Tailoring single-ion magnet properties of coordination polymer C11H18DyN3O9 (Dy-CP) using the radial effective charge model (RECM) and superposition model (SPM).

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 the radial effective charge model (RECM) and superposition model (SPM), and the computational packages SIMPRE and SPECTRE. The modelled CFPs enable the prediction of the energy levels and associated wave functions, which successfully explain the field-induced Dy-CP SIM properties. The so-calculated magnetic susceptibility and isothermal magnetization match the experimental data reasonably well. The smaller energy separations of the first (Δ0-1 ∼ 31 cm-1) and the second (Δ0-2 = 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 doublets. Low-symmetry CF components play a 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.

Eu3+ ions as a crystal-field probe for low-symmetry sites in doped phosphors - a case study: Eu3+ at triclinic sites in Li6RE(BO3)3 (RE = Y, Gd), YBO3 and ZnO and at trigonal sites in YAl3(BO3)4.

We present crystal-field (CF) calculations of energy levels (Ei) of Eu3+ ions doped in various hosts aimed at exploring the low-symmetry properties of CF parameters (CFPs) and reliability of CFP modelling with decreasing site symmetry. The hosts studied are: Li6Y(BO3)3, Li6Gd(BO3)3, YBO3, and ZnO with Eu3+ at triclinic sites; YAl3(BO3)4 with Eu3+ ions at trigonal D3 symmetry. Two independent CFP modelling approaches utilizing the hosts' structural data are employed: the exchange charge model (ECM) and the superposition model (SPM). We adopt the Eu3+ actual site symmetry and not the approximated one. The Ei values calculated using CFPs modelled by the ECM and SPM mutually agree with the observed ones. For triclinic symmetry, the ECM/CFPs and SPM/CFPs were numerically distinct, yet turned out to be physically equivalent yielding identical rotational invariants, Sk (k = 2, 4, 6) and Ei. For trigonal symmetry, both CFP sets agree numerically, thus Sk and Ei are identical. This disparity poses a dilemma, since the modified crystallographic axis system was used in both approaches. The standardization of the triclinic CFPs using the 3DD package was performed to solve this dilemma. It has enabled discussing standardization aspects in experimental and computed CFP sets and elucidating intricate low-symmetry aspects inherent in CFP sets. Understanding of low-symmetry aspects in CF studies may bring about a better interpretation of the spectroscopic and magnetic properties of rare-earth ion doped host crystals. Thus, our study could provide more deep insights into the importance of clear definitions of axis systems and adequate treatment of actual site symmetry in the modelling of CFPs for low-symmetry cases which is essential for technological applications and engineering of rare-earth activated phosphor materials.

EMR studies of the internal motion of Mn(4+) ions in the Sr overdoped (La(1-x)Sr(x))(Ga(1-y)Mn(y))O3 (x/y up to 8) supplemented by magnetic and optical spectroscopy measurements.

The effect of the Sr doping on electronic structure in single crystals of (La(1-x)Sr(x))(Ga(1-y)Mn(y))O3 solid solutions (LSGM) is investigated by means of electron magnetic resonance (EMR). The EMR results are supplemented by magnetic susceptibility and optical spectroscopy measurements. The compositions with small concentration of Mn doping (y<1%) and overdoped content of Sr (the ratio x(Sr)/y(Mn) up to 8) are used to maximally enhance the role of divalent doping. The experimental results provide evidence of the holes delocalization in the overdoped compound (x(Sr)/y(Mn)>1). This delocalization is accompanied by appearance of the new charge transfer transitions in the optical spectrum and dynamical valence change of manganese atoms. Additionally we observe the thermally activated narrowing of resonance EMR lines due to the internal motion, which is characterized by the energy barrier depending strongly on the ratio x(Sr)/y(Mn). The energy barrier is found to be associated with the charge carrier (hole) self-trapped energy. Fitting the EMR spectra in three orthogonal planes to an orthorhombic spin Hamiltonian enables extracting the zero-field splitting (ZFS) parameters and the Zeeman g-factors for Mn(4+) (S=3/2) ions in LSGM. The experimental ZFS parameters are modeled using superposition model analysis based on an orthorhombic symmetry approximation.

Comparative analysis of crystal-field parameters for rare-earth ions at monoclinic sites in AB(WO4)2 crystals: II. Pr3+ and Nd3+ ions in KRE(WO4)2 (RE = Y or Gd), Pr3+ ions in M+ Bi(XO4)2 (M+ = Li or Na and X = W or Mo), and Nd3+ ions in NaBi(WO4)2 and AgNd(WO4)2.

In part I, the crystal-field (CF) parameter (CFP) sets for important potential solid state laser systems Tm(3+), Ho(3+), and Er(3+) ions in KGd(WO4)2 and Tm(3+) ions in KLu(WO4)2 were thoroughly revisited using a general framework for the analysis of CF levels and CFP modeling. In this part the non-standard CFP sets for Pr(3+) and Nd(3+) ions in KR(WO4)2 (R = Y or Gd) and the standard CFP sets for Pr(3+) ions in M(+)Bi(XO4)2 (M(+) = Li or Na and X = W or Mo) and Nd(3+) ions in the related systems NaBi(WO4)2 and AgNd(WO4)2 are analyzed. Due to structural similarity of the hosts, the CFP values for a given trivalent rare-earth (RE(3+)) ion should be quite close in these systems. However, the fitted (and model) CFP sets appear disparate for the systems in question. The standardization criteria are utilized to ensure direct comparability of the apparently disparate CFP sets reported in the literature. The CFP sets standardized by us are compared with the originally standard CFP sets for Pr(3+) and Nd(3+) ions in related AB(XO4)2 systems. Following part I, we argue that meaningful analysis of the mixed CFP sets, i.e. standard and non-standard ones, must take into account the intrinsic features of CF Hamiltonians for orthorhombic and lower symmetry cases, which have not been fully recognized in the literature as yet. The model or fitted CFP sets that belong to disparate regions in the CFP space are intrinsically incompatible, i.e. such sets should not be directly compared. The correlated alternative CFP sets are calculated using monoclinic standardization transformations. The closeness of the standardized CFP sets is assessed in a quantitative way using the closeness factors and the norms ratios. Comparative analysis of the monoclinic CFP sets reported for the titled ion-host systems is carried out and several inconsistencies in the previous studies are clarified. The CFP sets determined by standardization are utilized as starting sets for applications of the multiple correlated fitting technique to independently obtain and additionally verify the fitted CFPs based on published energy levels data. Multiple correlated fittings offer an advantage over the single-fitting tactics by enabling an improved fine-tuning of the final fitted CFPs as well as their interpretation and comparability with the sets obtained by others. The present consistent methodology may enable better understanding of the intricate aspects inherent in the spectroscopic studies for other ion-host systems exhibiting orthorhombic, monoclinic, and triclinic site symmetry.

Comparative analysis of crystal-field parameters for rare-earth ions at monoclinic sites in AB(WO4)2 crystals: I. Tm3+ in KGd(WO4)2 and KLu(WO4)2, and Ho3+ and Er3+ ions in KGd(WO4)2.

The crystal-field (CF) parameters determined by various authors for rare-earth ions at monoclinic sites in AB(WO(4))(2) crystals are reanalyzed using a methodology incorporating several approaches, namely standardization, multiple-correlated fitting technique and closeness of CFP sets. In Part I recent spectroscopic data for Tm(3+) ions in KGd(WO(4))(2) (KGdW) and KLu(WO(4))(2) (KLuW), and Ho(3+) and Er(3+) ions in KGdW, which were interpreted using the free-ion (FI) and CF parameter (CFP) sets, are thoroughly revisited. Our reanalysis enables clarification of several doubtful aspects involved in the previous studies. The initial CFPs for fitting, calculated using the simple overlap model (SOM), differ markedly from the fitted CFPs for Tm(3+) ions in KGdW and KLuW. An inspection of the pertinent CFP sets reveals deeper intrinsic differences between the model and fitted CFPs. The model CFPs and the fitted CFPs for RE(3+) ions in both KGdW and KLuW crystals turn out to be non-standard. Importantly, the model and fitted CFP sets for Tm-KLuW belong to disparate regions of the CFP space and thus are intrinsically incompatible, i.e. such sets should not be directly compared. Thus the CFP sets reported in the literature require reconsideration in view of the intrinsic properties of monoclinic CF Hamiltonians previously not taken into account. Standardization of the originally non-standard CFP sets is carried out to ensure direct comparability of the CFP sets in question with other literature data. The correlated alternative CFP sets are calculated for each original set to facilitate future applications of the multiple correlated fitting technique, which enables improving overall reliability of the fitted CFPs. The closeness of the standardized CFP sets is assessed in a quantitative way. Our considerations indicate also the importance of proper definitions of the axis system used in the CFP model calculations and provide arguments for the nominal meaning of the axis systems assigned to the fitted CFPs. The consistent methodology proposed here may be considered as a general framework for analysis of CF levels and CFP modelling for rare-earth and transition-metal ions at monoclinic symmetry sites in crystals. CFP sets for other rare-earth ions in AB(WO(4))(2) crystals will be reanalyzed in Part II.