Anionic Open-Shell p Electron Systems

Introduction

The interplay between spin, orbital, charge, and lattice degrees of freedom has been explored widely in strongly correlated transition metal (TM) compounds with partially filled d shells. However, similar phenomena are also observed in open shell p electron systems. For instance, there is some analogy between the properties of TM compounds having Jahn-Teller t_2g⁶e_g³ (Cu²⁺) or t_2g³e_g¹ (Mn⁴⁺) electron configurations and alkali metal superoxides AO₂ with paramagnetic O₂^- ions having a degenerate (π*)³ electron configuration (Fig. 1 right). Here, the π* orbitals are antibonding molecular orbitals formed from O 2p_π atomic orbitals. A prototype system is CsO₂ where, similar as for the Cu²⁺ compound KCuF₃, a quasi-one-dimensional magnetic ordering was observed, which is driven by orbital ordering. The orbital ordering involves a reorientation of the molecular anions, which is an important degree of freedom in molecular anionic p electron systems.

Figure 1: Molecular orbital energy scheme for paramagnetic superoxide (right) and diamagnetic peroxide (left) ions — **Figure 1:** Molecular orbital energy scheme for paramagnetic superoxide (right) and diamagnetic peroxide (left) ions

**Figure 1:** Molecular orbital energy scheme for paramagnetic superoxide (right) and diamagnetic peroxide (left) ions

Recent Results

The main focus of our work is on alkali sesquioxides A₄O₆ ( A = Rb, Cs) which are anionic mixed valence compounds with two paramagnetic O₂^- anions and one diamagnetic O₂^2- anion (Fig. 1 left) per formula unit and thus feature also the charge degree of freedom. Surprisingly, the A₄O₆ compounds adopt a cubic crystal structure (space group I-43d) which only accommodates a single site for the O₂^n- anions. However, Raman and inelastic neutron experiments revealed the presence of distinct O₂^- and O₂^2- units. Recently we succeeded to isolate a single crystal of a tetragonal modification of Rb₄O₆ and to determine its crystal structure (space group I-4) [1]. In this phase charge ordering of distinct O₂^- and O₂^2- anions is clearly apparent (Fig. 2). As the molecular position and orientation fixes the orbital orientations and lifts the orbital degeneracy it is concluded that charge ordering may be associated with orbital ordering.

Figure 2: Illustration of the crystal structures of the cubic (left) and tetragonal (right) modification of Rb4O6, taken from Ref. [1]. The blue ellipsoids correspond to Rb+ ions. In the left picture the red dumbbells correspond to the indistinguishable O2- and O22- units, whereas in the right picture the red and yellow dumbbells correspond to O22- and O2- units, respectively — **Figure 2:** Illustration of the crystal structures of the cubic (left) and tetragonal (right) modification of Rb₄O₆, taken from Ref. [1]. The blue ellipsoids correspond to Rb⁺ ions. In the left picture the red dumbbells correspond to the indistinguishable O₂^- and O₂^2- units, whereas in the right picture the red and yellow dumbbells correspond to O₂^2- and O₂^- units, respectively

**Figure 2:** Illustration of the crystal structures of the cubic (left) and tetragonal (right) modification of Rb₄O₆, taken from Ref. [1]. The blue ellipsoids correspond to Rb⁺ ions. In the left picture the red dumbbells correspond to the indistinguishable O₂^- and O₂^2- units, whereas in the right picture the red and yellow dumbbells correspond to O₂^2- and O₂^- units, respectively

The formation of a tetragonal modification of A₄O₆ is also the clue to understand anomalies in the magnetic and structural properties of the sesquioxides, which has been established in depth for Cs₄O₆. Combining magnetization measurements with EPR and ¹³³Cs NMR measurements performed in collaboration with Denis Arcon, Ljubljana, we have shown that the magnetic properties and structural phases present depend on the chosen temperature protocol [2]. Powder neutron diffraction studies performed in collaboration with Kosmas Prassides (Durham University, UK, and Tohoku University, Sendai, Japan) have shown, that on rapid cooling the ambient temperature cubic phase is frozen, whereas on slow cooling a dominating tetragonal phase, similar to that of tetragonal Rb₄O₆, is forned. Anomalies seen in the magnetic susceptibility data on heating (Fig. 3) reflect the transformation of the frozen cubic to the tetragonal and the transformation back to the cubic phase, respectively. Our studies provide profound insights into the interplay between spin, orbital, charge, and molecular orientation degrees of freedom in these anionic p electron systems.

Figure 3: Inverse molar magnetic susceptibility cm-1 of Cs4O6 measured at 1 T as a function of temperature. Anomalies in data obtained on heating (red) are most evident in the product χm ∙T and reflect the structural transformations occurring in samples which were rapidly cooled from room temperature. The figure is taken from Ref [2] — **Figure 3:** Inverse molar magnetic susceptibility c_m^-1 of Cs₄O₆ measured at 1 T as a function of temperature. Anomalies in data obtained on heating (red) are most evident in the product χ_m ∙T and reflect the structural transformations occurring in samples which were rapidly cooled from room temperature. The figure is taken from Ref [2]

**Figure 3:** Inverse molar magnetic susceptibility c_m^-1 of Cs₄O₆ measured at 1 T as a function of temperature. Anomalies in data obtained on heating (red) are most evident in the product χ_m ∙T and reflect the structural transformations occurring in samples which were rapidly cooled from room temperature. The figure is taken from Ref [2]

Deeper insights into the low-dimensional physics of the prototype superoxide CsO₂emerged from recent ¹³³Cs NMR [3] and EPR [4] studies, which were performed in collaboration with Denis Arcon. The one-dimensional magnetic order is confirmed and the analysis of the data suggests that CsO₂ can be considered as a rare example for the Tomonaga-Luttinger liquid model. At higher temperatures the dimensionality of the system changes and an unusual variation of the exchange interactions with temperature is found.

Our work on alkali superoxides and sesquioxides has been part of the collaborative EU-Japan project LEMSUPER (Light Element Molecular Superconductivity, 2011 – 2015). It is of interest to compare the properties of these compounds having molecular O₂^n- units with those of superconducting fullerides containing C₆₀^n- units and superconducting rare earth sesquicarbides (Ln₂C₃) featuring C₂^2- units. In the latter hybridized π*-d states constitute the conduction band, whereas in case of the oxides no metal d states contribute to chemical bonding. Compared to the C₆₀ compounds the oxygen anion based solids are electronically more localized (U >> W, where U is the correlation energy and W the bandwidth). Application of high pressure could be a strategy to metallize these solids and possibly even induce superconductivity. Resistance measurements on Rb₄O₆ and Cs₄O₆, which were performed in collaboration with the group of Shimizu and Kagayama at Osaka University, revealed that the compounds stay insulating up to pressures of 80 GPa and 180 GPa, respectively. We have investigated the high pressure structural properties of Cs₄O₆ and Rb₄O₆ by synchrotron powder x-ray diffraction and by Raman experiments. Even at pressures of some GPa Cs₄O₆ transforms completely and Rb₄O₆ partially to the tetragonal charge ordered phase (Fig.4). Structural changes under pressure may prevent metallization which has been predicted by electronic structure calculations.