Two Acentric Aluminoborates Incorporated d¹⁰ Cations: Syntheses, Structures, and Nonlinear Optical Properties

Area of Science:

• Inorganic Chemistry
• Materials Science
• Crystallography

Background:

• Aluminoborates (ABOs) are a class of inorganic compounds with diverse structures and potential applications in nonlinear optics.
• Developing new ABO materials with enhanced second-harmonic generation (SHG) properties, particularly in the deep-ultraviolet (deep-UV) region, remains a key research objective.

Purpose of the Study:

• To synthesize and characterize two new acentric aluminoborates, [Zn(en)₂Al{B₅O₉(OH)}{BO(OH)₂}] (1) and [Cd(en)₂AlB₅O₁₀]·H₂O (2) (en = ethylenediamine).
• To investigate the structural transformation from a 2D layer to a 3D framework and understand its impact on material properties.
• To evaluate the nonlinear optical (NLO) properties, specifically second-harmonic generation (SHG), and optical transparency of the synthesized compounds.

Main Methods:

• Solvothermal synthesis was employed to prepare compounds 1 and 2.
• Single-crystal X-ray diffraction was used to determine the crystal structures, revealing a 2D wavy layer in 1 (hcb-type network) and a 3D porous framework in 2 (unc-type network).
• UV-vis diffuse reflectance spectroscopy was performed to assess optical transparency.
• Second-harmonic generation (SHG) measurements were conducted to quantify NLO responses.
• Density functional theory (DFT) calculations were utilized to elucidate the factors influencing optical properties.

Main Results:

• Two acentric aluminoborates, 1 and 2, were successfully synthesized.
• Compound 1 features a 2D wavy layer, while compound 2 exhibits a 3D porous framework, demonstrating a structural transformation driven by the elimination of hydroxyl groups.
• Both compounds display large powder SHG responses (2.2x and 2.7x KDP, respectively), among the highest for deep-UV ABOs.
• A wide transparency window below 190 nm was observed for both materials.
• DFT calculations confirmed the crucial role of B-O units and distorted d¹⁰ metal polyhedra in their optical properties.

Conclusions:

• The study successfully synthesized two novel acentric aluminoborates with distinct 2D and 3D structures.
• These compounds exhibit excellent deep-UV nonlinear optical properties and wide transparency, making them promising candidates for optical applications.
• The findings highlight the tunability of aluminoborate structures and their potential for designing advanced functional materials.