Wenlong Yin

Ba5CdGa6Se15, a congruently-melting infrared nonlinear optical material with strong SHG response

The quaternary selenide Ba5CdGa6Se15 was prepared by reaction of BaSe and CdGa2Se4 at 1273 K and forms as light yellow crystals. It adopts a noncentrosymmetric structure (orthorhombic, space group Ama2, Z = 4, a = 24.2458(8) A, b = 19.1582(7) A, c = 6.6028(2) A) featuring a three-dimensional anionic framework built up of corner-sharing tetrahedra centred by Cd and Ga atoms, with the intervening voids filled by Ba cations. The Cd atoms are partially disordered with Ga atoms within two out of five possible sites; the site preference can be understood in terms of optimizing bond valence sums. Band structure calculations predict that Ba5CdGa6Se15 should be a semiconductor in which substitution of Cd for Ga atoms reduces the electron count in a hypothetical all-Ga compound “Ba5Ga7Se15” so that the Fermi level falls within a band gap. Ba5CdGa6Se15 is a potential new infrared nonlinear optical material with several attractive properties. First, the large band gap of 2.60(2) eV, as determined from the UV-vis-NIR optical absorption spectrum, implies a high laser damage threshold. Second, strong second harmonic generation signals were observed from nonlinear optical measurements on powder samples, with intensities comparable to that of the benchmark material AgGaS2; type-I phase-matching behaviour was confirmed using 2090 nm as the fundamental laser wavelength. Third, this compound melts congruently at a relative low temperature of 866 °C, which could facilitate growth of large crystals eventually required for applications.

Na2MnGe2Se6: a new Mn-based antiferromagnetic chalcogenide with large Mn⋯Mn separation

A new one-dimensional (1D) Mn-based chalcogenide Na2MnGe2Se6 has been obtained by a solid-state reaction. It crystallizes in the tetragonal space group I4/mcm with unit cell parameters a = 7.7718(16) A, b = 7.7718(16) A, c = 19.077(7) A and Z = 2. In the structure, MnSe4 and GeSe4 tetrahedra are linked alternately via edge-sharing with a ratio of 1 : 2 to form 1D infinite [MnGe2Se6]2− anionic chains, which are further separated by Na+ cations. Such a spatial arrangement leads to large separations between magnetic ions, namely, 9.538(5) A for the nearest intrachain Mn⋯Mn distance and 5.495(5) A for the neighboring interchain Mn⋯Mn distance. Despite the large Mn⋯Mn separation, the temperature dependent susceptibility measurement and specific heat measurement still indicate an interesting antiferromagnetic interaction with the Neel temperature TN = 11 K for this compound. Such unusual magnetic properties have been seldom reported in other Mn chalcogenides. The magnetic interaction is investigated by the spin-polarized calculations. Besides, the UV-vis-NIR spectroscopy measurement indicates that Na2MnGe2Se6 has an indirect band gap of 1.93 eV.

Ba2M(C3N3O3)2 (M = Mg, Ca): Potential UV Birefringent Materials with Strengthened Optical Anisotropy Originated from (C3N3O3)3− Group

Demands for UV birefringent materials are growing dramatically owing to the rapid development of ultraviolet technology. Here, a new family of UV birefringent materials, Ba2Mg(C3N3O3)2 (BMCY) and Ba2Ca(C3N3O3)2 (BCCY), have been successfully discovered. It is the first time that the excellent birefringent properties of cyanurates have been studied. These materials exhibit an extremely large birefringence (Δn = 0.728–0.351 and 0.771–0.346 from 230 nm to 800 nm for BMCY and BCCY, respectively), much larger than that of the commercial UV birefringent crystal α-BaB2O4 (α-BBO) (Δn = 0.12@532 nm). Our study indicates that the impressive optical properties of BMCY and BCCY stem from the strengthened optical anisotropy of the planar (C3N3O3)3− group compared with the isoelectronic (B3O6)3− group. Besides, its congruent-melting properties make it feasible to grow a bulk crystal by the Bridgman–Stockbarger technique. The extraordinary properties of BMCY and BCCY may shed light on a new path to explore birefringent materials for practical application.