Temperature dependent band gap in PbX (X=S, Se, Te)

PbTe is an important thermoelectric material for power generation applications due its high conversion efficiency and reliability. Its extraordinary thermoelectric performance is attributed to band convergence of the light L and heavy Σ bands. However, the temperature at which these bands converge is disputed. In this letter, we provide direct experimental evidence combined with ab initio calculations that confirm an increasing optical gap up to 673 K and predict a band convergence temperature of 700 K, much higher than previous measurements showing saturation and band convergence at 450 K