Stable Lithium Argon compounds under high pressure

High pressure can fundamentally alter the bonding patterns of chemical elements. Its effects include stimulating elements thought to be “inactive” to form unexpectedly stable compounds with unusual chemical and physical properties. Here, using an unbiased structure search method based on CALYPSO methodology and density functional total energy calculations, the phase stabilities and crystal structures of Li−Ar compounds are systematically investigated at high pressure up to 300 GPa. Two unexpected Li(m)Ar(n) compounds (LiAr and Li(3)Ar) are predicted to be stable above 112 GPa and 119 GPa, respectively. A detailed analysis of the electronic structure of LiAr and Li(3)Ar shows that Ar in these compounds attracts electrons and thus behaves as an oxidizing agent. This is markedly different from the hitherto established chemical reactivity of Ar. Moreover, we predict that the P4/mmm phase of Li(3)Ar has a superconducting transition temperature of 17.6 K at 120 GPa

Structure of HgBa2CuO4+delta (0.06

The results of a neutron-diffraction study of the HgBa2CuO4+δ structure at ambient pressure and under external pressure at different extra oxygen concentrations are presented. The results have been analyzed together with the data of previous investigations. It is shown that in the cation-stoichiometric samples the Oδ oxygen is only present in the center of the mercury layer, Tc is parabolically dependent on δ, and Tc,max is obtained at δopt=0.13±0.01. The influence of pressure on the structure strongly depends on the doping level. At low oxygen content (δ≈0.06), the compression of the structure is practically uniform. An increase of the extra oxygen content to 0.19 (overdoped state) results in the larger compression of the apical Cu-O(2) and Ba-Oδdistances, while the HgO2 dumb-bell as well as the distance between Ba and O belonging to the (CuO2) layer become practically pressure independent. These results are in agreement with models, in which the effect of the charge transfer from the reservoir to the (CuO2) layers does not play a dominant role in the Tc increase with pressure at low and optimal δ values, while in the overdoped state the charge transfer is enhanced under pressure, thus inducing the Tc decrease. © 1999 The American Physical Society

Synthesis, neutron diffraction study and cation substitutions in Srn-1Cun+1O2n (n=3,5)

The n = 3 and n = 5 members of the homologous series Srn-1,Cun+1O2n have been synthesised by a high-pressure, high-temperature technique and characterized by X-ray powder diffraction and electron microscopy as well as EDS microanalysis. The solubility range of Sr4-xNdxCu6O10 solid solution was found to be <0.1. The crystal structure of Sr4Cu6O10 was refined from neutron powder diffraction data

Discovery of a second family of bismuth-oxide-based superconductors

The superconducting oxide BaPb(1-x)Bi(x)O3, discovered in 1975 (ref. 1), is an exotic system having an unusually high transition temperature (T(c)) of ~12K, despite a relatively low density of states at the Fermi level. The subsequent prediction that doping the electronically inactive barium donor sites, instead of the bismuth sites, might induce superconductivity with a higher T(c) led to the discovery in 1988 of superconductivity in the Ba(1-x)K(x)BiO3 system (T(c) ~30 K for x = 0.4). But it remains an open question why many of the superconducting properties of these materials are similar to those of the well-known copper oxide superconductors, despite their pronounced structural differences: the former have a three-dimensional bismuth-oxygen framework, whereas the structures of the latter are predominantly two-dimensional, consisting of copper-oxygen planes. Understanding of the copper oxide superconductors has gained immensely from the study of many different superconducting systems, and so it might be expected that the identification of bismuth oxide superconductors beyond the substituted BaBiO3 compounds will prove to be similarly fruitful. Here we report the synthesis of a second family of superconducting bismuth oxides, based on SrBiO3. We show that partial substitution of potassium or rubidium for strontium induces superconductivity with: T(c) values of ~12 K for Sr(1-x)K(x)BiO3 (x= 0.45-0.6) and ~13K for Sr(1-x)Rb(x)BiO3 (x = 0.5)

Unusual magnetic order in the pseudogap region of the superconductor HgBa2CuO4+delta

LetterInternational audienceThe pseudogap region of the phase diagram is an important unsolved puzzle in the field of high-transition-temperature (high-Tc) superconductivity, characterized by anomalous physical properties. There are open questions about the number of distinct phases and the possible presence of a quantum-critical point underneath the superconducting dome. The picture has remained unclear because there has not been conclusive evidence for a new type of order. Neutron scattering measurements for YBa2Cu3O6+delta (YBCO) resulted in contradictory claims of no6, 7 and weak8, 9 magnetic order, and the interpretation of muon spin relaxation measurements on YBCO10, 11 and of circularly polarized photoemission experiments on Bi2Sr2CaCu2O8+delta, has been controversial. Here we use polarized neutron diffraction to demonstrate for the model superconductor HgBa2CuO4+delta (Hg1201) that the characteristic temperature T* marks the onset of an unusual magnetic order. Together with recent results for YBCO, this observation constitutes a demonstration of the universal existence of such a state. The findings appear to rule out theories that regard T* as a crossover temperature rather than a phase transition temperature. Instead, they are consistent with a variant of previously proposed charge-current-loop order, that involves apical oxygen orbitals, and with the notion that many of the unusual properties arise from the presence of a quantum-critical poin