We propose to study the effect of pressure and temperature on the crystal structure and the elasticproperties of lead selenide, an important material for conversion of waste heat into electricalpower. We have successfully analysed and published results on the sibling phases, lead sulfideand lead telluride, and request ISIS beam time to complete this series. We propose to measurethe behaviour of lead selenide at low temperature and high pressure to derive useful thermoelasticproperties for this compound including thermal expansion and isothermal compression. We aim toresolve a long standing scientific problem in this compound as to the crystal structure of itsintermediate phase