Reconstructing nearly isotropic microstructures to construct a one-dimensional framework causing record birefringence in thiophosphates.

Infrared birefringent crystals that hold significant importance for optoelectronic application have been rarely reported. Traditional tetrahedral PS4, ethane-like P2S6, and octahedral InS6 units in thiophosphates typically manifest near isotropy, often resulting in extremely small birefringence. However, this study prepares α-Rb2InP2S7 (1), ß-Rb2InP2S7 (2), and Cs2InP2S7 (3), consisting of the aforementioned microstructures, notably exhibiting the highest refractive index difference or birefringence values (0.247, 0.298, and 0.250 at 546 nm, respectively) among thiophosphates, the middle one being larger than that of commercial birefringent materials. This unusual increase in birefringence can be primarily attributed to two key factors: (1) simultaneous stretching and compressing of the P-S and In-S covalent bond interactions, generating high polarizability anisotropy of InS6, PS4, and P2S6 polyhedral units; (2) the additional incorporation of alkali metals that further reduces the dimensionality of the crystal structure, creating one-dimensional [InP2S7]2- structures with increasing polarizability anisotropy. This study presents an alternative approach to enhance birefringent materials by reconstructing covalent bond interactions and specific spatial arrangements.

AIn4S6Cl (A = Rb and Cs) and Pb5Sn3Q10Cl2 (Q = S and Se): quaternary chalcohalides with mixed anionic coordination exhibit photocurrent responses.

Mixed-anionic compounds have caught considerable attention due to their flexible coordination manners and abundant physical properties. Four new chalcohalides RbIn4S6Cl (1), CsIn4S6Cl (2), Pb5Sn3S10Cl2 (3) and Pb5Sn3Se10Cl2 (4) were successfully obtained by the high-temperature halide salt flux method. Compounds 1 and 2 have layered structures that consist of octahedral InS6 and aliovalent-anionic InS3Cl units. Compounds 3 and 4 feature 3-D structural frameworks built by [Pb4SnQ8Cl4]6- and [PbSn2Q6]2- (Q = S and Se) polyhedral chains, in which partial Pb2+ cations are coordinated by Q2- and Cl- anions. Compounds 1-4 have optical band gaps close to the wavelength range of visible light and exhibit significant photocurrent responses of 28.75 nA cm-2, 55.12 nA cm-2, 19.58 mA cm-2, and 36.12 µA cm-2 with on/off ratios 30.0, 2.5, 15.7 and 2.6, respectively, implying their potential for photovoltaic applications. To the best of our knowledge, compound 3 has the largest photocurrent response among all non-oxides. In addition, the activation energies of 1-4 are well below 0.3 eV, which makes these compounds interesting for potential applications in electrochemical devices. This work sheds light on the exploration of promising photocurrent response materials in the mixed-anionic compound system.