Czesław Rudowicz’s research while affiliated with Adam Mickiewicz University in Poznań and other places

Herein we utilize computational methods for crystal field (CF) parameter (CFP) modelling to analyse experimental energy levels obtained from optical spectra of transition metal (TM) ions doped in crystalline hosts. These methods enable assessing the influence of structural distortions and chemical environments on the CF energy levels and thus optical properties of host-dopant systems. As a case study, we selected Ni3+ and Co2+ (3d7) ions doped into: (a) cubic pyrochlores A2Ti2O7 (A=Y, Gd), and (b) layered-hexagonal MCl2 (M = Cd, Mg). The interplay of octahedral and trigonal CF, and spin-orbit coupling leads to low-spin 2Eg for Ni3+ and high-spin 4T1g ground states for Co2+, impacting the optical properties of doped hosts. Semiempirical approaches: exchange charge model (ECM) and superposition model (SPM), are employed. For CFP modelling in titanates, the symmetry adapted axis system is adopted, whereas in chlorides - the modified crystallographic axis system. The predicted CF energy levels and g-factors compare well with experimental data. Ni3+ ion shows stronger cubic CF with larger CFPs and smaller distortions than Co2+ ion. For Co2+ ions across hosts, the SPM predicts more accurate CFPs and thus the CF energy levels than ECM. The behaviour of Co2+ differs substantially from that of other TM ions in doped hosts. Due to strong absorption/emission in the visible and infrared wavelength region, these hosts are promising for fabrication of infrared laser and sensor devices. This work highlights importance of fine-tuning structural properties and chemical environment, thus offering deeper insights into the structural and optical properties of the studied systems. The results may be useful for designing other TM ions doped hosts tailored for specific technological applications.

Studying magnetic properties of LiTmF4, a recognized insulating Van Vleck paramagnet, can hold promise for advancements in quantum computing, MRI, spintronics, material design, and potentially, single-photon technologies. This study may be pivotal due to challenges in simulating noncollinear magnetism using density functional theory (DFT), requiring more sophisticated spin configurations and time-consuming spin relaxations or embedded dynamical mean field theory. Instead, we utilize two distinct efficient and reliable schemes— ab initio DFT and a semiempirical superposition model, both integrated with crystal field (CF) theory (including Zeeman effect) and underpinned by statistical mechanics—to analyze noncollinear paramagnetic properties. At the core of this investigation is the S4 site symmetry of the Tm3+ ion, which admits several sets of six (seven) independent CF parameters (CFPs) under the reduced (complete) approach generated by suitable rotations of the coordinate system. By applying the Noether theorem, we show that these numerically distinct sets are physically equivalent. This is evidenced by computing the several conserved CFP quantities predicted by the Noether theorem, which exhibit notable coherence across different data sets. Using one of these equivalent sets of 7 CF parameters, as computed in [Phys. Rev. B 102, 045120 (2020)] under the complete approach, this study explores the theoretical analysis of multiplet splitting induced by the CF and the external magnetic field within the Tm3+ ion lattice in LiTmF4. We investigate the magnetic moment per ion and the temperature dependencies of magnetic susceptibility, utilizing a Hamiltonian, including the free ion, CF terms, and Zeeman interaction. The agreement of our findings with existing experimental data accentuates the efficacy of the proposed approach in reproducing magnetic properties in LiTmF4, providing a significant analytical tool for the analysis of EPR spectra in terms of the defined Zeeman g tensor. This research may stand as a pivotal guide in the accurate determination of magnetic properties, potentially influencing significant advancements in technology and materials science.

We investigate Dy-based coordination polymer C11H18DyN3O9 (Dy-CP) exhibiting single-ion magnet (SIM) properties, e.g., quantum tunnelling of magnetization (QTM), magnetic anisotropy, magnetic relaxation, and effective energy barrier (Ueff). To elucidate the underlying mechanisms, crystal field parameters (CFPs) for Dy3+ ions were modelled using radial effective charge model (RECM) and superposition model (SPM), and computational packages SIMPRE and SPECTRE. The modelled CFPs enable predicting energy levels and associated wave functions, which successfully explain the field-induced Dy-CP SIM properties. So-calculated magnetic susceptibility and isothermal magnetization match experimental data reasonably well. The smaller energy separations of the first (01 31 cm1) and the second (02 = 74 cm1) excited Kramers doublets suggest small Ueff = 65 cm1 for Dy-CP. The magnetic moments of Dy3+ ions exhibit an easy-axis type magnetic anisotropy in the ground state, but change orientation in the excited states due to mixing of states from different Kramers doublet. Low-symmetry CF components play crucial role in connecting different |MJ> states within the ground multiplet, resulting in QTM and magnetic relaxation to the ground state occurring via the excited states. The RECM and SPM calculated CFP sets are standardized employing the 3DD package to enable meaningful comparison and assessing their mutual equivalence. The results demonstrate the correlation between structural and electronic features of the molecule and site symmetry and distortion of the local coordination polyhedra with SIM properties, offering insights for rational design of new SIMs. The importance of considering low-symmetry aspects in CFP modelling for accurate predictions of magnetic properties is highlighted. This study provides deeper understanding of field-induced behaviour in rare-earth-based SIMs and approaches for rationalization of experimentally measured SIMs’ properties.

Effects of orbit-lattice interaction (OLI), structural distortions, and lattice strain on energy levels, crystal-field parameters (CFPs) and zero-field splitting parameter (ZFSPs) for metal ions in ligands polyhedra are largely unexplored. This study examines Ni2+ doped pyrochlores: Y2Ti2O7 and Gd2Ti2O7 involving static and dynamic structural distortions causing stress-strain effects in Ti/Ni-O6 polyhedra. Incorporation of OLI enables correlating lattice strains with changes in CFPs. Utilizing exchange charge model (ECM) and superposition model (SPM) for CFP modelling, OLI and stress-strain effects are investigated. So-obtained CFPs serve as input for CFA/MSH program predicting CF energies, states, and ZFS magnitude (equal ZFSP |D|) of Ni2+ ions. Relation between ZFS magnitude (and CF strength) and nephelauxetic ratio is studied. Results obtained with and without OLI, while including or excluding distortions, reveal profoundly impact of OLI on CF energy levels and ZFSP of Ni2+ ions. The variations of SPM/CFPs due to OLI and/or distortions are analyzed in terms of directional strain components. This computational study offers insights into the structural, spectroscopic, and magnetic properties of Ni2+:Y2Ti2O7 and Ni2+:Gd2Ti2O7. Our combined modelling approach is useful for exploring other hosts doped with transition or rare-earth ions at arbitrary symmetry sites, which are of interest for computational materials science.