Structure-Prediction-Oriented Synthesis of Thiophosphates as Promising Infrared Nonlinear Optical Materials.

Oriented synthesis of functional materials is a focus of attention in material science. As one of the most important function materials, infrared nonlinear optical materials with large second harmonic generation effects and broad optical band gap are in urgent need. In this work, directed by the theoretical structure prediction, the first series of non-centrosymmetric (NCS) alkali-alkaline earth metal [PS4]-based thiophosphates LiCaPS4 (Ama2), NaCaPS4 (P21), KCaPS4 (Pna21), RbCaPS4 (Pna21), CsCaPS4 (Pna21) were successfully synthesized. Comprehensive characterizations reveal that ACaPS4 could be regarded as promising IR NLO materials, exhibiting wide band gap (3.77-3.86 eV), moderate birefringence (0.027-0.064 at 1064 nm), high laser-induced damage threshold (LIDT, ~10×AGS), and suitable phase-matching second harmonic generation responses (0.4-0.6×AGS). Structure-properties analyses illustrate that the Ca-S bonds show non-ignorable covalent feature, and [PS4] together with [CaSn] units play dominant roles to determine the band gap and SHG response. This work indicates that Li-, Na- and K- analogs may be promising infrared nonlinear optical material candidates, and this is the first successful case of "prediction to synthesis" involving infrared (IR) nonlinear optical (NLO) crystals in the thiophosphate system and may provide a new avenue to the design and oriented synthesis of high-performance function materials in the future.

Na6Mg3P4S16 and RbMg2PS4Cl2: two Mg-based thiophosphates with ultrawide bandgaps resulting from [MgS6] and [MgSxCl6-x] octahedra.

Designing wide-bandgap chalcogenides is one of the most important ways of obtaining high-performance infrared (IR) functional materials. In this work, two Mg-based metal thiophosphates, namely Na6Mg3P4S16 (NMPS) and RbMg2PS4Cl2 (RMPSC), were successfully obtained by introducing [MgS6] and [MgSxCl6-x] octahedra into thiophosphates. In addition, their crystal structures were determined, a first for Mg-containing [PS4]-based thiophosphates to the best of our knowledge. Their bandgaps were investigated in theoretical ways and verified by taking experimental measurements, and determined to be 3.80 eV for NMPS and 3.93 eV for RMPSC, values greater than those of the other investigated thiophosphate halides. The wide bandgaps of NMPS and RMPSC were attributed, based on theoretical calculations, to the [MgSxCl6-x] (x = 0-6) octahedron.

[RbSr3X][(BS3)2] (X = Cl, Br): two salt-inclusion thioborates with large birefringence and structure transformation from centrosymmetric to asymmetric.

[RbSr3X][(BS3)2] (X = Cl, Br), two salt-inclusion chalcogenides with planar [BS3] as anionic units, were obtained. Structure analysis indicates that the size effect of halogens may adjust the arrangement between the [BS3] units and further lead to the CS-to-NCS structure transformation. Experimental characterizations reveal that they have wide bandgaps (3.64-3.70 eV), large birefringence (0.136-0.144) and high LIDT (12-14 × AgGaS2). This work indicates that the thioborate family is a rich source to explore structure chemistry and promising infrared functional materials.

Improved Birefringence Activated by Tetrahedra Decorated with a Single Linear Unit.

Performance enhancement induced by structural modification has long been the target in materials science fields. Direct evidence to witness the effectivity of one strategy is challenging and necessary. In this work, a tetrahedra-decoration strategy was proposed to improve the birefringent performance sharply, namely decorating the tetrahedra with a single linear [S2 ] unit. The strategy was verified by comprehensive characterization of two thiogermanates K2 BaGeS4 and K2 BaGeS5 , which crystallize in the same space group, have similar unit cells and the same units arrangements. Theoretical characterization verified that the [GeS5 ] group has much larger polarization anisotropy than [GeS4 ], further demonstrated that the linear [S2 ] led to the sharp birefringence enlargement of K2 BaGeS5 (0.19 vs 0.03 of K2 BaGeS4 ). This work provides a new guiding thought to improve the birefringence performance.

From BPS4 to AB3P2S10 (A = Na, K): cations inducing dimension reduction and bandgap enlargement.

Regulating the crystal structure by cations is one of the most effective methods to adjust the performances of optical materials and enrich the structural chemistry of solid-state inorganic crystals. In this work, only boron-thiophosphate BPS4 (BPS) was used as the template. By introducing alkali metal ions, Na+ and K+, the synthesis, structural revolution, and bandgap adjustment were studied. Namely, the first quaternary boron-thiophosphates, AB3P2S10 (A = Na, K) (ABPS) were obtained, whose anion skeleton decreased to zero dimension from one dimension (1D) in BPS. In addition, the bandgap showed obvious improvement from 3.30 (BPS) to 3.42 and 3.49 eV (ABPS). Structural studies and theoretical analyses indicated that the insert of cations separates the infinite chains to [B3P2S10] clusters, localizes the electrons around the S2- in [BS4] and [PS4] groups, and widens the bandgaps. This work could enrich the structural chemistry of boron-thiophosphates and reveal that ionic bonds can modulate the covalent skeleton and show the effect on the optical properties.

Chain-like [Sx ] (x=2-6) Units Realizing Giant Birefringence with Transparency in the Near-Infrared for Optoelectronic Materials.

Birefringent crystals are requisite optical devices in laser and modern opto-electronic fields. Development of excellent birefringent materials is still challenging. Herein, the linear or chain-like [Sx ] (x=2-6) species were theoretically proved to be the origin of the large birefringence, and could be regarded as birefringent genes. Besides, the metal polysulfide family was first proposed to be rich birefringent materials source, among which Cs2 S6 realizes giant birefringence 0.58@1064 nm together with a wide band gap of 1.70 eV (based on the generalized gradient approximation). Moreover, the first dual-anion group polysulfide Na4 Ba3 (S2 )4 S3 was obtained, showing wide infrared transmission range (0.5-6.2 µm), wide band gap (2.3 eV), and large birefringence (0.37 at 1064 nm). This work provides a new guiding thought for exploring large birefringence crystals in the future.

Unbiased Screening of Novel Infrared Nonlinear Optical Materials with High Thermal Conductivity: Long-neglected Nitrides and Popular Chalcogenides.

Traditional infrared (IR) nonlinear optical (NLO) materials such as AgGaS2 are crucial to key devices for solid-state lasers, however, low laser damage thresholds intrinsically hinder their practical application. Here, a robust strategy is proposed for unbiased high-throughput screening of more than 140 000 materials to explore novel IR NLO materials with high thermal conductivity and wide band gap which are crucial to intrinsic laser damage threshold. Via our strategy, 106 compounds with desired band gaps, NLO coefficients and thermal conductivity are screened out, including 8 nitrides, 68 chalcogenides, in which Sr2 SnS4 is synthesized to verify the reliability of our process. Remarkably, thermal conductivity of nitrides is much higher than that of chalcogenides, e.g., 5×AgGaS2 (5.13 W/m K) for ZrZnN2 , indicating that nitrides could be a long-neglected system for IR NLO materials. This strategy provides a powerful tool for searching NLO compounds with high thermal conductivity.

Second-Harmonic Generation-Positive Na2Ga2SiS6 with a Broad Band Gap and a High Laser Damage Threshold.

The development of high-power solid-state lasers is in urgent need of new infrared nonlinear optical (IR NLO) materials with a wide band gap and a high laser-induced damage threshold. A new infrared nonlinear optical material Na2Ga2SiS6 has been synthesized for the first time, crystallizing in the Fdd2 (no. 43) noncentrosymmetric space group. Its three-dimensional tunnel framework consists of two typical NLO active motifs [GaS4] and [SiS4], with Na+ cations located inside the tunnels. Na2Ga2SiS6 exhibits comprehensive optical properties, namely, a wide transmission range, a high laser-induced damage threshold (10 × AgGaS2), a type-I phase-matching second-harmonic generation response (0.2 × AgGaS2), and especially a wide band gap (3.93 eV), which is the largest in the A2MIII2MIVQ6 (A = alkali metals; MIII = IIIA elements; MIV = IVA elements; Q = S and Se) family. Therefore, Na2Ga2SiS6 does not produce two-photon absorption under a 1064 nm laser pump and could be used in high-energy laser systems, which makes Na2Ga2SiS6 a promising candidate for high-energy IR NLO applications.

A review of the ABIICIVD family as infrared nonlinear optical materials: the effect of each site on the structure and optical properties.

Inorganic infrared (IR) second-order nonlinear optical (NLO) crystals have become increasingly important for fulfilling the demands of modern laser technology through frequency conversion via optical parametric oscillation (OPO) and optical parameter amplification (OPA) technology. The AI2BIICIVDVI4 family contains a large number of compounds (to our best knowledge, 102 formula) and shows abundant structural diversity; it could be regarded as a potential source of IR second-order NLO materials with tunable structures and properties. This article summarizes the authors' contributions to the AI2BIICIVDVI4 family, together with other reported related results where the NLO properties have been provided (a total of 38 compounds). In the title family: (1) the AI site can be occupied by the univalent coinage metals Cu and Ag or the alkali metals Li and Na; (2) the BII site involves divalent metals, including the alkaline earth metals (AEM) Sr and Ba or the d10 metals Zn, Cd, and Hg; (3) the CIV site is prevalently IVA group semi-conducting Si and Ge, or Sn; and (4) the DVI site is S or Se. The structures and optical properties of these compounds are summarized and the influence of substitution at each site on the structures and properties is systematically analyzed.

Li2CdSiS4, a promising IR NLO material with a balanced Eg and SHG response originating from the effect of Cd with d10 configuration.

Infrared (IR) nonlinear optical (NLO) materials are of great importance in laser technology, which could be applied in many civil and military fields. However, the low laser damage threshold and two-photon absorption, caused by the small bandgap, have hindered the applications of commercial IR NLO materials. In this work, Li2CdSiS4, with a wide bandgap (3.76 eV) and a moderate second-harmonic generation (SHG) response (1 × AgGaS2) was successfully designed and fabricated by a high temperature flux method in vacuum and sealed quartz tubes. The compound with a wurtz-stannite structure type was constructed with hexagonal close packing tetrahedra and it meets the requirements of optimal IR NLO material properties. Theoretical results show that Li2CdSiS4 has an indirect bandgap, and the bandgap is determined by the S 3p and Si 3p orbitals, not by the massive Cd element, which enables Li2CdSiS4 to have a wider bandgap than those of other Cd-obtained chalcogenides. In addition, the SHG response originates mainly from the S2- and Cd2+ of CdS4 tetrahedra while the contributions of Li and Si in this compound are ignorable. All the above information indicates that Li2CdSiS4 is a promising NLO material in the IR field and Cd is a good choice for designing new IR NLO materials.

Ba4(BS3S)2S4: a new thioborate with unprecedented [BS3-S] and [S4] fundamental building blocks.

Ba4(BS3S)2S4 with unprecedented [BS3-S] and [S4] units has been synthesized successfully. The [BS3-S] unit named as (disulfido)bis(sulfido)borate(3-) is composed of a planar [BS3] group with a non-polar covalent S-S bond and stabilized by a large π-conjugation structure. The [S4] unit, formed by four S atoms, shows a helix chain-like structure. This work will enrich the structural diversity of boron chemistry.

[Ge2 S5 (S2 )]4- , A NLO-Active Unit Leading to an Asymmetric Structure Discovered in Li2 Cs4 Ge2 S5 (S2 )Cl2 : An Experimental and Theoretical study.

Metal-centered MS4 tetrahedra are fundamentally important building blocks for the structural construction of infrared (IR) nonlinear optical (NLO) mateirals, because they have a large effect on properties like second-harmonic generation, birefringence, band gap, etc. Therefore, rational design and selection of these tetrahedra could effectively help synthesize new materials and give satisfactory properties. In this work, we reported a new NLO-active [Ge2 S5 (S2 )]4- unit composed of two corner-shared GeS4 tetrahedra with a nonpolar covalent S-S bond, discovered in a new compound of Li2 Cs4 Ge2 S5 (S2 )Cl2 . To the best of our knowledge, this [Ge2 S5 (S2 )]4- unit has never been discovered. [Ge2 S5 (S2 )]4- units have intrinsic asymmetric features and lead to the whole structure being noncentrosymmetric (NCS). Further, real-space atom-cutting methods reveal that they make almost all the contribution to the second-harmonic generation response (SHG) for Li2 Cs4 Ge2 S5 (S2 )Cl2 . All of the above indicate that the [Ge2 S5 (S2 )]4- unit is a new functional unit and a good choice for designing new IR NLO materials.

Li3Ge3Se6: the first ternary lithium germanium selenide with interesting ∞[Ge6Se12]n chains constructed by ethane-like [Ge2Se6]6- clusters.

Li3Ge3Se6, the first compound of the ternary Li/Ge/Se system, has been synthesized. Note that interesting 1D ∞[Ge6Se12]n chains constructed by ethane-like [Ge2Se6]6- clusters were discovered in its structure. Investigations on the structures of all the [Ge2Se6]6- cluster-containing compounds have shown that only in Li3Ge3Se6 are there 1D chains composed of [Ge2Se6]6- clusters, which result from the space limitation within the tunnels surrounded by LiSe6 octahedra. Raman spectrum was obtained to demonstrate the existence of Ge-Ge bonds. UV-visible-NIR diffuse reflection spectrum showed an optical bandgap of 2.08 eV. Theoretical calculations based on first principles have also been performed for its band structure and density of states to analyze its structure-property relationship.

Na2CdGe2Q6 (Q = S, Se): two metal-mixed chalcogenides with phase-matching abilities and large second-harmonic generation responses.

In view of their inherent defects for commercial infrared nonlinear optical (IR NLO) materials, exploration for new IR NLO materials with excellent performance is an imperative and meaningful work. Herein, we report the successful design and synthesis of two metal-mixed chalcogenides containing divalent cations with d10 electronic configuration: Na2CdGe2S6 and Na2CdGe2Se6. Both of them crystallize in the polar Cc space group and exhibit three-dimensional tunnel structures constructed by the CdQ4 tetrahedra and ∞[GeQ3]n chains with Na+ located in the tunnels. Furthermore, an interesting space group transformation between the monoclinic (Cc) and tetragonal (I4/mcm) systems from Na2CdGe2Se6 to Na2ZnGe2Se6 was discovered, which may arise from the different connection types of their building blocks (more flexible corner-sharing type in Cc while tightly edge-sharing type in I4/mcm). Remarkably, they exhibit type-I phase-matching abilities and large second harmonic generation (SHG) responses (0.8 and 2 times that of benchmark AgGaS2 at 2.09 µm fundamental light). Notably, Na2CdGe2S6 satisfies the essential requirements (coexistence of large NLO response and high laser damage threshold) as one excellent IR NLO material. The structure-property relationship has also been investigated through theoretical calculations and the results indicate that the origin of their NLO effects can be attributed to CdQ4 and GeQ4 units.

Na2ZnGe2S6: A New Infrared Nonlinear Optical Material with Good Balance between Large Second-Harmonic Generation Response and High Laser Damage Threshold.

The development of frequency-conversion technology in the infrared region is in urgent need of new excellent infrared nonlinear optical (IR NLO) materials. How to achieve a good balance between laser damage threshold (LDT) and NLO coefficient (dij) for new IR NLO candidates is still a challenge. The combination of the highly electropositive alkali metal (Na) and Zn with d(10) electronic configuration into crystal structure affords one new IR NLO material, Na2ZnGe2S6. It exhibits excellent properties including a wide transparent region (0.38-22 µm), large band gap (3.25 eV), and especially a balance between a strong NLO coefficient (30-fold that of KDP) and a high LDT (6-fold that of AgGaS2), indicating a promising application in the IR region. Moreover, novel common-vertex-linked wavelike ∞[GeS3]n chains are interestingly discovered in Na2ZnGe2S6, which rarely exist in the reported thiogermanides containing alkali metals. In addition, calculated SHG density and dipole moment demonstrate that the large NLO response is mainly attributed to the cooperative effects of the [GeS4] and [ZnS4] units.