Crystal structure of Ba₂(La_0.727Ba_0.182M_0.091)MO₆(M = Nb, Sb, Bi): symmetry nuance identified in photoluminescence and IR spectroscopy studies

Abstract

A one-third lanthanum deficiency was created in BaLaMO compounds (LaM compounds) to form BaLaMO compounds (La2/3M compounds) for M = Nb, Sb, and Bi. The compounds were prepared by a gel-combustion method using citric acid as a fuel. All the compounds were characterized by powder X-ray diffraction (XRD). The XRD analysis showed that the space group of the La2/3M compounds remains the same for the Bi and Sb samples when compared to the reported LaM compounds, except for the Nb sample. La2/3Nb and La2/3Sb adopt a rhombohedral structure with the space group R3[combining macron], whereas La2/3Bi adopts a monoclinic structure with the space group I2/m. As the positron annihilation spectroscopy (PALS) technique is sensitive to cation deficiency, it was used to detect the presence of cation vacancies in the samples, which are formed due to the decrease in the lanthanum concentration. The PALS analyses indicated that the absence of cation deficiency in the La2/3M compounds is similar to that observed in the LaM compound. Thus, the crystal structure of the La2/3M compound was modeled, such that the cation deficiency at the La site is filled by Ba and M ions, and the crystal structure formula is given as Ba(LaBaM)MO. This model was confirmed by Rietveld refinement of the XRD data. The emission spectra of Eu showed a strong dependence on its local site symmetry in the host material, in which it is being doped and this can be used as a spectroscopic probe for detecting any differences in the symmetry. Comparison of the local symmetry around La cation was studied using photoluminescence (PL) by doping 2 atom% Eu in LaM and La2/3M compounds. Infrared spectroscopy (IRS) analyses were also carried out for LaM and La2/3M compounds. There was complete agreement between the PL and IRS results and they were also in concordance with the predicted crystal structure model. Interestingly in these La2/3M compounds, the equilibrium structure prefers large Ba ion to occupy the octahedral B-site rather than forming an octahedral vacancy at that site, making these perovskite compounds rare and novel in their class.

Probing crystal structure and site‐selective photo‐physical properties of various Eu3+‐doped niobates

“…In an asymmetric environment, the intensity of the EDT will be higher than the MDT and in a symmetric environment, the reverse is the case. Thus apart from orange–red emission, which has application in phosphor material, its sensitivity to local site in the host matrix is extensively used as local structure probe . PL study was performed on 2 at.…”

Effect of Molten Salt Synthesis Processing Duration on the Photo- and Radioluminescence of UV-, Visible-, and X-ray-Excitable La2Hf2O7:Eu3+ Nanoparticles

“…Their ground-state 7 F 0 and the most informative excited state 5 D 0 are nondegenerate. Therefore, they are not split by crystal-field effects. , Eu 3+ ions with the characteristic orange red emission at 592 and 614 nm are highly sensitive to the local surroundings . The Eu 3+ photoluminescence and spectral data can give highly reliable information on whether the dopant ions are localized on the surface of A 2 B 2 O 7 NPs or they have percolated either interstitially or substitutionally inside the A 2 B 2 O 7 lattice.…”