Local lattice distortions and ligand hyperfine interactions in Eu2+-and Gd3+-doped fluorites

A computer simulation of lattice distortions around an impurity ion Eu²⁺ in MeF₂ fluorites (Me=Ca, Sr, Ba) is reported. ENDOR data on displacements of F⁻ ions distant from an impurity center were used to determine the parameters of the Eu²⁺-F⁻ short-range interaction potential. A theoretical study of the effect of hydrostatic pressure on the impurity-center local structure has been made. A comparison with experimental data permits a conclusion that the calculated ligand displacements are reliable. An experimental ENDOR investigation of the ligand hyperfine interaction (LHFI) in MeF₂:Gd³⁺ crystals (Me=Ca, Sr, Pb, Ba) has been performed. The results obtained in the simulation are used to describe the LHFI of impurity ions with the nearest-neighbor fluorine environment. The contributions to LHFI associated with ligand polarization are shown to constitute 10–50% of the experimental LHFI constants. The inclusion of such contributions results in practically linear dependences of the remaining short-range part of the LHFI on distance. Fiz. Tverd. Tela (St. Petersburg) 40, 2172–2177 (December 1998)     

Theoretical study of local lattice structure of Fe-V cd and Fe-Li systems in RbCdF3 and CsCdF3 crystals

A theoretical method for investigating the inter-relation between the electronic and the molecular structures of a 3d⁵ ion in a tetragonal ligand-field has been established on the basis of a 252×252 complete energy matrix. By means of this method, the local structure of the Fe³⁺-V _cd and Fe³⁺-Li⁺ systems in RbCdF₃:Fe³⁺ and CsCdF₃:Fe³⁺ crystals are determined by the experimental EPR spectrum. Our calculation show that the local lattice structure around an octahedral Fe³⁺ center has a compression distortion along the crystalline axis in RbCdF₃ as well as in CsCdF₃ crystals, and that the compression magnitude of a tetragonal Fe³⁺-Li⁺ system is larger than that of the Fe³⁺-V _cd system. This may be ascribed to the fact that a Fe³⁺ ion replaces a Cd²⁺ ion and a Li⁺ ion substitutes for another Cd²⁺ ion next to the Fe³⁺ ion in the Fe³⁺-Li⁺ system, and the Li⁺ ion will shift to the Fe³⁺ ion, which pushes the F₁ ion toward the Fe³⁺ ion. Using this method, the experimental EPR parameters , and are also interpreted simultaneously.     

Energy levels and crystal-field analysis of Tm in YAl3(BO3)4 crystals

The electronic structure of the Tm³⁺ in YAl₃(BO₃)₄ crystals has been investigated by means of low temperature absorption and emission spectroscopy in the 5000-30,000 cm⁻¹ range. The assignment of the lines composing the observed manifolds to transitions between the Stark levels of Tm³⁺ is complicated by the presence of extra features having different origins. The energy levels scheme of the doping ion has been compiled after a careful analysis of the spectra by reproducing the observed transitions by means of theoretical calculations based on a Hamiltonian, including the free ion and the crystal field (CF) terms. The agreement between experimental and calculated energy values was reasonably good, the overall r.m.s. deviation being 16 cm⁻¹. The resulting CF parameters have been tabulated and compared with those reported in literature for other rare earth ions doped in YAB. The analysis of trends observed along the lanthanide series evidences some inconsistencies and the necessity of a systematic investigation of these systems.     

Inelastic light scattering and photoluminescence in the mixed valence systems Eu3O4, Eu3S4 and Sm3S4

The thermally activated valence fluctuations of Eu₃S₄ and Sm₃S₄ have been studied using light scattering and photoluminescence techniques and are compared with measurements on the non-fluctuating compound Eu₃O₄, Eu₃S₄ exhibits an anomalous vibrational mode which is associated with the frequency factor of hopping, ν₂=1.3×10¹³ sec^?1. In Sm₃S₄ electronic Raman scattering is observed within the ⁷F₉ multiplet of the Sm²⁺ ion. An anomalous frequency shift of the5d-4f photoluminescence emission band in Eu₃S₄ is related to the temperature dependent fluctuation rate which passes through the reference time scale of the photoluminescence. Intra-4f photoluminescence has also been observed in Eu₃S₄ and Sm₃S₄.     

Structure and magnetic properties of a new single-molecule magnet [Mn11Fe1O12 (CH3COO)16(H2O)4].2CH3COOH.4H2O

A new single-molecule magnet [Mn₁₁Fe₁O₁₂ (CH₃COO)₁₆(H₂O)₄]?2CH₃COOH?4H₂O (Mn₁₁Fe₁) has been synthesized.The structure has been studied by the single crystal x-ray diffraction. The difference of Jahn--Teller distortion between Fe³⁺ and Mn³⁺ ion reveals that Fe³⁺ ion substitutes for Mn³⁺ ion on the Mn(3) sites in the Mn₁₂ skeleton. The temperature dependence of the magnetization gives a blocking temperature T_B=1.9K for Mn₁₁Fe₁. Based on the magnetization process analysis of the crystal at T=2K, we suggest that Mn₁₁Fe₁ has the ground state with a total spin S= 11/2.     

KZnF3:Cr3+和KMgF3:Cr3+四角对称电子顺磁共振参量与晶格缺陷的研究

用Newman叠加模型研究了KZnF₃:Cr³⁺四角对称基态的零场分裂,证实了Zn²⁺空位和畸变的存在;并指出,空位对晶场的贡献不可忽略。计算得到:KZnF₃:cr³⁺晶体[0,01]方向的一个F^-配体向Cr³⁺移动Δ(KZnF₃)=0.0029-0.0043nm。还研究了KMgF₃:Cr^{3+关键词：}     

Microwave spectrum of (CH3)3CCN-SO3

Eight rotational transitions of the complex (CH₃)₃CCN-SO₃ have been recorded using pulsed-nozzle Fourier transform microwave spectroscopy and a series of ab initio calculations has been performed. The complex is a symmetric top with free or nearly free internal rotation of the SO₃ and (CH₃)₃CCN subunits. The nitrogen-sulfur bond distance is determined to be 2.394(19) Å. Calculations at the MP2/aug-cc-pVTZ level/basis, which are in excellent agreement with the experimental results, give a binding energy of 11.0 kcal/mol relative to (CH₃)₃CCN and SO₃. Physical properties of the system, including N-S bond length, N-S-O angle, binding energy, and the degree of electron transfer (obtained from Townes and Dailey analysis of the ¹⁴N nuclear quadrupole coupling constant) are compared with those of similar complexes. The proton affinity of the base is a useful parameter for ordering complexes in the series.     

Optical absorption spectrum and local symmetry of Tm ion in TmAl3(BO3)4 crystal

With the help of group theory analysis of absorption spectra of the transition ³H₆ → ³F₃ of Tm³⁺ ion in TmAl₃(BO₃)₄ crystal, measured at several temperatures from 1.8 till 293 K, it has been shown, that the local symmetry of the Tm³⁺ environment is C₃ and it decreases to C₁ at a low temperature. Effective selection rules and polarizations of lines at high enough temperatures (when the line-width is larger than the splitting in C₃ symmetry) have been obtained.     

Antiferromagnetic phase transition in garnet-type AgCa2Co2V3O12 and AgCa2Ni2V3O12

Antiferromagnetic phase transition in two vanadium garnets AgCa₂Co₂V₃O₁₂ and AgCa₂Ni₂V₃O₁₂ has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature T_N=6.39 K for AgCa₂Co₂V₃O₁₂ and 7.21 K for AgCa₂Ni₂V₃O₁₂, respectively. The magnetic susceptibilities exhibit broad maximum, and these T_N correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χ_max). The magnetic entropy changes from zero to 20 K per mol Co²⁺ and Ni²⁺ ions are 5.31 J K⁻¹ mol-Co²⁺-ion⁻¹ and 6.85 J K⁻¹ mol-Ni²⁺-ion⁻¹, indicating S=1/2 for Co²⁺ ion and S=1 for Ni²⁺ ion. The magnetic susceptibility of AgCa₂Ni₂V₃O₁₂ shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μ_eff=3.23 μ_B Ni²⁺-ion⁻¹ and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa₂Co₂V₃O₁₂. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state ²E(t₂⁶e) and the first excited state is spin quartet state ⁴T₁(t₂⁵e²) which locates extremely close to the ground state. The low spin state S=1/2 for Co²⁺ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.     

Fluorescence of Mn2+ in CaCO3

The fluorescence of Mn²⁺ ion as impurity in CaCO₃ has been investigated. Emission bands from the ⁴D(E_g), ⁴D(T_2g) and ⁴G(T_1g) levels have been observed. The analysis of excitation and emission spectra has allowed to obtain the values of field strength (Dq) for the excited energy levels of Mn²⁺ in CaCO₃ lattice. The temperature dependence of excitation and emission spectra yield an activation energy for thermal quenching of luminescence very close to theoretical calculation. The behaviour of luminescence lifetime with temperature has also been obtained.     

Crystal field parameters and energy level structure of the MnO4 tetroxo anion in Li3PO4, Ca2PO4Cl and Sr5(PO4)3Cl crystals

A comparative study concerning the electronic structure of the Mn⁵⁺ ion in the Li₃PO₄, Ca₂PO₄Cl, Sr₅(PO₄)₃Cl host lattices has been carried out in the framework of the exchange charge model. The crystal field parameters have been evaluated using the structural data as the only input information. The 10 K absorption spectra of the investigated compounds have been measured in order to verify the correspondence between experimental and calculated energy levels. A systematic trend of the crystal field splitting of the most intense transitions has been evidenced and discussed by considering the symmetry properties of the coordination polyhedra around Mn⁵⁺.     

Absolute sense of pyroelectric p3 and piezoelectric d33 coefficients in 3La (IO3)3.HIO3.7H2O

The pyroelectric coefficient p₃ in 3La(IO₃)₃.HIO₃.7H₂O has an average value 2.0×10^-5 Cm^-2 in the temperature range 152 to 240 K. The resistivity decreases from 10¹² to 10¹⁰ ohm-cm between 258 and 338 K. At 298 K, the piezoelectric coefficient d₃₃  19×10^-12CN^-1. Positive polarity is generated on (001) by increasing temperature or tensile stress. A displacement toward (001) by La³⁺ or H₃O⁺ ions of 1×10^-4 Å per K or 10⁶Nm^-2, or rotation of the water molecule or iodate ion dipoles by about 5 arc minutes per K or 10⁶Nm^-2, produces the observed polarity.     

Preparation,etudes optiques et dielectriques de monocristaux de Na5W3O9F5

Optical and dielectric measurements have been performed on Na₅W₃O₉F₅ single crystals which have a chiolite-related structure. The polar axis has been determined. The transition is of first order type. A strong conduction has been detected, following a direction parallel to the planes of the octahedra, it is due probably to a Na⁺ ion mobility.     

Dielectric properties and temperature stability of BaTiO3 co-doped La2O3 and Tm2O3

The influence of La₂O₃ and Tm₂O₃ co-doping on the dielectric properties and the temperature stability of BaTiO₃ was investigated. BaTiO₃ ceramics were prepared with the compositional formula of (Ba_1−xLa_x)(Ti_1-x/4−yTm_y)O₃. La₂O₃ and Tm₂O₃ co-doping in BaTiO₃ mainly had effects on an increase in the dielectric constant and the temperature stability, respectively. The increase of La₂O₃ concentration and the decrease of Tm₂O₃ concentration in BaTiO₃ resulted in a decrease of lattice parameter and tetragonality because La³⁺ ion substituting for Ba site is smaller than Ba²⁺ ion and Tm³⁺ ion substituting for Ti site is larger than Ti⁴⁺ ion. With the increase of La₂O₃ and the decrease of Tm₂O₃, the dielectric constant of BaTiO₃ was enhanced in spite of the reduction of tetragonality. P-E hysteresis measurements revealed that this phenomenon was based on the improvement of remanent polarization with the increase of La₂O₃ concentration. The introduction of excess Tm₂O₃ in BaTiO₃ suppressed the grain growth and BaTiO₃ ceramics showed higher temperature stability due to the stable tetragonal structure and the small grain size with the increase of Tm₂O₃ concentration.     

H+5团簇离子及其中性团簇产物H3和H4

报道了H⁺₅的实验结果.分析讨论了H⁺₅的 形成和分解途径.根据理论分析，以稳定的H⁺₃为核心与一个或多个氢分子结合可能形成稳定的H^{+_n氢团簇离子.另一方面，在高频离子源中, 有发生H+₃与H₂反应的条件.实 验中，从高频离子源引出的离子束被静电加速器加速，然后用9 关键词： +}₅团簇离子')" href="#">H⁺₅团簇离子 3中性团簇')" href="#">H₃中性团簇 4中性团簇')" href="#">H_{4中性团簇}     

Cross sections for slow ion production and charge transfer in H+3(D+3)−H2(D2) collisions

Slow ion production cross sections for collisions of H⁺₃ and D⁺₃ ions with H₂ and D₂ have been measured at collision energies between 100 eV and 500 eV. The values vary from 2 × 10^-17 cm² to 6 × 10^-17 cm². The smaller cross sections for D₃ projectiles may be explained as an internal energy effect.     

Far infrared absorption of Fe2+ in KMgF3

Far infrared absorption in cubic KMgF₃ doped with Fe²⁺ is reported . A line is observed at 87cm^?1, which is assigned to the (Γ_5g → Γ_3g, Γ_4g) transition in the Fe²⁺. The reduction in the spin-orbit coupling from the free ion value has been predicted by Ham, Schwarz and O'Brien.     

Soft x-ray absorption of FePS3 and NiPS3

With the partial yield technique we measured the absorption spectra of several core levels in FePS₃ and NiPS₃. The M_{2, 3} spectra of Fe and Ni are interpreted as localized transitions 3p⁶3d^m?3p⁵ 3d^m+1 of the transition metal ion, split into a multiplet by final state multiconfiguration interaction. The P L_{2, 3}, S L_{2, 3}, and S L₁ spectra are similar to each other and are interpreted in terms of the projected density of states of the conduction bands derived from the states of the (P₂S₆)^4? cluster.     

Etude par resonance paramagnetique electronique des verres du systeme B2O3-Na2O-Na3PO4

An EPR study of fast Na⁺ ion vitreous conductors of the system B₂O₃?Na₂O?Na₃PO₄ has been carried out. After X-ray irradiation two types of paramagnetic centers have been observed. The first one is of boron-oxygen hole center (B.O.H.C.) type and similar to that earlier observed for other alkali borate glasses. The second one is of PO₄^2? type. A simulation of its spectrum has been achieved and a defect model discussed.     

Optical absorption and thermoluminescence properties of ZnF2-MO-TeO2 (MO=As2O3, Bi2O3 and P2O5) glasses doped with chromium ions

Optical absorption, thermoluminescence, infrared spectra and differential thermal analysis of three different tellurite glass systems viz., ZnF₂-As₂O₃-TeO₂, ZnF₂-Bi₂O₃-TeO₂ and ZnF₂-P₂O₅-TeO₂ containing 0.4% of Cr₂O₃, have been investigated. Results have been analysed in the light of different oxidation states of chromium ion and the most suitable host for lasing Cr³⁺ ions has been identified and reported.