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.

Cd3(IO3)(IO4)F2·0.1CdO: A Nonlinear-Optical Crystal with the Introduction of Fluoride into Iodate Containing Both [IO3]- and [IO4]3- Groups.

A new d10 transition-metal iodate fluoride, namely, Cd3(IO3)(IO4)F2·0.1CdO, was successfully designed and synthesized via the mid-infrared hydrothermal method. It crystallizes in the polar space group R3m and features the coexistence of the [IO3]- and [IO4]3- groups. Cd3(IO3)(IO4)F2·0.1CdO has a strong second-harmonic-generation response of about 3.0 times that of KDP(KH2PO4), large birefringence (0.133 at 546.1 nm), and a wide energy band gap (4.00 eV). In addition, the power laser damage threshold (LDT) measurement indicated that it possesses a high LDT of 84.29 MW/cm2, which is about 30 times that of AgGaS2. These superior properties showed that Cd3(IO3)(IO4)F2·0.1CdO may be an excellent nonlinear-optical crystal for visible and mid-infrared application.

Na4CdGe2S7: A Sodium-Rich Quaternary Wide-Band-Gap Chalcogenide with Two-Dimensional [Ge2CdS7]∞ Layers.

Metal chalcogenides are an attractive subject for investigation owing to their wide application in optoelectronics. Here, we report the discovery of a new cadmium-based quaternary sulfide, Na4CdGe2S7, with a wide band gap of 3.35 eV. The microscopic structure of this compound features two-dimensional [Ge2CdS7]∞ layers, which were first found in metal sulfides. Remarkably, the Na/S ratio in Na4CdGe2S7 exceeds 50%, suggesting that it would have ecofriendly electrical applications, such as in sodium-ion-battery electrodes and fast ion conductors.

Nonlinear optical ASnX (A = Na, H; X = N, P) nanosheets with divalent tin lone electron pair effect by first-principles design.

We propose a series of novel ASnX (A = Na, H; X = N, P) layered structures based on first-principles modeling and simulation. These nanosheet materials are all derived from the rational design for the parent structures of NaSnX (X = N, P) with unique divalent tin lone electron pairs. First-principles calculations show that the polar divalent tin effect can lead to strong second-order nonlinear optical (NLO) and large birefringence response in bulk materials, and can also greatly enhance the second harmonic generation (SHG) effect in the corresponding two-dimensional (2D) nanostructures by hydrogen passivation or strain engineering. The correlation between structural evolution and bandgap is illustrated through detailed analysis of the electronic structure and charge transfer model. The SHG effect and its origin can be further evaluated based on the SHG-weighted density scheme, which will facilitate the NLO research of 2D materials in the nitride and phosphide systems.

Functional Chalcogenide Na2HgSn2Se6 and K2MnGe2Se6 Exhibiting Flexible Chain Structure and Intriguing Birefringence Tunability.

The two functional chalcogenides K2MnGe2Se6 and Na2HgSn2Se6, featuring a straight-chain structure, have been successfully prepared and fully characterized. K2MnGe2Se6 shows paramagnetic behavior. The birefringence of Na2HgSn2Se6 is as large as 0.3107 and derives from the superposition of the polarizabilities of its fundamental building blocks, on the basis of first-principles calculations. Moreover, the flexible framework of the A2MIIMIV2Se6 family enables a variety of heterogeneous substitutions and thus offers possible birefringence tunability, which may inspire the design and exploratory synthesis of IR birefringent materials.

An unprecedented planar π-conjugated [B2P5]5- group with ultra-large birefringence and nonlinearity: an ab initio study.

Searching for new nonlinear optical active (NLO-active) units would greatly promote the development of NLO materials. Herein, we theoretically investigate a newly reported [B2P5]5- group which consisted of two corner-sharing [BP3]3- trigonal planar units. Attributed to the coplanar π-conjugated configuration and the unique connection mode, the [B2P5]5- group can achieve both the largest birefringence (Δn∼ 0.68) and nonlinearity (d15∼ 57.14 pm V-1) among all inorganic planar NLO-active groups, which makes it a good NLO-active unit. Our analysis also reveals that the modulation of the electronegativity of constituent elements upon replacing with the elements in the same period can effectively improve the NLO responses, which would inspire a new direction for designing high performing NLO materials.

Ba8SrPb24O24Cl18: the first alkali-earth metal lead(ii) oxyhalide with an intriguing multimember-ring layer.

The first alkali-earth metal lead(ii) oxyhalide Ba8SrPb24O24Cl18 characterized by fascinating multimember-ring layers has been discovered. Theoretical and experimental investigations illustrate that Ba8SrPb24O24Cl18 exhibits a moderate band gap of 3.09 eV, incongruent melting behavior and birefringence of 0.014@1064 nm. This discovery may offer new ideas for regulating the optical properties of oxyhalides and broadening their structural diversity.

SnGa2GeSe6, a benign addition to the AMMIVQ6 family: synthesis, crystal structure and nonlinear optical performance.

A new selenide, SnGa2GeSe6, in the AMIII2MIVQ6 family was synthesized for the first time by a high-temperature solid-state reaction. It crystallized in the non-centrosymmetric space group Fdd2 with cell dimensions of a = 47.195(9) Å, b = 7.5213(15) Å, c = 12.183(2) Å, and Z = 16. SnGa2GeSe6's crystal structure is characterized by a crisscross network of two types of infinite chains (i.e. the 1∞[GaSe3] chain and the 1∞[M3Se7] chain, where M represents the two metal sites randomly occupied by Ga and Ge atoms in a 1 : 1 ratio), which is similar to SnGa2GeS6 and diverges strongly from its Ba analogue owing to the substitution of Ba with Sn atoms that contain stereochemically active lone pair electrons. Careful experimental research has revealed that SnGa2GeSe6 exhibits an optical band gap of 1.98 eV and incongruent melting behavior. Furthermore, the second harmonic generation (SHG) intensity of the SnGa2GeSe6 powder sample is about 1.7 × AgGaS2 at a particle size of 150-200 µm with a 2 µm laser as the fundamental light.

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.

Na3Ca4(TeO3)(PO4)3: a new noncentrosymmetric tellurite phosphate with fascinating multimember-ring architectures and intriguing nonlinear optical performance.

A new functional compound Na3Ca4(TeO3)(PO4)3 has been successfully synthesized and fully characterized. It is the first example of a tellurite phosphate that exhibits a fascinating multimember-ring configuration. Moreover, a careful theoretical and experimental study also reveals the intriguing nonlinear optical performance of Na3Ca4(TeO3)(PO4)3.