57Fe and 119Sn probe M?ssbauer spectroscopy investigation of perovskite-type double manganite family CaCuxMn7?xO12 (x = 0, 0.15, 3)

CaCu _x Mn_7−x O₁₂ (x = 0, 0.15, 3) manganites were studied by ⁵⁷Fe and ¹¹⁹Sn probe M?ssbauer spectroscopy. It was established that the ⁵⁷Fe and ¹¹⁹Sn probe cations are stabilized in the octahedral positions of the manganite structures by substituting the manganese cations. The magnetic and structure phase transition temperatures of CaMn₇O₁₂ and CaCu_0.15Mn_6.85O₁₂ manganites fell upon the introduction of ⁵⁷Fe probe cations into their structures. It was concluded that intrasublattice Mn⁴⁺-O-Mn⁴⁺ exchange interactions play a noticeable role in the formation of the magnetic structure of CaCu₃Mn₄O₁₂ manganite.     

Anomalous effect of magnetic field on the low-temperature thermal expansion of the Bi2Sr2?xLaxCuO6+δ and Nd2?xCexCuO4?δ HTSC systems in the non-superconducting phase

The thermal expansion of single crystals of the Bi₂Sr_2−x La _x CuO_6+δ high-temperature superconducting (HTSC) system in the insulating phase with compositions having no superconducting transition to a temperature of 1.8 K (x ≥ 0.8) is measured in an arbitrary direction in the (ab) plane in the temperature range of 7–50 K. Temperature regions of material compression upon heating are found. The study of anomalies in magnetic fields of 3 and 6 T, parallel and perpendicular to the c-axis revealed an anisotropic and nonmonotonic effect of the field on thermal expansion. Such anomalies for the n-type Nd_2−x Ce _x CuO_4−δ HTSC sample also having no superconducting transition are detected for the first time. The results show that the anomaly nature is caused by anisotropic electronic ordering, probably, by the charge density wave in the CuO₂ plane and superconductivity fluctuations in the insulating phase.     

Anti‐Stokes Luminescence of Cadmium Telluride Nanocrystals

For the first time, the antiStokes luminescence in colloidal solutions of CdTe nanocrystals on excitation below the absorption edge has been discovered. The maximum spectral shift to the shortwave region relative to the excitation energy E _con ^max = 319 meV is obtained for meansized nanocrystals (2.5 nm). The conversion efficiency of the absorbed radiant energy is 1.3·10^–2%. The rise in the antiStokes photoluminescence intensity with increasing temperature and the linear dependence on the exciting radiation intensity have been established. It is shown that the effect observed cannot be caused by twophoton excitation or by Auger recombination. It is assumed that the basic mechanism of the luminescence observed is the radiative recombination through the energy levels of the states attributable to the disturbance of the crystal structure in the nanocrystals.     

Specific features of the structural transformations in La1 − x Ca x Mn0.98 57Fe0.02O3 + δ (x = 0.05−0.50)

The specific features of the structural transformations in La_{1 ? x} Ca _x Mn_0.98 ⁵⁷Fe_0.02O_{3 + δ} (x = 0.05?0.50) at different Ca concentrations have been studied using Mössbauer spectroscopy and X-ray diffraction. Variations of the heat treatment conditions (annealing in vacuum and in air) cause reversible phase transitions. For calcium concentrations to 20%, a whole set of phases with reversible structural transitions, such as PnmaII ? PnmaI ? $R\bar 3c$ , can be obtained using the corresponding heat treatment. The phases are successively suppressed with increasing calcium concentration. The rhombohedral phase is suppressed for x = 0.20. When the Ca concentration exceeds 20%, the PnmaII phase is suppressed; as a result, the PnmaI phase with equal concentrations of Mn⁴⁺ and calcium stable during any heat treatments remains. The specific features of phase formation in the La_{1 ? x} Ca _x Mn_0.98 ⁵⁷Fe_0.02O_{3 + δ} compound doped with calcium and in the basis LaMnO_{3 + δ} compound have been compared.     

Mössbauer and magnetic studies of nanocomposites containing iron oxides and humic acids

A sample of volcanic ashes emanated from the Osorno volcano, southern Chile, was characterized with X-ray fluorescence, X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy, in an attempt to identify the iron-bearing minerals of that geologically recent magmatic deposit. X-ray patterns indicated that the sample is mainly constituted of anorthite, Fe-diopside-type and Ca-magnetite. The crystallographic structures of these dominant iron minerals are proposed on basis of their chemical composition and corresponding Mössbauer data to support models refined by fitting powder X-ray diffraction data with the Rietveld algorithm.     

Luminescence Monitoring of Lanthanide Catalysts for Diene Polymerization

Changes in the luminescence characteristics of Tb³⁺ and Ce³⁺ lanthanide ions in the interaction of LnCl₃ · 3(BuO)₃PO complexes with iso-Bu₃Al in the absence and presence of piperylene were studied. As a result, a new luminescence approach was proposed to study reaction mechanisms in the synthesis of lanthanide catalysts for diene polymerization.     

Synthesis and phase formation in M 0.5(1+x) 2+ FexTi2−x (PO4)3 phosphate series

New complex phosphates of titanium, iron, and alkaline-earth metals have been synthesized. X-ray powder diffraction, differential thermal analysis (DTA), and IR spectroscopy are used to study phase formation in the series of M_0.5(1+x)Fe_xTi_2?x (PO₄)₃ (M = Mg, Ca, Sr, Ba) phosphates. Individual compounds and solid solutions are found to crystallize in the NaZr₂(PO₄)₃ and K₂Mg₂(SO₄)₃ structure types. Their crystal parameters are calculated. CaFeTi(PO₄)₃ is studied using Mössbauer spectroscopy. Its structure is refined by the Rietveld method: space group $R\bar 3$ c, Z = 6, a = 8.5172(1), Å, c = 21.7739(4) Å, V = 1367.91(4) ų.     

The special features of the crystal structure and properties of oxides with mixed conductivity based on lanthanum gallate

The paper presents a review of works on the synthesis, structural composition effects, phase transitions, and electrical conductivity properties of multicomponent solid solutions based on heterosubstituted lanthanum gallate (La,A)(Ga,M)O_{3 ? y} . High-temperature phase transitions and structural and charge ordering effects were studied. The presence of iron cations in different valence states was proved; the relative contents of these cations depended on the x parameter and nonstoichiometry parameter y of the base composition. For M = Fe, antiferromagnetic ordering was observed; its temperature interval was determined by the concentration of iron cations in the high-spin state. The total conductivity was found to increase as the concentration of transition metal cations grew because of an increase in the electronic conductivity component. The data on structural parameters and dc and ac conductivity substantiated the conclusion that the highest ionic conductivity and permeability to oxygen were characteristic of iron-containing oxides. The results obtained are evidence that crystal chemical factors play a determining role in the formation of the ion-conducting properties of anion-deficient perovskite-like oxides.     

Whitlockite solid solutions Ca9?xMxR(PO4)7 (x = 1, 1.5; M = Mg, Zn, Cd; R = Ln, Y) with antiferroelectric properties

Whitlockite solid solutions Ca_9−x M _x R(PO₄)₇ (M = Mg, Zn, Cd; R = Ln, Y) were synthesized as powders and ceramics using solid-phase synthesis. Dielectric investigations and second harmonic generation (SHG) tests showed that ferroelectric (FE) phase transitions existing in samples with x = 0 change to antiferro-electric (AFE) transitions between two centrosymmetrical phases in samples with x = 1 or 1.5. The calcium-ion solid electrolyte conductivity in Ca_9−x M _x R(PO₄)₇ at high temperatures appears either as a result of an antiferroelectric-paraelectric (AFE-PE) phase transition (for x = 1) or as a result of a separate phase transition near 1173 K (for x = 1.5). The appearance of dielectric properties in whitlockites is discussed with reference to the features of their polar and centrosymmetrical structures. Original Russian Text ? A.V. Teterskii, S.Yu. Stefanovich, B.I. Lazoryak, D.A. Rusakov, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 3, pp 357–363.     

Mössbauer Study of Magnetite Formation by Iron- and Sulfate-Reducing Bacteria

Hyperfine Interactions - Mössbauer investigations of the influence of physicochemical factors and the inert organic substances on the magnetite formation by thermophilic dissimilatory...