Stress-Driven Evolution on Mismatched Ca2Co2O5 Oxide Material: From Geometry to the Electronic States

Abstract

The geometrical structures, phase stabilities, electron energy band structures, electron density of states, and atom recombination together with the electron conduction behaviors of the sandwiched Ca₂Co₂O₅ with external stress of 1 GPa are intensively studied by the density functional theory method. The studying results show that the symmetry remains undisturbed; the strain to the stress response is anisotropic. The strain of microarchitecture induced by external stress is also anisotropic. There is stronger covalent binding between Co and O. The binding between Co and O within CdI₂ like CoO₂ is very much even covalent, and it is weakened under external stress. But the covalent Co-O binding within the rock salt like CaCoO layer is enhanced. The Ca-O binding strength is insensitive to external stress. An energy gap of 0.1 eV below Fermi level for the spin-up electron band disappears, and the two energy gaps are narrowed for the spin-down electron bands. The p orbital electrons form primarily the bands below Fermi level and the d orbital electrons form primarily the bands above Fermi level. The transitions from p orbital electrons to d orbital electrons produce the conduction. The CdI₂ like CoO₂ layer has been enhanced in terms of participating in the conduction properties with external stress of 1 GPa, and the capability of Co is enhanced while the capability of O is decreased.

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1. Introduction

The multioxide framework materials with complicated layered crystal structures such as NaCoO, CaCoO, and BiSrCoO are very much diverse in physical properties, as well as the related sensitivity to structure, spintronics, topology, preparation procedures, and so on [1–6] when compared to pure CoO [Philosophical Magazine 96 (30), 3211–3226]. The family of transitional metal Co-based CaCoO oxide materials which show similar valuable properties are the research focus in recent years. For example, the Ca₂Co₂O₅ and Ca₃Co₄O₉ type layered oxide materials exhibit especially complex crystal structure, spin topology, preparation variety, and anisotropic transport phenomena [2, 3]. They are similarly composed of rock salts like the CaCoO layer and CdI₂ like the CoO₂ layer that are stacked along c axis with the Sandwich framed crystal structure. The Ca₂Co₂O₅ crystalline oxide material was first discovered and reported in terms of its unique sandwiched structure by Vidyasagar et al. in 1984 [4]. Its anisotropic semiconductor conduction and positive temperature-dependent thermopower of 100 μV·K⁻¹ at 100 K were then demonstrated by Funahashi et al. [2, 3]. The Ca₃Co₄O₉ crystalline oxide material was discovered and reported in terms of its sandwiched structure and anisotropic transport by Shikano and Funahashi in 2003 [5]. The sensitivity of physical properties to preparation procedures was also investigated during the past years. For instance, the grain alignment together with conduction is very dependent on the external stress of preparation.

The polycrystalline materials of sandwiched CaCoO oxide have been more widely studied in contrast to their single-crystal materials for the sake of preparation cost, fabrication easiness, product scale, etc. In addition, they have been intensively studied experimentally in terms of the transport properties for the intrinsic as well as the regulated materials in recent years [7–11]. In order to recover the performance of single-crystal materials, some fabrication methods have been adopted. For instance, isostatic pressing is one of these ways. In this way, stress ranging from tens of MPas to several GPas is applied to the crystalline bulk materials when preparing. The resulting bulk materials should then be consolidated and regulated with regard to their density and grain alignment in order to get the bulk texture. We have also reported the stress-dependent transport properties of this sandwiched CaCoO crystalline oxide material. The grain alignment and the transport property thereafter can be regulated by external stress ranging from 30 MPa to 500 MPa [9, 12]. The fundamental background physical properties are determined by the geometry structure as well as the electronic states thereafter. The evolution of geometry and electronic states with external stress merits investigation. Unfortunately, a theoretical study in this sandwiched CaCoO crystalline oxide material is moderately lacking.

The transitional metal Cobalt has several d orbital electrons where varieties of spin alignments can be configured. We have demonstrated and reported that the antiferromagnetic aligned Ca₂Co₂O₅ crystalline oxide material is most stable among the antiferromagnetic phase and ferromagnetic phase [13]. In the present work, the geometrical structures, microarchitectures, stabilities, electron energy band structures, the electron density of states, and species recombination together with the electron conduction properties of the sandwiched Ca₂Co₂O₅ crystalline oxide material with external stress of 1 GPa are intensively studied via the density functional theory (DFT) calculational and analyzing method for the first time to our knowledge.

2. Computational Methods and Details

The sandwiched Ca₂Co₂O₅ crystalline oxide material is composed of rock salts like the CaCoO sublayer and CdI₂ like the CoO₂ sublayer along c axis with space group of P1M1. The rock salt like CaCoO and CdI₂ like CoO₂ has the same lattice parameters along b and c axis; their sublattices are mismatched along a axis. The cell angles α, β, and γ are 90°, 90°, and 98.13°, the a, b, and c of the cell are 4.56 Å, 9.66 Å, and 10.84 Å, respectively [2–4]. The schematical crystal structure of this sandwiched Ca₂Co₂O₅ crystalline oxide material and the projections onto several planes are shown in Figures 1 and 2.

[figure omitted; refer to PDF]

Figure 10 shows the detailed density of states of Ca, Co, and O within the rock salt like CaCoO near Fermi energy level of the sandwiched Ca₂Co₂O₅ crystalline oxide material with external stress of 1 GPa. It is observed that the total density of state values of Ca, Co, and O at Fermi level is 0.0197, 0.565, and 1.7465; they contribute 0.8%, 24%, and 75.2% to the total density of state value of 2.3742 of this layer. Nevertheless, for the counterpart Ca₂Co₂O₅, the total density of state values of Ca, Co, and O at the Fermi level is 0.09, 2.08, and 2.30; they contribute 2%, 47%, and 51% to the total density of state value. It can be indicated that the capability of contributing to electronic properties of Ca and Co is decreased; the capability of contributing to electronic properties of O is enhanced. It can be concluded from Figures 9 and 10 that the total density of states of the CaCoO layer at the Fermi energy level is mainly composed of the orbital electrons of Co d and O p. This is the same as the counterpart Ca₂Co₂O₅. It is inferred that these two kinds of orbital electrons within this layer contribute to the conduction process.

[figures omitted; refer to PDF]

Figure 11 shows the detailed density of states of Co and O within the CdI₂ like CoO₂ layer near Fermi energy level of the sandwiched Ca₂Co₂O₅ crystalline oxide material with external stress of 1 GPa. It is observed that the total density of state values of Co and O at the Fermi level is 0.1854 and 0.3237; they contribute 36% and 64% to the total density of state value of 0.5093 of this layer. However, for the counterpart Ca₂Co₂O₅, the total density of state values of Co and O at Fermi level is 0.36 and 0.48; they contribute 43% and 57% to the total density of state value of this layer. It can be indicated that the capability of contributing to electronic properties of Co is enhanced; the capability of contributing to electronic properties of O is decreased. It can be concluded from Figures 9–11 that the total density of states of the CaCoO layer at the Fermi energy level is mainly composed of the orbital electrons of Co d and O p, too. This is the same as the counterpart Ca₂Co₂O₅. It is inferred that these two kinds of orbital electrons within this layer contribute to the conduction process.

[figures omitted; refer to PDF]

4. Conclusions

In conclusion, the geometrical structures, microarchitectures, phase stabilities, electron energy band structures, electron density of states, species recombination, and the electron conduction properties of the sandwiched Ca₂Co₂O₅ with external stress of 1 GPa are intensively studied within the framework of density functional theory calculational and analyzing method. The symmetry type is not influenced, and the space group remains undisturbed. The strain-to-stress response of geometry is sensitive along a direction; it is insensitive along the c direction. The strain induced by external stress of microarchitecture is anisotropic, indicating the different binding characteristics. The distances between Ca and O are larger than those between Co and O in common, and there is stronger covalent binding for the Co and O. The bindings between Co and O within CdI₂ like CoO₂ are very much covalent than those between Co and O within the rock salt like CaCoO layer. The covalent Co-O binding within the rock salt like CaCoO layer is enhanced; nevertheless, the covalent Co-O binding within the CdI₂ like CoO₂ layer is weakened under the external stress. The Ca-O binding strength is insensitive to external stress. The intrinsic sandwiched Ca₂Co₂O₅ is more stable. An energy gap of 0.1 eV below Fermi level for spin-up electron band disappears, and the two energy gaps are decreased to 1.1089 eV and 0.6047 eV for the spin-down electron bands, respectively. The p orbital electrons form largely the bands below Fermi energy level and the d state electrons form largely the bands above Fermi energy level. The transitions from p orbital electrons to d orbital electrons produce the conduction process. The CdI₂ like CoO₂ layer has been enhanced in terms of involving the transport properties with external stress of 1 GPa. Nevertheless, the rock salt like the CaCoO layer exhibits contrary characteristics. For the CdI₂ like CoO₂ layer, the capability of contributing to transport properties for Co is enhanced, but the capability of contributing to transport properties for O is decreased. For the rock salt like the CaCoO layer, the capability of contributing to transport properties for Ca and Co is decreased; the capability of contributing to transport properties for O is enhanced.

Acknowledgments

The authors would like to thank the support provided by the Henan Provincial Natural Science Foundation under grant no. 162300410007. The fruitful discussions on band formation and related electron alignments with Prof. J. J. Zhao and Dr. S. J. Qin are also acknowledged.

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