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At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Crystal structure of a new variety of lead dodecaborate Pb{sub 6}(Li{sub 0.65}Na{sub 0.19})[B{sub 12}O{sub 24}]I{sub 0.84} {center_dot} 0.168H{sub 2}O and its comparison with beryl and cordierite

Crystals of a new representative of ring-radical dodecaborates Pb{sub 6}(Li{sub 0.65}Na{sub 0.19})[B{sub 12}O{sub 24}]I{sub 0.84} {center_dot} 0.168H{sub 2}O, space group R3bar m , are obtained under hydrothermal conditions. The structure is determined with-out preliminary knowledge of the chemical formula. It is close to that of the Pb{sub 6}[B{sub 12}O{sub 24}] {center_dot} H{sub 2}O dodecaborate studied earlier, but unlike the latter structure it contains admixtures of iodide anion, lithium cation, and water molecule, which incompletely populate positions in channels. The formation of the second variety, which brings to light ion-exchange properties of the crystals, is due to mineralizing ions available in the concen-trated solution in the course of crystallization. The new compound is compared with beryl and cordierite, which have close structures with channels capable of capturing various groups. Structures of synthetic Na and Ag dodecaborates with analogous but distorted ring dodecaborate radicals are discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Fe{sub 2.5}[BP{sub 2}O{sub 7}(OH){sub 2}][PO{sub 3}(OH)][PO{sub 3}(O{sub 0.5}OH{sub 0.5})] · H{sub 2}O, a new phosphate-borophosphate with a microporous structure

A new phosphate-borophosphate Fe{sub 2.5}{sup 3+}[BP{sub 2}O{sub 7}(OH){sub 2}][PO{sub 3}(OH)][PO{sub 3}(O{sub 0.5}OH{sub 0.5})] · H{sub 2}O, space group P12{sub 1}/n, is obtained under hydrothermal conditions. Blocks (Fe{sup 3+}(PO{sub 4}){sub 6}){sup 15−} consisting of vertex-sharing (Fe1,Fe2)O{sub 6} octahedra and six PO{sub 4} tetrahedra are distinguished in the structure of the new phase, which was determined without preliminary knowledge of the chemical formula. Such blocks are known for many phosphates (borophosphates), germanates, gallates, and silicates. Blocks form layers connected by BO{sub 4} tetrahedra into a framework with large pores reaching ∼9.5 Å, which are occupied by water molecules. The out-of-layer octahedral position of the Fe3 atom is split and occupied statistically. The anion radical is characterized as a phosphate-borophosphate: it consists of two isolated PO4 tetrahedra and a borophosphate soro group [BP{sub 2}O{sub 7}(OH){sub 2}] first found in NaIn[BP{sub 2}O{sub 8}(OH)]. A layer of octahedra is characterized by higher local symmetry corresponding to the orthorhombic group Pm2{sub 1}n.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Structure of Gd{sub 0.95}Bi{sub 0.05}Fe{sub 3}(BO{sub 3}){sub 4} single crystals at 293 and 90 K

The structure of GdFe{sub 3}(BO{sub 3}){sub 4} single crystals has been studied by X-ray diffraction at 293 and 90 K. The crystals are grown from a flux in the Bi{sub 2}Mo{sub 3}O{sub 12}–B{sub 2}O{sub 3}–Li{sub 2}MoO{sub 4}–Gd{sub 2}O{sub 3}–Fe{sub 2}O{sub 3} system. The results of chemical analysis and structural study show that these crystals contain bismuth as an impurity. It is found that bismuth atoms are located at gadolinium sites in the structure. A decrease in the temperature is accompanied by a lowering of the symmetry from sp. gr. R32 (at 293 K) to sp. gr. P3{sub 1}21 (at 90 K). The presence of two types of iron chains with different geometries at 90 K promotes a change in the magnetic properties of these crystals with a decrease in the temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Magnetic-resonance and thermophysical studies of the magnetic phase diagram for a GdFe{sub 2.1}Ga{sub 0.9}(BO{sub 3}){sub 4} single crystal

The antiferromagnetic resonance, heat capacity, magnetic properties, and magnetic phase diagram of a GdFe{sub 3}(BO{sub 3}){sub 4} crystal in which some of the iron ions were substituted by diamagnetic gallium ions have been investigated. It has been found that the Neel temperature upon diamagnetic substitution decreased to 17 K compared to 38 K in the unsubstituted crystal. The effective exchange and anisotropy fields for GdFe{sub 2.1}Ga{sub 0.9}(BO{sub 3}){sub 4} have been estimated from the field dependences of magnetization and resonance measurements. The magnetic phase diagram of the crystal has been constructed from magnetic and resonance measurements. In GdFe{sub 2.1}Ga{sub 0.9}(BO{sub 3}){sub 4}, there is no spontaneous reorientation and, in the absence of a magnetic field, the crystal remains an easy-axis one in the entire domain of magnetic ordering. The critical field of the reorientation transition to an induced easy-plane state in a magnetic field along the trigonal axis has been found to increase compared to that in the unsubstituted crystal.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Crystal structure and magnetic anisotropy of ludwigite Co{sub 2}FeO{sub 2}BO{sub 3}

Co{sub 3}O{sub 2}BO{sub 3} and Co{sub 2}FeO{sub 2}BO{sub 3} single crystals with a ludwigite structure are fabricated, and their crystal structure and magnetic properties are studied in detail. Substituted ludwigite Co{sub 2}FeO{sub 2}BO{sub 3} undergoes two-stage magnetic ordering at the temperatures characteristic of Fe{sub 3}O{sub 2}BO{sub 3} (T{sub N1} Almost-Equal-To 110 K, T{sub N2} Almost-Equal-To 70 K) rather than Co{sub 3}O{sub 2}BO{sub 3} (T{sub N} = 42 K). This effect is explained in terms of preferred occupation of nonequivalent crystallographic positions by iron, which was detected by X-ray diffraction. Both materials exhibit a pronounced uniaxial magnetic anisotropy. Crystallographic direction b is an easy magnetization axis. Upon iron substitution, the cobalt ludwigite acquires a very high magnetic hardness.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Magnetic phase transitions in Nd{sub 1-x}Dy{sub x}Fe{sub 3}(BO{sub 3}){sub 4} ferroborates

The magnetic properties of ferroborate single crystals with substituted compositions Nd{sub 1-x}Dy{sub x}Fe{sub 3}(BO{sub 3}){sub 4} (x = 0.15, 0.25) with competing exchange Nd-Fe and Dy-Fe interactions are investigated. For each composition, we observed a spontaneous spin-reorientation transition from the easy-axis to the easy-plane state and step anomalies on the magnetization curves for the spin-flop transition induced by a magnetic field B vertical bar c. The measured parameters and effects are interpreted using a unified theoretical approach based on the molecular field approximation and on calculations performed in the crystal-field model for the rare-earth ion. The experimental temperature dependences of the initial magnetic susceptibility from T = 2 K to T = 300 K, anomalies on the magnetization curves for B vertical bar c in fields up to 1.8 T, and their evolution with temperature, as well as temperature and field dependences of magnetization in fields up to 9 T are described. In the interpretation of experimental data, the crystal-field parameters in trigonal symmetry for the rare-earth subsystem are determined, as well as the parameters of Nd-Fe and Dy-Fe exchange interactions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Magnetoelectric and magnetoelastic properties of easy-plane ferroborates with a small ionic radius

The magnetic, magnetoelectric, and magnetoelastic properties of RFe{sub 3}(BO{sub 3}){sub 4} ferroborates are studied. The measurement of the field dependences of the magnetoelectric polarization along the a axis in holmium ferroborate and in the mixed composition Ho{sub 0.5}Sm{sub 0.5}Fe{sub 3}(BO{sub 3}){sub 4} revealed the following dependences for easy-plane ferroborates: (a) the longitudinal and transverse magnetoelectric effects have the opposite signs and (b) magnetically induced polarization changes its sign in a field close to the field of exchange between rare-earth and iron ions. These dependences agree well with theoretical predictions based on the symmetry of the compounds. The relatively low f-d exchange field in holmium ferroborate (about 20 kOe), which magnetizes the rare-earth subsystem, causes smaller polarization jumps (about 30 {mu}C/m{sup 2}) in fields lower than 10 kOe as compared to the jumps in other easy-plane ferroborates (R = Sm, Nd). The increase in the electric polarization induced in HoFe{sub 3}(BO{sub 3}){sub 4} in magnetic fields higher than 100 kOe (200-300 {mu}C/m{sup 2}) is found to be significantly smaller than in neodymium ferroborate, which indicates a substantial dependence of the magnetoelectric effects on the electronic structure of a rare-earth ion.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Vibrational spectra and elastic piezoelectric and polarization properties of the {alpha}-SrB{sub 4}O{sub 7} crystal

The crystal lattice vibrational frequencies in the center of the Brillouin zone have been determined with the Raman spectroscopy method. The lattice vibrational frequencies, the phonon density of states, elastic and piezoelectric moduli, Born dynamic charges, and high-frequency dielectric constant have been calculated using the density functional method. All calculated quantities have been compared to the experimental data. A model of a nonpolar paraelectric phase for this compound, as well as a mechanism of the formation of domain in it, has been proposed. The polarization in the experimentally observed polar phase has been calculated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Magnetic Properties of Sm{sub 0.7}Ho{sub 0.3}Fe{sub 3}(BO{sub 3}){sub 4}

The magnetic properties of a ferroborate single crystal of substituted composition Sm{sub 0.7}H{sub 0.3}Fe{sub 3}(BO{sub 3}){sub 4} with competing Sm-Fe and Ho-Fe exchange interactions are studied. The measured properties and effects are interpreted in terms of a general theoretical approach based on the molecular field approximation and calculations using the crystal field model for a rare-earth ion. The experimental temperature dependences of the initial magnetic susceptibility in the temperature range 2-300 K, the anomalies in the magnetization curves for B Double-Vertical-Line c and B Up-Tack c in fields lower than 1.2 T, and the field and temperature dependences of magnetization in fields lower than 9 T are described. The crystal field parameters and the parameters of the R-Fe and Fe-Fe exchange interactions are determined during the interpretation of the experimental data.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Magnetic properties of the rare-earth ferroborate SmFe{sub 3}(BO{sub 3}){sub 4}

The magnetic properties of trigonal antiferromagnet SmFe{sub 3}(BO{sub 3}){sub 4} are studied experimentally and theoretically. The measured characteristics are considered in terms of a theoretical approach based on the molecular field approximation and a crystal field model for a rare-earth ion. The temperature dependences of the initial magnetic susceptibility and the field and temperature dependences of magnetization in fields up to 5 T are described, and the anomaly in the magnetization curve for B Up-Tack c near 1 T, which points to a first-order phase transition, is analyzed.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Magnetic properties of the Nd{sub 0.95}Dy{sub 0.05}Fe{sub 3}(BO{sub 3}){sub 4} ferroborate with small substitution in the rare-earth element subsystem

The magnetic properties of a substituted Nd{sub 0.95}Dy{sub 0.05}Fe{sub 3}(BO{sub 3}){sub 4} ferroborate single crystal with competing Nd-Fe and Dy-Fe exchange interactions are studied experimentally and theoretically. A spontaneous spin-reorientation transition is detected near T = 4.3 K, and anomalies are observed in the low-temperature magnetization curves along trigonal axis c and in basal plane ab. The measured properties and the detected effects are interpreted in terms of a general theoretical approach, which is based on the molecular field approximation and crystal field calculations for a rare-earth ion. The experimental temperature dependences of the initial magnetic susceptibility in the range 2–300 K, the anomalies in the magnetization curves for B ‖ c and B ⊥ c in fields up to 1.5 T, and the field and temperature dependences of magnetization in fields up to 9 T are described. The effect of small substitution in the rare-earth subsystem on the magnetic properties is analyzed. The crystal field parameters and the parameters of the R-Fe and Fe-Fe exchange interactions are determined from the experimental data.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Electron paramagnetic resonance of Cr{sup 3+} ions in ABO{sub 3} (A = Sc, Lu, In) diamagnetic crystals

A magnetic resonance method is applied to the investigation of a number of isostructural diamagnetic compounds ABO{sub 3} (A = Sc, Lu, In) with small additions of Cr{sup 3+} ions (S = 3/2) sufficient to observe single-ion spectra. It is shown that the resonance spectra for isolated Cr{sup 3+} ions can be described to a good accuracy by the ordinary axial spin Hamiltonian for 3d ions in octahedral oxygen environment. The parameters of the spin Hamiltonian are determined. It is established that Cr{sup 3+} ions in these crystals are characterized by easy-axis-type anisotropy.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

X-ray natural circular dichroism in copper metaborate

The local electronic structure of copper ions in a copper metaborate CuB{sub 2}O{sub 4} crystal is studied on the ESRF synchrotron using X-ray absorption polarization-dependent spectroscopy. The X-ray natural circular dichroism near the K absorption edge of copper is measured in the direction that is perpendicular to crystal axis c. The data obtained indicate the presence of hybridized p–d electronic states of copper. Theoretical calculations are used to separate the contributions of the two crystallographically nonequivalent positions of copper atoms in the unit cell of CuB{sub 2}O{sub 4} to the absorption and X-ray circular dichroism spectra of the crystal.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Excitons at the center of the Brillouin zone in CuB{sub 2}O{sub 4} magnetoelectric

The possibility of exciton excitation in CuB{sub 2}O{sub 4} magnetoelectric at point Γ(0, 0, 0) of the Brillouin zone has been analyzed using group theory. All possible orientations of the electric and magnetic fields that permit the excitation of these excitons have been determined.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Superexchange Interaction in Magnetic Insulators with Spin Crossover

We present the derivation of a microscopic superexchange Hamiltonian for undoped magnetic insulators with an arbitrary spin. It is established that the sign of the (ferromagnetic or antiferromagnetic) superexchange between magnetic ions in the d{sup n} configuration depends on the spin nature of virtual multielectron states d{sup n± 1}, viz., low-spin or high-spin partners with S ± 1/2 relative to ground state of the d{sup n} configuration with spin S. A macroscopic substantiation is given for the Goodenough–Kanamori rules and simple mean-field estimates connecting the magnetic ordering temperature with the exchange constant. The conventional Anderson superexchange for magnetic materials with spin S = 1/2 and the P/T magnetic phase diagram for ferroborate FeBO{sub 3} with spin crossover S = (5/2 ↔ 1/2) at the Fe{sup 3+} ion under a high pressure are also reproduced as a test.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Electron Paramagnetic Resonance of Cr{sup 3+} Ions in ABO{sub 3} (A = Sc, In, Ga) Diamagnetic Crystals

A magnetic resonance method is applied to the investigation of a number of isostructural diamagnetic compounds ABO{sub 3} (A = Sc, In, and Ga) with small additions of Cr{sup 3+} ions (S = 3/2) sufficient to observe single-ion and pair spectra. It is shown that the resonance spectra for isolated Cr{sup 3+} ions can be described to a good accuracy by the ordinary axial spin Hamiltonian for 3d ions in octahedral oxygen environment. The parameters of the spin Hamiltonian are determined for single Cr{sup 3+} ion and Cr{sup 3+}–Cr{sup 3+} pair. Lattice distorsions of the parent ABO{sub 3} crystals caused by the Cr{sup 3+} impurities is discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Weak Ferromagnetism along the Triad Axis and the Basal Anisotropy Caused by the Dzyaloshinskii–Moriya Interaction and the Cubic Electric Field of the FeBO{sub 3} Crystal

Based on the spin Hamiltonian and taking into account the cubic invariant of the crystal field and the Dzyaloshinskii–Moriya interaction, the weak ferromagnetic moment along the triad axis and the basal anisotropy of FeBO{sub 3} crystals are calculated in the approximation of second-order perturbation theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗