Lattice collapse in mixed-valence samarium fulleride Sm2.75C60 at high pressure†
Sm2.75C60 displays large negative thermal expansion behaviour in the temperature range 4.2–32 K at ambient pressure as a result of a quasi-continuous valence change from the larger Sm2+ towards the smaller Sm2.3+ ion. Here we use the powder synchrotron X-ray diffraction technique at ambient temperature and elevated pressures to study the compression behaviour in the pressure range 0–6 GPa. An abrupt hysteretic phase transition, accompanied by a dramatic volume decrease (≈6.0%) and a change in colour from black to golden was found at ≈4 GPa induced by a sudden Sm valence transition from +2.3 towards +3. Such behaviour is typical of highly correlated Kondo insulators like SmS and makes Sm2.75C60 the first known molecular-based member of this fascinating class of materials.