In 1992, Gabriel Aeppli and Zachary Fisk found a descriptive way to explain the physical properties of Ce 3 Bi 4 Pt 3 and CeNiSn. They called the materials Kondo insulators, showing Kondo lattice behavior near room temperature, but becoming semiconducting with very small energy gaps (a few Kelvin to a few tens of Kelvin) when decreasing the temperature.