arXiv subjects

Stanislav Savvin

Publications and source records attributed to Stanislav Savvin.

5 recordsLinked to original sources

arXiv2026

Oxygen-nonstoichiometry-driven phase transition in $\mathrm{Sr}_{1-x}\mathrm{Nd}_{x}\mathrm{CoO}_{3-\delta}$ ($x = 0.1, 0.2, 0.3$) perovskites

We report a systematic study of the interplay between oxygen nonstoichiometry, crystal structure, and magnetic/electrotransport properties in $\mathrm{Sr}_{1-x}\mathrm{Nd}_{x}\mathrm{CoO}_{3-\delta}$ ($x = 0.1, 0.2, 0.3$). High-resolution neutron powder diffraction combined with synchrotron x-ray powder diffraction reveals that increasing the oxygen content induces a structural transition from a layered $I4/mmm$ ($2a_p \times 2a_p \times 4a_p$) to an oxygen-deficient orthorhombic $Pmmm$ ($a_p \times a_p \times 2a_p$) phases with preferential oxygen-vacancy occupation. This transition is accompanied by a crossover from G-type antiferromagnetic with a weak ferromagnetic component to a ferromagnetic state, and a drastic decay in resistivity. The evolution of the magnetic and transport properties is discussed in terms of changes in the Co spin state, enhanced Co $3d$ - O $2p$ orbital overlap upon oxygen uptake, and a magnetically inhomogeneous ferromagnetic state associated with residual oxygen vacancies and mixed $\mathrm{Co}^{3+}/\mathrm{Co}^{4+}$ valence. Our findings experimentally confirm that the stabilization of the layered "314" structure is driven by the presence and ordering of oxygen vacancies rather than A-site cation ordering, whereas the oxygen-deficient oxidized compounds represent an intermediate orthorhombic state preceding fully stoichiometric phases.

cond-mat.mtrl-sci↗

arXiv2026

The effect of pressure in the crystal and magnetic structure of FeWO4

The temperature dependence of the structural and magnetic properties of wolframite-type FeWO4 were studied in situ by high pressure neutron diffraction. Neutron diffraction measurements were performed at the XtremeD instrument at the Institut Laue Langevin up to a maximum pressure of 8.7(4) GPa and a minimum temperature of 30.0(5) K. The diffraction data were analyzed via Rietveld refinements. We found that despite of producing a contraction of 5% of the volume, the maximum pressure applied in this study does not modify the Shubnikov space group below magnetic order. However, the orientation of magnetic moments and the N\'eel temperature, are slightly modified with the pressure, which is expected according to the preexistent understanding of magnetism in wolframites. We also determined a pressure-volume equation of state of FeWO4 at 300 K, which is compared with previous X-ray diffraction studies and density-functional theory calculations.

cond-mat.mtrl-sci↗

arXiv2026

Cryogenic stabilization of molecular hydrogen in dense cubic ice

Hydrogen is widely regarded as a cornerstone of future low-carbon energy technologies, yet the lack of safe, efficient, and reversible solid-state storage materials remains a major barrier to its large-scale deployment. Although porous frameworks and metal hydrides have been extensively explored, far less is known about the ability of dense molecular solids to stabilize hydrogen at near-ambient pressure. Here we show that fully crystalline cubic ice, despite its non-porous nature, can retain molecular hydrogen as an interstitial guest following controlled decompression from a high-pressure hydrogen hydrate precursor. Using synchrotron X-ray diffraction, neutron diffraction, and Raman spectroscopy, we demonstrate that hydrogen is retained within the ice structure up to about 130 K, producing reproducible lattice expansion and distinct spectroscopic signatures. We further show that pure cubic ice can be partially refilled with hydrogen at 0.18 GPa and 130 K, while fully hydrogen-filled cubic structure can be preserved at the same pressure up to 90 K. The retained hydrogen content reaches several percent of the parent hydrate composition, corresponding to gravimetric and volumetric storage densities comparable to those of interstitial hydrogen in metals. These results reveal an unexpected ability of a dense hydrogen-bonded crystal structure to host molecular hydrogen without permanent porosity or chemical bonding, establishing cubic ice as a minimal model for hydrogen-lattice interactions. More broadly, our findings identify dense hydrogen-bonded solids as an unexplored class of materials for hydrogen storage physics, with implications extending from energy materials to planetary and astrophysical ice environments.

cond-mat.mtrl-sci↗

arXiv2025

Disorder-driven magnetic duality in the spin-$\frac{1}{2}$ system ktenasite, Cu$_\text{2.7}$Zn$_\text{2.3}$(SO$_\text{4}$)$_\text{2}$(OH)$_\text{6}\cdot$6H$_\text{2}$O

Disorder in frustrated quantum systems can critically influence their magnetic ground states and drive exotic correlated behavior. In the $S = \frac{1}{2}$ system ktenasite, Cu$_\text{2.7}$Zn$_\text{2.3}$(SO$_\text{4}$)$_\text{2}$(OH)$_\text{6}\cdot$6H$_\text{2}$O, we show that structural disorder drives an unexpected dimensional crossover and stabilizes a rare coexistence of distinct magnetic states. Neutron diffraction reveals significant Cu/Zn mixing at the Cu2 site, which tunes the Cu$^{2+}$ sublattice from a two-dimensional scalene-distorted triangular lattice into a one-dimensional spin-chain network. Magnetic susceptibility, neutron diffraction, ac susceptibility, and specific heat measurements collectively indicate magnetic duality: a coexistence of incommensurate long-range magnetic order below $T_\text{N} = 4\,$K and a cluster spin-glass state with $T_\text{f} = 3.28\,$K at $\nu = 10\,$Hz. Our findings highlight ktenasite as a rare platform where structural disorder tunes the effective dimensionality and stabilizes coexisting ordered and glassy magnetic phases, offering a unique opportunity to explore the interplay of frustration, disorder, and dimensional crossover in quantum magnets.

cond-mat.str-el↗

arXiv2025

Incommensurate and commensurate antiferromagnetic orders in the kagome compound UV$_{6}$Sn$_{6}$

We report on the synthesis of single crystals of the kagome compound, UV$_6$Sn$_6$, and present the results of magnetization, electrical resistivity, heat capacity, x-ray, and neutron diffraction experiments to characterize the structure and magnetic properties. UV$_6$Sn$_6$ crystallizes in a large supercell of the HfFe$_6$Ge$_6$ parent structure with an hexagonal symmetry in which some of the U atoms are shifted by $c/2$ in an ordered fashion. Below $T_{N1}\approx$ 29 K, an incommensurate magnetic structure with a temperature-dependent wave vector $(0,0,k_z)$ is observed. Below $T_{N2}=$ 23.5 K, the wave vector locks in to $(0,0,0.5)$, forming an antiferromagnetic ground state. The U moments align along the $c$ axis retaining a large magnetic anisotropy. These findings highlight the role of the $5f$ orbitals from uranium in this structural family in driving both magnetic ordering and structural modulation and distinguish UV$_6$Sn$_6$ from its lanthanide-based analogs.

cond-mat.str-el↗