The extraordinary thermal stability of [NH₂CHNH₂]₃Bi₂I₉ (FA₃Bi₂I₉) crystals, up to around 600 K, has been enhanced by using the evaporating method. DSC measurements have revealed reversible structural phase transitions: at 203 K (Phase I → Phase II), 173 K (II → III), 134.7 K (III → IV), 131.4 K (IV → V), and 120 K (V → VI). The crystal structures of FA₃Bi₂I₉ adopt the polar space groups P6₃mc at 220 K (Phase I) and Cmc2₁ at 190 K (Phase II). The pyroelectric current (I_pyr) measured in the temperature range covering all the phases of FA₃Bi₂I₉ confirmed their polar nature. The reversibility of the spontaneous polarization in each phase has been confirmed by the observed hysteresis loops. All the phase transitions are dielectrically active. The dielectric response (ε*(ω,T) close to 134.7 and 131.4 K) is characteristic of ferroelectrics with a critical slowing-down process. Optical bandgaps estimated from the UV-vis measurements and calculated using the DFT method are equal to 1.85 and 1.89 eV, respectively. On the basis of the structural, dielectric, and spectroscopic results, the molecular mechanisms of the phase transitions have been proposed.

We report the physicochemical characteristics of a novel Sb-based formamidinium ferroelectric material, (NH₂CHNH₂)₃Sb₂I₉ (FAIA). FAIA crystallizes at room temperature in the hexagonal P6₃mc space group. The crystal structure of FAIA is isomorphic to that of its bismuth analog, [NH₂CHNH₂]₃Bi₂I₉ (FAIB), which is characterized by a zero dimensional (0D) anionic network (discrete bioctahedral units). From the DSC results three phase transitions (PT) have been found for FAIA: at 187.4/189 K (phase I ↔ phase II), 177.6/177.9 K (II ↔ III) and 148.2/152.3 K (III ↔ IV). The compound exhibits ferroelectricity in the room temperature phase I and in the successive temperature phases (II and III). The spontaneous polarization (P_S = 0.5 × 10⁻² μC cm⁻²) reversibility has been confirmed by the presence of the ferroelectric hysteresis loops and pyroelectric measurements. The origin of ferroelectricity is discussed in terms of the cationic dynamics. This material may be considered as a semiconductor (σ ∼ 10⁻⁶ S m⁻¹, 300 K).

Gadolinium-doped nanoparticles (NPs) are regarded as promising luminescent probes. In this report, we studied details of toxicity mechanism of low doses of NaGdF₄-based fluorescent nanoparticles in activated RAW264.7, J774A.1 macrophages. These cell lines were specifically sensitive to the treatment with nanoparticles. Using nanoparticles of three different sizes, but with a uniform zeta potential (about −11 mV), we observed rapid uptake of NPs by the cells, resulting in the increased lysosomal compartment and subsequent superoxide induction along with a decrease in mitochondrial potential, indicating the impairment of mitochondrial homeostasis. At the molecular level, this led to upregulation of proapoptotic Bax and downregulation of anti-apoptotic Bcl-2, which triggered the apoptosis with phosphatidylserine externalization, caspase-3 activation and DNA fragmentation. We provide a time frame of the toxicity process by presenting data from different time points. These effects were present regardless of the size of nanoparticles. Moreover, despite the stability of NaGdF₄ nanoparticles at low pH, we identified cell acidification as an essential prerequisite of cytotoxic reaction using acidification inhibitors (NH₄Cl or Bafilomycin A1). Therefore, approaching the evaluation of the biocompatibility of such materials, one should keep in mind that toxicity could be revealed only in specific cells. On the other hand, designing gadolinium-doped NPs with increased resistance to harsh conditions of activated macrophage phagolysosomes should prevent NP decomposition, concurrent gadolinium release, and thus the elimination of its toxicity.

Novel lead-free ferroelastic materials, (C ₃ N ₂ H ₅ ) ₃ Sb ₂ I ₉ and (C ₃ N ₂ H ₅ ) ₃ Bi ₂ I ₉ , exhibit rich sequences of phase transitions and reveal semiconducting properties.

Development of protocols for transferring of hydrophobic quantum dots (QDs) into aqueous solution is of special importance for their biomedical applications. Particularly, hydrophilization of Ag₂S without quenching of photoluminescence is a great challenge. Application of standard protocol for amphiphilic polymer coating of hydrophobic Ag₂S nanocrystals failed, whereas ligand exchange and direct synthesis of Ag₂S in aqueous solution leads to poorly emitting materials. In this paper we present the facile method for transferring of hydrophobic Ag₂S QDs into aqueous solution employing the phase transfer catalysed hydrolysis of a commercially available polymer - poly(maleic anhydride-alt-1-octadecene), (PMAO) in the presence of tetramethylammonium hydroxide. Because the original surface ligands are retained and we do not use solvents (like THF) detrimental for Ag₂S emission, the modification does not deteriorate photoluminescence properties and quantum yield of modified QDs in aqueous solution reaches 60% of that for hydrophobic QDs in chloroform. The hydrodynamic diameter of modified, water soluble Ag₂S QDs is about 10 nm and is only slightly larger than their original size. Moreover, the polymer coated nanocrystals are not aggregated and are stable for months. Surface characterization of QDs by NMR and IR spectroscopy indicates that polymer chains intercalate alkyl chains of dodecanethiol (DDT) bound to the surface of Ag₂S. The cytotoxicity studies indicate that the presented method should be regarded as a notable progress towards the biocompatible Ag₂S NIR-II emitting nanoprobes.