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At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Laser patterning and preferential orientation of two-dimensional planar {beta}-BaB{sub 2}O{sub 4} crystals on the glass surface

The laser-induced crystallization method is applied to pattern two-dimensional planar {beta}-BaB{sub 2}O{sub 4} crystals on the surface of Sm{sub 2}O{sub 3}-BaO-B{sub 2}O{sub 3} glass. By scanning Yb:YVO{sub 4} fiber lasers (wavelength: 1080 nm) continuously with a small step (0.5 {mu}m) between laser irradiated areas, homogeneous planar {beta}-BaB{sub 2}O{sub 4} crystals are patterned successfully, and a preferential growth orientation of {beta}-BaB{sub 2}O{sub 4} crystals is confirmed from linearly polarized micro-Raman scattering spectrum and second harmonic intensity measurements. It is found that the crystal growth direction is perpendicular to the laser scanning direction. This relation, i.e., the perpendicular relation, is different from the behavior in discrete crystal line patterning, where the crystal growth direction is consistent with the laser scanning direction. The present study proposes the possibility of the control of crystal growth direction in laser-induced crystallization in glasses. - Graphical abstract: This figure shows confocal scanning laser microscope and polarized optical microscope photographs for {beta}-BaB{sub 2}O{sub 4} crystals obtained by laser irradiations. The laser scanning was repeated with a step of 0.5 {mu}m between the lines using the condition of the power of P=0.8 W and a laser scanning speed of S=8 {mu}m/s. It is suggested that {beta}-BaB{sub 2}O{sub 4} crystals in the overlapped laser-irradiated region are highly oriented and the c-axis direction of {beta}-BaB{sub 2}O{sub 4} crystals is perpendicular to the laser scanning direction. Highlights: Black-Right-Pointing-Pointer Laser-induced crystallization method is applied to pattern {beta}-BaB{sub 2}O{sub 4} crystals. Black-Right-Pointing-Pointer Two-dimensional planar crystals are patterned on the glass surface. Black-Right-Pointing-Pointer Preferential growth orientation of {beta}-BaB{sub 2}O{sub 4} crystals is confirmed. Black-Right-Pointing-Pointer Crystal growth direction is perpendicular to the laser scanning direction.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Li{sub 2}B{sub 3}O{sub 4}F{sub 3}, a new lithium-rich fluorooxoborate

The new lithium fluorooxoborate, Li{sub 2}B{sub 3}O{sub 4}F{sub 3}, is obtained by a solid state reaction from LiBO{sub 2} and LiBF{sub 4} at 553 K and crystallizes in the acentric orthorhombic space group P2{sub 1}2{sub 1}2{sub 1} (no. 19) with the cell parameters a=4.8915(9), b=8.734(2), and c=12.301(2) A. Chains of fluorinated boroxine rings along the b axis consists of BO{sub 3} triangles and BO{sub 2}F{sub 2} as well as BO{sub 3}F tetrahedra. Mobile lithium ions are compensating the negative charge of the anionic chain, in which the fourfold coordinated boron atoms bear a negative formal charge. Annealing Li{sub 2}B{sub 3}O{sub 4}F{sub 3} at temperatures above 573 K leads to conversion into Li{sub 2}B{sub 6}O{sub 9}F{sub 2}. The title compound is an ionic conductor with the highest ion conductivity among the hitherto know lithium fluorooxoborates, with conductivities of 1.6 Multiplication-Sign 10{sup -9} and 1.8 Multiplication-Sign 10{sup -8} S cm{sup -1} at 473 and 523 K, respectively. - Graphical abstract: Repetition unit of Li{sub 2}B{sub 3}O{sub 4}F{sub 3}. Highlights: Black-Right-Pointing-Pointer Li{sub 2}B{sub 3}O{sub 4}F{sub 3} is the third member within the family of lithium fluorooxoborates. Black-Right-Pointing-Pointer It shows the highest lithium ion conductivity among them. Black-Right-Pointing-Pointer Chains of interconnected fluorinated boroxine rings run along the b axis. Black-Right-Pointing-Pointer Acentric space group meets the requirement for second harmonic generation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Structure, growth and properties of a novel polar material, KSr{sub 4}B{sub 3}O{sub 9}

A novel polar material, KSr{sub 4}B{sub 3}O{sub 9}, has been obtained by the high temperature solution method for the first time. It crystallizes in the polar space group Ama2. The material exhibits a three-dimensional structure consisting of interconnecting KO{sub 10} groups, SrO{sub x} (x=8, 9) units and isolated BO{sub 3} triangles. DSC/TG curve shows that KSr{sub 4}B{sub 3}O{sub 9} melts incongruently. The calculated band structures and the density of states of KSr{sub 4}B{sub 3}O{sub 9} suggest that its direct gap is 3.897 eV. The absorption spectrum indicates the absorption edge of KSr{sub 4}B{sub 3}O{sub 9} is about 302 nm. It produces SHG intensity about as large as that of KH{sub 2}PO{sub 4} (KDP) and is phase matchable. - Graphical abstract: Viewed along the c-axis, B(2)O{sub 3} triangles and B(3)O{sub 3} triangles are parallel with the (0 1 0) face and opposite to each other. However, all the B(1)O{sub 3} triangles have the same direction along the c-axis. So the main SHG efficiency of the compound comes from B(1)O{sub 3} triangles according to the anion group theory. Highlights: Black-Right-Pointing-Pointer A polar material, KSr{sub 4}B{sub 3}O{sub 9}, has been obtained by high temperature solution method. Black-Right-Pointing-Pointer The material consists of KO{sub 10} units, SrO{sub x}(x=8, 9) units and isolated BO{sub 3} units. Black-Right-Pointing-Pointer KSr{sub 4}B{sub 3}O{sub 9} produces SHG intensity as large as that of KDP and is phase matchable. Black-Right-Pointing-Pointer The absorption edge of KSr{sub 4}B{sub 3}O{sub 9} is about 302 nm.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Effects of scandium substitution on the crystal structure and luminescence properties of LuBO{sub 3}: Ce{sup 3+}

The calcite phase of LuBO{sub 3} and ScBO{sub 3} polycrystalline powders were synthesized by solid state reaction method, and the Lu{sub 1-x}Sc{sub x}BO{sub 3}:Ce (x=0.2, 0.5, 0.7) single crystals were grown by the Czochralski method. A large composition deviation between the initial polycrystalline powders and final single crystal was confirmed by electron probe micro-analysis. Raman spectroscopy revealed that moderate lattice disorder was induced by scandium substitution. However, based on the single crystal X-ray study, we finally concluded that the crystal structure of lutetium scandium orthoborate still crystallized in the rhombohedral system belonging to R3{sup -}c. Furthermore, the relationship between the energies of the five 5d levels of Ce{sup 3+} and the crystalline environment was revealed. The total redshift, total crystal field splitting, and centroid shift of Lu{sub 1-x}Sc{sub x}BO{sub 3}:Ce{sup 3+} were calculated based on their VUV excitation spectra. The variations trend of these observed spectroscopic parameters was in accordance with the predicted ones. - Graphical abstract: The crystal structure of Lu{sub 1-x}Sc{sub x}BO{sub 3}:Ce is rhombohedral system with R3{sup -}c space group. The relationship between the energies of the five Ce{sup 3+} 5d levels and the crystalline environment is established. Highlights: Black-Right-Pointing-Pointer Moderate lattice disorder is induced by scandium doping. Black-Right-Pointing-Pointer The crystal structure of Lu{sub 1-x}Sc{sub x}BO{sub 3}:Ce is rhombohedral system with R3{sup -}c space group. Black-Right-Pointing-Pointer Relationship between energies of Ce{sup 3+} 5d levels and crystalline environment is established. Black-Right-Pointing-Pointer The spectroscopic parameters are experimentally and theoretically calculated.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Structure of Li{sub 4}B{sub 2}O{sub 5}: High-temperature monoclinic and low-temperature orthorhombic forms

Crystal structures of both polymorphs of Li{sub 4}B{sub 2}O{sub 5} were determined. The high temperature phase crystallized in a monoclinic space group I2/a with lattice constants a=10.2269(18) A, b=4.6988(5) A, c=8.7862(16) A, and {beta}=93.562(14) Degree-Sign , while the low temperature one crystallized in an orthorhombic space group Pca2{sub 1} with lattice constants a=10.1497(8) A, b=4.7365(5) A, c=17.5880(14) A. Though the lattice of the low temperature phase is of higher symmetry than that of the high temperature one, the structure itself loses symmetry elements when it transforms from the high temperature phase to the low temperature polymorph. The bond strain associated with the high temperature structure is supposed to be the reason for the phase transition. A slight increase in volume per formula unit was observed when Li{sub 4}B{sub 2}O{sub 5} transforms from the high temperature polymorph to low temperature structure, which is quite unusual and has to be studied further. - Graphical abstract: The monoclinic high temperature polymorph of Li{sub 4}B{sub 2}O{sub 5} loses symmetry elements when it transforms into the orthorhombic low temperature phase to relieve the bond strain. Highlights: Black-Right-Pointing-Pointer Structures of two polymorphs of Li{sub 4}B{sub 2}O{sub 5} are determined. Black-Right-Pointing-Pointer The high and low temperature phases are monoclinic and orthorhombic, respectively. Black-Right-Pointing-Pointer A slight larger molar volume was observed for the low temperature phase. Black-Right-Pointing-Pointer Symmetry elements are lost in the phase transition from high to low temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Electronic structure and optical properties of the nonlinear optical crystal Pb{sub 4}O(BO{sub 3}){sub 2} by first-principles calculations

Pb{sub 4}O(BO{sub 3}){sub 2} has a layered-type arrangement with optimally aligned BO{sub 3} triangles. The optical band gap is 3.317 eV obtained via the extrapolation method from the UV-vis-IR optical diffuse reflectance spectrum, consequently the absorption edge is about 374 nm. Density functional calculations using a generalized gradient approximation were utilized to investigate the electronic structures and optical properties of Pb{sub 4}O(BO{sub 3}){sub 2}. The calculated band structures show a direct gap of 2.608 eV, which is in agreement with the experimental optical band gap. A delocalized {pi} bonding of BO{sub 3} triangles and the stereo-effect of the lone pair 6s{sup 2} of lead cations are studied in electron densities. The birefringence is about 0.039-0.061 with the wavelength larger than about 375 nm. The calculated second-order susceptibility d{sub 24}=3.5 d{sub 36} (KDP) which is well consistent with the powder SHG intensity. - Graphical abstract: The density of state (DOS) show that the bottom of the valence bands is mainly derived from of the lone pair 6s{sup 2} of Pb{sup 2+}, and the top of the valence band is attributed to the hybridization orbitals from B-O groups. Calculated electronic structures indicate that the BO{sub 3} group with typical delocalization {pi} orbitals and strongly distorted lead oxygen polyhedra with highly asymmetric lobes on lead cations make a large SHG effect in Pb{sub 4}O(BO{sub 3}){sub 2}. Highlights: Black-Right-Pointing-Pointer Lone pair effect on Pb{sup 2+} and delocalization {pi} orbital in BO{sub 3} group is studied. Black-Right-Pointing-Pointer The combination of PbO{sub n} (n=3,4,5) and BO{sub 3} group makes Pb{sub 4}O(BO{sub 3}){sub 2} a large SHG effect. Black-Right-Pointing-Pointer Pb{sub 4}O(BO{sub 3}){sub 2} is a direct gap material with the gap 2.608 eV by the ab initio method. Black-Right-Pointing-Pointer The calculated birefringence is about 0.039-0.061 with the wavelength of about 375 nm. Black-Right-Pointing-Pointer The second-order susceptibility d{sub 24}=3.5d{sub 36} (KDP) in accordance with the experimental result.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Hydrothermal syntheses, crystal structures and characterization of new vanadoborates: The novel decorated cage cluster [V₆B₂₂O₄₄(OH)₁₀]

Three new vanadoborates [H₃teta]₃[V₁₂B₁₈O₅₄(OH)₆(H₂O)]·(H₃O)·5H₂O (1, teta=triethylenetetramine), [H₄teta]₂[V₆B₂₂O₄₄(OH)₁₀] ·4.5H₂O (2) and [H₄tepa]₂[V₆B₂₂O₄₄(OH)₁₀] ·4H₂O (3, tepa=tetraethylenepentamine) have been hydrothermally synthesized and structurally characterized. 1 contains cage cluster [V₁₂B₁₈O₅₄(OH)₆(H₂O)]¹⁰⁻ based on one puckered B₁₈O₃₆(OH)₆ ring sandwiched between two V₆O₁₅ clusters. Both 2 and 3 contain new decorated cage cluster [V₆B₂₂O₄₄(OH)₁₀]⁸⁻ built up from a central band of [V₆O₁₈] capped top and bottom by two raft-like polyborate [B₁₁O₂₀(OH)₄] units. The magnetic susceptibility of 3 demonstrates the presence of antiferromagnetic interaction between V{sup IV} cations. The theoretical band structure and spectroscopic properties have been also studied. - Graphical abstract: The circular [V₆O₁₈] fragment sandwiched by the two raft-like polyborate [B₁₁O₂₀(OH)₄] units via sharing O atoms form a new cage cluster [V₆B₂₂O₄₄(OH)₁₀]⁸⁻, which represents a new type. Highlights: • Novel decorated cage cluster [V₆B₂₂O₄₄(OH)₁₀] is observed. • The mixed valence vanadoborate cluster is found. • The theoretical band structure and spectroscopic properties have been also studied.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Crystal structure, growth and characterization of LiPbB₉O₁₅: A new congruent melting nonlinear optical crystal

A new congruent-melting crystal LiPbB₉O₁₅ (LPBO) has been grown and its structure was determined by single crystal X-ray diffraction. LPBO crystallizes in a polar space group R3c with a large unit cell of a=b=21.649(11) Å, c=17.193(11) Å, and Z=24. The basic anionic unit, (B₃O₇)⁵⁻ group, is connected by terminal O atoms to build an infinite [(B₃O₅)⁻]ₙ helical chain along the c axis. Second harmonic generation (SHG) measurements of the title compound indicate that LPBO can be phase matchable and its SHG coefficient is almost equal to that of KDP, which is about 3.3 times that of its analog BaLiB₉O₁₅. This significant enhancement of the SHG effect can be tentatively attributed to the lone pair Pb²⁺ cations with severely distorted coordination. LPBO has a wide transparent region from 410 to 2500 nm with a UV absorption edge about 261 nm according to the reflectance spectra. - Graphical abstract: The crystal structure of LiPbB₉O₁₅ composed of the (B₃O₇)⁵⁻ anionic group and the lone pair Pb and Li polyhedra along c axis. Highlights: • A new congruent-melt NLO crystal LiPbB₉O₁₅ (LPBO) has been grown. • The basic structural unit of LPBO is B₃O₇ group same as that found in LiB₃O₅. • LPBO is phase matchable with SHG signal 10 times that of its Ba-analog. • LPBO is highly transparent within 410–1700 nm with a UV cut-off of 261 nm.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Syntheses, crystal structures, and optical properties of Pb{sub 6}B{sub 3}O{sub 10}X (X=F, Cl, Br)

A series of lead-containing halogen oxyborates, Pb{sub 6}B{sub 3}O{sub 10}X (X=F, Cl, Br), have been grown by high-temperature solution method and their crystal structures were determined by single-crystal X-ray diffraction. They are isostructural and crystallize in the space group Pbcm of the orthorhombic crystal system. The crystal structures are dominated by one-dimensional {sub ∞}[(Pb{sub 4}O)(BO{sub 3}){sub 3}] “Zig-Zag”-chains, while the remaining Pb atoms and X (X=F, Cl, Br) atoms are filled to balance the charge. Compared with the previously reported compound Pb{sub 4}O(BO{sub 3}){sub 2} (the molecular formula Pb{sub 4}O(BO{sub 3}){sub 2} can be regarded as Pb{sub 6}B{sub 3}O{sub 10.5}) with the space group Aba2, the structures of Pb{sub 6}B{sub 3}O{sub 10}X (X=F, Cl, Br) are completely different from that of Pb{sub 4}O(BO{sub 3}){sub 2}. IR spectroscopy, UV–vis-NIR diffuse reflectance spectroscopy, thermal analysis, and theoretical calculations were also performed on the reported materials. Highlights: • A new family of lead-containing halogen oxyborates, Pb{sub 6}B{sub 3}O{sub 10}X (X=F, Cl, Br), have been grown by high-temperature solution method. • Owing to the introduction of X atoms into Pb{sub 4}O(BO{sub 3}){sub 2}, the structures of Pb{sub 6}B{sub 3}O{sub 10}X are completely different. • The crystal structures are dominated by one-dimensional {sub ∞}[(Pb{sub 4}O)(BO{sub 3}){sub 3}] “Zig-Zag”-chains.

36 MATERIALS SCIENCE↗
DOE OSTI2014

A new copper borophosphate with novel polymeric chains and its structural correlation with raw materials in molten hydrated flux synthesis

A novel copper borophosphate, Cu{sub 3}[B{sub 2}P{sub 3}O{sub 12}(OH){sub 3}] has been prepared by the molten hydrated flux method. Its crystal structure was determined by the single-crystal X-ray diffraction (monoclinic, Cc, a=6.1895 Å, b=13.6209 Å, c=11.9373 Å, β=97.62°, V=997.5 Å{sup 3}, Z=4). The three-dimensional framework of the titled compound, is composed by two kinds of polymeric chains and isolated PO{sub 4} tetrahedral. One novel 4-membered tetrahedral rings has been observed in borophosphates. Magnetic measurements indicate that the title compound exits antiferromagnetic interactions. Due to the special reaction medium created by the molten hydrated flux method, a possible structural correlation between the final solids and the raw materials has been noted. - Graphical abstract: The 3D structure consists of a framework composed of CuO{sub x} polyhedra, BO{sub 4} and PO{sub 4} tetrahedra. A intersection angle between the metal chains and borophosphate chains can be noted. Display Omitted - Highlights: • A novel copper borophosphate has been prepared by the molten hydrated flux method. • One novel 4-membered tetrahedral ring has been observed firstly in borophosphates. • A possible structural correlation between the final solids and the raw materials has been noted.

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

Isolation and characterization of nanosheets containing few layers of the Aurivillius family of oxides and metal-organic compounds

Nanosheets containing few-layers of ferroelectric Aurivillius family of oxides, Bi{sub 2}A{sub n−1}B{sub n}O{sub 3n+3} (where A=Bi{sup 3+}, Ba{sup 2+} etc. and B=Ti{sup 4+}, Fe{sup 3+} etc.) with n=3, 4, 5, 6 and 7 have been prepared by reaction with n-butyllithium, followed by exfoliation in water. The few-layer samples have been characterized by Tyndall cones, atomic force microscopy, optical spectroscopy and other techniques. The few-layer species have a thickness corresponding to a fraction of the c-parameter along which axis the perovskite layers are stacked. Magnetization measurements have been carried out on the few-layer samples containing iron. Few-layer species of a few layered metal-organic compounds have been obtained by ultrasonication and characterized by Tyndall cones, atomic force microscopy, optical spectroscopy and magnetic measurements. Significant changes in the optical spectra and magnetic properties are found in the few-layer species compared to the bulk samples. Few-layer species of the Aurivillius family of oxides may find uses as thin layer dielectrics in photovoltaics and other applications. - Graphical abstract: Exfoliation of the layered Aurivillius oxides into few-layer nanosheets by chemical Li intercalation using n-BuLi followed by reaction in water. Exfoliation of the layered metal-organic compounds into few-layer nanosheets by ultrasonication. - Highlights: • Few-layer nanosheets of Aurivillius family of oxides with perovskite layers have been generated by lithium intercalation. • Few-layer nanosheets of few layered metal-organic compounds have been generated by ultrasonication. • Few-layer nanosheets of the Aurivillius oxides have been characterized by AFM, TEM and optical spectroscopy. • Aurivillius oxides containing Fe show layer dependent magnetic properties. • Exfoliated few-layer metal-organic compounds show changes in spectroscopic and magnetic properties compared with bulk materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Hydrothermal phase stability study of Li{sub 2}B{sub 4}O{sub 7}

Growth of Li{sub 2}B{sub 4}O{sub 7} by the hydrothermal method was attempted in 10{sup −6} M LiOH, by both spontaneous nucleation and transport growth. Li{sub 2}B{sub 4}O{sub 7} grew out of the first spontaneous nucleation reaction, while the seed dissolved and γ-LiBO{sub 2} formed during the transport growth reactions. A species thermodynamic stability study showed that γ-LiBO{sub 2} formed exclusively from 350 °C to 550 °C, and Li{sub 3}B{sub 5}O{sub 8}(OH){sub 2} was observed at 250 °C; Li{sub 2}B{sub 4}O{sub 7} was not observed. A solubility study on Li{sub 2}B{sub 4}O{sub 7} did not achieve equilibrium conditions, but did demonstrate that γ-LiBO{sub 2} is significantly more thermodynamically stable than Li{sub 2}B{sub 4}O{sub 7} under the tested conditions. Formation of Li{sub 2}B{sub 4}O{sub 7} hydrothermally was shown to result from kinetic growth. - Graphical abstract: Every boron in lithium γ-metaborate is tetrahedrally coordinated with oxygen, and each tetrahedron is linked to four other tetrahedrons, creating a strong B-O lattice surrounding the lithium atoms. - Highlights: • Li{sub 2}B{sub 4}O{sub 7} was grown hydrothermally via spontaneous nucleation in 10{sup −6} M LiOH. • γ-LiBO{sub 2} formed from 350 °C to 550 °C and Li{sub 3}B{sub 5}O{sub 8}(OH){sub 2} was observed at 250 °C. • A solubility study showed γ-LiBO{sub 2} to be more thermodynamically stable than Li{sub 2}B{sub 4}O{sub 7}. • Hydrothermal formation of Li{sub 2}B{sub 4}O{sub 7} was shown to result from kinetic growth.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Variations of thermoelectric properties of Mg{sub 2.2}Si{sub 1−y}Sn{sub y−0.013}Sb{sub 0.013} materials with different Si/Sn ratios

Mg{sub 2}Si–Mg{sub 2}Sn solid solutions are a promising class of thermoelectric materials. Mg{sub 2.2}Si{sub 1−y}Sn{sub y−0.013}Sb{sub 0.013} compounds with different Si/Sn ratios were prepared by B{sub 2}O{sub 3} “flux” method followed by hot-pressing. The effect of Si/Sn ratio on the thermoelectric properties of the Mg{sub 2}Si–Mg{sub 2}Sn solid solutions at a fixed Sb doping level were measured in the temperature range 300–760 K. With the increase of Sn content (0.3≤y≤0.7), power factors are improved and lattice thermal conductivity decreased. Band convergence induced by Si/Sn ratio leads to enhanced effective mass and the materials parameter β which qualifies a kind of thermoelectric material is improved evidently. A maximum ZT of ∼1.1 at ∼760 K was achieved for the Mg{sub 2.2}Si{sub 0.3}Sn{sub 0.7–0.013}Sb{sub 0.013} sample arising from a high power factor of ∼4×10{sup -3} W m{sup −1} K{sup −2} and a low lattice thermal conductivity of ∼1.6 W m{sup −1} K{sup −1} at ∼760 K. - Graphical abstract: (a)Temperature dependence of power factor of Mg{sub 2.2}Si{sub 1−y}Sn{sub y−0.13}Sb{sub 0.013} samples. (b)Temperature dependence of lattice thermal conductivity of Mg{sub 2.2}Si{sub 1−y}Sn{sub y−0.13}Sb{sub 0.013} samples. (c)Temperature dependence of dimensionless figure of merit ZT of Mg{sub 2.2}Si{sub 1−y}Sn{sub y−0.13}Sb{sub 0.013} samples. - Highlights: • Mg{sub 2.2}Si{sub 1−y}Sn{sub y−0.013}Sb{sub 0.013} was prepared by B{sub 2}O{sub 3} ''flux'' method followed by hot-pressing. • Band convergence leads to enhanced m{sup ⁎} and improved materials parameter β. • ZT{sub max} of ∼1.1 at ∼760 K was achieved for the Mg{sub 2.2}Si{sub 0.3}Sn{sub 0.7−0.013}Sb{sub 0.013} sample.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Thermally induced formation of 2D hexagonal BN nanoplates with tunable characteristics

We have investigated a thermally induced combustion route for preparing 2D hexagonal BN nanoplates from B{sub 2}O{sub 3}+(3+0.5k)Mg+kNH{sub 4}Cl solid system, for k=1–4 interval. Temperature–time profiles recorded by thermocouples indicated the existence of two sequential exothermic processes in the combustion wave leading to the BN nanoplates formation. The resulting BN nanoplates were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy, PL spectrometry, and Brunauer–Emmett–Teller surface area analysis. It was found that B{sub 2}O{sub 3} was converted into BN completely (by XRD) at 1450–1930 °C within tens of seconds in a single-step synthesis process. The BN prepared at a k=1–4 interval comprised well-shaped nanoplates with an average edge length ranging from 50 nm to several micrometer and thickness from 5 to 100 nm. The specific surface area of BN nanoplates was 13.7 g/m{sup 2} for k=2 and 28.4 m{sup 2}/g for k=4. - Graphical abstract: 2D hexagonal BN nanoplates with an average edge length ranging from 50 nm to several micrometer and thickness from 5 to 100 nm were prepared by combustion of B{sub 2}O{sub 3}+(3+0.5k)Mg+kNH{sub 4}Cl solid mixture in nitrogen atmosphere. - Highlights: • Thermally induced combustion route was developed for synthesizing BN nanoplates from B{sub 2}O{sub 3}. • Mg was used as reductive agent and NH{sub 4}Cl as an effective nitrogen source. • Temperature–time profiles and the combustion parameters were recorded and discussed. • BN with an average edge length from 50 nm to several micrometer and thickness from 5 to 100 nm were prepared. • Our study clarifies the formation mechanism of BN in the combustion wave.

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

Pressure-induced phase transitions in acentric BaHf(BO{sub 3}){sub 2}

High-pressure Raman scattering studies revealed that BaHf(BO{sub 3}){sub 2} is more compressible than calcite-type orthoborates and calcite, aragonite or dolomite carbonates. It undergoes a first-order reversible pressure-induced phase transition in the 3.9–4.4 GPa pressure range. Second structural change is observed at 9.2 GPa. The intermediate phase is most likely trigonal. However, Raman results suggest increase in the number of distinct BO{sub 3} groups from two in the ambient pressure phase to at least three in the intermediate phase. This intermediate phase is also strongly compressible and strong pressure dependence of the lattice modes proves that the main changes under pressure occur within the layers built from BaO{sub 6} and HfO{sub 6} octahedra. The second phase transition leads most likely to lowering of the trigonal symmetry, as evidenced by significant increase of the number of observed bands. The pressure coefficients of the Raman bands of the high-pressure phase are relatively small, suggesting more dense arrangement of the metal–oxygen polyhedra and BO{sub 3} groups in this phase. It is worth noting that the high-pressure phase was not reached in the second compression experiment up to 10 GPa. This behavior can be most likely attributed to worse hydrostatic conditions of the first experiment. - Graphical abstract: Raman spectra of BaHf(BO{sub 3}){sub 2} recorded at different pressures during compression showing onset of pressure-induced phase transitions. - Highlights: • High-pressure Raman spectra were measured for BaHf(BO{sub 3}){sub 2.} • BaHf(BO{sub 3}){sub 2} undergoes a reversible first-order phase transition at 3.9–4.4 GPa into a trigonal phase. • The intermediate trigonal phase is strongly compressible second structural transformation is observed at 9.2 GPa under non-perfect hydrostatic conditions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Enhancement of redox- and phase-stability of thermoelectric CaMnO{sub 3−δ} by substitution

Redox Reactivity and structural phase transitions have a major impact on transport and me-chemical properties of thermoelectric CaMnO{sub 3−δ}. In this study series of Ca{sub 1−x}A{sub x}Mn{sub 1−y}B{sub y}O{sub 3−δ} (0≤x,y≤0.8) compounds, each with A-site (Dy{sup 3+}, Yb{sup 3+}) or B-site (Nb{sup 5+}, Ta{sup 5+} and Mo{sup 6+}, W{sup 6+}) substitution, were synthesized and crystallographically analyzed. It was found that the high-temperature oxygen content is widely independent from the substituent. Subsequently, with increasing temperature the differences in the Seebeck coefficient vanish above 1200 K. With increasing substitution the orthorhombic distortion of the perovskite-like phase increases. The orthorhombic distortion and the upper temperature limit of the stability of the orthorhombic crystal structure show an almost linear dependency. Accordingly, the mechanical stability of all-oxides thermoelectric converters at temperatures exceeding 1000 K will be increased employing materials with high substitution level and substituents inducing a high orthorhombic distortion. - Graphical abstract: Thermoelectric n-type CaMn{sub 0.98}W{sub 0.02}O{sub 3−δ}—Transport properties and expansion coefficient of: Oxygen loss (green region) and upper stability limit of the orthorhombic phase (yellow region) strongly affect the transport properties. Both features also cause lattice expansion, which leads to cracking of thermoelectric all-oxide converters. We report how the upper limit for application can be shifted to even higher temperatures. - Highlights: • Level of Mn{sup 3+} at RT determines reduction behavior of Ca{sub 1−x}A{sub x}Mn{sub 1−y}B{sub y}O{sub 3−δ} at HT. • Differences in Seebeck coefficient vanish at T>1200 K independent from substitution. • Substitution increases orthorhombicity of Ca{sub 1−x}A{sub x}Mn{sub 1−y}B{sub y}O{sub 3−δ}. • Linear dependence of orthorhombicity and phase stability. • Design guidelines for increase of mechanical stability of all-oxide converters.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Growth, structure, defects and polarized absorption spectral properties of Er:Yb:YCa{sub 4}O(BO{sub 3}){sub 3} crystals

YCa{sub 4}O(BO{sub 3}){sub 3} (YCOB) crystals co-doped with 3 at% Er{sup 3+} and 20 at% Yb{sup 3+} were successfully grown by the Czochralski method. X-ray powder diffraction (XRPD) results show that the as-grown Er:Yb:YCOB crystal belongs to the monoclinic system with space group Cm. And the lattice parameters are a=8.076(8) Å, b=16.023(7) Å, c=3.528(4) Å and β=101.15(4)°. Crystal defects were revealed by chemical etching experiments. The density of etch pits, attributed to dislocations and observed on (010) planes, was found to be not uniform along the crystal diameter. The detailed polarized absorption spectra were measured. The polarized absorption cross sections at 977 nm are 1.01×10{sup −20}, 1.22×10{sup −20} and 1.05×10{sup −20} cm{sup 2} for E//X, E//Y and E//Z, respectively. And the polarized absorption cross sections at 1538 nm is about 1.86×10{sup −20} cm{sup 2} for both E//X and E//Z, but for E//Y the result is 1.03×10{sup −20} cm{sup 2}, which is much smaller. The relationship between the crystal structure and absorption spectra was discussed. - Graphical abstract: The as-grown bulk Er:Yb:YCOB crystal (the long axis is along the b-axis) and LeBail fit to the XRPD pattern of Er:Yb:YCOB (top) compared to the diffraction pattern of YCOB from the JCPDS data bank (bottom). Reflection markers indicate the calculated reflection positions. The middle trace gives the difference between observed and calculated intensity. Reflection indices corresponding to the strongest diffraction peaks are marked on the experimental pattern. - Highlights: • Er:Yb:YCOB crystal with a new composition was grown. • XRPD and FT-IR showed a certain degree of deformation in the crystal structure. • Detailed polarized absorption cross sections at 977 nm and 1538 nm were calculated. • Crystal defects were clearly revealed by chemical etching experiments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Formation, stability and crystal structure of mullite-type Al{sub 6−x}B{sub x}O{sub 9}

Mullite-type Al{sub 6−x}B{sub x}O{sub 9} compounds were studied by means of powder diffraction and spectroscopic methods. The backbones of this structure are chains of edge-connected AlO{sub 6} octahedra crosslinked by AlO- and BO-polyhedra. Rietveld refinements show that the a and b lattice parameters can be well resolved, thus representing an orthorhombic metric. A continuous decrease of the lattice parameters most pronounced in c-direction indicates a solid solution for Al{sub 6−x}B{sub x}O{sub 9} with 1.09≤x≤2. A preference of boron in 3-fold coordination is confirmed by {sup 11}B MAS NMR spectroscopy and Fourier calculations based on neutron diffraction data collected at 4 K. Distance Least Squares modeling was performed to simulate a local geometry avoiding long B-O distances linking two octahedral chains by planar BO{sub 3} groups yielding split positions for the oxygen atoms and a strong distortion in the octahedral chains. The lattice thermal expansion was calculated using the Grüneisen first-order equation of state Debye-Einstein-Anharmonicity model. - Graphical abstract: Local distortion induced by boron linking the octahedral chains. - Highlights: • Decreasing lattice parameters indicate a solid solution for Al{sub 6−x}B{sub x}O{sub 9} (1.09≤x≤2). • B-atoms induce a local distortion of neighboring AlO{sub 6} octahedra. • A preference of boron in BO{sub 3} coordination is confirmed by {sup 11}B MAS NMR spectroscopy. • An optimized structural model for Al{sub 6−x}B{sub x}O{sub 9} is presented.

36 MATERIALS SCIENCE↗