Bismuthates: BaBiO₃ and related superconducting phases

BaBiO₃ has the perovskite structure, but tilting of the BiO₆ octahedra destroy the ideal cubic symmetry except at temperatures above 820 K. BaBiO₃ is a diamagnetic semiconductor due to a charge density wave (CDW), which is equivalent to a Ba₂Bi³⁺Bi⁵⁺O₆ representation. Recent calculations and experimental results confirm that there is no significant deviation from the oxidation states of 3+ and 5+. Superconductivity with a T[subscript c] as high as 13 K occurs for BaPb₁₋ₓBiₓO₃ phases where the 6s band is about 25% filled, and superconductivity with a T[subscript c] as high as 34 K occurs for Ba₁₋ₓKₓBiO₃ phases where the 6s band is about 35% filled. Structures in these two solid solutions can have cubic, tetragonal, orthorhombic, or monoclinic symmetry. However, superconductivity has only been observed when the symmetry is tetragonal.Keywords: Superconductor, Bismuthate, Perovskite structur

Covalency-driven Structural Evolution in the Polar Pyrochlore Series Cd2Nb2O7-xSx

The arrangement of cations on the triangular pyrochlore lattice leads to a wealth of interesting physical phenomena influenced by geometric frustration. Although uncommon, several pyrochlore materials overcome this frustration and exhibit polar structures. Unraveling the origin of such behavior is key to understanding how broken inversion symmetry arises in complex crystal structures. Here, we investigate the effect of varying degrees of covalency in the pyrochlore lattice through a detailed structural and lattice dynamical analysis of the pyrochlore oxysulfide series Cd2Nb2O7-xSx above and below the ferroelectric transition temperatures (TC) using synchrotron X-ray diffraction and first principles calculations. All compositions exhibit the cubic Fd3 m pyrochlore aristotype above TC, whereas the amplitude and character of various structural distortions are found to be composition-dependent below TC. For x = 0, large Cd and Nb cation displacements occur to produce the polar Ima2 structure accompanied by a change in translational symmetry. Our symmetry and lattice dynamical calculations indicate that Cd2Nb2O7 undergoes a proper ferroelectric transition through TC. Analysis of the sulfur-substituted niobates indicates that although the polar space group Fdd2 is adopted by the nominal x = 0.25 sample, the transition into the polar phase is improper. For the nominally x = 0.7 composition, the lattice remains nearly cubic, but exhibits a high degree of structural disorder in the pyrochlore channel, with a deviation from the linear Cd-X′-Cd bond by nearly 15° to accommodate the large size of S while preventing extreme stretching of the Nb-O bond. This highly distorted Cd-X′ network is accompanied by a highly distorted NbO6 network, which is accommodated by the polarizable NbO6 coordination environment. This sheds light on the limited existence of oxysulfide pyrochlores; for example, the lack of reported S substitution in the case of the similar yet less-polarizable Cd2Ta2O7. Our work provides a new understanding of how inversion-symmetry lifting displacements arise and how anion substitution, which tunes covalent cation-anion interactions, is a useful strategy for manipulating polar behavior in a pyrochlore lattice

Prognostic model to predict postoperative acute kidney injury in patients undergoing major gastrointestinal surgery based on a national prospective observational cohort study.

Background: Acute illness, existing co-morbidities and surgical stress response can all contribute to postoperative acute kidney injury (AKI) in patients undergoing major gastrointestinal surgery. The aim of this study was prospectively to develop a pragmatic prognostic model to stratify patients according to risk of developing AKI after major gastrointestinal surgery. Methods: This prospective multicentre cohort study included consecutive adults undergoing elective or emergency gastrointestinal resection, liver resection or stoma reversal in 2-week blocks over a continuous 3-month period. The primary outcome was the rate of AKI within 7 days of surgery. Bootstrap stability was used to select clinically plausible risk factors into the model. Internal model validation was carried out by bootstrap validation. Results: A total of 4544 patients were included across 173 centres in the UK and Ireland. The overall rate of AKI was 14·2 per cent (646 of 4544) and the 30-day mortality rate was 1·8 per cent (84 of 4544). Stage 1 AKI was significantly associated with 30-day mortality (unadjusted odds ratio 7·61, 95 per cent c.i. 4·49 to 12·90; P < 0·001), with increasing odds of death with each AKI stage. Six variables were selected for inclusion in the prognostic model: age, sex, ASA grade, preoperative estimated glomerular filtration rate, planned open surgery and preoperative use of either an angiotensin-converting enzyme inhibitor or an angiotensin receptor blocker. Internal validation demonstrated good model discrimination (c-statistic 0·65). Discussion: Following major gastrointestinal surgery, AKI occurred in one in seven patients. This preoperative prognostic model identified patients at high risk of postoperative AKI. Validation in an independent data set is required to ensure generalizability