A systematic first-principles investigation of the structural, electronic, optical, thermodynamic and transport properties of lead free pyrochlore oxides Q2Sb2O7 (Q= be, Ca, Sr) for low-Cost energy applications

A systematic study has been performed on structural, optical and thermophysical properties of newly designed pyrochlore oxides Q2Sb2O7 (Q = Be, Ca, Sr) employing FP-LAPW based first-principles calculations. The GGA approach was used to treat exchange and correlation potentials. The investigated E-V plots reveals that Sr2Sb2O7 is the most stable structure compared to Be2Sb2O7/Ca2Sb2O7. A direct energy bandgap of 0.29 eV is evident from band structure plot of Be2Sb2O7, however, Ca2Sb2O7/Sr2Sb2O7 possess indirect energy bandgaps of magnitude 1.47/1.467 eV. The studied materials show maximum absorption of incoming photons in near UV region as shown in ε2(ω) plots, however, considerable absorption in visible region is also present. Ca2Sb2O7/Sr2Sb2O7 are effective optical material with a n(ω) value between 1.0 and 2.0. Optical properties of pyrochlore oxides reveal that these materials are potential candidates for shielding materials in upper UV region. A photon reflection of up to 60% is evident from the R(ω) in UV region, however, in IR and visible regions, the reflectance is negligible. Based on calculated values of Seebeck coefficient (S), we can state that Be2Sb2O7 is n-type semiconductor whereas Ca2Sb2O7/Sr2Sb2O7 are p-type semiconductors. The most effective thermoelectric material among Q2Sb2O7 (Q = Be, Ca, Sr) is Ca2Sb2O7 as its ZT value is highest (∼1.05) in the entire temperature range. Thermodynamic properties of Q2Sb2O7 (Q = Be, Ca, Sr) are also evaluated to check dynamical stability and appropriateness of these materials in thermal applications. The investigated outcomes show that these pyrochlore oxides are potential candidates for thermoelectric and optoelectronic device applications.