Evidence of lattice strain as a precursor to superconductivity in BaPb_0.75Bi_0.25O₃

Abstract

In this work, we have investigated the precursor effects to superconductivity in BaPb0.75Bi0.25O3 using temperature dependent resistivity, x-ray diffraction technique and photoemission spectroscopy. The present compound exhibits superconductivity around 11 K (TC). The synthesis procedure adopted is much simpler as compared to the procedure available in the literature. In the temperature range (10 K–25 K) i.e. above TC, our results show an increase in both the orthorhombic and tetragonal strain. The well screened features observed in Bi and Pb 4f7/2 core levels are indicative of the metallic nature of the sample. The compound exhibits finite intensity at the Fermi level at 300 K and this intensity decreases with decrease in temperature and develops into a pseudogap; the energy dependence of the spectral density of states suggests disordered metallic state. Furthermore, our band structure calculations reveal that the structural transition upon Pb doping results in the closing of the band gap at the Fermi level.closing of the band gap at the Fermi level.

Pseudogap in BaPb x Bi1‒x O3 (x = 0.7, 0.75 and 1.0)

“…Figures 4(a)-(c) shows the variation of the strain as a function of temperature in the three planes that bound the unit cell-ab, ac and bc planes. In case of BPBO75 compound, the orthorhombic strain becomes a maximum as the temperature nears the T C in the ab plane, as noted previously [22]. On the other hand, in the ac and bc planes, the strain reduces with temperature, attaining a minimum near T C .…”

“…It is evident from the graph that it shows semiconductor-like behaviour. The BPBO75 compound, on the other hand, shows a sharp transition at ∼11 K, indicating a superconducting transition, the details of which are reported elsewhere [22].…”

“…Thus, to account for this, we have performed TB mBJ calculations for these two compounds. Further more, the BPBO75 compound is diphasic, and to account for it, the calculations were performed on both the phases, and have been added in 70:30 ratio (tetragonal:orthorhombic), as was observed experimentally [22]. When the TB-mBJ calculations for BPBO70 and BPBO75 are compared, we observe an additional feature B 0 in the latter.…”

“…For the preparation of BPBO70 and BPO samples, proper stoichiometric ratios of the raw materials were ground thoroughly in an agate mortar for 6-10 h. This finely ground powder was then calcined in a box furnace at 900 • C for 48 h. The heating rate was maintained at 5 • C min −1 and the samples were allowed to cool down to room temperature naturally inside the box furnace. The calcined powders were then sintered in the box furnace at 900 • C for around 48-96 h, with intermediate grinding every 24 h. In the case of BPBO75 sample, a similar procedure was adopted, the details of which are published elsewhere [22]. After each grinding, the powders were then pressed into pellets of 10 mm diameter under a hydrostatic pressure of 5 tons.…”

“…We were able to index all the peaks with a single Ibmm phase, indicating that the compound is single phase. On the other hand, a two phase refinement was carried out for the BPBO75 compound using I4/mcm and Ibmm phases, whose crystal structure properties have been reported in our previous work [22]. The BPO compound stabilises in Ibmm space group of the orthorhombic crystal structure.…”