Unusual mixed valence of Eu in two new materials EuSr2Bi2S4F4 and Eu2SrBi2S4F4: M\"ossbauer and X-ray photoemission Spectroscopy investigations

TL;DR

This study synthesizes and characterizes two new Eu-based compounds, revealing mixed valence states of Eu and their structural and electronic properties through spectroscopy, magnetization, and resistivity measurements.

Contribution

It is the first detailed investigation of Eu valence states and distribution in these new compounds using M"ossbauer, XPS, and magnetic studies.

Findings

Eu atoms in both compounds exhibit mixed valence states around 2.6-2.7.

Eu2SrBi2S4F4 shows Eu valence shifting towards 2+ in magnetic fields.

Both compounds display semi-metallic behavior down to 2 K.

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 M\"ossbauer 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 ~ 2.6 - 2.7. From our magnetization studies in an applied H = 9 Tesla, we infer that the valence of Eu-atoms in Eu2SrBi2S4F4 at the 2a-sites exhibits a shift towards 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 semi-metallic nature for both compounds.