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In this work, frequency-dependent dielectric properties of polycrystalline samples with nominal compositions FeTeSe ( x = 0.0-0.5) were investigated. The samples were synthesized via solid-state reaction method with intermittent grinding at ambient pressure. The phase formation, lattice properties and chemical compositions of the samples were analysed. Dielectric constants ( $\varepsilon ^{\prime }$ , $\varepsilon ^{\prime \prime }$ ), dielectric loss (tan δ) and alternating current (AC) conductivity ( σ ) as a function of frequency ranging from 100 Hz to 10 MHz were measured at room temperature. X-ray diffraction (XRD) data showed the presence of impurity phases of FeO, FeTe and hexagonal FeSe/FeSe. Both a and c lattice parameters decreased with the substitution of Se. Energy-dispersive x-ray spectroscopy confirmed the increasing ratio of Se/Te with x. The measured negative values of real dielectric constant ( $\varepsilon ^{\prime } )$ for x = 0.0-0.5 indicate the conductive nature of these samples. As the Se content was increased, the $\varepsilon ^{\prime } $ became more negative as a result of better grain connectivity as shown by the higher AC conductivity and dielectric loss. [ABSTRACT FROM AUTHOR]

In this study, polycrystalline samples with nominal composition FeTeSe ( x=0.1−0.5) were synthesized by solid-state reaction method at ambient pressure. In order to minimize oxidation, argon gas flow was maintained throughout the heat treatment. The phase formation of FeTeSe samples was checked by X-ray diffraction (XRD). The polycrystalline FeTeSe samples were indexed to a tetragonal structure with space group of P4/nmm. The lattice parameters a- and c-axes shrink significantly with the substitution of Se. As shown by the scanning electron microscope (SEM) images, the samples developed a plate-like grain structure gradually with the increase of Se concentration. Measurements of temperature dependence of magnetic moment showed that the onset of superconducting transition temperature, T, increased with Se concentration. The T is about 10.6 and 13.5 K for x= 0.1 and 0.5, respectively. All the samples exhibit ferromagnetic behavior asshown by the field-dependent magnetization measured at room temperature. [ABSTRACT FROM AUTHOR]