Crystal structures of the double perovskites Ba2Sr1−xCaxWO6

Studies of the Structure and Optical Properties of BaSrMgWO6 Thin Films Deposited by a Spin-Coating Method

Highly transparent thin films with the chemical formula BaSrMgWO₆ were deposited by spin coating using a solution of nitrates of Ba, Sr, and Mg and ammonium paratungstate in dimethylformamide with a Ba:Sr:Mg:W ratio = 1:1:1:1. XRD, SEM, EDX, and XPS investigations evidenced that annealing at 800 °C for 1 h results in an amorphous structure having a precipitate on its surface, and that supplementary annealing at 850 °C for 45 min forms a nanocrystalline structure and dissolves a portion of the precipitates. A textured double perovskite cubic structure (61.9%) was found, decorated with tetragonal and cubic impurity phases (12.7%), such as BaO₂, SrO₂, and MgO, and an under-stoichiometric phase (24.4%) with the chemical formula Ba_2-(x+y) Sr_xMg_yWO₅. From transmittance measurements, the values of the optical band gap were estimated for the amorphous (E_gdir = 5.21 eV, E_gind = 3.85 eV) and nanocrystalline (E_gdir = 4.69 eV, E_gind = 3.77 eV) phases. The presence of a lattice disorder was indicated by the high Urbach energy values and weak absorption tail energies. A decrease in their values was observed and attributed to the crystallization process, lattice strain diminution, and cation redistribution.

On how the mechanochemical and co-precipitation synthesis method changes the sensitivity and operating range of the Ba2Mg1-xEuxWO6 optical thermometer

The suitability of Ba₂MgWO₆ (BMW) double perovskite doped with Eu³⁺ for the construction of an optical thermometer was tested. It has been shown that by controlling the conditions of BMW synthesis, the sensitivity of the optical thermometer and the useful range of its work can be changed. Pure BMW and doped with Eu³⁺ samples were prepared using the mechano-chemical and co-precipitation methods. Both the absolute sensitivity and the relative sensitivity in relation to the synthesis route were estimated. The findings proved that the relative sensitivity can be modulated from 1.17%K⁻¹ at 248 K, to 1.5%K⁻¹ at 120 K for the co-precipitation and the mechanochemical samples, respectively. These spectacular results confirm the applicability of the Ba₂MgWO₆: Eu³⁺ for the novel luminescent sensors in high-precision temperature detection devices. The density-functional theory was applied to elucidate the origin of the host emission.

The role of π-bonding on the high temperature structure of the double perovskites Ba2CaUO6and BaSrCaUO6

High temperature synchrotron X-ray and neutron diffraction powder diffraction studies of the uranium perovskites Ba₂CaUO₆and BaSrCaUO₆reveal unusual phase transition behavior associated with the progressive loss of cooperative octahedral tilting.