Recent advances in nonlinear optical rare earth structures

A Rare Earth Chalcogenide Nonlinear Optical Crystal KLaGeS4: Achieving Good Balance among Band Gap, Second Harmonic Generation Effect, and Birefringence

Midinfrared nonlinear optical (NLO) rare earth chalcogenides have attracted extensive research interest in recent several decades. Employing charge-transfer engineering strategy in the early stage, rigid tetrahedral [GeS4] was introduced into rare-earth sulfides to synthesize KYGeS4, which had an enlarged band gap while maintaining a strong second harmonic generation (SHG) effect. Based on KYGeS4, La was equivalently substituted to successfully synthesize KLaGeS4 with a stronger SHG effect (dij = 1.2 × AgGaS2) and lower cost. Meanwhile, a larger band gap (Eg = 3.34 eV) was retained and realized phase matching (Δn = 0.098 @ 1064 nm). KLaGeS4 enabled an effective balance among band gap, SHG effect, and birefringence, making it a promising candidate for infrared NLO optical materials among various rare-earth sulfides.

Rare-earth-based chalcogenides and their derivatives: an encouraging IR nonlinear optical material candidate

With the continuous development of laser technology and the increasing demand for lasers of different frequencies in the infrared (IR) spectrum, research on infrared nonlinear optical (NLO) crystals has garnered growing attention. Currently, the three main commercially available types of borate materials each have their drawbacks, which limit their applications in various areas. Rare-earth (RE)-based chalcogenide compounds, characterized by the unique f-electron configuration, strong positive charges, and high coordination numbers of RE cations, often exhibit distinctive optical responses. In the field of IR-NLO crystals, they have a research history spanning several decades, with increasing interest. However, there is currently no comprehensive review summarizing and analyzing these promising compounds. In this review, we categorize 85 representative examples out of more than 400 non-centrosymmetric (NCS) compounds into four classes based on the connection of different asymmetric building motifs: (1) RE-based chalcogenides containing tetrahedral motifs; (2) RE-based chalcogenides containing lone-pair-electron motifs; (3) RE-based chalcogenides containing [BS3] and [P2Q6] motifs; and (4) RE-based chalcohalides and oxychalcogenides. We provide detailed discussions on their synthesis methods, structures, optical properties, and structure-performance relationships. Finally, we present several favorable suggestions to further explore RE-based chalcogenide compounds. These suggestions aim to approach these compounds from a new perspective in the field of structural chemistry and potentially uncover hidden treasures within the extensive accumulation of previous research.

Rare Earths-The Answer to Everything

Rare earths, scandium, yttrium, and the fifteen lanthanoids from lanthanum to lutetium, are classified as critical metals because of their ubiquity in daily life. They are present in magnets in cars, especially electric cars; green electricity generating systems and computers; in steel manufacturing; in glass and light emission materials especially for safety lighting and lasers; in exhaust emission catalysts and supports; catalysts in artificial rubber production; in agriculture and animal husbandry; in health and especially cancer diagnosis and treatment; and in a variety of materials and electronic products essential to modern living. They have the potential to replace toxic chromates for corrosion inhibition, in magnetic refrigeration, a variety of new materials, and their role in agriculture may expand. This review examines their role in sustainability, the environment, recycling, corrosion inhibition, crop production, animal feedstocks, catalysis, health, and materials, as well as considering future uses.

Co-substitution design: a new glaserite-type rare-earth phosphate K2RbSc(PO4)2 with high structural tolerance

An alkali rare-earth phosphate K2RbSc(PO4)2 was successfully obtained as a derivative of glaserite-type K3Na(SO4)2 by co-substitution of K(1)O12 → RbO12, K(2)O10 → KO7, NaO6 → ScO6 and SO4 → PO4, while maintaining the original anionic framework. K2RbSc(PO4)2 exhibits a layered [Sc(PO4)2]∞ framework built from ScO6 octahedra and PO4 tetrahedra, with K and Rb residing in the interlayers. Its isostructural lanthanide analogues K2RbEr(PO4)2 and K2RbLu(PO4)2, inspired by an elemental substitution strategy, were also prepared by a high-temperature solid state reaction. The successful substitution indicates that the skeleton of K2RbSc(PO4)2 is stable with high structural tolerance, which can provide a possibility for substitution of resident ions to obtain diverse structural types and applications.

Exploring Short-Wavelength Phase-Matching Nonlinear Optical Crystals by Employing KBe2BO3F2 as the Template

Exploration of nonlinear optical (NLO) crystals that are competent in generating short-wavelength ultraviolet (UV, λ ≤ 266 nm, and even deep-UV, λ ≤ 200 nm) coherent light output by direct second harmonic generation (SHG) remains a formidable challenge. Herein, four UV/deep-UV NLO crystals, M₂B₄SO₁₀ (M = K, Rb, and Cs) and Rb₃B₁₁PO₁₉F₃, were successfully synthesized by evolving the KBe₂BO₃F₂ (KBBF) structure into mixed-anionic borosulfate and fluoroborophosphate systems. They display functional [B₄SO₁₀]_∞ or [B₁₁PO₁₉F₃]_∞ KBBF-type layers that are composed of [BO₃], [BO₄], and [SO₄] groups or [BO₃], [BO₄], [BO₃F], and [PO₄] groups, respectively. Experimental characterization and numerical computation results indicate that these crystals possess exceptional NLO performance, including large SHG responses (0.9-1.7 × KDP at 1064 nm and 0.1-0.3 × β-BBO at 532 nm) and adequate birefringence to fulfill the SHG phase-matching (PM) condition at 266 nm. In particular, the shortest type-I PM wavelength (λ_PM) of Rb₃B₁₁PO₁₉F₃ reaches 180 nm, which implies that Rb₃B₁₁PO₁₉F₃ can become a prospective deep-UV NLO crystal. In addition, single crystals of K₂B₄SO₁₀, Rb₂B₄SO₁₀, and Cs₂B₄SO₁₀ are easily obtained by the high-temperature solution approach. This work will facilitate the discovery of short-wavelength PM NLO crystals by using the KBBF structure as the template.

Eu2 P2 S6 : The First Rare-Earth Chalcogenophosphate Exhibiting Large Second-Harmonic Generation Response and High Laser-Induced Damage Threshold

Metal chalcogenophosphates are receiving increasing interest, specifically as promising infrared nonlinear optical (NLO) candidates. Here, a rare-earth chalcogenophosphate Eu₂ P₂ S₆ crystallizing in the monoclinic noncentrosymmetric space group Pn was synthesized using a high-temperature solid-state method. Its structure features isolated [P₂ S₆ ]^4- dimer, and two types of EuS₈ bicapped triangular prisms. Eu₂ P₂ S₆ exhibits a phase-matchable second-harmonic generation (SHG) response ≈0.9×AgGaS₂ @2.1 μm, and high laser-induced damage threshold of 3.4×AgGaS₂ , representing the first rare-earth NLO chalcogenophosphate. The theoretical calculation result suggests that the SHG response is ascribed to the synergetic contribution of [P₂ S₆ ]^4- dimers and EuS₈ bicapped triangular prisms. This work provides not only a promising high-performance infrared NLO material, but also opens the avenue for exploring rare-earth chalcogenophosphates as potential IR NLO materials.

AgGaGeSe4: An Infrared Nonlinear Quaternary Selenide with Good Performance

The symmetry of crystals is an extremely important property of crystals. Crystals can be divided into centrosymmetric and non-centrosymmetric crystals. In this paper, an infrared (IR) nonlinear optical (NLO) material AgGaGeSe4 was synthesized. The related performance analysis, nonlinear optical properties, and first-principle calculation of AgGaGeSe4 were also introduced in detail. In the AgGaGeSe4 structure, Ge4+ was replaced with Ga3+ and produced the same number of vacancies at the Ag+ position. The low content of Ge doping kept the original chalcopyrite structure and improved its optical properties such as the band gap. The UV-Vis diffuse reflection spectrum shows that the experimental energy band gap of AgGaGeSe4 is 2.27 eV, which is 0.48 eV larger than that of AgGaSe2 (1.79 eV). From the perspective of charge-transfer engineering strategy, the introduction of Group IV Ge elements into the crystal structure of AgGaSe2 effectively improves its band gap. The second harmonic generation (SHG) effect of AgGaGeSe4 is similar to that of AgGaSe2, and at 1064 nm wavelength, the birefringence of AgGaGeSe4 is 0.03, which is greater than that of AgGaSe2 (∆n = 0.02). The results show that AgGaGeSe4 possessed better optical properties than AgGaSe2, and can been broadly applied as a good infrared NLO material.

Centrosymmetric Rb[Te2O4(OH)5] and noncentrosymmetric K2[Te3O8(OH)4]: metal tellurates with corner and edge-sharing (Te4O18)12- anion group

New nonlinear optical (NLO) crystals have become urgent need for extending laser wavelengths. By cation substitution between alkali metal (K+, Rb+), anion group arrangement transition were realized, and centrosymmetric Rb[Te2O4(OH)5]...

Applying band gap engineering to tune the linear optical and nonlinear optical properties of noncentrosymmetric chalcogenides La4Ge3SexS12−x (x = 0, 2, 4, 6, 8, 10)

Band gap engineering was applied to tune the properties of a phase-matchable La4Ge3S12 by replacing S with Se, which uncovered five new compounds: La4Ge3SexS12−x (x = 2, 4, 6, 8, 10).