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We report on synthesis, structural characterization, resistivity, magnetic and thermal expansion measurements on the as yet unexplored $delta$-phase of FeSe$_{1-x}$, here synthesized under ambient- (AP) and high-pressure (HP) conditions. We show that in contrast to $beta$-FeSe$_{1-x}$, monophasic superconducting $delta$-FeSe$_{1-x}$ can be obtained in off-stoichiometric samples with excess Fe atoms preferentially residing in the van der Waals gap between the FeSe layers. The AP $delta$-FeSe$_{1-x}$ sample studied here ($T_c$ $simeq$ 8.5,K) possesses an unprecedented residual resistivity ratio RRR $simeq$ 16. Thermal expansion data reveal a small feature around $sim$90,K, which resembles the anomaly observed at the structural and magnetic transitions for other Fe-based superconductors, suggesting that some kind of magnetic state is formed also in FeSe. %indicative of a fluctuating magnetic ordering. For HP samples (RRR $simeq$ 3), the disorder within the FeSe layers is enhanced through the introduction of vacancies, the saturated magnetic moment of Fe is reduced and only spurious superconductivity is observed.
Superconductivity was achieved in PrFeAsO by partially substituting Pr^{3+} with Sr^{2+}. The electrical transport properties and structure of this new superconductor Pr_{1-x}Sr_xFeAsO at different doping levels (x = 0.05$sim$ 0.25) were investigated
We report the successful synthesis of FeSe$_{1-x}$S$_{x}$ single crystals with $x$ ranging from 0 to 1 via a hydrothermal method. A complete phase diagram of FeSe$_{1-x}$S$_{x}$ has been obtained based on resistivity and magnetization measurements. T
We combine the results of magnetic and transport measurements with neutron diffraction data to construct the structural and magnetic phase diagram of the entire family of SrMn$_{1-x}$Ru$_{x}$O$_3$ ($0 leqslant x leqslant 1$) perovskites. We have foun
Temperature (12K $le$ T $le$ 300K) dependent extended X-ray absorption fine structure (EXAFS) studies at the Fe K edge in FeSe$_{1-x}$Te$_x$ (x = 0, 0.5 and 1.0) compounds have been carried out to understand the reasons for increase in T$_C$ upon Te
Local structure of FeSe(1-x)Te(x) has been studied by extended x-ray absorption fine-structure (EXAFS) measurements as a function of temperature. Combination of Se and Fe K edge EXAFS has permitted to quantify the local interatomic distances and thei