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Abstract Complex correlated states emerging from many-body interactions between quasiparticles (electrons, excitons and phonons) are at the core of condensed matter physics and material science. In low-dimensional materials, quantum confinement affects the electronic, and subsequently, optical properties for these correlated states. Here, by combining photoluminescence, optical reflection measurements and ab initio theoretical calculations, we demonstrate an unconventional excitonic state and its bound phonon sideband in layered silicon diphosphide (SiP 2 ), where the bound electron–hole pair is composed of electrons confined within one-dimensional phosphorus–phosphorus chains and holes extended in two-dimensional SiP 2 layers. The excitonic state and emergent phonon sideband show linear dichroism and large energy redshifts with increasing temperature. Our ab initio many-body calculations confirm that the observed phonon sideband results from the correlated interaction between excitons and optical phonons. With these results, we propose layered SiP 2 as a platform for the study of excitonic physics and many-particle effects. 

Materials with an abrupt volume collapse of more than 20 % during a pressure‐induced phase transition are rarely reported. In such an intriguing phenomenon, the lattice may be coupled with dramatic changes of orbital and/or the spin‐state of the transition metal. A combined in situ crystallography and electron spin‐state study to probe the mechanism of the pressure‐driven lattice collapse in MnS and MnSe is presented. Both materials exhibit a rocksalt‐to‐MnP phase transition under compression with ca. 22 % unit‐cell volume changes, which was found to be coupled with the Mn²⁺(d⁵) spin‐state transition fromS=5/2 toS=1/2 and the formation of Mn−Mn intermetallic bonds as supported by the metallic transport behavior of their high‐pressure phases. Our results reveal the mutual relationship between pressure‐driven lattice collapse and the orbital/spin‐state of Mn²⁺in manganese chalcogenides and also provide deeper insights toward the exploration of new metastable phases with exceptional functionalities.

 

The discovery of electrides, in particular, inorganic electrides where electrons substitute anions, has inspired striking interests in the systems that exhibit unusual electronic and catalytic properties. So far, however, the experimental studies of such systems are largely restricted to ambient conditions, unable to understand their interactions between electron localizations and geometrical modifications under external stimuli, e.g., pressure. Here, pressure‐induced structural and electronic evolutions of Ca₂N by in situ synchrotron X‐ray diffraction and electrical resistance measurements, and density functional theory calculations with particle swarm optimization algorithms are reported. Experiments and computation are combined to reveal that under compression, Ca₂N undergoes structural transforms fromRmsymmetry toI2dphase via an intermediateFdmphase, and then toCcphase, accompanied by the reductions of electronic dimensionality from 2D, 1D to 0D. Electrical resistance measurements support a metal‐to‐semiconductor transition in Ca₂N because of the reorganizations of confined electrons under pressure, also validated by the calculation. The results demonstrate unexplored experimental evidence for a pressure‐induced metal‐to‐semiconductor switching in Ca₂N and offer a possible strategy for producing new electrides under moderate pressure.