In a recent article published in the journal Coordination Chemistry Reviews, researchers highlight the pivotal role of high-performance nonlinear optical (NLO) crystals in advancing modern photonic technologies, particularly in the infrared (IR) region. Among the emerging materials, chalcophosphates stand out as promising candidates thanks to their distinctive structural features that support desirable optical behaviors. By combining elements from both the chalcogenide and phosphate families, these compounds naturally favor the formation of non-centrosymmetric crystal structures, an essential requirement for NLO activity. Their structural diversity and chemical versatility enable fine-tuning of optical properties, making chalcophosphates a focal point for research aimed at filling the existing gaps in IR NLO materials.
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Background
NLO materials are foundational for applications such as laser systems, medical diagnostics, environmental monitoring, and molecular spectroscopy, where efficient frequency conversion and phase matching are essential. Historically, the chalcopyrite-type crystals like AgGaQ (Q = S, Se) and ZnGeP have been the primary IR NLO sources, but these face limitations such as low damage thresholds, narrow transparency windows, and defect issues. Consequently, developing new materials with superior optical properties, including broad IR transparency, large second-order nonlinearity, chemical stability, and high laser-induced damage thresholds, is a pressing challenge in the field.
To meet emerging requirements, researchers are increasingly turning to crystal engineering strategies that exploit the structural diversity of chalcophosphates and their intricate composition–structure–property relationships. These approaches aim to systematically tailor bandgaps, optimize phase-matching conditions, and enhance second-harmonic generation efficiencies, thereby unlocking the full potential of chalcophosphates as next-generation infrared NLO materials.
The review summarizes the development of design principles, focusing on the importance of the fundamental building blocks and the influence of metal cations on the polymerization and dimensionality of NLO-active units like P–Q (Q = S, Se). Structural parameters such as transparency range, birefringence, band gap, and thermal stability are critically evaluated, as they directly impact the optical performance relevant to IR laser applications.
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Studies Highlighted in this Review
This review compiles recent studies focused on the synthesis, structural characterization, and optical assessment of chalcophosphates with potential IR NLO applications. The key highlight is the identification of over 40 NLO-active [P Q] units, where modifications in composition and structure influence optical properties profoundly. Several classes of chalcophosphates are examined, especially thiophosphates and selenophosphates, which demonstrate promising nonlinearity, transparency, and stability.
The research emphasizes structure-property correlations, showing how the dimensionality and polymerization of P–Q units affect optical responses. Design strategies such as the "diamond-like" template approach, assembly of polyhedral units, mixed anionic strategies involving combined sulfide and selenide groups, and molecular-based crystal construction are analyzed. These strategies aim to optimize properties like broad IR transparency, high second-harmonic generation, and resistance to laser-induced damage.
Several compounds stand out due to their high second-harmonic generation (SHG) efficiencies (≥ 1.0 × AGS), wide IR transparent regions, and stability profiles suitable for high-power laser applications. The studies demonstrate that by controlling the structural motifs and chemical composition, it is possible to tune the band gap, birefringence, and nonlinearity, thus pushing towards practical, high-performance IR NLO crystals.
Discussion
The discussion highlights both the opportunities and challenges in advancing chalcophosphates as IR NLO materials. A central difficulty lies in balancing key properties, large second-order nonlinearity, broad IR transparency, high laser-damage thresholds, and robust chemical stability, since these traits often involve intrinsic trade-offs. For instance, materials with strong SHG responses typically exhibit smaller band gaps, which in turn lowers their resistance to optical damage. Likewise, efforts to enhance birefringence for improved phase-matching conditions can inadvertently narrow transparency windows, underscoring the delicate optimization required for practical applications.
The review emphasizes that structural design is key to overcoming these challenges. It highlights strategies such as the deliberate assembly of fundamental building blocks, the incorporation of diverse anionic groups, and the use of molecular templates to precisely tune optical properties. Looking ahead, the authors advocate exploring mixed-anionic systems and novel functionalization methods as promising routes to further improve stability, expand transparency windows, and strengthen nonlinear responses in chalcophosphates.
In addition, the discussion notes the potential of computational modeling and high-throughput screening to predict promising candidates, accelerating the discovery process. Emphasis is placed on understanding the detailed structure-property relationships, which will facilitate the rational design of materials capable of meeting the stringent demands for high-performance IR NLO crystals.
Conclusion
Chalcophosphates stand out as highly promising candidates for next-generation IR NLO materials, thanks to their structural diversity, versatile composition–structure–property relationships, and intrinsic tendency to form non-centrosymmetric crystals. The review underscores that strategic structural design can yield compounds with strong SHG efficiencies, wide IR transparency windows, and robust stability, key advances over current commercial crystals. Despite this progress, balancing these critical parameters remains a central challenge. Moving forward, integrating advanced design strategies, computational modeling, and deeper insights into structure–property correlations will be essential. With continued innovation in chemistry and materials engineering, chalcophosphates hold considerable promise for meeting the increasing demands of high-power IR laser technologies.
Journal Reference
Kong Y, et al. (2025). Chalcophosphates: A plentiful source of infrared nonlinear optical materials. Coordination Chemistry Reviews. DOI: 10.1016/j.scienta.2025.112345, https://www.sciencedirect.com/science/article/abs/pii/S0010854525000943