A. G. Ivanova

An investigation of the growth of β-BaB2O4 crystals in the BaB2O4-NaF system and new fluoroborate Ba2Na3[B3O6]2F

The phase formation in the BaB2O4-NaF cut has been investigated by the methods of solid-phase synthesis, visual polythermal analysis, and spontaneous crystallization on a platinum loop. A range of primary crystallization of the new compound Ba2Na3[B3O6]2F (hexagonal system, P63/m, a = 7.346(1) Å, c = 12.636(2) Å) is revealed in this cut, which is used to grow single crystals of low-temperature barium borate modification β-BaB2O4.

Spin transition of Fe[superscript 2+] in ringwoodite (Mg,Fe)[subscript 2]SiO[subscript 4] at high pressures

Abstract Electronic spin transitions of iron in the Earth’s mantle minerals are of great interest to deep-Earth researchers because their effects on the physical and chemical properties of mantle minerals can significantly affect our understanding of the properties of the deep planet. Here we have studied the electronic spin states of iron in ringwoodite (Mg0.75Fe0.25)2SiO4 using synchrotron Mössbauer spectroscopy in a diamond-anvil cell up to 82 GPa. The starting samples were analyzed extensively using transmission and scanning electron microscopes to investigate nanoscale crystal chemistry and local iron distributions. Analyses of the synchrotron Mössbauer spectra at ambient conditions reveal two non-equivalent iron species, (Fe2+)1 and (Fe2+)2, which can be attributed to octahedral and tetrahedral sites in the cubic spinel structure, respectively. High-pressure Mössbauer measurements show the disappearance of the hyperfine quadrupole splitting (QS) of the Fe2+ ions in both sites at approximately 45-70 GPa, indicating an electronic high-spin (HS) to low-spin (LS) transition. The spin transition exhibits a continuous crossover nature over a pressure interval of ~25 GPa, and is reversible under decompression. Our results here provide the first experimental evidence for the occurrence of the spin transition in the spinel-structured ringwoodite, a mantle olivine polymorph, at high pressures.

Spin transition of Fe2+ in ringwoodite (Mg,Fe)2SiO4 at high pressures

Abstract Electronic spin transitions of iron in the Earth’s mantle minerals are of great interest to deep-Earth researchers because their effects on the physical and chemical properties of mantle minerals can significantly affect our understanding of the properties of the deep planet. Here we have studied the electronic spin states of iron in ringwoodite (Mg0.75Fe0.25)2SiO4 using synchrotron Mössbauer spectroscopy in a diamond-anvil cell up to 82 GPa. The starting samples were analyzed extensively using transmission and scanning electron microscopes to investigate nanoscale crystal chemistry and local iron distributions. Analyses of the synchrotron Mössbauer spectra at ambient conditions reveal two non-equivalent iron species, (Fe2+)1 and (Fe2+)2, which can be attributed to octahedral and tetrahedral sites in the cubic spinel structure, respectively. High-pressure Mössbauer measurements show the disappearance of the hyperfine quadrupole splitting (QS) of the Fe2+ ions in both sites at approximately 45-70 GPa, indicating an electronic high-spin (HS) to low-spin (LS) transition. The spin transition exhibits a continuous crossover nature over a pressure interval of ~25 GPa, and is reversible under decompression. Our results here provide the first experimental evidence for the occurrence of the spin transition in the spinel-structured ringwoodite, a mantle olivine polymorph, at high pressures.

Specific features of sample preparation from amorphous aluminum alloys for transmission electron microscopy

An aluminum amorphous alloy doped with transition (Fe and Ni) and rare earth (La) metals has been used as an object of systematic study of the structural transformations that are characteristic of different methods of sample preparation for transmission electron microscopy (the mechanical tearing of ribbons, electrochemical thinning, and Ar+-ion etching under different conditions). The results of X-ray diffraction analysis and a calorimetric study of the structure in comparison with electron microscopy data made it possible to determine the optimal method of sample preparation, which ensures minimum distortions in the structure of metastable amorphous alloys with a low crystallization temperature.

Copper rubidium diphosphate, Rb2Cu3(P2O7)2: synthesis, crystal structure, thermodynamic and resonant properties

A new compound, Rb2Cu3(P2O7)2, has been obtained from the melt in the Rb–Cu–P–O system. Its monoclinic crystal structure was determined by single-crystal X-ray diffraction: space group P21/c, Z = 2, a = 7.7119(8) A, b = 10.5245(9) A, c = 7.8034(9) A, β = 103.862(5)° at 293 K, R = 0.030. The copper ions show coordination number (CN) 6 (4+2, distorted tetragonal bipyramidal). Trimers of [CuO6] polyhedra sharing cis-edges form together with diphosphate groups of two tetrahedra [P2O7] a microporous 3D framework with channels open along the c direction. The rubidium ions positioned in the channels show CN 10. The new phase is isotypic to Cs2Cu3(P2O7)2. The regular changes in cell dimensions in the row Cs2Cu3(P2O7)2 → Rb2Cu3(P2O7)2 are caused by the compression of channel volumes due to decrease of the Cu–O–P angles in the framework windows. An electron spin resonance study indicates appearance of short range magnetic correlations below ∼120 K, long range magnetic order takes place at TN = 9.2 K as follows from magnetization and specific heat measurements. First principles calculations of the magnetic exchanges indicate that the effective Cu–Cu hopping interactions corresponding to super–super-exchange paths involving P atoms are much stronger than those within the edge-sharing Cu2–Cu1–Cu2 trimer units.

Ordering of cations in the voids of the anionic framework of the crystal structure of catapleiite

The pseudohexagonal crystal structure of the mineral catapleiite Na1.5Ca0.2[ZrSi3(O,OH)9] · 2(H2O,F) from the Zhil’naya Valley in the central part of the Khibiny alkaline massif (Kola Peninsula, Russia) is studied by X-ray diffraction (XCalibur-S diffractometer, R = 0.0346): a = 20.100(4), b = 25.673(5), and c = 14.822(3) Å; space group Fdd2, Z = 32, and ρcalcd = 2.76 g/cm3. Fluorine atoms substituting part of H2O molecules in open channels of the crystal structure have been found for the first time in the catapleiite composition by microprobe analysis. The pattern of distribution of Na and Ca atoms over the voids of the mixed anionic framework consisting of Zr-octahedra and three-membered rings of Si-tetrahedra accounts for the pronounced pseudoperiodicity along the a and c axes of the pseudohexagonal unit cell and for the lowering of crystal symmetry to the orthorhombic one. It is shown that part of the hydrogen atoms of water molecules is statistically disordered; their distribution correlates with the pattern of the population of large eight-vertex polyhedra by Na and Ca atoms.

Microstructure of the Al-La-Ni-Fe system

The microstructure of alloys based on the Al-La-Ni-Fe system, which are characterized by a unique ability to form metal glasses and nanoscale composites in a wide range of compositions, has been investigated. Al85Ni7Fe4La4 and Al85Ni9Fe2La4 alloys have been analyzed by electron microscopy (including high-resolution scanning transmission electron microscopy), energy-dispersive X-ray microanalysis, electron diffraction (ED), and X-ray diffraction (XRD). It is found that, along with fcc Al and Al4La (Al11La3) particles, these alloys contain a ternary phase Al3Ni1 − xFex (sp. gr. Pnma) isostructural to the Al3Ni phase and a quaternary phase Al8Fe2 − xNixLa isostructural to the Al8Fe2Eu phase (sp. gr. Pbam). The unit-cell parameters of the Al3Ni1 − xFex and Al8Fe2 − xNixLa compounds, determined by ED and refined by XRD, are a = 0.664(1) nm, b = 0.734(1) nm, and c = 0.490(1) nm for Al3Ni1 − xFex and a = 1.258(3) nm, b = 1.448(3) nm, and c = 0.405(8) nm for Al8Fe2 − xNixLa. In both cases Ni and Fe atoms are statistically arranged, and no ordering is found. Al8Fe2 − xNixLa particles contain inclusions in the form of Al3Fe δ layers.

Low-Temperature P–T Phase Diagram of the (Mg, Fe)SiO3 Perovskite

The electron spin states of iron in minerals of the Earth’s mantle at high pressures mostly determine the physicochemical properties of deep layers of the Earth and are of great interest not only for geophysics but also for fundamental physics of strongly correlated electron systems. In this work, using Raman and synchrotron Mössbauer nuclear forward scattering (NFS) spectroscopies, iron-containing magnesium–silicate perovskite (Mg, Fe)SiO3 (10% Fe) has been studied in the cryogenic temperature range of 35–300 K and at high pressures up to 48 GPa, which are created in diamond anvil cells. The analysis of NFS spectra has indicated that iron ions are in a nonmagnetic (para- or diamagnetic) state in the entire region of temperatures and pressures and the electronic properties can be controlled by means of the quadrupole splitting parameter. It has been found that an increase in the pressure and a decrease in the temperature are accompanied by a significant increase in the parameter Δ from 2 mm/s to ~4 mm/s, which indicates that the electronic state of Fe2+ ions changes. The maximum Δ value has been observed at P > 20 GPa, but the pressure behavior of a transition strongly depends on the temperature. Possible mechanisms of the transition have been discussed.

Thermal Expansion of EuF 2 + x Single Crystals and Their Thermal Stability

AbstractThermal expansion of an EuF2.136 nonstoichiometric crystal with the fluorite structure type (Eu0.8642+Eu0.1363+F2.136, lattice parameter 5.82171(5) Å) has been experimentally investigated in the temperature range of 9–500 K. The coefficient of thermal expansion is α = 15.8 × 10–6 K–1 at T = 300 K. The observed anomalies in the behavior of the coefficient of thermal expansion at T > 400 K are related to the oxidation processes with partition of Eu2+ ions. It is established by differential scanning calorimetry that the onset temperature of EuF2 + x oxidation in air is 430 K and that this process occurs in three stages. X-ray diffraction analysis shows that the oxidation is accompanied by the formation of a phase mixture based on two modifications of the Eu1– y3+Euy2+F3–y solid solution with the structure types of tysonite (LaF3), orthorhombic β-YF3 phase, and europium oxyfluorides of variable composition EuO1–xF1 + 2x, with dominance of the latter.

Structural phase transitions and the equation of state of SnTe at high pressures up to 2 mbar

Synchrotron X-ray diffraction studies of the structure of SnTe have been performed at room temperature and high pressures under the conditions of quasihydrostatic compression up to 193.5 GPa created in diamond anvil cells. Two structural phase transitions have been detected at P ≈ 3 and 23 GPa. The first phase transition is accompanied by a stepwise decrease in the volume of the unit cell by 4% because of the orthorhombic distortion of the initial SnTe-B1 cubic structure of the NaCl type. It has been found that two intermediate rhombic phases of SnTe with the space groups Cmcm and Pnma coexist in the pressure range of 3–23 GPa. The second phase transition at 23 GPa occurs from the intermediate rhombic modification to the SnTe-B2 cubic phase with the CsCl structure type. This phase transition is accompanied by an abrupt decrease in the volume of the unit cell by 8%. The pressure dependence of the volumes per formula unit at room temperature has been determined.