RESUMO
We describe the synthesis, crystal structure and semiconducting properties of a number of hexacyanidometallates with the formula A2[MFe(CN)6]·xH2O (A = Na, K; M = Mg, Ca, Sr and Ba). All crystal structures were studied via single-crystal or powder X-ray diffraction. The unexpectedly low-symmetric structures in these ferrocyanides are described and contrasted with analogous transition-metal compounds which have been reported to be strictly or nearly cubic. The amount of crystal water in the structure for powder samples was determined by the thermogravimetric analysis (TGA), supported by IR and Raman spectroscopy. Electronic-structure calculations of K2[MgFe(CN)6] and K2[CaFe(CN)6] are compared with experimental UV-Vis measurements. The large band gaps by advanced theory indicate that the smaller experimental band gaps are due to surface effects of impurity states. Mott-Schottky curves of K2[MgFe(CN)6], K2[CaFe(CN)6] and K2[BaFe(CN)6]·3H2O exhibit positive slopes, which characterizes these compounds as n-type semiconductors.
RESUMO
Transition-metal-based chalcogenides are a series of intriguing semiconductors with applications spanning various fields because of their rich structure and numerous functionalities. This paper reports the crystal structure and basic physical properties of a new quaternary chalcogenide In4Pb5.5Sb5S19. The crystal structure of In4Pb5.5Sb5S19 was determined by both powder and single-crystal X-ray diffraction techniques. In4Pb5.5Sb5S19 crystallizes in the monoclinic system with I2/m space group, and the structure parameters are a = 26.483 Å, b = 3.899 Å, c = 32.696 Å, and ß = 111.86°. The polyhedral double chains of Sb3+ and Sb/Pb2+ as the main cations are parallel to each other and form a Jamesonite-like mineral structure through the short chain links of the distorted In, Pb, and Sb polyhedron. In4Pb5.5Sb5S19 exhibits a moderate experimental band gap of 1.42 eV, indicating its potential for application in solar cells and photocatalysis. In addition, In4Pb5.5Sb5S19 exhibits good ambient stability, and differential scanning calorimetry tests demonstrate that it is stable up to 892 K in a nitrogen atmosphere. Moreover, In4Pb5.5Sb5S19 exhibits extremely low thermal conductivity (0.438-0.478 W m-1 K-1 ranging from 300 to 700 K) compared with binary counterparts such as PbS and In2S3. Future chemical manipulation via elemental doping or defect engineering may make the title compound a potential thermoelectric or thermal insulating material.
RESUMO
Pressure-driven spin-crossover (PSCO) is a collective quantum phenomenon frequently observed in transition-metal-based systems. According to the crystal-field theory, PSCO highly depends on the surrounding coordination environment of a given magnetic ion; nevertheless, it has never been verified experimentally up to now. Herein, we report the observation of a site-specific PSCO phenomenon in Lu1-xScxFeO3, in which octahedrally coordinated Fe3+ in orthorhombic LuFeO3 and trigonal-bipyramidally coordinated Fe3+ in hexagonal Lu0.5Sc0.5FeO3 show distinct PSCO response to external pressure. X-ray emission spectra and DFT calculations reveal the key role of coordination environment in a PSCO process and predict the occurrence of PSCO for trigonal-bipyramidally coordinated Fe3+ above 100 GPa, far beyond that of 50 GPa for octahedrally coordinated Fe3+ in LuFeO3. The demonstration of site-specific PSCO sheds light on the state-of-the-art design of PSCO materials for directional applications.
RESUMO
We report for the first time the discovery of reversible n-p conduction type switching in a chalcogenide, NaCu5S3, without structural transition. AC impedance and first-principles simulations of the ionic migration confirmed the local melting trends of the hexagonal copper lattice at high temperatures, which could result in superionic conductivity within NaCu5S3.
