Structural evolution and photovoltaic band gaps in the EuREAgS3 series (RE = Tb, Dy, Ho, Tm, Yb)
https://doi.org/10.1016/j.inoche.2026.116140
We report the first successful synthesis of polycrystalline EuREAgS3 (RE = Tb, Tm, Yb) compounds. These new quaternary chalcogenides extend an ongoing series of materials produced via a sealed-ampoule method from EuS, RE2S3, Ag and S at 1170 K. Powder X-ray diffraction reveals that the compounds adopt high-temperature polymorphs with two distinct structures. EuTbAgS3 crystallizes in cubic space group Fm3¯m, with a unit cell parameter of a = 5.7267(6) Å. In contrast, EuTmAgS3 and EuYbAgS3 form in trigonal space group R3¯m, with unit cell parameters of a = 3.9837(4) Å, c = 9.982(1) Å and a = 3.9763(2) Å, c = 9.9700(7) Å, respectively. Both structure types feature a single crystallographic site for the cations (Eu2+/RE3+/Ag+) and one for the S2– anions, forming a three-dimensional framework of edge-sharing (Eu/RE/Ag)S6 octahedra. The structural distinction arises from the different local symmetry at the shared cation site. A clear structural contraction is observed across the EuREAgS3 series (RE = Tb, Dy, Ho, Tm, Yb), manifesting as a decrease in both unit cell volume and the average bond length d(Eu/RE/Ag–S) with the decreasing ionic radius, r(RE3+). Electronic structure analysis confirms that all synthesized compounds are semiconductors, with the measured direct and indirect band gaps ranging from 1.48 to 1.70 eV and from 0.62 and 1.34 eV, respectively.