Structure and thermoelectric characterization of Ba8Al14Si31

A molten Al flux method was used to grow single crystals of the type I clathrate compound Ba8Al14Si31. Single-crystal neutron diffraction data for Ba8Al14Si31 were collected at room temperature using the SCD instrument at the Intense Pulsed Neutron Source, Argonne National Laboratory. Single-crystal neutron diffraction of Ba8Al14Si31 confirms that the Al partially occupies all of the framework sites (R1 = 0.0435, wR2 = 0.0687). Stoichiometry was determined by electron microprobe analysis, density measurements, and neutron diffraction analysis. Solid-state (27)Al NMR provides additional evidence for site preferences within the framework. This phase is best described as a framework-deficient solid solution Ba8Al14Si31, with the general formula, Ba(8)Al(x)Si(42-3/4x)[](4-1/4x) ([] indicates lattice defects). DSC measurements and powder X-ray diffraction data indicate that this is a congruently melting phase at 1416 K. Temperature-dependent resistivity reveals metallic behavior. The negative Seebeck coefficient indicates transport processes dominated by electrons as carriers.     

Crystal and Electronic Structure and Optical Properties of AE2SiP4 (AE = Sr,Eu, Ba) and Ba4Si3P8

Three new compounds in the AE‐Si‐P (AE = Sr, Eu, Ba) systems are reported. Sr₂SiP₄ and Eu₂SiP₄, the first members of their respective ternary systems, are isostructural to previously reported Ba₂SiP₄ and crystallize in the noncentrosymmetric I4 2d (no. 122) space group. Ba₄Si₃P₈ crystallizes in the new structure type, in P2₁/c (no. 14) space group, mP‐120 Pearson symbol, Wyckoff sequence e³⁰. In the crystal structures of Sr₂SiP₄ and Eu₂SiP₄ all SiP₄ tetrahedral building blocks are connected via formation of P–P bonds forming a three‐dimensional framework. In the crystal structure of Ba₄Si₃P₈, Si‐P tetrahedral chains formed by corner‐sharing, edge‐sharing, and P–P bonds are surrounded by Ba cations. This results in a quasi‐one‐dimensional structure. Electronic structure calculations and UV/Vis measurements suggest that the AE₂SiP₄ (AE = Sr, Eu, Ba) are direct bandgap semiconductors with bandgaps of ca. 1.4 eV and have potential for thermoelectric applications.     

Synthesis, structure, and high-temperature thermoelectric properties of boron-doped Ba8Al14Si31 clathrate I phases

Single crystals of boron-doped Ba8Al14Si31 clathrate I phase were prepared using Al flux growth. The structure and elemental composition of the samples were characterized by single-crystal and powder X-ray diffraction; elemental analysis; and multinuclear (27)Al, (11)B, and (29)Si solid-state NMR. The samples' compositions of Ba8B0.17Al14Si31, Ba8B0.19Al15Si31, and Ba8B0.32Al14Si31 were consistent with the framework-deficient clathrate I structure Ba8Al(x)Si(42-3/4x)cube(4-1/4x) (x = 14, cube = lattice defect). Solid-state NMR provides further evidence for boron doped into the framework structure. Temperature-dependent resistivity indicates metallic behavior, and the negative Seebeck coefficient indicates that transport processes are dominated by electrons. Thermal conductivity is low, but not significantly lower than that observed in the undoped Ba8Al14Si31 prepared in the same manner.     

A5Sn2As6 (A = Sr, Eu). Synthesis, crystal and electronic structure, and thermoelectric properties

Two new ternary Zintl phases, Sr(5)Sn(2)As(6) and Eu(5)Sn(2)As(6), have been synthesized, and their structures have been accurately determined through single-crystal X-ray diffraction. Both compounds crystallize in orthorhombic space group Pbam (No. 55, Z = 2) with cell parameters of a = 12.482(3)/12.281(5) ?, b = 14.137(3)/13.941(5) ?, and c = 4.2440(10)/4.2029(16) ? for Sr(5)Sn(2)As(6) (R1 = 0.0341; wR2 = 0.0628) and Eu(5)Sn(2)As(6) (R1 = 0.0324; wR2 = 0.0766), respectively. Their structure belongs to the Sr(5)Sn(2)P(6) type, which can be closely related to the Ca(5)Ga(2)As(6) type. Electronic band structure calculations based on the density functional theory reveal an interesting electronic effect in the structure formation of these two types of Zintl phases, which substantially affect their corresponding electronic band structure. Related studies on the thermal stability, magnetism, and thermoelectric properties of Eu(5)Sn(2)As(6) are presented as well.     

Nitridophosphate-Based Ultra-Narrow-Band Blue-Emitters: Luminescence Properties of AEP8N14:Eu2+ (AE=Ca,Sr, Ba)

The nitridophosphates AEP₈N₁₄ (AE=Ca, Sr, Ba) were synthesized at 4–5 GPa and 1050–1150 °C applying a 1000 t press with multianvil apparatus, following the azide route. The crystal structures of CaP₈N₁₄ and SrP₈N₁₄ are isotypic. The space group Cmcm was confirmed by powder X-ray diffraction. The structure of BaP₈N₁₄ (space group Amm2) was elucidated by a combination of transmission electron microscopy and diffraction of microfocused synchrotron radiation. Phase purity was confirmed by Rietveld refinement. IR spectra are consistent with the structure models and the chemical compositions were confirmed by X-ray spectroscopy. Luminescence properties of Eu²⁺-doped samples were investigated upon excitation with UV to blue light. CaP₈N₁₄ (λ_em=470 nm; fwhm=1380 cm⁻¹) and SrP₈N₁₄ (λ_em=440 nm; fwhm=1350 cm⁻¹) can be classified as the first ultra-narrow-band blue-emitting Eu²⁺-doped nitridophosphates. BaP₈N₁₄ shows a notably broader blue emission (λ_em=417/457 nm; fwhm=2075/3550 cm⁻¹).     

Structure and Luminescence Characteristics of Europium Activated Strontium Silicates Sr（Eu ,Bi）SiO3 and Sr（Eu ,Bi）2SiO4

Sr2SiO4 ： Eu^3 , Bi^3 and SrSiO3 ：Eu^3 , Bi^3 samples were synthesized at high temperature and high pressure. The effect of high pressure on the structure and luminescence properties of the samples were stud-ied. As a comparison, the samples were also prepared by the method of sol-gel at high temperature and atmo-spheric pressure. The SrSiO3 ： Eu^3 , Bi^3 prepared at atmosphere has a hexagonal phase structure; in the pressure range of 2. 34—4. 10 GPa, it is transformed into a pseudo-orthorhombic structure (monoclinic), and in the pressure range of 4. 10—4. 15 GPa, the structure change of Sr2SiO4 ： Eu^3 , Bi^3 has not been ob-served, it maintains the monoclinic structure of the samples synthesized at an atmospheric pressure. High pressure makes the luminescence properties of the samples changed obviously. The intensity and the relative quantum luminescent efficiency decrease, the half-width increases obviously and the red shift occurs. The changes of the luminescence properties result from the pressure-induced changes of the crystal structures.     

EuAuGe Type Indides RAgIn (R = Ca,Sr, La,Eu)

The equiatomic intermetallic phases CaAgIn [a = 482.75(7), b = 750.0(1), c = 835.5(1) pm], SrAgIn [a = 495.86(5), b = 794.71(9), c = 851.89(9) pm], LaAgIn [a = 489.99(5), b = 767.93(9), c = 837.53(9) pm], and EuAgIn [a = 493.02(7), b = 781.6(1), c = 844.2(1) pm] were synthesized from the elements in sealed niobum containers. They crystallize with the EuAuGe type structure, space group Imm2. The four structures were refined from single‐crystal X‐ray data. The silver and indium atoms build up orthorhombically distorted, puckered Ag₃In₃ hexagons, which are stacked in AA′ sequence, leading to direct Ag–Ag and In–In interlayer bonding (e.g. 303 and 304 pm in CaAgIn). Temperature dependent magnetic susceptibility measurements show a magnetic moment of 7.40(1) μ_B per europium atom. EuAgIn orders antiferromagnetically at 5.7(5) K. The divalent nature of europium is also evident from ¹⁵¹Eu Mössbauer spectra: δ = –10.50(1) mm · s^–1 at 78 K.     

Hydrogen sites in A2BHy (A = Ca,Sr, Eu; B = Ir,Rh, Ru)

A geometric model has been applied to the A₂BH_y hydrides (deuterides), Eu₂IrD₅, Ca₂IrH₅, Sr₂IrD₅, Ca₂RhH₅, Sr₂RhH₅, Ca₂RuH₆, and Sr₂RuD₆, none of which can be synthesized directly by reaction of hydrogen (deuterium) gas with an A₂B compound. Hole radii and intersite distances were calculated for the two types of interstices in each compound. There are two very large cubical interstices per formula unit. These are coordinated by eight atoms of type A, but they must remain unoccupied in A₂BH_y with y = 5 (or 6), because of their proximity to the square pyramidal interstices, of which there are six per formula unit. The geometric model allows rationalization of the occupation of these pyramidal sites. Despite the very significant chemical differences between the compounds considered here and those for which the model was initially developed, the present results showed no inconsistency with geometric criteria requiring that occupied interstices in stable hydrides have minimum hole radii of 0.40 Å and minimum hydrogen-hydrogen distances of 2.10 Å. Published results indicate that the seemingly related compound Mg₂NiD₄ does not conform to these empirical rules, and this case is discussed.     

Synthesis,characterization, and bonding properties of polymeric fullerides AC70.nNH3 (A = Ca,Sr, Ba,Eu, Yb)

Reduction of C70 with alkaline earth and rare earth metals dissolved in liquid ammonia results in metal fulleride solvates AC70.nNH3 (A = Ca, Sr, Ba, Eu, Yb) containing linear polymeric, anionic chains infinity 1 [C70(2-)]. The compounds were characterised by means of Raman spectroscopy and single-crystal structure determination. The accurate crystal structure of [Sr(NH3)8]C70.3NH3, determined with atomic resolution, allowed for a comparison with results of quantum chemical calculations. The nature of the C-C bonds in the fulleride is analysed in detail leading to a model explaining the unexpected polymerisation of C70(2-).     

Phase range of the type-I clathrate Sr8Al(x)Si(46-x) and crystal structure of Sr8Al10Si36

Samples of the type-I clathrate Sr(8)Al(x)Si(46-x) have been prepared by direct reaction of the elements. The type-I clathrate structure (cubic space group Pm3n) which has an Al-Si framework with Sr(2+) guest atoms forms with a narrow composition range of 9.54(6) ≤ x ≤ 10.30(8). Single crystals with composition A(8)Al(10)Si(36) (A = Sr, Ba) have been synthesized. Differential scanning calorimetry (DSC) measurements provide evidence for a peritectic reaction and melting point at ～1268 and ～1421 K for Sr(8)Al(10)Si(36) and Ba(8)Al(10)Si(36), respectively. Comparison of the structures reveals a strong correlation between the 24k-24k framework sites distances and the size of the guest cation. Electronic structure calculation and bonding analysis were carried out for the ordered models with the compositions A(8)Al(6)Si(40) (6c site occupied completely by Al) and A(8)Al(16)Si(30) (16i site occupied completely with Al). Analysis of the distribution of the electron localizability indicator (ELI) confirms that the Si-Si bonds are covalent, the Al-Si bonds are polar covalent, and the guest and the framework bonds are ionic in nature. The Sr(8)Al(6)Si(40) phase has a very small band gap that is closed upon additional Al, as observed in Sr(8)Al(16)Si(30). An explanation for the absence of a semiconducting "Sr(8)Al(16)Si(30)" phase is suggested in light of these findings.     

Synthesis, Crystal Structure, Transport, and Magnetic Properties of Novel Ternary Copper Phosphides, A(2)Cu(6)P(5) (A = Sr, Eu) and EuCu(4)P(3)

Three new ternary copper phosphides, Sr(2)Cu(6)P(5), Eu(2)Cu(6)P(5), and EuCu(4)P(3), have been synthesized from the elements in evacuated silica capsules. Eu(2)Cu(6)P(5) and Sr(2)Cu(6)P(5) adopt the Ca(2)Cu(6)P(5)-type structure, while EuCu(4)P(3) is isostructural to BaMg(4)Si(3) and still remains the only representative of this structure type among the ternary Cu pnictides. All three materials show metallic conductivity in the temperature range 2 K ≤ T ≤ 290 K, with no indication for superconductivity. For Eu(2)Cu(6)P(5) and EuCu(4)P(3), long-range magnetic order was observed, governed by 4f local moments on the Eu atoms with predominant ferromagnetic interactions. While Eu(2)Cu(6)P(5) shows a single ferromagnetic transition at T(C) = 34 K, the magnetic behavior of EuCu(4)P(3) is more complex, giving rise to three consecutive magnetic phase transitions at 70, 43, and 18 K.     

Ternary phases MPtSi (M = Ca,Eu, Sr,Ba) with the LalrSi-type structure

Four ternary phases MPtSi (M = Ca, Eu, Sr, Ba) have been shown to crystallize in the LaIrSi-type structure (space group P2₁3). This ternary structure is a derivative structure of the binary SrSi₂-type structure (space group P4₃32 or P4₁32). In the MPtSi series the LaIrSi-type structure has a stability range for metals with radii from r_Ca = 1.973 Å to r_Ba = 2.243 Å in contrast to MSi₂ compounds which exist with the SrSi₂-type structure only from r_Sr = 2.151Å to r_Ba 2.243 Å. From a single-crystal investigation on CaPtSi remarkably short PtSi distances of 2.30 Å (3x) are obtained. Structural relations are discussed.     

Structure determination and relative properties of novel cubic borates MM'4(BO3)3 (M = Li, M' = Sr; M = Na, M' = Sr, Ba)

A series of novel borates, MM'4(BO3)3 (M = Li, M' = Sr; M = Na, M' = Sr, Ba), have been successfully synthesized by standard solid-state reaction. The crystal structures have been determined from powder X-ray diffraction data. They crystallize in the cubic space group Iad with large lattice parameters: a = 14.95066(5) A for LiSr4(BO3)3, a = 15.14629(6) A for NaSr4(BO3)3, and a = 15.80719(8) A for NaBa4(BO3)3. The structure was built up from 64 small cubic grids, in which the M' atoms took up the corner angle and the BO3 triangles or MO6 cubic octahedra filled in the interspaces. The isolated [BO3]3- anionic groups are perpendicular to each other, distributed along three 100 directions. The anisotropic polarizations were counteracting, forming an isotropic crystal. Sr and Ba atoms were found to be completely soluble in the solid solution NaSr(4-)xBax(BO3)3 (0 < or = x < or = 4). The photoluminescence of samples doped with the ions Eu2+ and Eu3+ was studied, and effective yellow and red emission was detected, respectively. The results are consistent with the crystallographic study. The DTA and TGA curves of them show that they are chemically stable and congruent melting compounds.     

Origin of Thermal Degradation of Sr(2-x)Si(5)N(8):Eu(x) Phosphors in Air for Light-Emitting Diodes

The orange-red emitting phosphors based on M(2)Si(5)N(8):Eu (M = Sr, Ba) are widely utilized in white light-emitting diodes (WLEDs) because of their improvement of the color rendering index (CRI), which is brilliant for warm white light emission. Nitride-based phosphors are adopted in high-performance applications because of their excellent thermal and chemical stabilities. A series of nitridosilicate phosphor compounds, M(2-x)Si(5)N(8):Eu(x) (M = Sr, Ba), were prepared by solid-state reaction. The thermal degradation in air was only observed in Sr(2-x)Si(5)N(8):Eu(x) with x = 0.10, but it did not appear in Sr(2-x)Si(5)N(8):Eu(x) with x = 0.02 and Ba analogue with x = 0.10. This is an unprecedented investigation to study this phenomenon in the stable nitrides. The crystal structural variation upon heating treatment of these compounds was carried out using the in situ XRD measurements. The valence of Eu ions in these compounds was determined by electron spectroscopy for chemical analysis (ESCA) and X-ray absorption near-edge structure (XANES) spectroscopy. The morphology of these materials was examined by transmission electron microscopy (TEM). Combining all results, it is concluded that the origin of the thermal degradation in Sr(2-x)Si(5)N(8):Eu(x) with x = 0.10 is due to the formation of an amorphous layer on the surface of the nitride phosphor grain during oxidative heating treatment, which results in the oxidation of Eu ions from divalent to trivalent. This study provides a new perspective for the impact of the degradation problem as a consequence of heating processes in luminescent materials.     

Sr2Si5N8:Eu2+荧光粉的制备与发光性能

以α-Si3N4,SrCO3,Eu2O3为原料,采用碳热还原氮化法制备了Sr2Si5N8:Eu2+荧光粉.研究了材料的结构与光谱特性,分析了影响材料发光性能的工艺因素.结果表明,石墨粉含量和助熔剂的用量对Sr2Si5N8相的形成和发光性能有重要影响.当nc/nSr=1.5,助熔剂用量为4wt%时,合成样品的主晶相为正交晶系Sr2Si5N8,在400～550 nm可见光激发下,可发射峰值波长位于 609nm荧光.激发带的位置与Eu2+离子浓度无关,为400～550 nm之间的宽带激发;但发射强度随Eu2+离子浓度的增加而增加.Eu2+离子浓度达到5mol%时发射强度达最大值,在Eu2+离子浓度为2mol%～5mol%之间,观察到发射峰的红移现象.     

Growth, spectral and thermal characterization of vanillin semicarbazone (VNSC) single crystals

Vanillin semicarbazone (VNSC) has been synthesized from 4-hydroxy 3-methoxy benzaldehyde and semicarbazide hydrochloride using sodium acetate as catalyst. Good quality single crystals of VNSC were successfully grown by slow evaporation method at room temperature using DMF as solvent. The grown crystals have been characterized using melting point, elemental analysis, FT-IR, UV–Visible, ¹H NMR, ¹³C NMR, and X-Ray diffraction analysis. The compound crystallizes into an orthorhombic Pca21 space group. Intermolecular hydrogen bonding interactions facilitate unit cell packing in the crystal lattice. The UV–Visible spectrum confirmed the transparency of the compound between the wavelengths 420 and 1,200 nm, which is characteristic to property of an NLO material. The thermal decomposition of the compound under static air atmosphere was investigated by simultaneous TG/DTG at a heating rate of 10 °C/min.     

Low-lying isomers of Si(n)(+) and Si(n)(-) (n=31-50) clusters

We carry out a systematic search for the atomic structures of silicon cluster cations and anions in the size range n=31-50 using density functional theory in the generalized-gradient approximation. The obtained lowest-energy candidates feature cagelike structures. We find that the computed binding energies and the dissociation pathways as well as the mobilities of our lowest-energy isomers of the cations are all in good agreement with the measured data from experiments. Furthermore, based on these isomers, we reveal that the steplike feature appearing in the measured high-resolution mobilities can be correlated with the corresponding fullerenes explicitly, which strongly support the notion that endohedral silicon fullerenelike structures are the most favored growth pattern for silicon clusters in the range n=31-50. Our calculation and analysis suggest that the proposed isomers are probably very close to the major-abundance isomers observed in experiments.