New stable rare earth Ti-based semiconductor pyrochlore oxides for low-cost energy applications

This study investigates the structural, optoelectronic, magnetic and thermophysical properties of three newly designed semiconductor pyrochlore oxides, namely RE₂Ti₂O₇ (RE = Er, Eu, Tb), using ab-initio calculations within the density functional theory framework. The values of indirect bandgaps are 3.8, 2.2 and 3.77 eV for the spin up channel of Er₂Ti₂O₇, Eu₂Ti₂O₇, and Tb₂Ti₂O₇, respectively. While the values of indirect bandgaps for the spin down channel of Er₂Ti₂O₇, Eu₂Ti₂O₇, and Tb₂Ti₂O₇ are 3.56, 3.65 and 2.1 eV, respectively. The paramagnetic magnetic moments can be inferred by analyzing the distinct band shapes observed in the energy band structures of the studied compounds corresponding to the spin up and spin down states. The magnetic moments of Er₂Ti₂O₇, Eu₂Ti₂O₇, and Tb₂Ti₂O₇ have significant magnitudes, specifically measuring 12.00, 24.51, and 24.00  \({\mu }_{B}\) ${\mu }_B$ , respectively. Significant absorption of incident photons by the studied compounds can be noted in near UV region in both spin channels. Low reflectivity (~ 30%) by RE₂Ti₂O₇ (RE = Er, Eu, Tb) is evident from the \(R(\omega )\) $R(\omega )$ spectra in an energy range of 1.0–10.0 eV. However, these compounds exhibit ~ 50% reflectance of the incident photons in upper UV region (above 10.0 eV). The analysis of the Seebeck coefficient spectra reveals that RE₂Ti₂O₇ (RE = Er, Eu, Tb) exhibits p-type semiconductor behavior, however, Eu₂Ti₂O₇ also shows n-type behavior from 200 to 450 K. The presented thermodynamic characteristics reveal that the pyrochlore oxides RE₂Ti₂O₇ (RE = Er, Eu, Tb) exhibit thermal stability.