Long‐range interactions in embedded ionic cluster calculations

The effect of long‐range Coulomb interactions on bulk properties is studied for the ionic solids NaBr and NaCl. The embedded cluster approach in the framework of density functional theory is employed. The Madelung potential is calculated with the Evjen cube summation method. To explore the effects of the long‐range interactions on the electron densities and the Madelung constant, the Evjen cube size is varied from 310 to 19650 point charges for 33 atom clusters. To study the size effect of the quantum region, all‐electron clusters with 33 to 87 atoms, embedded in Evjen cubes of 6859 point charges, are investigated. The results show that for the 81 and 87 atom clusters the Madelung potential is constant from the center up to the second neighbor shell. For the same atoms, in all clusters, the electron density at the nuclei has nearly the same value. The largest difference found for the positive ions was 0.54%, and for the negative ions, 0.14%. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Structure and thermoelectric properties of the new quaternary bismuth selenides A(1-x)M(4-x)Bi(11+x)Se21 (A = K and Rb and Cs; M = Sn and Pb)--members of the grand homologous series Km(M6Se8)m(M(5+n)Se(9+n))

Several members of the new family A(1-x)M(4-x)Bi(11+x)Se21 (A = K, Rb, Cs; M = Sn, Pb) were prepared by direct combination of A2Se, Bi2Se3, Sn (or Pb), and Se at 800 degrees C. The single-crystal structures of K(0.54)Sn(3.54)Bi(11.46)Se21, K(1.46)Pb(3.08)Bi(11.46)Se21, Rb(0.69)Pb(3.69)Bi(11.31)Se21, and Cs(0.65)Pb(3.65)Bi(11.35)Se21 were determined. The compounds A(1-x)M(4-x)Bi(11+x) Se21 crystallize in a new structure type with the monoclinic space group C2/m, in which building units of the Bi2Te3 and NaCl structure type join to give rise to a novel kind of three-dimensional anionic framework with alkali-ion-filled tunnels. The building units are assembled from distorted, edge-sharing (Bi,Sn)Se6 octahedra. Bi and Sn/Pb atoms are disordered over the metal sites of the chalcogenide network, while the alkali site is not fully occupied. A grand homologous series Km(M6Se8)m(M(5+n)Se(9+n)) has been identified of which the compounds A(1-x)M(4-x)Bi(11+x)Se21 are members. We discuss here the crystal structure, charge-transport properties, and very low thermal conductivity of A(1-x)M(4-x)Bi(11+x)Se21.     

Madelung Constants

Madelung constants are simple numbers which depend on the type of structure investigated. They are needed for the calculation (using the Born-Haber cycle) of lattice energies and enthalpies of formation of ionic compounds. Each Madelung constant is the sum of partial Madelung constants which represent the contributions of the individual ions to the total lattice energy. The partial Madelung constants depend on the ionic charge and, clearly though not stringently, on the coordination number. On the other hand, each Madelung constant can be represented by a sum of Madelung constants for related simple primitive AB structures. Surprisingly, these Madelung constants are numerically interrelated in a simple manner, and are related to the partial Madelung constants of interstitial sites. – Madelung constants of parameter-dependent structures (e.g. of the rutile or anatase type) and their variations with the structure-determining quantities are of particular interest. Madelung constants also yield information in the case of complex compounds and – surprisingly – of non-metal compounds (e.g. XeF₂, XeF₄, XeF₂·XeF₄).     

Chemical Manipulation of Magnetic Ordering in Mn(1-x)Sn(x)Bi(2)Se(4) Solid-Solutions

Several compositions of manganese-tin-bismuth selenide solid-solution series, Mn(1-x)Sn(x)Bi(2)Se(4) (x = 0, 0.3, 0.75), were synthesized by combining high purity elements in the desired ratio at moderate temperatures. X-ray single crystal studies of a Mn-rich composition (x = 0) and a Mn-poor phase (x = 0.75) at 100 and 300 K revealed that the compounds crystallize isostructurally in the monoclinic space group C2/m (no.12) and adopt the MnSb(2)Se(4) structure type. Direct current (DC) magnetic susceptibility measurements in the temperature range from 2 to 300 K indicated that the dominant magnetic ordering within the Mn(1-x)Sn(x)Bi(2)Se(4) solid-solutions below 50 K switches from antiferromagnetic (AFM) for MnBi(2)Se(4) (x = 0), to ferromagnetic (FM) for Mn(0.7)Sn(0.3)Bi(2)Se(4) (x = 0.3), and finally to paramagnetic (PM) for Mn(0.25)Sn(0.75)Bi(2)Se(4) (x = 0.75). We show that this striking variation in the nature of magnetic ordering within the Mn(1-x)Sn(x)Bi(2)Se(4) solid-solution series can be rationalized by taking into account: (1) changes in the distribution of magnetic centers within the structure arising from the Mn to Sn substitutions, (2) the contributions of spin-polarized free charge carriers resulting from the intermixing of Mn and Sn within the same crystallographic site, and (3) a possible long-range ordering of Mn and Sn atoms within individual {M}(n)Se(4n+2) single chain leading to quasi isolated {MnSe(6)} octahedra spaced by nonmagnetic {SnSe(6)} octahedra.     

Ta3AlC2 and Ta4AlC3--single-crystal investigations of two new ternary carbides of tantalum synthesized by the molten metal technique

Single crystals of the new ternary carbides Ta4AlC3 and Ta3AlC2 were synthesized from molten aluminum and characterized XRD, EDX, and WDX measurements. Crystal structures were refined for the first time on the basis of single-crystal data. Both compounds crystallize in a hexagonal structure with space group P63/mmc and Z = 2. The lattice constants are a = 3.1131(3) A and c = 24.122(3) A for Ta4AlC3 and a = 3.0930(6) A and c = 19.159(4) A for Ta3AlC2. The crystal structures can be explained with a building block system consisting of two types of partial structures. The intermetallic part with a composition TaAl is a two layer cutting of a hexagonal closest packing. The carbide partial structure is a fragment of the binary carbide TaC (NaCl type). It consists of three (Ta4AlC3) or two layers (Ta3AlC2) of CTa6-octahedra linked via common corners and edges. Both compounds are members of the series (TaC)nTaAl. The crystal quality of Ta3AlC2 is improved by using a Al/Sn melt for crystal growth leading to small quantities of Sn in the crystal: Ta3Al1-xSnxC2, x approximately 0.04. On the basis of reliable data a detailed discussion of structural parameters is possible. According to the building principle structure models can be developed for the whole series (MX)nMM' including coordinates for all atoms.     

Tropochemical cell-twinning in the new quaternary bismuth selenides KxSn(6-2x)Bi(2+x)Se9 and KSn5Bi5Se13

The quaternary K(x)Sn(6-2x)Bi(2+x)Se(9) and KSn(5)Bi(5)Se(13) were discovered from reactions involving K(2)Se, Bi(2)Se(3), Sn, and Se. The single crystal structures reveal that K(x)Sn(6-2x)Bi(2+x)Se(9) is isostructural to the mineral heyrovskyite, Pb(6)Bi(2)S(9), crystallizing in the space group Cmcm with a = 4.2096(4) A, b = 14.006(1) A, and c = 32.451(3) A while KSn(5)Bi(5)Se(13) adopts a novel monoclinic structure type (C2/m, a = 13.879(4) A, b = 4.205(1) A, c = 23.363(6) A, beta = 99.012(4) degrees ). These compounds formally belong to the lillianite homologous series xPbS.Bi(2)S(3), whose characteristic is derivation of the structure by tropochemical cell-twinning on the (311) plane of the NaCl-type lattice with a mirror as twin operation. The structures of K(x)Sn(6-2x)Bi(2+x)Se(9) and KSn(5)Bi(5)Se(13) differ in the width of the NaCl-type slabs that form the three-dimensional arrangement. While cell-twinning of 7 octahedra wide slabs results in the heyrovskyite structure, 4 and 5 octahedra wide slabs alternate in the structure of KSn(5)Bi(5)Se(13). In both structures, the Bi and Sn atoms are extensively disordered over the metal sites. Some physicochemical properties of K(x)Sn(6-2x)Bi(2+x)Se(9) and KSn(5)Bi(5)Se(13) are reported.     

铽锰基化合物TbMn6Sn6的结构和磁性

研究了重稀土金属化合物ＴｂＭｎ６Ｓｎ６的晶体结构、内禀磁性及其矫顽力随和外场变化的规律。ＴｂＭｎ６Ｓｎ６为ＨｆＦｅ６Ｇｅ６型六角型结构，晶格常数为：ａ＝０．５５６１ｎｍ，ｃ＝０．９０４７７ｎｍ，单胞体积为０．２４１ｎｍ＾３，居里温度ＴＣ＝４２１Ｋ。ＴＭｎ６Ｓｎ６化合物在外场较弱时，热磁曲线呈亚铁磁性；当外场较强时，则表现出铁磁行为，具有变磁性，ＴｂＭｎ６Ｓｎ６的矫顽力随温度变化，当温度Ｔ＝２００Ｋ     

A new method applicable to study solid compounds with multiple polyhedral structures

A new direct summation method, named as polyhedron method, is proposed to calculate Madelung energy. This method calculates sums of electrostatic interactions over sets of neutral polyhedron unit pairs rather than conventional ion pairs; this gives Madelung constant in a matrix. With robustly rapid convergence, polyhedron method is generally applicable for complex compounds containing multiple polyhedral building‐blocks and numerical polyhedral connection modes. The matrical analysis suggests face‐sharing between octahedral pairs and edge‐sharing between tetrahedral pairs can be electrostatically stable, against Pauling's third rule. Further, the matrical calculation of Madelung energies offers a unique advantage to evaluate enormous configurations of cation distributions in a given lattice in a high‐throughput manner. That is applicable to study solid solution composites, polymorphism, and defect structures, including but not limited to intermediate phase of delithiated cathode compounds, charge order or antisite defects, and extensively magnetic order. © 2016 Wiley Periodicals, Inc.     

89Y magic-angle spinning NMR of Y2Ti2-xSnxO7 pyrochlores

The yttrium local environment in the series of pyrochlores Y2Ti2-xSnxO7 was studied using 89Y NMR. Oxides with the pyrochlore structure exhibit a range of interesting physical and chemical properties, resulting in many technological applications, including the encapsulation of lanthanide- and actinide-bearing radioactive waste. The use of the nonradioactive Y3+ cation provides a sensitive probe for any changes in the local structure and ordering with solid solution composition, through 89Y (I = 1/2) NMR. We confirm that a single pyrochlore phase is formed over the entire compositional range, with Y found only on the eight-coordinated A site. A significant (approximately 15 ppm) chemical shift is observed for each Sn substituted into the Y second neighbor coordination environment. The spectral signal intensities of the possible combinations of Sn/Ti neighbors match those predicted statistically assuming a random distribution of Sn4+/Ti4+ on the six-coordinated pyrochlore B site.     

Study of Surface Cell Madelung Constant and Surface Free Energy of Nanosized Crystal Grain

1 INTRODUCTION 2. 1 Madelung constant of crystal Surface energy of crystal grain has crucial influ- The Madelung constant, which is used to calculate ence on the electrical and mechanical performances lattice energy and so on[1], is of central importance in of material, especially for material making up of na- the theory of ionic crystal and property of crystal nosized crystal grains because all outstanding per- structure. There is no special difficulty in calculating formances of the mat…     

Kaliumtriamidostannat(II), K[Sn(NH2)3] – Synthese und Kristallstruktur

Potassium Triamidostannate(II), K[Sn(NH₂)₃] – Synthesis and Crystal Structure Rusty‐red crystals of K[Sn(NH₂)₃] were obtained by the reaction of SnBr₂ and KNH₂ in a 1 : 3 molar ratio in liquid ammonia at 233 K in the form of platelets. The structure was determined from single crystal X‐ray diffractometer data: Space group P3; Z = 2; a = 6.560(1) Å, c = 7.413(2) Å. The structure contains trigonal pyramidal complex anions [Sn(NH₂)₃]^– and potassium cations. These ions are arranged to one another following the motif of a strongly distorted hexagonal close packing of sequence A(Sn) B(Sn) A′(K) B′(K) …     

Nanospheres of a new intermetallic FeSn5 phase: synthesis, magnetic properties and anode performance in Li-ion batteries

We synthesized monodisperse nanospheres of an intermetallic FeSn(5) phase via a nanocrystal-conversion protocol using preformed Sn nanospheres as templates. This tetragonal phase in P4/mcc space group, along with the defect structure Fe(0.74)Sn(5) of our nanospheres, has been resolved by synchrotron X-ray diffraction and Rietveld refinement. Importantly, FeSn(5), which is not yet established in the Fe-Sn phase diagram, exhibits a quasi-one dimensional crystal structure along the c-axis, thus leading to interesting anisotropic thermal expansion and magnetic properties. Magnetization measurements indicate that nanospheres are superparamagnetic above the blocking temperature T(B) = 300 K, which is associated with the higher magnetocrystalline anisotropy constant K = 3.33 kJ m(-3). The combination of the magnetization measurements and first-principles density functional theory calculations reveals the canted antiferromagnetic nature with significant spin fluctuation in lattice a-b plane. The low Fe concentration also leads Fe(0.74)Sn(5) to enhanced capacity as an anode in Li ion batteries.     

Similar K@Au10Sn10 polyhedra in the markedly different structures of KAu4Sn6 and KAu3Sn3. syntheses and characterization

These compounds were synthesized by high-temperature reactions of the elements in welded Ta tubes and characterized by X-ray diffraction methods and linear muffin-tin orbital (LMTO) calculations. AAu4Sn6 (A = K, Rb) have a new structural type (Fddd, Z = 8), and KAu3Sn3 (Pmmn, Z = 2) is isostructural with SrAu3In3. Both orthorhombic structures contain similar condensed K@Au10Sn10 polyhedral building blocks, which can be described as overall 6-8-6 arrangements of planar rings or, alternatively, as hexagonal prisms centered by K and augmented about the waists by 8-rings of Au and Sn. However, the 3D Au-Sn networks differ appreciably in both composition and the modes of condensation. In KAu3Sn3, the prisms stack by sharing both hexagonal faces with like neighbors along a, whereas those in KAu4Sn6 condense in a complex zigzag network. Compared with related indium systems, the structure change from KAu4In6 ( P_6m2, Z = 1) to KAu4Sn6 apparently illustrates the effect of complex factors such as atom size and valence electron counts on structure, whereas the SrAu3In3 and KAu3Sn3 pair are isotypic. Both compounds are Pauli-paramagnetic and inert to water at room temperature for several days. Tight-binding electronic structure (LMTO) calculations emphasize the dominance and strength of the heteroatomic Au-Sn bonding.     

Studies on fluorescein-VII The fluorescence of fluorescein as a function of pH

The relative fluorescence of fluorescein over the pH range 3-12 has been measured at 516 nm, with excitation at 489 nm. The relative fluorescence is essentially zero at pH 3, increases slowly between pH 4 and 5, rises rapidly between pH 6 and 7, reaches a maximum at pH 8, and remains constant at above pH 8. The curve of relative fluorescence as a function of pH lies somewhat above the corresponding curve describing the fraction of fluorescein present as the doubly charged anion, Fl(2-), indicating much weaker fluorescence of the singly charged anion, HFl(-), and very much weaker fluorescence by the neutral species, H(2)Fl. The fluorescence data have been used to calculate a value for the third dissociation constant. Because of the complexity of the system, one unknown dissociation constant and three (relative) fluorescence constants, a series of three variable regressions on the data was made. The final values were K(HFl) = 4.36 x 10(-7) (mu = 0.10) for the third dissociation constant and K(H(2)Fl) = 0.8; kappa(HFl) = 5.7; kappa(Fl) = 100.0 for the relative fluorescence constants.     

Crystal structures of Sr4Sn2Se9 and Sr4Sn2Se10 and the oxidation state of tin in an unusual geometry

The new ternary selenostannates Sr(4)Sn(2)Se(9) and Sr(4)Sn(2)Se(10) have been synthesized by heating the elements at 1023 K in an argon atmosphere. Their structures were determined by single-crystal X-ray methods. Sr(4)Sn(2)Se(9) crystallizes in a new structure type (Pbam, a = 12.042(2) A, b = 16.252(3) A, c = 8.686(2) A, Z = 4) with Sn(2)Se(6)(4-), SnSe(4)(4-), and Se(2)(2-) subunits. Sr(4)Sn(2)Se(10) (P2(1)2(1)2, a = 12.028(2) A, b = 16.541(3) A, c = 8.611(2) A, Z = 4) has a similar structure with Se(3)(2-) triangles instead of Se(2)(2-) dumbbells. Strontium is 8-fold-coordinated by selenium in both cases. The opening angles between tin and the terminal selenium atoms in the Sn(2)Se(6) subunits are close to 160 degrees , which is nearer a typical Sn(2+) coordination geometry than classical SnSe(4) tetrahedra. This result suggests the tin oxidation state in the Sn(2)Se(6) units to be lower than the expected Sn(4+). This question is examined by self-consistent LMTO and LAPW band structure calculations expanded by the Bader analysis of the charge density to assign reliable atomic charges.     

Substrate evaporation induced neck-shape evolution of a liquid/solid interface

The wetting behavior and interface interaction in the CaF₂/Me and NaCl/Me systems were studied using the sessile drop method. It was observed that liquid Bi, In, Sn and Ga do not wet the CaF₂ and NaCl substrates at 1000 K and liquid Cu does not wet the CaF₂ substrate in the 1423–1573 K temperature range. Nevertheless, different spreading behavior was observed during experiments. For the CaF₂/Me systems, at 1000 K, the contact angle remains constant with time, while for the NaCl/Me systems, a non-monotonic spreading behavior was detected. For these systems the contact angle increases initially and then rapidly decreases. For the CaF₂/Cu system at 1423 K the contact angle remains constant, while it increases with time at 1573 K. It was established that the wetting behavior is attributed to the evaporation of the solid substrate, which leads to the formation of a neck-shape interface. The experimental results were well accounted for by a model, which considered the geometrical characteristic of the metal/ceramic interface and thermo-physical properties of the metals and the substrates.     

Synthesis and characterization of A3Na10Sn23 (A = Cs, Rb, K) with a new clathrate-like structure and of the chiral clathrate Rb5Na3Sn25

Four new compounds Cs17.4(1)Na60.6(1)Sn138 (1), Rb19.1(1)Na58.9(1)Sn138 (2), K21.3(1)Na56.7(1)Sn138 (3), and Rb20Na12Sn100 (4) were synthesized by fusion of the corresponding elements. The structures were determined by single-crystal X-ray diffraction. Compounds 1-3 are isostructural and crystallize in a new structure type (rhombohedral, R3m, Z = 1, a = 12.4567(9) A, c = 51.533(3) A for 1; a = 12.465(1) A, c = 51.085(3) A for 2; a = 12.456(2) A, c = 50.559(4) A for 3). The structure contains layers of fused pentagonal dodecahedra of tin that alternate with layers of isolated tin tetrahedra. It is an intergrowth between the structure of clathrate-II (A24Sn136) with the same layers of pentagonal dodecahedra and the Zintl phase ASn with Sn4(4-) tetrahedra. Compound 4 is a new chiral clathrate (cubic, P4(1)32, Z = 1; a = 16.4127(7) A) with stoichiometry that corresponds to an electronically balanced Zintl phase.     

Fine-tuning of optical properties with salts of discrete or polymeric, heterobimetallic telluride anions [M4micro4-Te)(SnTe4)4](10-) (M = Mn, Zn, Cd, Hg) and 3{[Hg4(micro4-Te)(SnTe4)3](6-)}

Compounds with first discrete M/Sn/Te anions--exhibiting a series of optical absorption energies in the semiconductor range-are obtained by reactions of K+ or Rb+ salts of [SnTe4](4-) with MCl2 (M = Mn, Zn, Cd, Hg) in H2O or H2O-MeOH; larger Cs+ cations provoke the formation of a polymeric derivative of the ternary anionic structure.     

The chemical changes occurring upon cycling of a SnO2 negative electrode for lithium ion cell: In situ Mössbauer investigation

Electrochemical reduction of a SnO₂ electrode for a lithium ion cell is known to result in formation of Li_4.4Sn alloy+2Li₂O. In order to determine to which extent such an electrode can be considered as reversible, we studied the electrochemical oxidation of a previously reduced SnO₂ electrode, using in situ ¹¹⁹Sn Mössbauer spectroscopy. Contrary to what could be expected, the first step does not consist in extraction of lithium from Li_4.4Sn for β-Sn to be obtained. In fact, simple lithium extraction proceeds only down to the Li_1.4Sn composition. Further oxidation (second step) involves formation of unusual species (Sn(0) and oxygen-surrounded Sn(II), both probably in interaction with Li₂O). Then (third step), red SnO-like Sn(II) species are formed, along with some Sn(IV). Especially during the second and third steps, the working electrode is far from thermodynamic equilibrium despite the low oxidation rate. This non-equilibrium behavior is probably related to the ultrafine particle size resulting from electrochemical grinding.     

Computation of the Madelung constant for hypercubic crystal structures in any dimension

A new method of computing the Madelung constants for hypercubic crystal structures in any dimension \(n\ge 2\) is given. It is shown for \(n\ge 3\) that the Madelung constant may be obtained in a simple, efficient and unambiguous way as the Hadamard finite part of the integral representation of the potential within the crystal which is divergent at any point charge location. Such a regularization method fails in the bidimensional case due to the logarithmic nature of singularities for the potential. In that case, a specific approach is proposed taking in account the scale invariance of the Poisson equation and the existence of a finite horizon for each point charge in the plane. Since a closed-form exact solution for the 2D electrostatic potential may be derived, one shows that the Madelung constant may be defined via an appropriate limit calculation as the mean value of potential energies of charges composing the unit cell.