Deepak Prajapat

Rare-earth elements are key components in a wide range of functional materials, including solid-state lasers, superconductors and high-performance permanent magnets, owing to their localized 4 f electrons that give rise to distinctive electronic and magnetic properties. Nuclear resonant scattering of synchrotron radiation using Mössbauer isotopes provides a powerful isotope-specific probe of these properties, enabling access to vibrational dynamics as well as electronic and magnetic hyperfine interactions. Its application, however, requires dedicated high-resolution monochromators tailored to the ultra-narrow nuclear resonances of individual Mössbauer isotopes. The 8.41 keV nuclear resonance of ¹⁶⁹ Tm was among the first resonances excited at synchrotron sources, but has not been revisited since the early pioneering experiments. Here, a narrow-band high-resolution monochromator optimized for the ¹⁶⁹ Tm nuclear resonance at the High-Resolution Dynamics Beamline P01 of PETRA III (DESY, Hamburg) is presented. The monochromator provides a tunable energy resolution between 1.4 and 0.33 meV. Its performance is demonstrated by nuclear forward scattering and nuclear inelastic scattering measurements on Tm ₂ O ₃ and metallic Tm, allowing the determination of hyperfine parameters and the phonon density of states, and derived thermodynamic quantities. The instrument establishes ¹⁶⁹ Tm nuclear resonant scattering as a probe of vibrational, electronic and magnetic properties in Tm-based materials.

Resistive switching in SrCoO _2.5 has been variably attributed to localized filamentary conduction and lattice‐scale oxygen redox; as a topotactic oxide capable of reversible oxygen exchange with minimal lattice disruption, it provides a model system to resolve this distinction, which is critical because the underlying mechanism governs switching stability, reproducibility, and device design. Here, we address this issue by directly comparing Au/SrCoO _2.5 /Nb‐SrTiO ₃ and SrRuO ₃ /SrCoO _2.5 /Nb‐SrTiO ₃ heterostructures using electrical transport measurements combined with operando bias‐dependent x‐ray absorption and hard x‐ray photoelectron spectroscopy, providing direct sensitivity to lattice‐oxygen‐driven redox processes. Devices with an oxygen‐inert Au electrode exhibit polarity‐restricted switching, whereas the oxygen‐active SrRuO ₃ electrode exhibits switching under both bias polarities within the investigated voltage window, consistent with participation of both interfaces in the bias‐induced redox response. Operando spectroscopy reveals electrode‐dependent modulation of the Co valence, while complementary structural and vibrational measurements show no macroscopic lattice change but clear local rearrangements of the oxygen coordination, indicating an interface‐controlled, spatially distributed redox mechanism. These results identify electrode oxygen‐exchange capability as an important parameter governing switching polarity and redox dynamics in topotactic transition‐metal oxides. The combined electrical and spectroscopic observations further support the contribution of a distributed redox mechanism to the switching process.

Here we report results of orientation dependent nuclear inelastic scattering (NIS) experiments on a single crystal of a chemical ferrous iron complex performed at T = 10 K. For that purpose, we have constructed a sample holder which is equipped with a magnetic coupling to accommodate single crystal loops as used in protein crystallography. The sample holder is compatible with the sample rod of a helium cryostat at beamline P01, PETRA III, DESY Hamburg, which allows NIS experiments down to liquid He temperatures and below. For our experiments we chose a ⁵⁷ Fe enriched single crystal of the compound [Fe ^II (b(bdpa))](PF ₆ ) ₂ , with (b(bdpa)) = N, N‘-bis(benzyl)-N, N’-bis(2-pyridylmethyl)-6,6’-bis(aminomethyl)-2,2’-bipyridine). Crystals of this complex have unit cells containing two crystallographically independent molecular sites. Our single crystal NIS experiments show that the two irons of the unit cell are in their low spin (S = 0) state at 10 K. This is in contradiction with previous findings (Brady at al. Inorg. Chem. 2004, 43, 14, 4289–4299) which show significant residual high spin (S = 2) contributions up to 25% as obtained from Mössbauer Spectroscopy and susceptibility measurements at room temperature.

Maturation of [FeFe]‐hydrogenase depends on the HydF maturase, a [4Fe–4S]‐containing scaffold protein that assembles and delivers the organometallic [2Fe] subsite of the H‐cluster. Despite extensive research, the function of the HydF [4Fe–4S] cluster and its interaction with the [2Fe] cofactor remains unresolved, with conflicting evidence regarding cyanide linkage isomerism and the functional role of the cubane. Using ⁵⁷ Fe nuclear resonance vibrational spectroscopy in combination with selective isotopic labeling, we show that the [2Fe] subsite binds adjacent to the [4Fe–4S] cluster without forming a covalent cyanide bridge, while remaining electronically coupled. Complementary protein structure predictions support this arrangement and reconcile prior spectroscopic, mutagenesis, and structural studies. Extending this framework to earlier biosynthetic steps, additional structure predictions suggest that the [4Fe–4S] cluster contributes to assembly of the CH ₂ –NH–CH ₂ bridge of the [2Fe] site via interactions with the lipoate cofactor of the aminomethyl‐lipoyl H‐protein, thereby positioning reactive components in proximity to the [2Fe] precursor. Together, these results provide a coherent structural and mechanistic framework for HydF function across distinct stages of H‐cluster biosynthesis.

Maturation of [FeFe]‐hydrogenase depends on the HydF maturase, a [4Fe–4S]‐containing scaffold protein that assembles and delivers the organometallic [2Fe] subsite of the H‐cluster. Despite extensive research, the function of the HydF [4Fe–4S] cluster and its interaction with the [2Fe] cofactor remains unresolved, with conflicting evidence regarding cyanide linkage isomerism and the functional role of the cubane. Using ⁵⁷ Fe nuclear resonance vibrational spectroscopy in combination with selective isotopic labeling, we show that the [2Fe] subsite binds adjacent to the [4Fe–4S] cluster without forming a covalent cyanide bridge, while remaining electronically coupled. Complementary protein structure predictions support this arrangement and reconcile prior spectroscopic, mutagenesis, and structural studies. Extending this framework to earlier biosynthetic steps, additional structure predictions suggest that the [4Fe–4S] cluster contributes to assembly of the CH ₂ –NH–CH ₂ bridge of the [2Fe] site via interactions with the lipoate cofactor of the aminomethyl‐lipoyl H‐protein, thereby positioning reactive components in proximity to the [2Fe] precursor. Together, these results provide a coherent structural and mechanistic framework for HydF function across distinct stages of H‐cluster biosynthesis.

Fe(II) containing spin crossover (SCO) complexes can be reversibly switched between a low-spin (S = 0) and a high-spin (S = 2) state. Switching between the two spin states can be performed e.g. via change of temperature. In some SCO complexes, it is also possible to trigger a light induced excited spin state trapping (LIESST) effect by irradiating the SCO material with light. Here, we report on the vibrational properties of the LIESST state of the SCO complex Fe(PM-BiA) ₂ (NCS) ₂ investigated via nuclear inelastic scattering (NIS). For our NIS experiments, we used a liquid Helium cryostat at beamline P01, PETRA III, DESY Hamburg, dedicated to NIS experiments down to LHe temperature. Using a modified sample rod which allows illumination of samples within the synchrotron beam down to LHe temperatures we could stabilize the LIESST state of Fe(PM-BiA) ₂ (NCS) ₂ for nearly 100% of the iron centres at a sample temperature of 25 K.

Iron-sulfur clusters fulfill numerous roles throughout biology. The reduced [2Fe-2S]⁺ cluster offers unique electronic and magnetic properties due to its mixed-valent nature and can serve as an essential model for understanding electron transfer, electron delocalization, and accessible spin states not only in mixed-valent dimers, but potentially larger iron sulfur clusters. Recently a series of mixed-valent diiron dichalcogenide complexes [L₂Fe₂Q₂]⁻ (Q = S (1), Se (2), Te (3), L = 2,6-diisopropylphenyl β-diketiminate ligand) were synthesized and characterized, where complex 1 showed a typical S = 1/2 spin state, while complexes 2 and 3 exhibited intermediate S = 3/2 spin states, potentially enabled by the minimization of vibronic coupling. Here we studied the vibrational dynamics of the Fe and Te centers in these complexes using ⁵⁷Fe and ¹²⁵Te nuclear resonance vibrational spectroscopy (NRVS), coupled with DFT calculations. The findings suggest that heavy character of larger chalcogen atoms results in decreased vibronic coupling. The observation of an intermediate spin state is shown to be unattainable for lighter Fe₂Q₂ cores. This highlights the crucial role of vibronic coupling in modulating the electronic structure of mixed-valence systems and should enhance understanding of the electronic structure in more complex biological Fe-S clusters.