versus temperature (R−T) characteristics, hopping conduction mechanisms in MCO compounds (x = 2.3 and 2.5) are attributed to variable range hopping (VRH) motion with a parabolic distribution of the density of states (DOS) near the Fermi level. However, when Co content increases up to 2.7, transition in the hopping motion occurs from VRH to the nearest neighboring hopping (NNH) motion, which can be responsible for a huge increase of the resistance accompanied by decrease of the factor of thermal sensitivity (B value) in MCO compounds (x = 2.7). Also, hopping distance and activation energies for MCO (x = 2.3 and 2.5) compounds following VRH conduction are calculated as a function of temperature, indicating that higher B value observed in MCO (x = 2.5) compound is due to the larger hopping distance compared to that of MCO (x = 2.3) compound.
■ INTRODUCTIONMixed transition metal oxides with spinel structures (AB 2 O 4 , A, B = Ni, Co, Mn, etc.) play an increasingly important role in a variety of applications such as electronics, energy storage devices, and medical treatments due to their exceptional electronic properties. 1−5 To be specific, great attention has been focused on spinel oxides for the use of negative temperature coefficient (NTC) thermistors 6 thanks to its high absolute temperature coefficient of resistance (TCR, ∼ −4%/K) with moderate resistivity (10 < ρ (Ω·cm) < 10 3 ) at room temperature. 7 In the spinel crystal structure, oxygen ions are close-packed with face-centered cubic configuration, and one-eighth of the tetragonal A sites and one-half of the octahedral B sites are occupied by cations. The cations in A and B sites can have oxidation states of +2 to +3 and +2 to +4, respectively. Electrical conduction in spinel oxides mainly occurs via hopping conduction process within different cations rather than electronic conduction. 8 The hopping conduction in spinel oxides is attributed to the mixed valence states of different cations. Thus, electrical properties of spinel oxides are greatly affected by cation distribution and oxidation states in the two crystallographic sites, A and B. 3,7 Among a number of spinel oxides, the (Mn 3−x Co x )O 4 (0 ≤ x ≤ 3, MCO) system has been widely investigated for the NTC thermistor application given that the strong electron correlations in metal Mn can induce semiconductive characteristics with moderate resistivity at room temperature. Also, when Co content increases over 1, Co cations start to occupy octahedral B sites leading to significant change of cation
