In this paper, we present a study of the temperature-dependent characteristics of electromagnetic fields at the atomic scale in multiferroic bismuth ferrite (${\mathrm{BiFeO}}_3$ or BFO). The study was performed using time differential perturbed angular correlation (TDPAC) spectroscopy on implanted ${}^{111}\mathrm{In}$ (${}^{111}\mathrm{Cd}$) probes over a wide temperature range. The TDPAC spectra show that substitutional ${}^{111}\mathrm{In}$ on the ${\mathrm{Fe}}^{3+}$ site experiences local electric polarization, which is otherwise expected to essentially stem from the ${\mathrm{Bi}}^{3+}$ lone pair electrons. Moreover, the TDPAC spectra show combined electric and magnetic interactions below the Néel temperature $T_N$. This is consistent with simulated spectra. X-ray diffraction (XRD) was employed to investigate how high-temperature TDPAC measurements influence the macroscopic structure and secondary phases. With the support of ab initio DFT simulations, we can discuss the probe nucleus site assignment and can conclude that the ${}^{111}\mathrm{In}$ (${}^{111}\mathrm{Cd}$) probe substitutes the Fe atom at the B site of the perovskite structure.

It is still an open question if magnetoelectric coupling occurs at the atomic scale in multiferroic BiFeO 3 . Nuclear solid-state techniques monitor local fields at the atomic scale. Using such an approach, we show that, contrary to our own expectation, ferroelectric and magnetic ordering in bismuth ferrite ( BiFeO 3 or BFO) decouple at the unit-cell level. Time differential perturbed angular correlation (TDPAC) data at temperatures below, close, and above the magnetic Néel temperature show that the coupling of the ferroelectric order to magnetization is completely absent at the bismuth site. It is common understanding that the antiferromagnetic order and the cycloidal ordering due to the Dzyaloshinskii-Moriya interaction generate a net zero magnetization of the sample canceling out any magnetoelectric effect at the macroscopic level. Our previous data show that a very large coupling of magnetic moment and electrical distortions arises on the magnetic sublattice (Fe site). The oxygen octahedra around the iron site experience a large tilt due to the onset of magnetic ordering. Nevertheless, the Bi-containing complementary sublattice carrying the largest part of ferroelectric order is practically unaffected by this large structural change in its direct vicinity. The magnetoelectric coupling thus vanishes already at the unit cell level. These experimental results agree well with an density functional theory (DFT) calculation. Published by the American Physical Society 2025

In the last decade, organic–inorganic hybrid halide perovskite materials have developed into a very large research area in photovoltaics and optoelectronics as promising light harvesters. Lead-free double perovskites have recently been investigated as an environmentally friendly alternative to the lead-containing compositions. However, lead-free organic–inorganic hybrid halide double perovskites have so far rarely been produced due to a certain complexity in their synthesis. A number of small molecular cations have been investigated, but compositions containing azetidinium, which is a 4-membered heterocyclic molecular ring, on the A-site have hardly been considered. This study investigates the potential of [(CH₂)₃NH₂]₂AgBiBr₆ as an optical absorber in photovoltaics or optoelectronics. The use of this alternative cation changes the crystal symmetry significantly. Columns of alternating metal cation form which are separated by the organic ions. While crystal symmetry is rather different from the perovskites, the overall properties as an absorber are similar. It is thus worthwhile to further investigate alternate hybrid compositions which form into other symmetries than the perovskite base structure.

In this work, we report on the hyperfine parameters of the foreign ¹⁸¹Ta probe in the rutile structure of the single crystal TiO₂ using the e−γ and γ−γ time differential perturbed angular correlation (TDPAC) technique. We implanted ¹⁸¹Hf ions into a sample of single crystal rutile TiO₂ in the Bonn Isotope Separator. The implanted sample was then thermally annealed at a temperature of 873 K for 315 min in a vacuum. The ¹⁸¹Hf radioisotopes decayed by β⁻ emission, followed by a cascade to the ground of γ rays or conversion electrons into a stable state ¹⁸¹Ta. The ¹⁸¹Ta probe substitutes the Ti lattice site with a unique nuclear quadrupole interaction, allowing for the precise measurement of the largest electric field gradient (V_zz) and asymmetry parameter (η). The hyperfine parameters obtained from the e−γ TDPAC spectroscopy agree with those of the γ−γ TDPAC spectroscopy at room temperature, apart from a calibration factor, both from our experiments and the literature. This suggests that the electronic recombination following the internal conversion of the L shell electron takes less time (ps) than the intermediate lifetime of the metastable ¹⁸¹Ta state (ns).

The temperature-dependent electric field gradient (EFG) tensor at a probe nucleus reflects the combined effects of local symmetry, bonding, and lattice dynamics. However, determining the primary atomic distortions responsible for this response can be difficult. Here, we combine electron-gamma (e-γ) time-differential perturbed angular correlation (TDPAC) measurements on Ta 181 probes produced by the Hf 181 → Ta 181 decay at substitutional Ti sites in rutile TiO 2 with symmetry-resolved local frozen-phonon density functional theory (LFP-DFT) calculations. Atomic displacements of the TiO 6 octahedron are decomposed into site-symmetry-adapted local modes, and their contributions to the EFG are thermally weighted via Boltzmann averaging. We find that, in pristine rutile TiO 2 , the temperature evolution of the EFG shape is primarily governed by two orthogonal B 1 g equatorial-oxygen shear distortions. For Ta substituting the Ti site, the same shear sector remains active but preserves a finite in-plane anisotropy, resulting in a comparatively weak temperature dependence of the asymmetry parameter η. Measurements between 30 and 470 K, together with earlier high-temperature gamma-gamma ( γ − γ ) data, show weak low-temperature curvature and high-temperature quasilinearity of the principal EFG component V 33 ( T ) , while η(T) remains comparatively robust. The calculations reproduce these qualitative trends and provide a microscopic, symmetry-based interpretation of the thermal evolution of hyperfine tensors in solids.