Abstract: Porous materials and single crystals both hold significant application value in modern science and technology. Introducing porosity into macroscopic single crystals offers a promising route to a new class of materials that combine an open porous architecture with a continuous single-crystalline skeleton, namely macroscopic porous single crystal (PSC). In PSC, the continuous single-crystalline skeleton maintains long-range crystallographic order and effectively reduces grain-boundary effects on transport processes and structural stability, while the porous architecture provides mass-transport channels and accessible internal surfaces. However, conventional crystal growth processes generally tend toward densification, and pores are usually regarded as inclusions or defects. Therefore, the controllable introduction of porous structures into single crystals remains a fundamental challenge. This review focuses on lattice reconstruction strategies driven by solid-solid phase transformations. We discuss the basic mechanism by which a high-density single-crystalline mother phase is transformed in situ into a low-density target single-crystalline skeleton through selective component removal, lattice rearrangement, and phase-boundarymigration, accompanied by pore formation. Using typical oxide systems such as TiO2 and CeO2 as examples, we further analyze how the crystallographic orientation, composition, relative density variation, and heat-treatment conditions of the mother phase regulate the facet orientation, porosity, pore size, and surface structure of the resulting PSC. Finally, with photoelectrochemical energy conversion as a representative application, we discuss the synergistic effects of the continuous single-crystalline skeleton, three-dimensionally interconnected pore channels, and tunable defect structures. We also outline future opportunities for these materials in structure-sensitive functional processes, including electron-phonon transport decoupling and isotope separation.

Key words: Single-crystalline materials, Porous materials, Single-crystalline structure, Porous architecture, Solid-solid phase transformation