Orthopositronium

Orthopositronium was a form of positronium, an atom-like system consisting of an electron and a positron. As the electron and positron are oppositely charged, the particle behaves like a Hydrogen atom; however, the positron has a much smaller mass than the proton it replaces. As the electron and positron are anti-particles of each other, any form of positronium was very unstable and its lifespan is measured in nanoseconds.

Orthopositronium was used to coat the mirrors of Mariah Learman's time machine. (BFA: The Time of the Daleks)