We propose to study the effect of pressure and temperature on the crystal structure and the elastic properties of lead selenide, an important material for conversion of waste heat into electrical power. We have successfully analysed and published results on the sibling phases, lead sulfide and lead telluride, and request ISIS beam time to complete this series. We propose to measure the behaviour of lead selenide at low temperature and high pressure to derive useful thermoelastic properties for this compound including thermal expansion and isothermal compression. We aim to resolve a long standing scientific problem in this compound as to the crystal structure of its intermediate phase.