Insulator-metal-superconductor transition in the medium-entropy van der Waals compound MPSe3 (M = Fe, Mn, Cd, and In) under high pressure

MPX3 (M=metals; X=SorSe) represents a large family of van der Waals (vdW) materials featuring P-P dimers of similar to 2.3 angstrom separation. A dramatic alteration of its electrical transport properties, such as metal-insulator transition, has not been realized by intentional chemical doping and ionic intercalation. Here, we employ an entropy-enhancement strategy to successfully obtain a series of medium-entropy MPSe3 (M=Fe, Mn, Cd, and In), in which the electrical and magnetic properties change simultaneously. Lone-pair electrons of P emerge due to the dissociation of the dimers as evidenced by a 35% elongation in the P-P interatomic distance. The band gap widens from 0.1 to 0.7 eV by this dissociation. Under external physical pressure up to similar to 50 GPa, a giant collapse of up to 15% in the c axis happens, which is in contrast to the in-plane shrinkage of their counterparts Fe/MnPSe3. It leads to the recombination of P3- with lone-pair electrons into a P-P dimer and the smallest bulk modulus of 28 GPa in MPX3. The medium-entropy MPSe3 transitions from a spin-glass insulator to metal, and to superconductor, which is rarely observed in MPX3. Our findings highlight the P-P dimer as an indicator to probe diverse electronic structure and the effectiveness of entropy enhancement in materials science.