Structural Stability and Properties of Marokite-Type γ-Mn₃O₄

Abstract

We synthesized single crystals of marokite (CaMn2O4)-type orthorhombic manganese (II,III) oxide, γ-Mn3O4, in a multianvil apparatus at pressures of 10–24 GPa. The magnetic, electronic, and optical properties of the crystals were investigated at ambient pressure. It was found that γ-Mn3O4 is a semiconductor with an indirect band gap E g of 0.96 eV and two antiferromagnetic transitions (T N) at ∼200 and ∼55 K. The phase stability of the γ-Mn3O4 crystals was examined in the pressure range of 0–60 GPa using single-crystal X-ray diffraction and Raman spectroscopy. A bulk modulus of γ-Mn3O4 was determined to be B 0 = 235.3(2) GPa with B′ = 2.6(6). The γ-Mn3O4 phase persisted over the whole pressure range studied and did not transform or decompose upon laser heating of the sample to ∼3500 K at 60 GPa. This result seems surprising, given the high-pressure structural diversity of iron oxides with similar stoichiometries. With an increase in pressure, the degree of distortion of MnO6 polyhedra decreased. Furthermore, there are signs indicating a limited charge transfer between the Mn3+ ions in the octahedra and the Mn2+ ions in the trigonal prisms. Our results demonstrate that the high-pressure behavior of the structural, electronic, and chemical properties of manganese oxides strongly differs from that of iron oxides with similar stoichiometries.

Pressure Tuning the Jahn–Teller Transition Temperature in NaNiO2

“…This is comparable with a similar JT-distorted material with edge-sharing octahedra, CuMnO 2 , which has a bulk modulus of 116(2) GPa . It is, however, substantially less than the reported bulk modulus for ZnMn 2 O 4 of 197(5) GPa, and although there are several different reported values for Mn 3 O 4 depending on the phase and morphology, − all are higher than what we report for NaNiO 2 . LaMnO 3 is not entirely comparable owing to the LaO 12 units and corner-sharing octahedra, but for reference, it has a reported bulk modulus of 108(2) GPa …”

“…Throughout the measurement, the bond length distortion index{effective coordination} of NaO 6 octahedra remains very near its high-symmetry value of 0{6}; for example, at 293 K, the value of the bond length distortion index{effective coordination} in NaO 6 octahedra is 0.00581 (11) {5.99232(19)}, compared with 0.05463(14){5.309(3)} in NiO 6 octahedra. This is indicative of much greater distortion in bond lengths for NiO 6 octahedra, consistent with the JT distortion.…”

“…While the latter is a well-documented property of JT-distorted materials, the former is in contrast to the JT distortion in LaMnO 3 . NaNiO 2 is more resistant to pressure than other similar materials, having a higher bulk modulus [ B = 119.6(5) GPa at 290 K] than similar perovskites, , Prussian Blue analogues, and layered honeycomb structures, although its bulk modulus is very similar to that of JT-distorted edge-sharing CuMnO 2 and is less than those of Mn 3 O 4 , − NiO, and ZnMn 2 O 4 . NaNiO 2 also displays a much smaller magnitude of than LaMnO 3 , with LaMnO 3 shifting T JT from ∼750 K at ambient pressure to room temperature in 12 GPa compared with a very small shift from ∼480 K at ambient pressure in NaNiO 2 .…”

“…Here we describe three different examples of classes, all containing JT-active d 4 Mn 3+ . Mn 3 O 4 , a spinel containing both Mn 3+ and Mn 2+ , has been found to exhibit different pressure dependence of JTdistorted octahedra depending on morphology; for instance, in single-crystal Mn 3 O 4 , the JT disortion survives to 60 GPa, 11 whereas there are observed transitions to JT-free phases at much lower pressures in powdered 12 and nanorod 13 Mn 3 O 4 . ZnMn 2 O 4 , also with spinel-type structure but with Zn 2+ on the Mn 2+ of Mn 3 O 4 , has been studied to a very high pressure (∼52 GPa), 14 with a transition reported at ∼23 GPa, which has been alternately described as a transition from JT elongation to a slight JT compression 14 or a spin-crossover transition resulting in an insulator → metal transition.…”

Electronic properties of single-crystalline Fe4O5

“…Other details were similar to those reported elsewhere. 39,64 We analyzed Fe-O bond lengths for all cations using a common bond valence sum (BVS) method. 54 Since bond lengths tend to reduce with pressure, the calculated "nominal" BVS values for Fe ions were accordingly renormalized at each pressure.…”