Structural evolution and photovoltaic band gaps in the EuREAgS3 series (RE = Tb, Dy, Ho, Tm, Yb)
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