  Understanding the phase behavior of semiconductor materials is important for
applications in solid state physics and nanoscience. Accurate experimental data
is often difficult to obtain due to strong kinetic effects. In this work, we
calculate the temperature-pressure phase diagram of lead selenide (PbSe) using
the random phase approximation (RPA), an accurate wavefunction based many-body
technique. We consider three crystalline phases, the low pressure B1 phase
(NaCl-type), the intermediate B33 phase (CrB-type), and the high pressure B2
phase (CsCl-type). The electronic contributions to the free energy (at T=0K)
are calculated in the Born-Oppenheimer approximation using the RPA, whereas
phononic contributions are computed in the quasi-harmonic approximation using
DFT and the PBEsol functional. At room temperature, we find transition
pressures of 4.6 +/- 0.3 GPa for the B1-B33 transition and 18.7 +/- 0.3 GPa for
the B33-B2 transition, in good agreement with experiments. In contrast to the
interpretation of recent experiments, we observe a negative Clapeyron slope for
both transitions. Gibbs free energy differences between competing structures
have small gradients close to coexistence, consistent with pronounced
hysteresis observed in experiments. The phase diagram presented in this work
can serve as a reference for future studies of PbSe and should prove useful in
the development of accurate and efficient force fields.
