Unexpected giant negative area compressibility in palladium diselenide.

Negative area compressibility (NAC) is a counterintuitive 'squeeze-expand' behavior in solids that is very rare but attractive due to possible pressure-response applications and coupling with rich physicochemical properties. Herein, NAC behavior is reported in palladium diselenide with a large magnitude and wide pressure range. We discover that, apart from the rigid flattening of layers that has been generally recognized, the unexpected giant NAC effect in PdSe2 largely comes from anomalous elongation of intralayer chemical bonds. Both structural variations are driven by intralayer-to-interlayer charge transfer with enhanced interlayer interactions under pressure. Our work updates the mechanical understanding of this anomaly and establishes a new guideline to explore novel compression-induced properties.

Sr14.06Gd14.63(BO3)24: A Gadolinium-Rich Borate with Magnetic Refrigeration Performance.

A single crystal of Sr14.06Gd14.63(BO3)24 has been successfully grown through a high-temperature solution technique with K2O-KF-B2O3 as the flux. It crystallizes in the Pnma space group with parameters a = 22.3153(5) Å, b = 15.9087(4) Å, c = 8.7507(2) Å, and Z = 2. Sr14.06Gd14.63(BO3)24 has a three-dimensional (3D) framework built from [GdO] chains, in which the isolate [BO3]3- groups and Sr2+ ions fill in the space of the 3D framework. The magnetic measurements revealed that the title compound exhibits a large magnetocaloric effect with the magnetic entropy change of -ΔSm = 42.2 J kg-1 K-1 at 2 K for 7 T, which is higher than that of the commercial material, Gd3Ga5O12 (GGG), with -ΔSm of 38.4 J kg-1 K-1 under the same conditions. Moreover, the infrared spectrum (IR), UV-vis-NIR diffuse reflectance spectrum, and thermal stability were investigated.

sp2 to sp3 Hybridization Transformation in Ionic Crystals under Unprecedentedly Low Pressure.

Under cold pressure sp1 /sp2 -to-sp3 hybridization transformation has been exclusively observed in covalent or molecular crystals overwhelmingly above ≈10 GPa, and the approaches to lower the transition pressure are limited on external heat-treatment and/or catalyzers. Herein we demonstrate that, by internal-lattice stress-transfer from ionic to covalent groups, the transformation can be significantly prompted, as shown in a crystal of LiBO2 under 2.85 GPa for the first case in ionic crystals. This unprecedentedly low transformation pressure is ascribed to the enhanced localized stress on covalent B-O frames transferred from ionic Li-O bonds in LiBO2 , and accordingly the corresponding structural feature is summarized. This work provides an internal structural regulation strategy for pressure-reduction of the s-p orbital hybridization transformation and extends the sp1 /sp2 -to-sp3 transformation landscape from molecular and covalent compounds to ionic systems.

Integration of negative, zero and positive linear thermal expansion makes borate optical crystals light transmission temperature-independent.

Negative and zero thermal expansion (NTE and ZTE) materials are widely adopted to eliminate the harmful effect from the "heat expansion and cool contraction" effect and frequently embrace novel fundamental physicochemical mechanisms. To date, the manipulation of NTE and ZTE materials has mainly been realized by chemical component regulation. Here, we propose another method by making use of the anisotropy of thermal expansion in noncubic single crystals, with maximal tunability from the integration of linear NTE, ZTE and positive thermal expansion (PTE). We demonstrate this concept in borate optical crystals of AEB2O4 (AE = Ca or Sr) to make the light transmission temperature-independent by counterbalancing the thermal expansion and thermo-optics coefficient. We further reveal that such a unique thermal expansion behavior in AEB2O4 arises from the synergetic thermal excitation of bond stretching in ionic [AEO8] and rotation between covalent [BO3] groups. This work has significant implications for understanding the thermal excitation of lattice vibrations in crystals and promoting the functionalization of anomalous thermal expansion materials.

The synthesis and structure-property relation analysis of metal chalcohalide crystals Cs2InPS4X2 (X = Cl, Br) with mixed anions.

Inorganic metal chalcohalides are significant semiconductive materials for photovoltaics, photodetetion and infrared optics. Thus it is considerably rewarding to develop a new synthetic strategy to provide more degrees of freedom for atomic coordination to tune the optical and electronic properties of metal chalcohalides. In this work, the mixed-anion strategy is performed to synthesize two new metal chalcohalides Cs2InPS4X2 (X = Cl, Br) with mixed-anion structure by the reaction of InPS4 and CsX. Single-crystal X-ray diffraction analysis shows that they are isostructural and crystallize in the centrosymmetric space group P21/n, consisting of zero-dimensional structure [In2P2S8X4]4- (X = Cl, Br) built from tetrahedral [PS4]3- and octahedral [InS4X2]7- (X = Cl, Br) through edge-sharing, with Cs cations filling in intervening voids. The UV-vis-NIR diffuse reflectance spectroscopy measurement reveals that Cs2InPS4Cl2 and Cs2InPS4Br2 exhibit large optical bandgaps of 3.21 eV and 3.12 eV, respectively. The electronic structure calculations show that the bandgap mainly originates from the [InS4X2]7- (X = Cl, Br) mixed-anion groups. First-principles calculations indicate that the birefringence of Cs2InPS4Cl2 and Cs2InPS4Br2 is ∼0.08 and ∼0.05 at 2090 nm, respectively. Furthermore, thermal analysis reveals that the Cs2InPS4X2 (X = Cl, Br) are thermostable up to 400 °C. This discovery enriches the structural diversity of inorganic chalcohalides and provides an insight for the exploration of new semiconductive materials.

Breaking through the "3.0 eV wall" of energy band gap in mid-infrared nonlinear optical rare earth chalcogenides by charge-transfer engineering.

Increasing the energy band gap under the premise to maintain a large nonlinear optical (NLO) response is a challenging issue for the exploration and molecular design of mid-infrared nonlinear optical crystals. Utilizing a charge-transfer engineering method, we designed and synthesized a rare earth chalcogenide, KYGeS4. With an NLO effect as large as that in AgGaS2, KYGeS4 breaks through the limitation of energy band gap, i.e., the "3.0 eV wall", in NLO rare earth chalcogenides, and thus exhibits an excellent comprehensive NLO performance. First-principles electronic structure analysis demonstrates that the large band gap in KYGeS4 is ascribed to the decreased covalency of Y-S bonds by transferring charge from [YS7] to [GeS4] polyhedra. The charge-transfer engineering strategy would have significant implications for the exploration of good-performance NLO crystals.

Cs2ZnSn3S8: A Sulfide Compound Realizes a Large Birefringence by Modulating the Dimensional Structure.

Birefringence, an important optical performance parameter for optoelectronic functional materials, is mainly influenced by the types of anion groups and their spatial arrangement. Inspired by the relationship between the structure and properties of chalcogenides, combined with the dimensional transformation, we successfully synthesized a sulfide compound (Cs2ZnSn3S8) with a two-dimensional layered structure and a large birefringence. The experimental results showed that, compared with Rb10Zn4Sn4S17, Cs2ZnSn3S8 achieved the structural transition from a zero-dimensional arrangement to a two-dimensional lamellar arrangement and achieved a breakthrough of birefringence from 0 to 0.12, which was determined by both experiments and first-principles calculations. These findings demonstrated that Cs2ZnSn3S8 was a potential birefringent material and provided instructions for the study of the synthesis of birefringent materials.

Na3Bi(IO3)6: An Alkali-Metal Bismuth Iodate with Intriguing One-Dimensional [BiI6O18] Chains and Pressure-Induced Structural Transition.

An alkali-metal bismuth iodate, Na3Bi(IO3)6, was successfully obtained by the hydrothermal method for the first time and contains intriguing one-dimensional [BiI6O18] chains. High-pressure Raman spectra were carried out to investigate the structural transition of Na3Bi(IO3)6. Electronic states and anisotropic optical responses were also investigated by theoretical calculations.

EuHgGeSe4 and EuHgSnS4: Two Quaternary Eu-Based Infrared Nonlinear Optical Materials with Strong Second-Harmonic-Generation Responses.

Metal chalcogenides play a critical role in the infrared (IR) nonlinear optical (NLO) field. However, Eu-based chalcogenide-type IR NLO materials are still scarce up to now. In this paper, two new quaternary Eu-based chalcogenides, EuHgGeSe4 and EuHgSnS4, containing the "NLO active groups" [HgQ4]6- (Q = S, Se) and [GeSe4]4-/[SnS4]4- were synthesized through traditional high-temperature solid-state reactions. They possess noncentrosymmetric structures, crystallizing in the Ama2 space group, and exhibit strong phase-matchable second-harmonic-generation (SHG) responses (3.1× and 1.77× that of AgGaS2 for EuHgGeSe4 and EuHgSnS4, respectively). Meanwhile, the optical band gaps of EuHgGeSe4 (1.97 eV) and EuHgSnS4 (2.14 eV) were determined from UV-vis-NIR diffuse reflectance spectra. Differential scanning calorimetry (DSC) analyses reveal the congruent-melting behavior of EuHgGeSe4. Furthermore, structural analysis and theoretical calculations verify the critical driving effects of [HgQ4]6- tetrahedra on the strong SHG activity. The overall results demonstrate that EuHgGeSe4 and EuHgSnS4 are potential IR NLO materials.

Anomalous mechanical materials squeezing three-dimensional volume compressibility into one dimension.

Anomalous mechanical materials, with counterintuitive stress-strain responding behaviors, have emerged as novel type of functional materials with highly enhanced performances. Here we demonstrate that the materials with coexisting negative, zero and positive linear compressibilities can squeeze three-dimensional volume compressibility into one dimension, and provide a general and effective way to precisely stabilize the transmission processes under high pressure. We propose a "corrugated-graphite-like" structural model and discover lithium metaborate (LiBO2) to be the first material with such a mechanical behavior. The capability to keep the flux density stability under pressure in LiBO2 is at least two orders higher than that in conventional materials. Our study opens a way to the design and search of ultrastable transmission materials under extreme conditions.

Two Mixed-Anion Units of [GeOSe3] and [GeO3S] Originating from Partial Isovalent Anion Substitution and Inducing Moderate Second Harmonic Generation Response and Large Birefringence.

Highly polarizable mixed-anion structural building units (SBUs) have been demonstrated as promising candidates for high-performing optical crystals. In this work, two new mixed-anion SBUs of [GeOSe3] and [GeO3S] are first designed through partial isovalent substitution of chalcogen atoms by O atoms in the classical [GeQ4] (Q = S, Se) tetrahedra. On the basis of these SBUs, two new quaternary oxychalcogenides, Sr3Ge2O4Se3 and SrGe2O3S2, are successfully synthesized. Sr3Ge2O4Se3 crystallizes in the noncentrosymmetric space group R3m and possesses unique zero-dimensional [Ge2O4Se3]6- units consisting of highly distorted [GeOSe3] tetrahedra and [GeO4] tetrahedra through a shared O atom. It displays intriguing potential as an infrared nonlinear optical material with a wide band gap (2.96 eV) and moderate second harmonic generation intensity (0.8 × AgGaS2). SrGe2O3S2 belongs to the centrosymmetric space group P21/c and features 2∞[Ge2O3S2]2- layers formed by the corner-shared [GeO3S] tetrahedra. Moreover, the large birefringence of SrGe2O3S2 (calculated Δn = 0.22-0.17 from 0.4 to 4.0 µm) gives it a potential as a birefringent material. Theoretical calculations revealed the crucial effects of mixed-anion [GeOSe3] and [GeO3S] units on the moderate second harmonic generation response and large birefringence. The discovery of new mixed-anion SBUs of [GeOSe3] and [GeO3S] will guide the exploration of new functional oxychalcogenides.

New quaternary chalcogenide Ba4HgAs2S10 originating from the combination of linear [HgS2]2- and tetrahedral [AsS4]3- modules.

A new quaternary chalcogenide Ba4HgAs2S10 has been successfully synthesized with the aid of a KI flux. The compound crystallizes in the space group C2/c (no. 9) of the monoclinic system [a = 22.7787(6) Å, b = 6.4712(2) Å, c = 25.0606(7) Å, ß = 90.101(2)° and Z = 8]. It is the first example of tetrahedral [AsS4]3- and linear [HgS2]2- units coexisting in a single compound. The [AsS4]3- tetrahedra and [HgS2]2- units are totally separated by Ba2+. The UV-visible diffuse reflectance spectrum reveals a large bandgap of 2.98 eV for Ba4HgAs2S10 and DSC measurement demonstrates the incongruent melting nature of the compound. Moreover, based on first-principles calculations, Ba4HgAs2S10 is a direct bandgap semiconductor with the optical property related to the electron transition from the S-3p orbital to As-4p and Ba-5d orbitals.

Intrinsic Isotropic Near-Zero Thermal Expansion in Zn4B6O12X (X = O, S, Se).

Zero thermal expansion (ZTE) materials, keeping size constant as temperature varies, are valuable for resisting the deterioration of the performance from environmental temperature fluctuation, but they are rarely discovered due to the counterintuitive temperature-size effect. Herein, we demonstrate that a family of borates with sodalite cage structure, Zn4B6O12X (X = O, S, Se), exhibits intrinsic isotropic near-ZTE behaviors from 5 to 300 K. The very low thermal expansion is mainly owing to the coupling rotation of [BO4] rigid groups constrained by the bonds between Zn and cage-edged O atoms, while the central atoms in the cage have a negligible contribution. Our study has significant implications on the understanding of the ZTE mechanism and exploration of new ZTE materials.

Gadolinium-Rich Borate Gd17.33(BO3)4(B2O5)2O16 Exhibiting a Magnetocaloric Effect.

A gadolinium-rich borate Gd17.33(BO3)4(B2O5)2O16 was successfully grown by the high-temperature solution method using the Rb2O-B2O3 flux. The crystal crystallizes in centrosymmetric space group C2/m with lattice constants a = 18.4300(2) Å, b = 3.7400(4) Å, c = 14.2047(16) Å, and ß = 119.8550(12)°. Two different honeycomb-like [GdO] layers alternately arrange in the order ABAB forming the three-dimensional framework, in which the isolated [B2O5] and [BO3] units fill in channels of the 12-membered and 10-membered [GdO] polyhedral rings, respectively. The UV cutoff edge of Gd17.33(BO3)4(B2O5)2O16 is less than 240 nm. The maximum -ΔSm,max is 26.53 J kg-1 K-1 or 174.70 mJ cm-3 K-1 (T = 4.4 K and ΔH = 7 T) as investigated by the isothermal magnetization method.

Two rare-earth-based quaternary chalcogenides EuCdGeQ4 (Q = S, Se) with strong second-harmonic generation.

Two new rare-earth-based chalcogenides EuCdGeQ4 (Q = S, Se) have been designed and constructed by using Eu2+ and the classical NLO-active SBUs of [CdQ4] and [GeQ4]. They crystallize in a non-centrosymmetric Ama2 (no. 40) space group. Benefiting from the synergistic effects of [GeQ4] and highly distorted [CdQ4] tetrahedra, both compounds possess type-I phase-matching behaviour and large powder second harmonic generation (SHG) effects at 2.09 µm (2.6 and 3.8 × AgGaS2 for sulfide and selenide), as well as large direct band gaps (2.5 eV and 2.25 eV). Besides, they melt congruently at relatively low temperatures (997 °C for EuCdGeS4 and 882 °C for EuCdGSe4), which is suitable for bulk crystal growth by the Bridgman method. In addition, their electronic structures and some optical coefficients are calculated by first-principles.

Co-crystal LiCl·(H3C3N3O3): a promising solar-blind nonlinear optical crystal with giant nonlinearity from coplanar π-conjugated groups.

Compact and reliable all-solid-state solar-blind lasers based on nonlinear optical conversion are opening the door to new industrial applications of lasers. Herein, we proposed the cyanuric acid molecule, benefiting from its wide band gap, considerable optical anisotropy, and strong binding interaction resulting from hydrogen bonds, to be a superior building block for solar-blind NLO crystals. Taking the co-crystal LiCl·(H3C3N3O3) as an example, our first-principles studies predicted this crystal to display a short UV transparent edge of 215 nm, strong optical anisotropy (Δn ∼ 0.28), and a second-order coefficient (d22 = 4.15 pm V-1) greater than those of all other investigated solar-blind NLO crystals.

A new ultraviolet transparent hydra-cyanurate K2(C3N3O3H) with strong optical anisotropy from delocalized π-bonds.

A new metal hydra-cyanurate, K2(C3N3O3H), was synthesized by a slow evaporation method in aqueous solution. It possesses a short ultraviolet cutoff wavelength at 240 nm. First-principles calculations reveal that its birefringence (∼0.35@800nm) is two times larger than that of commercial calcite crystals. Therefore, K2(C3N3O3H) and metal cyanurates, are very promising ultraviolet birefringent materials in future optical devices.

Growth, Crystal Structures, and Characteristics of Li5ASrMB12O24 (A = Zn, Mg; M = Al, Ga) with [MB12O24] Frameworks.

Two aluminoborates (Li5ZnSrAlB12O24 and Li5MgSrAlB12O24) and one galloborate (Li5ZnSrGaB12O24) were successfully obtained for the first time, and the centimeter-sized crystals were grown by the Kyropoulos method. These crystals possess the [MB12O24] (M = Al, Ga) basic frameworks in their crystal structures, and the [GaB12O24] framework is a new framework in galloborates. The synthesis, single crystal growth, characterizations, and theoretical calculation are reported for these compounds.

The Anisotropic Thermal Expansion of Non-linear Optical Crystal BaAlBO3F2 Below Room Temperature.

Thermal expansion is a crucial factor for the performance of laser devices, since the induced thermal stress by laser irradiation would strongly affect the optical beam quality. For BaAlBO3F2 (BABF), a good non-linear optical (NLO) crystal, due to the highly anisotropic thermal expansion its practical applications are strongly affected by the "tearing" stress with the presence of local overheating area around the laser spot. Recently, the strategy to place the optical crystals in low-temperature environment to alleviate the influence of the thermal effect has been proposed. In order to understand the prospect of BABF for this application, in this work, we investigated its thermal expansion behavior below room temperature. The variable-temperature XRD showed that the ratio of thermal expansion coefficient between along c- and along a(b)- axis is high as 4.5:1 in BABF. The Raman spectrum combined with first-principles phonon analysis revealed that this high thermal expansion anisotropy mainly ascribe to progressive stimulation of the respective vibration phonon modes related with the thermal expansion along a(b)- and c-axis. The good NLO performance in BABF can be kept below room temperature. The work presented in this paper provides an in-depth sight into the thermal expansion behavior in BABF, which, we believe, would has significant implication to the manipulation in atomic scale on the thermal expansion of the materials adopted in strong-field optical facility.