ABSTRACT
A series of new compounds MI(OCN) were prepared from the mixtures of MI2 and K(OCN) (M = Sr, Eu or Ba) by solid-state reactions that were controlled by differential scanning calorimetry (DSC) and differential thermal analysis (DTA) techniques. The presence of two phases is highlighted for BaI(OCN) which crystallizes in the Pnma (α-BaI(OCN)) and Cmcm (ß-BaI(OCN)) space groups. The SrI(OCN), EuI(OCN) and ß-BaI(OCN) compounds crystallize in the same orthorhombic Cmcm space group and the structure consists of a layered arrangement of [M2I2]2+ blocks separated by single layers of cyanate ions that are found to be related to the Sillén-type structure. The polymorphism of BaI(OCN) was also observed by analyzing the infrared (IR) spectra. This analysis showed for ß-BaI(OCN) splitting bands, which were ascribed to the presence of more than one resonant cyanate form that can be induced by the dynamical disorder of OCN units. In contrast, the α-BaI(OCN) vibration modes suppose the existence of one cyanate form that adopts a defined orientation. The SrI(OCN) and BaI(OCN) compounds were doped with Eu2+. The luminescence spectra recorded under excitation at 340 and 350 nm revealed a blue emission of Eu2+ at 431 nm for the ß-BaI(OCN) and SrI(OCN) phases and at 427 nm for the α-BaI(OCN) phase that was ascribed to the 4f65d1â8S7/2 (4f7) transition.
ABSTRACT
Yellow transparent single crystals of the new compound K2Pb(OCN)I3 were synthesized from a mixture of lead iodide and potassium cyanate through a solid-state reaction, which was controlled by the differential scanning calorimetry (DSC) technique. The single crystal X-ray refinement showed that K2Pb(OCN)I3 crystallizes in the cubic system (Fd3[combining macron]m, a = 16.295(2) Å, V = 4327(1) Å3, Z = 16 and R1 = 0.0167). The structure is basically composed of two frameworks built from potassium tetrahedra capped by OCN- units and corner-sharing lead iodide octahedra. The infrared spectrum of K2Pb(OCN)I3 reveals the presence of vibration modes that were assigned to cyanate ions. The band structure and total and partial density of states calculated by DFT with the PBE functional (CASTEP code) showed that this compound exhibits a direct band gap of 2.19 eV, which is in good agreement with the experimental value obtained from a diffuse reflectance measurement (2.4 eV). The investigation of the electrical properties of this material demonstrates that it is highly resistive and has large permittivity (relative values of up to 104 at 150 °C). However, it does not display ferroelectric behavior.
