Spin-Hamiltonian parameters and tetragonal distortion due to the Jahn–Teller effect for Cu2+ centres in trigonal Zn(BrO3)·6H2O crystal

The spin-Hamiltonian parameters (g factors g_//, g_⊥ and hyperfine structure constants ⁶⁵A_// and ⁶⁵A_⊥) for the tetragonal Cu²⁺ centres in trigonal Zn(BrO₃)·6H₂O crystal are calculated from the complete diagonalization (of energy matrix) method (CDM) based on the cluster approach. In the CDM, the Zeeman and hyperfine interaction terms are added to the Hamiltonian in the conventional CDM and the contributions to the spin-Hamiltonian parameters from both the spin-orbit coupling parameter of central d⁹ ion and that of ligand ion are included. The calculated spin-Hamiltonian parameters of Zn(BrO₃)·6H₂O: Cu²⁺ show good agreement with the experimental values and the tetragonal elongation (characterized by ΔR=R_// ? R _⊥, where R_// and R _⊥ represent the metal-ligand distances parallel with and perpendicular to the C₄ axis) due to the static Jahn–Teller effect is obtained from the calculations. The results are discussed.     

EPR of Cu2+-doped Cs2CO3 and CsHCO3: evidence of the Jahn–Teller effect

Cu²⁺-doped Cs₂CO₃ and CsHCO₃ single crystals were investigated by electron paramagnetic resonance between 113–273 and 173–313 K, respectively. For both single crystals, two sites were observed for the Cu²⁺ at ambient temperature for arbitrary orientations of the single crystals in the magnetic field. However, when the temperature is varied, the spectra indicate the equivalence of the two sites at 225 and 240 K for the single crystals, respectively, to the above order. Below and above these temperatures two sites for Cu²⁺ appear, and below 133 and 173 K the signals do not vary and two sites were always observed. This is attributed to the transition of the dynamic Jahn–Teller effect to a static situation at lower temperatures. Cu²⁺ seems to replace Cs⁺ and the charge compensation is fulfilled by another Cs⁺. Spin-Hamiltonian parameters for both single crystals at ambient temperature are reported and discussed.     

Zero-field splitting parameters and local structures for tetragonal Cr2＋ centers in Cr2＋-doped ZnSe semiconductors

The inter-relation between zero-field splitting （ZFS） parameters and local lattice structures of the （CrSe4）6 clusters in ZnSe semiconductors has been established by using the complete diagonalization （of the energy matrix） method. On the basis of this, the local lattice distortions, the ZFS parameters D, a, F and the optical spectrum for Cr2＋ ions doped into ZnSe are theoretically investigated, and the contributions of the spin singlets have been taken into account. The calculated ZFS parameters are in good agreement with the experimental values. From our calculations, the tetragonal distortion parameters AR = 0.091A and Aθ = 4.28° of Cr2＋ in ZnSe are acquired, and the results suggest that there exists a tetragonal expansion distortion for the local lattice structure of （CrSe4）6- clusters in ZnSe crystals. The influence of the spin singlets on ZFS parameters is also discussed, indicating that the contributions to ZFS parameters a and F cannot be ignored.     

Jahn–Teller effect in the 4T2g excited state of Cr3+ ion in Cs2NaYF6 crystal

Calculations of the fine structure of Cr³⁺ energy levels in Cs₂NaYF₆ accompanied by estimations of the Jahn–Teller (JT) stabilization energy in the first excited ⁴T_2g state of Cr³⁺ ion are presented. Two independent approaches—effective second-order spin–orbit Hamiltonian and analysis of the potential energy surfaces—are used. The JT energy was estimated to be 216 and 257 cm^–1 in the first and the second models, respectively. It is shown that the octahedral [CrF₆]^3? complex undergoes an equatorial expansion by 0.09 Å and an axial elongation by 0.02 Å due to the combined effect of the a_1g and e_g normal modes.     

Determining the Parameters of the Jahn–Teller Effect in Impurity Centers from Ultrasonic Experiments: Application to the ZnSe : Ni2+ Crystal

Physics of the Solid State - The earlier developed methodology of ultrasonic investigation for determining the parameters of the Jahn–Teller effect (JTE) in impurity centers in crystals is...     

On the guiding principles for understanding of geometrical structures of the CaMn4O5 cluster in oxygen-evolving complex of photosystem II. Proposal of estimation formula of structural deformations via the Jahn–Teller effects

ABSTRACT

Atmospheric oxygenation and evolution of aerobic life on our earth are a result of water oxidation by oxygenic photosynthesis in photosystem II (PSII) of plants, algae and cyanobacteria. The water oxidation in the oxygen-evolving complex (OEC) in PSII is expected to proceed through five oxidation states, known as the S _i (i = 0, 1, 2, 3 and 4) states in the Kok cycle, with the S₁ being the most stable state in the dark. The OEC in PSII involves the active catalytic site made of four Mn ions and one Ca ion, namely the CaMn₄O₅ cluster. Past decades, molecular structures of the CaMn₄O₅ cluster in OEC in PSII have been investigated by the extended X-ray absorption fine structure (EXAFS). The magneto-structural correlations were extensively investigated by electron paramagnetic resonance (EPR) spectroscopy. Recently, Kamiya and Shen groups made great breakthrough for determination of the S₁ structure of OEC of PSII by the X-ray diffraction (XRD) and X-ray free-electron laser (XFEL) experiments, providing structural foundations that are crucial for theoretical investigations of the CaMn₄O₅ cluster. Large-scale quantum mechanics/molecular mechanics calculations starting from the XRD structures elucidated geometrical, electronic and spin structures of the CaMn₄O₅ cluster, indicating an important role of the Jahn–Teller (JT) effect of Mn(III) ions. This paper presents theoretical formulas for estimation of the JT deformations of the CaMn₄O₅ cluster in OEC of PSII. Scope and applicability of the formulas are examined in relation to several different structures of the CaMn₄O₅ cluster proposed by XRD, XFEL, EXAFS and other experiments. Implications of the computational results are discussed for further refinements of geometrical parameters of the CaMn₄O₅ cluster.     

Higher borides and oxygen-enriched Mg–B–O inclusions as possible pinning centers in nanostructural magnesium diboride and the influence of additives on their formation

The study of high pressure (2 GPa) synthesized MgB₂-based materials allows us to conclude that higher borides (with near MgB₁₂ stoichiometry) and oxygen-enriched Mg–B–O inclusions can be pinning centers in nanostructural magnesium diboride matrix (with average grain sizes of 15–37 nm). It has been established that additions of Ti or SiC as well as manufacturing temperature can affect the size, amount and distribution of these inclusions in the material structure and thus, influence critical current density. The superconducting behavior of materials with near MgB₁₂ stoichiometry of matrix is discussed.     

Effect of Cu–Al double substitution on the electrical properties of Bi4V2O11

Bi₂Cu_0.1?xAl_xV_0.9O_5.35?x/2?δ, 0.02 ≤ x ≤ 0.08, were synthesized by standard solid-state reaction route. Structural and electrical properties of samples are characterized by X-ray diffraction (XRD), differential thermal analysis (DTA), Fourier transform infrared (FT-IR) and alternating current (AC) impedance spectroscopy. The tetragonal γ′ phase structure is preserved to room temperature with compound x = 0.02. The stabilization of β orthorhombic phase is observed for compositions 0.04 ≤ x ≤ 0.05. As the Al content increases, the monoclinic α phase is evidenced for materials 0.06 ≤ x ≤ 0.08. The electrical investigation of Bi₂Cu_0.1?xAl_xV_0.9O_5.35?x/2?δ system has been performed in the frequency range from 20 Hz to 1 MHz using AC impedance spectroscopy. The impedance spectra indicate the two semicircle arcs associated with the bulk and grain boundary resistances at temperature below ～450 ^○C. The conductivity generally changes when Al is substituted. The highest conductivity at 300 ^○C (σ = 2.55 × 10^?4 S cm^?1) is shown for x = 0.02.     

Investigation of the EPR parameters of the rhombic Cu2+ center in (NH4)2Mg(SO4)2·6H2O single crystal

The electron paramagnetic resonance (EPR) parameters (g factors g_x, g_y, g_z and hyperfine structure constants A_x, A_y, A_z) for Cu²⁺ in (NH₄)₂Mg(SO₄)₂·6H₂O (DHMS) crystal are theoretically investigated using the high-order perturbation formulas of these parameters. In the calculations, the ligand orbital and spin–orbit coupling for the impurity Cu²⁺ are taken into account; the required crystal-field parameters are estimated from the superposition model which enables correlation of the crystal-field parameters and hence the EPR parameters with the local structure of the impurity center. The ligand orbital and the spin–orbit coupling contributions are included on the basis of the cluster approach. Based on the calculation, the theoretical EPR parameters show good agreement with the observed values. The results are discussed.     

Ultrasonic study on the Jahn–Teller effect near different phase transitions in La1/3Sr2/3MO3 (M=Mn,Fe, Co)

The longitudinal ultrasonic velocity (V_l), magnetization and resistivity of polycrystalline La_1/3Sr_2/3MO₃ (M=Mn, Fe, Co) have been measured between 20 and 300 K. Dramatic anomalies in V_l were observed near the temperature of the charge-ordering transition (CO), charge disproportionation transition (CD), and ferromagnetic transition (FM), which are explained by the Jahn–Teller effect originating from the M ions (M=Mn, Fe, Co). However, the detailed form of these anomalies is different, which is strongly depended on the M ion's unique electronic structure. For La_1/3Sr_2/3MO₃ (M=Mn, Fe), the V_l exhibits a valley around the CO transition temperature because of the localization of the Jahn–Teller active ions (Mn³⁺, Fe⁴⁺). And due to the instability of Fe⁴⁺, the CD transition occurs in La_1/3Sr_2/3FeO₃, which results in another softening in V_l, while only normal increase is observed in La_1/3Sr_2/3MnO₃. For La_1/3Sr_2/3CoO₃, the local lattice distortion via the Jahn–Teller effect of intermediate spin Co³⁺ leads to the velocity anomaly in the ferromagnetic metallic state.     

Study of the effect of the substitution of O by F,S, and Cl on the superconductivity of Y1Ba2Cu3O7−δ

We have studied the superconducting transitions of Y₁Ba₂Cu₃D _x O_7–x– samples where O has been replaced by D=F, S, or Cl andx=0.2-2. No single phase compounds were obtained forx>0.2. No dramatic increase inT _c was registered. Generally the results can be interpreted as a dilution of the highT _c 1 2 3 phase by other phases as the dopant concentration increases, ultimately suppressingT _c completely. Possibly there is an increase ofT _c for an intermediate dopant range,x, in the Cl series.