ABSTRACT
Recently, we reported the synthesis and investigations of EuSr2Bi2S4F4 and Eu2SrBi2S4F4. We have now been able to induce superconductivity in EuSr2Bi2S4F4 by Se substitution at the S site (isovalent substitution) with Tc = 2.9 K in EuSr2Bi2S2Se2F4. The other compound, Eu2SrBi2S4F4, shows a significant enhancement of Tc. In Se-substituted Eu2SrBi2S4-xSexF4, we find Tc = 2.6 K for x = 1.5 and Tc = 2.8 K for x = 2, whereas Tc = 0.4 K in the Se-free sample. In addition to superconductivity, an important effect associated with Se substitution is that it gives rise to remarkable changes in the Eu valence. Our 151Eu Mössbauer and X-ray photoemission spectroscopic measurements show that Se substitution in both of the compounds Eu2SrBi2S4F4 and EuSr2Bi2S4F4 gives rise to an increase in the Eu2+ component in the mixed-valence state of Eu.
ABSTRACT
We synthesized double perovskite Pr2FeCrO6 by solid-state method. Analysis of its X-ray powder diffraction shows that the compound crystallizes in a centrosymmetric structure with space group Pbnm. Our X-ray photoelectron spectroscopy (XPS) studies show that all the cations are present in +3 oxidation state. Magnetization studies of Pr2FeCrO6 show that the material is paramagnetic at room temperature and undergoes a magnetic transition below TCM = 250 K. We observe clear magnetic hysteresis loop, for example, below 150 K. A low remnant magnetization Mr, â¼0.05 µB/f. u., is inferred from the observed magnetic hysteresis loop. 57Fe Mössbauer study at 25 K shows a high hyperfine magnetic field of â¼53 T at the Fe nucleus, which corresponds to a magnetic moment of â¼6-7 µB/Fe. These two results together suggest a ferrimagnetic (nearly compensated or canted) ordering of the Fe moments. Mössbauer studies close to the ferrimagnetic ordering temperature suggest interesting magnetic relaxation effects. A dielectric anomaly observed at TCE = 453 K signals a ferroelectric â paraelectric phase transition. We observe at room temperature a clear and well-defined ferroelectric hysteresis loop, PS = 1.04 µC/cm2, establishing ferroelectricity in the material. From these results, we conclude that Pr2FeCrO6 is a type I multiferroic (TCE > TCM).
ABSTRACT
We have synthesized two new Eu-based compounds, EuSr2Bi2S4F4 and Eu2SrBi2S4F4, which are derivatives of Eu3Bi2S4F4, an intrinsic superconductor with Tc = 1.5 K. They belong to a tetragonal structure (SG: I4/mmm, Z = 2), similar to the parent compound Eu3Bi2S4F4. Our structural and 151Eu Mössbauer spectroscopy studies show that, in EuSr2Bi2S4F4, Eu-atoms exclusively occupy the crystallographic 2a-sites. In Eu2SrBi2S4F4, 2a-sites are fully occupied by Eu-atoms and the other half of Eu-atoms and Sr-atoms together fully occupy 4e-sites in a statistical distribution. In both compounds Eu atoms occupying the crystallographic 2a-sites are in a homogeneous mixed valent state â¼2.6-2.7. From our magnetization studies in an applied H ≤ 9 T, we infer that the valence of Eu-atoms in Eu2SrBi2S4F4 at the 2a-sites exhibits a shift toward 2+. Our XPS studies corroborate the occurrence of valence fluctuations of Eu and after Ar-ion sputtering show evidence of enhanced population of Eu2+-states. Resistivity measurements, down to 2 K, suggest a semimetallic nature for both compounds.
ABSTRACT
The crystal structure and properties of a new member of the oxybismuth sulfide family SmO(0.5)F(0.5)BiS(2) are reported here. The compounds SmO(1-x)F(x)BiS(2) (x = 0.0 and 0.5) are isostructural with LaOBiS(2) and crystallize in the CeOBiS(2)-type structure (P4/nmm). Sm substitution in LaO(0.5)F(0.5)BiS(2) (La1-ySmyO(0.5)F(0.5)BiS(2)) leads to a gradual decrease in the a-lattice constant; however, the c-lattice constant does not show such a gradual trend. Enhancement in T(c) is achieved upon partial substitution of La by the smaller Sm ion. A maximum T(c) â¼ 4.6 K was observed for composition with y = 0.8. Disobeying this trend, Tc disappears unexpectedly in the composition SmO(0.5)F(0.5)BiS(2) (y = 1.0). Both the undoped and F-doped (x = 0.0 and 0.5) compounds are paramagnetic, exhibiting semiconducting behavior down to 2 K.
