  The temperature dependence of the local structure of PbSe has been
investigated using pair distribution function (PDF) analysis of x-ray and
neutron powder diffraction data and density functional theory (DFT)
calculations. Observation of non-Gaussian PDF peaks at high temperature
indicates the presence of significant anharmonicity, which can be modeled as Pb
off-centering along [100] directions that grows on warming similar to the
behavior seen in PbTe and PbS and sometimes called emphanisis. Interestingly,
the emphanitic response is smaller in PbSe than in both PbS and PbTe indicating
a non-monotonic response with chalcogen atomic number in the PbQ (Q=S, Se, Te)
series. The DFT calculations indicate a correlation between band gap and the
amplitude of [100] dipolar distortion, suggesting that emphanisis may be behind
the anomalous composition and temperature dependencies of the band gaps in this
series.
