Hydrothermally obtained α-MnO₂ nanowire characterizations confirm the tetragonal crystalline structure that is several micrometers long and 20–30 nm in diameter with narrow distributions in their dimensions. The absorption calculated from diffuse reflectance of α-MnO₂ occurred in the visible region ranging from 400 to 550 nm. The calculated band gap with Quantum Espresso using HSE approximation is ~2.4 eV for the ferromagnetic case, with a slightly larger gap of 2.7 eV for the antiferromagnetic case, which is blue-shifted as compared to the experimental. The current work also illustrates the transformations that occur in the material under heat treatment during TGA analysis, with the underlying mechanism. Electrochemical studies on graphite supports modified with α-MnO₂ compositions revealed the modified electrode with the highest electric double-layer capacitance of 3.444 mF cm⁻². The degradation rate of an organic dye—rhodamine B (RhB)—over the compound in an acidic medium was used to examine the catalytic and photocatalytic activities of α-MnO₂. The peak shape changes in the time-dependent visible spectra of RhB during the photocatalytic reaction were more complex and progressive. In two hours, RhB degradation reached 97% under sun irradiation and 74% in the dark.

We report the synthesis, crystal structures, and thermal, optical and magnetic properties of three new metal dicyanamide frameworks comprising benzyltrimethylammonium cations (BeTriMe⁺) as labile guests. These compounds crystallize in two-dimensional noncentrosymmetric structures, in space group Pna2₁ of the orthorhombic system, in which metal cations coordinate to five N atoms from dicyanamide anions and one oxygen atom from the water molecule. Magnetic studies indicate that Mn, Co and Ni compounds order at 2.5, 2.8 and 7.0 K, respectively. Second harmonic generation (SHG) and density functional theory (DFT) studies performed for BeTriMe[Mn(dca)₃(H₂O)] confirm a noncentrosymmetric structure. DFT computations also provide insights into the experimentally observed low-temperature magnetic order and insulating behavior. Thus, the synthesized compounds are rare examples of dicyanamide frameworks exhibiting coexistence of polar and magnetic orders, and additionally – SHG activity. Optical studies indicate that BeTriMe[Mn(dca)₃(H₂O)] also exhibits red emission under 450 nm excitation. Thermal studies reveal that BeTriMe[M(dca)₃(H₂O)] networks dehydrate near 350–380 K resulting in structural transformations. The anhydrous BeTriMeMn also exhibits SHG activity, which is about 4 times smaller compared to the hydrated analogue. On direct contact with air the anhydrous BeTriMeMn spontaneously rehydrates, restoring the SHG response to initially observed intensity. On the other hand, if the dehydrated BeTriMeMn is sealed from air, its SHG response is essentially irreversible and stable for days. Taking BeTriMeMn as an example we demonstrate that noncentrosymmetric compounds with effectively irreversible temperature-induced change of SHG response can be potentially employed as remote nonlinear optical (NLO) markers (NLO threshold temperature sensors). The usefulness of non-contact NLO markers of this kind is based on the fact that they can inform on the thermal history of a given object, specifically, by showing whether a certain threshold temperature was achieved or not.

We present a detailed study on the magnetization under high magnetic fields of Tb₅Si₂Ge₂ and Gd₅Si₂Ge₂ compounds. From the Arrott plot construction (A. Arrott, Phys. Rev. 108, 1394 (1957)), we were able to estimate the T_C of each structure (M and O(I)) experimentally and found that the T_C of the O(I) phase can be directly obtained by extrapolating the T_C curve of this phase in the respective phase diagram. Using a physical model based on free energy considerations, one explains the (de)coupling of the magnetic and structural transitions in R₅(Si_xGe_1−x)₄ (R = Tb, Gd) compounds.

The hyperfine interaction between the quadrupole moment of atomic nuclei and the electric field gradient (EFG) provides information on the electronic charge distribution close to a given atomic site. In ferroelectric materials, the loss of inversion symmetry of the electronic charge distribution is necessary for the appearance of the electric polarization. We present first-principles density functional theory calculations of ferroelectrics such as BaTiO3, KNbO3, PbTiO3 and other oxides with perovskite structures, by focusing on both EFG tensors and polarization. We analyze the EFG tensor properties such as orientation and correlation between components and their relation with electric polarization. This work supports previous studies of ferroelectric materials where a relation between EFG tensors and polarization was observed, which may be exploited to study the ferroelectric order when standard techniques to measure polarization are not easily applied.

Ga₂O₃ bulk single crystals have been implanted with 300 keV Europium ions to fluences ranging from 1×10¹³ to 4×10¹⁵ at/cm². The damage build-up and Eu-incorporation was assessed by Rutherford Backscattering Spectrometry in the channeling mode (RBS/C). RBS/C results suggest that implantation causes a mixture of defect clusters and extended defects such as dislocations. Amorphisation starts at the surface for fluences around 1×10¹⁵ at/cm² and then proceeds to deeper regions of the sample with increasing fluence. Amorphous regions and defect clusters are efficiently removed during rapid thermal annealing at ~1100 °C; however, Eu diffuses towards the surface. Nevertheless, Eu ions are optically activated and show cathodoluminescence at room temperature. Results in bulk samples are compared to those in Eu-implanted Ga₂O₃ nanowires and despite strong similarities in the structural properties differences were found in the optical activation. Furthermore, damage and dopant incorporation studies were performed using the Perturbed Angular Correlation technique, which allows probing the immediate lattice surroundings of an implanted radioactive probe at the atomic level.

Work of numerous research groups has shown different outcomes of studies of the transition from the ferroelectric $\alpha$-phase to the high temperature $\beta$-phase of the multiferroic, magnetoelectric perovskite Bismuth Ferrite (${\mathrm{BiFeO}}_3$ or BFO). Using the perturbed angular correlation (PAC) method with ${}^{111m}\mathrm{Cd}$ as the probe nucleus, the $\alpha$ to $\beta$ phase transition was characterized. The phase transition temperature, the change of the crystal structure, and its parameters were supervised with measurements at different temperatures using a six detector PAC setup to observe the $\gamma -\gamma$ decay of the ${}^{111m}\mathrm{Cd}$ probe nucleus. The temperature dependence of the hyperfine parameters shows a change in coordination of the probe ion, which substitutes for the bismuth site, forecasting the phase transition to $\beta$-BFO by either increasing disorder or formation of a polytype transition structure. A visible drop of the quadrupole frequency ${\omega }_0$ at a temperature of about $T_c\approx {820}^{\circ }\mathrm{C}$ indicates the $\alpha -\beta$ phase transition. For a given crystal symmetry, the DFT-calculations yield a specific local symmetry and electric field gradient value of the probe ion. The $Pbnm$ ($\beta$-BFO) crystal symmetry yields calculated local electric field gradients, which very well match our experimental results. The assumption of other crystal symmetries results in significantly different computed local environments not corresponding to the experiment.