Lichens are symbiotic organisms with an extraordinary capability to colonise areas of extreme climate and heavily contaminated sites, such as metal-rich habitats. Lichens have developed several mechanisms to overcome the toxicity of metals, including the ability to bind metal cations to extracellular sites of symbiotic partners and to subsequently form oxalates. Calcium is an essential alkaline earth element that is important in various cell processes. Calcium can serve as a metal ligand but can be toxic at elevated concentrations. This study investigated calcium-rich and calcium-poor sites and the lichen species that inhabit them (Cladonia sp.). The calcium content of these lichen species were analyzed, along with localized calcium oxalate formed in thalli collected from each site. The highest concentration of calcium was found in the lichen squamules, which can serve as a final deposit for detoxification. Interestingly, the highest content of calcium in Cladonia furcata was localized to the upper part of the thallus, which is the youngest. The produced calcium oxalates were species-specific. Whewellite (CaC₂O₄∙H₂O) was formed in the case of C. furcata and weddellite (CaC₂O₄∙2H₂O) was identified in C. foliacea.

Sr₃Fe₂WO₉ and Ba₃Fe₂WO₉ triple perovskites have been prepared in nanocrystalline form using a modified sol–gel route with citric acid as a chelating agent. These materials have been studied by X-ray powder diffraction (PXRD), energy-dispersive X-ray (EDX) spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and unpolarized Raman spectroscopy, while their magnetic properties have been determined using SQUID measurements. At room temperature, the crystal structure of 6H-perovskite Ba₃Fe₂WO₉ is hexagonal, space group P6₃/mmc, with a = 5.7577(2) and c = 14.1095(5) Å, whereas Sr₃Fe₂WO₉ presents a tetragonal lattice, space group I4/m, with a = 5.5803(1) and c = 7.8745(3) Å. Both compounds are nanocrystalline with average crystallite sizes of 25.6(1) nm for Ba₃Fe₂WO₉ and 35.9(1) nm for Sr₃Fe₂WO₉ with moderate microstrain. They exhibit a disordered perovskite structure, in which Fe³⁺ and W⁶⁺ cations distribute randomly at the B-sites of the perovskite lattice: Sr₂[Fe_0.72W_0.28]^2a[Fe_0.61W_0.39]^2bO₆ and Ba₆[Fe_1.71W_0.29]^2a[Fe_2.29W_1.71]^4fO₁₈. The detected hysteresis loops with non-zero remanent magnetization and a rather large coercive field reveal ferrimagnetic ordering with ordering temperatures of 373 K for Sr₃Fe₂WO₉ and 330 K for Ba₃Fe₂WO₉. Magnetic measurements show the onset of ferrimagnetic ordering at relatively high temperatures of 717 and 711 K for the Sr and Ba compounds, respectively; however the magnetization isotherms at 2 K exhibit, for H = 50 kOe, maximum magnetization values close to 0.8 μ_B (f.u.)⁻¹ and 0.35 μ_B (f.u.)⁻¹ for Sr and Ba compounds, respectively, although full saturation is not reached.

Suitable single crystals of the title compound, barium dicobalt(II) bis[orthoarsenate(V)], were prepared under hydrothermal conditions. This phase belongs to a series of compounds with general formula AM ₂(XO₄)₂, where A = alkaline earth metal, M = Mg or a divalent first-row transition element, and X = P, As or V. BaCo₂(AsO₄)₂ is isotypic with BaNi₂(XO₄)₂ (X = P, V or As) and is characterized by brucite-like sheets of edge-sharing CoO₆ octahedra (3 symmetry) parallel to (001), with one-third of the octahedral positions being vacant. The sheets are capped above and below by AsO₄ tetrahedra (3 symmetry) and are interconnected by distorted BaO₁₂ cuboctahedra (\overline{3} symmetry).

The magnetic properties of NiAs₂O₄and CoAs₂O₄single crystals were investigated. The unexpected behaviour of ZFC-FC curves of NiAs₂O₄is detected around 20 K, where the transition to the ferromagnetic state has been clearly observed.