Role of f-electrons in determining insulator to metal phase transitions of Ca(La1−xCex)2S4 (0 ≤ x ≤ 1) solid solution: A DFT + U study

Abstract

For efficient usage of the rare earth-based materials in electronic or optoelectronic applications, their 4f electrons' behaviors must be understood properly. We have presented Hubbard U corrected density functional theory (DFT+U) study of structural, magnetic, electronic, and optical properties of a rare-earth chalcogenide system, Ca(La1−xCex)2S4 (0 ≤ x ≤ 1). A unique site selection technique based on local magnetic moment arrangements was applied to build the atomic arrangements for a Ce doped Ca(La1−xCex)2S4 solid solution. The incorporation of f-electrons by Ce doping modifies the properties of the parent compound, CaLa2S4. In conjunction with the DFT + U method, we applied spin–orbit coupling to determine the magnetic ground state. The inclusion of 25% Ce transforms the non-magnetic parent compound to an antiferromagnetic (AFM) compound, and AFM magnetic ordering remains unaltered throughout the whole solid solution series. In addition, these compounds also undergo insulator to semiconducting to metallic phase transitions as Ce concentration increases. While CaLa2S4 is an insulator, Ca(La1−xCex)2S4 with x = 0.25 and 0.50 are n-type semiconductors, and on the other hand, compounds with x = 0.75 and 1.0 are found to have metallic band structures. The Ce atoms in these materials were found to be in a mixed valence state, Ce3+/4+. We explained these phase transitions from the calculated electronic structures. In addition, we have presented an explanation for the experimentally observed red-orange colors of Ca(La0.25 Ce0.75)2S4 and CaCe2S4 compounds.