Positronium atom in solids. Peculiarities of formation and interconnection with free volume nanostructure

The recent several decades have seen the spectacular advances in a new area at the junction of elementary particle physics and chemistry, the so called chemistry of new atoms in which either electron is replaced by another negative particle or a proton by another positive particle. The area of research does not confine itself to chemical studies alone. External influences on the properties of new atoms provide useful, sometimes unique information about the structure of solids. In this review, we survey briefly properties of the new atoms and illustrate the last statement on example of positronium, the simplest hydrogen-like atom, where pick-off annihilation process gives valuable information of the free volume nanostructure of solids. (author)