Novel Superconductors - Discovery and Mechanisms

Novel Superconductors - Discovery and Mechanisms

Perovskite antimonates: 5s analogues of bismuthates with positive DCT

Quantum materials with unprecedented properties are often stabilized under high pressures. Here we present the discovery of superconductivity at Tc = 15 K in new perovskite antimonates Ba1−xKxSbO3 (BKSO) synthesized at a high pressure of 12 GPa. The sibling perovskites Ba1−xKxBiO3 (BKBO) are known as anionic high-Tc superconductors with even higher Tc = 30 K. The distribution of valence electrons onto the cations (Sb or Bi) and ligands (O) is slightly different in BKSO compared to BKBO. Metal-oxygen covalency is suggested to become more important in BKSO.


High-pressure syntheses of solid-state compounds


By applying pressure in the synthesis of solids, the structures of the products can be changed significantly. More densely packed structures with elements in higher valence states are possible, which also have higher coordination numbers and altered hybridization of the valence electrons. The associated changes in the physical properties of these high-pressure polymorphs are an incentive to search for new quantum materials at high pressures and high temperatures.


One 500-ton and two 1000-ton presses are currently available for these syntheses. The pressure is applied to the reaction mixture via a Multi-anvil Walker module (Fig. 1). With this arrangement, pressures of up to 20 GPa and temperatures of up to 2500 K are possible, as they also occur in the outer mantle. A whole series of compounds, e.g. superconductors, quantum magnets, quantum spin liquids have already been synthesized in this way. The cationic perovskite superconductor BKSO with a transition temperature of Tc = 15 K deserves special mention.


Hole doped and oxygen holes


In this article, two technical terms are used, which should be briefly explained. Hole-doped means that the number of valence electrons is reduced compared to the parent compound. For example, in BaBiO3, the divalent barium is partially replaced by monovalent potassium: Ba1−xKxBiO3 (BKBO). Here x indicates the degree of doping of the monovalent K and thus the proportion of holes in the electronic structure.


Oxygen holes, on the other hand, characterize the local distribution of the electrons in the crystal. In general, the compounds considered here are ionic, i.e. valence electrons are transferred from the more electropositive element (Ba / K or Bi / Sb) to the more electronegative element (O). The latter can absorb a maximum of two electrons (O2-) in order to obtain a filled inert gas shell. If the charge transfer is not complete, oxygen holes occur.


Superconducting bismuthates with oxygen holes


Hole-doped perovskite-type crystallizing bismuthates (BKBO) are high-temperature superconductors with Tc up to ~30 K (−243 ºC) [1]. Apart from the cuprates, this is the highest transition temperature measured for oxides. However, the causes of such a high transition temperature have not yet been finally clarified.


If the parent compound BaBiO3 (O2-, Ba2+) is considered purely ionic, the valence state of bismuth would be 4+, i.e. the 6s orbital would be half-occupied. However, Bi4+ is unstable and tends to disproportionate 2Bi4+ (6s1) → Bi3+ (6s2) + Bi5+ (6s0). As a result, Bi-O coordination polyhedra appear with different distances and lead to the formation of a charge density wave (CDW). For BaSbO3, this is shown in Fig. 2(a). From a certain doping with K (hole-doping), the CDW is suppressed and superconductivity is observed [Fig. 2(b)] [2].


Bi4+ disproportionation as the cause of BKBO behavior is one view. Another considers the influence of the theory of relativity on the behavior of the electrons, which promotes stabilization of the 6s orbital of Bi. This makes it energetically more favorable to transfer electrons from the O2- anion to the Bi cation, corresponding to a negative value for DCT (charge transfer energy) [3]. Oxygen holes are thus formed in the ligand shell, which can be responsible for the formation of the CDW and, after doping, for the superconductivity.


Perovskite antimonates: 5s analogues of bismuthates with positive DCT


Antimony (Sb) is one period higher than Bi in group 15 of the periodic table. It has the same valence electron configuration, but the influence of the theory of relativity is significantly lower. A smaller or even positive value for DCT is expected, combined with a reduced oxygen hole character and stronger Sb disproportionation. The bonds should have a significantly higher covalent character.


BKBO-analogous Sb compounds can contribute significantly to understanding. However, it has not yet been possible to synthesize corresponding Sb compounds with a perovskite structure under normal conditions. BSO and BKSO with different degrees of doping could only be obtained using high-pressure techniques.


The valence states of Sb can be determined using 121Sb Mössbauer spectroscopy [4]. At doping levels from x=0.0 to x=0.5, two signals are actually measured that can be assigned to Sb3+ and Sb5+. At the same time, X-ray absorption measurements at the O K edge show a significantly reduced proportion of oxygen holes. This is in line with expectations. At a doping level of x=0.65, only one Mössbauer signal is observed, which corresponds to a valence of +4.5. The compound is metallic. The phase diagram of BKSO shows a transition from an insulator with CDW to a cationic metal.


Superconductivity in the cationic metal BKSO and consequences


The metallic sample with doping level x=0.65 becomes superconducting at Tc = 15 K [5], lower than the maximum temperature in BKBO (Tc ≈ 30 K). However, comparing BKBO and BKSO at the same doping level, the Tc is higher in the Sb compound. The superconductivity in BKSO seems to be enhanced by the more covalent character, but at the same time, it is suppressed by a more stable charge density wave [Fig. 2(c)].


Oxygen holes are not a necessary condition for achieving high Tc. Rather, the metal-oxygen covalence seems to be of importance, but it also leads to unfavorable developments such as the formation of the CDW. The results offer new insights to understand superconductivity in the mentioned compounds and spur further theoretical and experimental studies. High-pressure synthesis techniques are a tool to create new, unusual materials.




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