Advances in aliovalent substitution strategy for the design and synthesis of nonlinear optical materials: d0 transition metal/gallium iodates and selenites

As an important class of solid-state functional materials, nonlinear optical (NLO) crystals have attracted enormous academic and industrial interest due to their unique ability to generate lasers at a variety of inaccessible wavelengths. A promising NLO material must meet several requirements, including noncentrosymmetric structure, a strong second-harmonic generation (SHG) response, sufficient birefringence (Δn), and a wide transparency window. The rarity of noncentrosymmetric structures and the three rigorous yet contradictory performance criteria pose significant challenges for the development of NLO materials with desired properties. Therefore, it is imperative to develop effective strategies to achieve the goal of rational design of NLO candidates. Among many, chemistry-directing substitution from existing prototypes has proven to be a promising and accessible approach. In this Review, our aim is to provide useful insights for the discovery of new NLO materials by highlighting how to implement precise aliovalent chemical substitution tactics targeting B-site cations to design novel functional NLO materials. For this purpose, we chose early d0-transition metal (d0-TM) and gallium cations-based iodates and selenites due to their structural diversity and abundant sources. The B-site cations, d0-TM and gallium ions, have sequentially variable oxidization states from hexavalent to trivalent states, providing an ideal platform for inequivalent substitution. In the first section, we describe the three types of distortion of d0-TM cation-centered octahedra and briefly explain the aliovalent substitution strategy. To differentiate the concept of fundamental building units (FBUs) or basic building units (BBUs), we adopt secondary building units (SBUs) to specifically describe the structural fragments generated by the connections of [MX6] (M = TM or Ga cations, X = O or F ligands) octahedra with iodate or selenite groups. Next, we provide seven sets of examples to meticulously illustrate the precise aliovalent chemical substitution on B-site cations and discuss the crystal structure and functional properties. We also rationally design and predict several new NLO candidates based on the above-mentioned pathways. Finally, we discuss several factors that affect the functional properties of NLO materials.