Similarity of structural changes in HgBa2CuO4+δ induced by extra oxygen and by high pressure

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Abstract

The structural changes in HgBa2CuO4+δ (Hg-1201) induced by the extra oxygen atoms in the Hg plane and by high pressure were investigated using computer simulation techniques. Without partial replacement of Hg by Cu, the most favorable oxygen interstitial position is the O3 site (0.5,0.5,0), while with partial replacement of Hg by Cu, the extra oxygen atom can be located at the O4 site (0.5,0,0.087). These results agree excellently with experimental results. Under high pressure, the Cu–O2 bond is compressed much more and the Hg–O2 bond is compressed much less compared with the unit cell in the c direction. The Ba–O2 interlayer distance is found to have the largest compressibility, four times as much as that of the lattice parameter c. The Ba–O2 interlayer distance becomes zero at 87 GPa, implying occurrence of structural transition. Our simulation found the structural changes induced by O3 oxygen and by high pressure are similar: the Cu–O2 bond and Ba–O2 interlayer distance are shortened while the Hg–O2 bond is relatively elongated. This result agrees with the experimental result.

Introduction

HgBa2Can−1CunO2n+2+δ superconductors are attractive targets for the investigation of the relationship between compound structure and superconducting properties as these compounds have a remarkably high critical temperature and relatively simple crystal structures [1], [2], [3]. The first member of this mercury-bearing superconducting family, HgBa2CuO4+δ (Hg-1201), is considered one of the most convenient compounds for structural studies of high-Tc superconductor because of the simplicity of the Hg-1201 structure (see Fig. 1). There has been great interest in investigating its structure with different extra oxygen content (δ) [2], [3], [4], [5], [6], [7], [8], [9] and its pressure-induced structural changes [10], [11], [12], [13], as both affect the Tc value of the Hg-1201.

Although there is, in general, a good agreement between the refined structural parameters of Hg-1201, a discrepancy concerning position of the extra oxygen atoms in Hg plane exists. The extra oxygen atom is reported to be located in the centre of the Hg basal plane at the O3 site (0.5, 0.5, 0) [3], [4], [5], [9]. It is reported that the extra oxygen can be located at the (0.4486, 0.4486, 0) position [6]. The partial substitution of Hg atoms by Cu atoms result in the additional interstitial oxygen at the O4 site (0.5,0, z) where z=0.043 [3] and z=0.10 [8]. It is reported that the O4 oxygen can also be located at the (0.086, 0.5, 0) site.

The pressure-induced structural changes in Hg-1201 have been investigated using the neutron diffraction method. Because the changes in the interatomic distances are very small, it is not easy to achieve sufficient accuracy in the experimental data. An experimental work showed quite an unexpected result [10] that the most compressible interatomic distance in Hg-1201 and Hg-1212 is the apical Cu–O bond while in the Hg-1223 the Hg–O interatomic distance has the greatest compressibility. This reverse change in Cu–O bond in Hg-1223 has been attributed to the presence of impurities in the compound, whose behavior under pressure could systematically damage the determination of interatomic distances. A recent work [13] reported that interatomic distance modulations are strongly inhomogeneous: approximately 4% for the apical Cu–O bond and only about 0.4% for the apical Hg–O bond. It was found that the decrease of Ba–O2 interlayer distance under pressure is approximately four times as much as the relative shortening of the unit cell along the c-axis. The linear extrapolation of the change in Ba–O2 interlayer distance under pressure shows that Ba–O2 interlayer distance will become zero at a pressure range of 30–60 GPa if the behavior of fast changes in the Ba–O2 interlayer distance is assumed to be the same in the whole pressure range. This may be an indication of possible structural transitions at very high pressures.

The pressure-induced Tc enhancement is observed in many hole-doped superconducting copper oxides in which high pressure changes the interatomic distances. It is natural to suppose that these structural changes result in free charge concentrations in the superconducting CuO2 layers leading to changes in Tc. The extra oxygen in Hg-1201 will also result in a change in Tc. If the amount of extra oxygen (represented by the value of δ) is appropriate, the Tc of Hg-1201 will be increased with the increasing δ value. It is interesting to know whether the extra oxygen and high pressure result in similar structural changes in Hg-1201. If so, it may help in the understanding of the mechanism of Tc enhancement in Hg-1201 by high pressure and by adding appropriate amount interstitial oxygen. To our knowledge, no such work has been reported. In this work, we used computer simulation technique to study if there is any correlation between structural changes caused by the extra oxygen and by high pressure in Hg-1201.

Section snippets

Computational method

Our simulation is based on the shell model generalisation of the Born model which treats solids as a collection of point ions with short-range repulsive forces acting between them. This approach has achieved a wide range of success and has been used in the study of high Tc superconductors, such as La2CuO4 [14], [15], [16], [17], [18], [19], [20], Ba1−xKxBiO3 [21], YBaCu3O7−x [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], YBa2Cu4O8 [32], [33], [34], [35], [36], HgBa2Can−1CunO2n+2+δ

Structural changes caused by extra oxygen

We calculated the energy needed to insert an interstitial oxygen atom into various positions in the Hg plane. Our calculations show that without partial replacement of Hg by Cu, defect energy of oxygen interstitial at the O3 site is −16.448 eV whereas defect energy of oxygen interstitial at the O4 site is −13.996 eV (Table 3). This result indicates that the extra oxygen atom is energetically more favorable to be located at the O3 site than at the O4 site. This result is in excellent agreement

Conclusions

The structural changes in Hg-1201 induced by the extra oxygen in Hg-plane and by high pressure was studied using atomistic simulation techniques. It is found that without the partial replacement of Hg by Cu, the most favorable extra oxygen position is the exact O3 site (0.5, 0.5, 0), which agrees well with experimental results. With partial replacement of Hg by Cu, the extra oxygen is energetically favorable to be located at the O4 site (0.5, 0, 0.087). This result is also in good agreement

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