Thermodynamics of Cobalt (II, III) Oxide (Co3O4): Evidence of Phase Transition (original) (raw)
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Physical Review B
In this paper we investigate the high-temperature structure of Co3O4, a compound that has been studied extensively over the last 60 years due to its unresolved high-temperature structure. In situ thermal analysis and X-ray diffraction confirm previously reported high-temperature structural changes and show that these changes are unrelated to the high-temperature decomposition to CoO. Raman active peaks are also extinguished over the same temperature range. By considering the changing lattice parameter, A-O, and B-O bond lengths as well as cation size we are able to calculate degree of inversion which reaches a maximum of 0.6. To further study the structure in this experimentally inaccessible range we quench samples and perform ex situ measurements including redox titration, X-ray photoelectron spectroscopy, and neutron diffraction. We do not observe any evidence of large oxygen vacancy concentrations or octahedral Co 3+ B ions with high spin state. However, we do show an evolution in the magnetic moment from magnetic structure refinement from (2.4 µB) to (2.7 µB) that coincides exactly with the high-temperature anomaly and suggests partial inversion (0.46) of the spinel structure in fairly good agreement with the inversion calculated from bond lengths. orbitals that account for the observed magnetic moment 62 of 2.6 µ B to 3.25 µ B. 3-5 63 In contrast, the details of the high-temperature crys-64 tal structure of Co 3 O 4 are not well understood and have 65 been an active area of research. A variety of obser-66 vations including a sudden non-linear increase in lat-67 tice parameter, 6-8 excess thermal expansion, 9 electromo-68 tive force (EMF) measurements on the oxygen partial 69 pressure, 7,10 opposite changes in the A-O and B-O bond 70 lengths with increasing temperature, 8 electrical conduc-71 tivity and thermopower, 9,11 and a phase transformation 72 from heat capacity measurements, 12 all indicate a high-73 temperature structural anomaly. 74 The exact origin of the high-temperature structural 75 anomaly is obfuscated by two potential mechanisms as 76 well as the incipient decomposition to rock-salt type CoO 77 (1165 K to 1270 K). 7 One mechanism is cation anti-site 78 disorder, termed inversion of the spinel structure where 79 Co 3+ ions sit on the tetrahedral A site and Co 2+ ions on 80 the octahedral B site. The other mechanism is a low-spin 81 (LS) to high-spin (HS) spin state transition of Co 3+ ions 82 on the octahedral sites between 1000 K and 1200 K that 83 has been suggested by many researchers. 7,8,10,12 There is 84 currently no consensus on how much, if any, cation anti-85 site disorder is present. Chen 13 summarizes that some 86 authors suggest a complete inversion of the structure 7 87 (i.e. Co 3+ A and Co 2+ B Co 3+ B), while others favor complete 88 anti-site disorder, 8 and still others propose a retention of 89 the normal spinel structure with only 5-10% disorder. 12 90 High-temperature in situ magnetic susceptibility, X-91 ray photoelectron spectroscopy (XPS), and neutron 92 diffraction measurements to settle the ambiguity are 93 hampered by the low Néel temperature ≈40 K of Co 3 O 4 94 and lack of high-temperature instrumentation. Never-95 theless, there are several in situ techniques remaining to 96 study the high-temperature structure of Co 3 O 4 includ-97 ing Raman spectroscopy and thermogravimetric analysis 98 (TGA) discussed in this work, and thermopower which 99 will be discussed in detail in a subsequent contribution. 14 100 Furthermore, in this contribution we attempt to preserve 101 the high-temperature structure via quenching and then 102 use ex situ XPS, neutron diffraction, and redox titra-103 tion measurements on the samples to gain additional in-104 sight. Finally, in situ phase composition is monitored 105 via spectroscopy and diffraction to ensure that the high-106 temperature anomaly is unrelated to the transformation 107 from Co 3 O 4 to CoO. 108 II. EXPERIMENTAL 109 Disc-shaped pellets were fabricated from nanometer-110 sized Co 3 O 4 powder (Sigma Aldrich, 99.9 wt.%, 50 nm 111 particles). The powders were first cold-pressed with a 112 6 ton load in a 1 2 inch die and then sintered in air. 113 Pure phase, dense samples of Co 3 O 4 and CoO were ob-114 tained by firing the samples from room temperature up 115 to 1148 K over 2 hours and holding for 6 hours, ramping 116 to 1373 K over 1 hour and holding for 6 hours, ramp-117 ing back to 1148 K over 2 hours and holding for 2 hours.
Influence of CaO on the thermodynamic and transport properties of cobaltous oxide
Solid State Ionics, 2022
The electrical conductivity of Ca-doped Co1-xO single crystals was measured as a function of oxygen partial pressure, over the temperature range 1273-1673 K. The results were analyzed using Seebeck coefficient measurements, microstructural characterizations, EELS and X-ray diffraction experiments. From this set of results, we have shown that the influence of calcium on the thermodynamic and transport properties of cobaltous oxide is due to the reducing behavior of these cations, leading to both the shift of the Co/CoO phase boundary to higher P O2 and the formation of singly ionized cobalt cations (Co +) in the stability range of CoO.
Thermodynamic model of the coupled valence and spin state transition in cobaltates
Journal of physics. Condensed matter : an Institute of Physics journal, 2020
A class of cobalt-based oxides exhibits a peculiar type of transition, entangling valence and spin state degrees of freedom of 4f and 3d elements. It constitutes one of the most spectacular illustrations of the interplay between charge, spin and lattice degrees of freedom in strongly correlated materials. In this work, we present a thermodynamic model capable to reproduce the main features of this transition. Our approach is based on the minimization of a free energy combining the contributions of two sublattices and the interaction between them. The coupling energies introduced in the model are related to well-known chemical pressure effects in the perovskite structure. The results of this model are compared to experimental data derived from x-ray absorption spectroscopy.
Journal of Physics: Condensed Matter, 2013
The electric, magnetic, and thermal properties of three perovskite cobaltites with the same 30% hole doping and ferromagnetic ground state were investigated down to very low temperatures. With decreasing size of large cations, the ferromagnetic Curie temperature and spontaneous moments of cobalt are gradually suppressed-T C = 130 K, 55 K and 25 K and m = 0.68 µ B , 0.34 µ B and 0.23 µ B for Nd 0.7 Sr 0.3 CoO 3 , Pr 0.7 Ca 0.3 CoO 3 and Nd 0.7 Ca 0.3 CoO 3 , respectively. The moment reduction with respect to moment of the conventional ferromagnet La 0.7 Sr 0.3 CoO 3 (T C = 230 K, m = 1.71 µ B) in so-called IS/LS state for Co 3+ /Co 4+ , was originally interpreted using phase-separation scenario. Based on the present results, mainly the analysis of Schottky peak originating in Zeeman splitting of the ground state Kramers doublet of Nd 3+ , we find, however, that ferromagnetic phase in Nd 0.7 Ca 0.3 CoO 3 and likely also Pr 0.7 Ca 0.3 CoO 3 is uniformly distributed over all sample volume, despite the severe drop of moments. The ground state of these compounds is identified with the LS/LS-related phase derived theoretically by Sboychakov et al. [Phys. Rev. B 80, 024423 (2009)]. The ground state of Nd 0.7 Sr 0.3 CoO 3 with an intermediate cobalt moment is inhomogeneous due to competing of LS/LS and IS/LS phases. In the theoretical part of the study, the crystal field split levels for 4f 3 (Nd 3+), 4f 2 (Pr 3+) and 4f 1 (Ce 3+ or Pr 4+) are calculated and their magnetic characteristics are presented.
Structural study of layered cobaltate Lax/3CoO2 (x~1) at temperatures up to 800 K
Journal of Solid State Chemistry, 2015
The layered cobaltate La x=3 CoO 2 (x=3 ¼ 0:33ð2Þ) was prepared by an ionic exchange from the Na x CoO 2 precursor. Its crystal structure has been studied by neutron powder diffraction at room and low temperatures and by X-ray powder diffraction at elevated temperatures. Compared to Na þ in the parent system, the La 3 þ ions occupy only one-third of available sites, forming a 2-dimensional superstructure in the ab-plane. The absence of long-range ordering of La cations along the c-axis was confirmed by a simulation using the stacking faults approach. No significant deviations of the basic structure from the hexagonal P6 3 =mmc symmetry were observed up to the decomposition temperature of 800 K. The 2dimensional superstructure of La 3 þ is essential for the overall structure stability and is thus preserved up to the temperature of decomposition. The thermal expansion of La x=3 CoO 2 is similar in the a-and cdirections, in contrast to the highly anisotropic thermal expansion of the parent Na x CoO 2. This difference is attributed to a higher covalency of La-O bonds compared to more ionic Na-O bonds.
Structure and Properties of Alkali Cobalt Double Oxides A0.6CoO2 (A = Li, Na, and K)
Inorganic Chemistry, 2009
Lamellar A x CoO 2 cobalt double oxides with A = Li, Na, and K (x ∼ 0.6) have been synthesized and their chemical (alkali content, oxidation state, and structure) and physical (resistivity, thermopower, magnetization, and specific heat) properties have been studied. All the three materials exhibit strong electron correlation emphasized by their behavior ranging from Fermi liquid to spin-polarized system. Our results show that both the dimensionality of the interactions and the nature of the alkali play a determining role on the properties.
Oxygen nonstoichiometry and cobalt valence in misfit-layered cobalt oxides
Journal of Solid State Chemistry, 2004
The hexagonal CoO 2 layer is the main building unit of the newly established category of misfit-layered and related oxides showing-as a general feature-excellent thermoelectric characteristics. Here we use high-quality samples of the three prototype phases, Na 0.77 CoO 2+d , Ca 3 Co 3.95 O 9+d and (Bi,Pb) 2 Sr 2 Co 2 O 8+d , and apply both precise wet-chemical redox analysis techniques and thermogravimetric annealing experiments to approach the not-yet-addressed questions concerning oxygen nonstoichiometry and the valence of Co in these phases. In terms of the oxygen-stoichiometry tunability, substantial variation in the overall oxygen content upon reducing/oxidizing annealing is observed only for Ca 3 Co 3.95 O 9+d . The valence of Co in all the samples is found to lie in a range of 3.0-3.3, being significantly lower than the commonly believed values of 3.3-3.5. r 2004 Elsevier Inc. All rights reserved.