EFFECT OFTHE SUBSTITUTION OF Bi→Sb ON THE STRUCTURE CHANGES WITHIN THE AgBi1-xSbxS2 (x = 0 - 1) SOLID SOLUTION

Quaternary compounds Er2.34Се(Pr)0.66Ge1.28S7 are formed in the quasi-ternary systems Er2S3–R2S3–GeS2 (R–Ce, Pr) at 770 K. The structure of these compounds was determined from data sets obtained from homogeneous samples weighing 0.8 g recorded at a DRON 4-13 X-ray diffractometer, CuKα radiation, in the range of 10°≤2θ≤100°, scan step 0.05°, 20 s exposure at each point. The computation of the crystal structure of the quaternary compounds Er2.34Се(Pr)0.66Ge1.28S7 was peformed using WinCSD software package. It was determined by powder method that these compounds crystallize in the Dy3Ge1.25S7 structure type (space group P63). The new compounds are formed by substituting Dy atoms in the Dy3Ge1.25S7 structure with atoms of the statistical mixture (2.34 Er + 0.66 Се) or (2.34 Er + 0.66 Pr): r(Dy+3) = 0.0910 nm, averaged radius r(Rr+3 + Ce+3) = 0.0916 nm, r(Er+3 + Pr+3) = 0.0912 nm (was calculated on the basis of the REM content of the statistical mixture). The replacement of Ce atoms with Pr atoms leads to a decrease in the unit cell parameters which is explained mainly by a corresponding decrease in the atomic radii of the above elements. The unit cell contains 22.5 atoms. The coordinates of the atoms and the isotropic displacement parameters have satisfactory values. The 2a site contain Ge2 atoms with 0.28 occupation ratio.The atoms of the statistical mixture (Er + R) form in the structure of the quaternary compounds trigonal antiprisms [(Er + R)7S] that are spherically embedded around the [Ge26S] octahedral; the next sphere around these antiprisms is composed of the [Ge14S] tetrahedra. One of the features of such crystal structure is the formation of two-dimensional networks of sulfur atoms parallel to ab plane. The Er2.34Се(Pr)0.66Ge1.28S7 compounds have mixed covalent ionic nature. The closest packing of sulfur atoms in the Er2.34Се(Pr)0.66Ge1.28S7 structure contains one octahedral void and two tetrahedral voids per unit cell; these are occupied by Ge2 and Ge1 atoms, respectively. A row of prismatic voids that are occupied by the mixture (Er + R) is located between the rows of octahedral and tetrahedral voids. This arrangement of atoms in the structure forms the particular properties of these materials that will be a promising object for the study of physical properties.

Solid electrolytes with superionic conductivity are required as a main component for all-solid-state batteries. Here we present a novel solid electrolyte with three-dimensional conducting pathways based on "lithium-rich" phosphidosilicates with ionic conductivity of σ > 10-3 S cm-1 at room temperature and activation energy of 30-32 kJ mol-1 expanding the recently introduced family of lithium phosphidotetrelates. Aiming toward higher lithium ion conductivities, systematic investigations of lithium phosphidosilicates gave access to the so far lithium-richest compound within this class of materials. The crystalline material (space group Fm3m), which shows reversible thermal phase transitions, can be readily obtained by ball mill synthesis from the elements followed by moderate thermal treatment of the mixture. Lithium diffusion pathways via both tetrahedral and octahedral voids are analyzed by temperature-dependent powder neutron diffraction measurements in combination with maximum entropy method and DFT calculations. Moreover, the lithium ion mobility structurally indicated by a disordered Li/Si occupancy in the tetrahedral voids plus partially filled octahedral voids is studied by temperature-dependent impedance and 7Li NMR spectroscopy.

A review of experimental X-ray diffraction data on the structure of welding flux melts has been performed. It is shown that the structure factors (SF) of the investigated melts obtained from the diffraction data are similar, regardless of differences in their chemical composition. This indicates that the structure of oxide melts is based on certain atomic clusters with a similar local atomic order. It is shown that such clusters are dense non-crystalline packings of oxygen atoms, which play the main role in forming the structure of welding flux melts. The structural model of oxide melts consistent with the high-temperature X-ray diffraction data has been considered. According to this model, nano-sized clusters of oxygen atoms (nanoclusters) with dense packing are uniformly located in a medium ("quasi-gas"), which is characterized by less dense atomic packing and much smaller sizes of atomic clusters. The dense packing of oxygen atoms in such nanoclusters has octahedral and tetrahedral voids, which are partially occupied with cations. Small cations (Si4+) occupy tetrahedral voids, while large cations (Mg2+, Mn2+, Fe2+) occupy only octahedral voids. The Al3+ cations can occupy both tetrahedral and octahedral voids.

All solid-state batteries are considered to extend the limits of lithium ion batteries in terms of power density, cycle life stability and device safety. One key component for this technology is the solid electrolyte. Heavy research on oxide and sulfide based lithium ion conductors has brought out some highly optimized systems, which show ionic conductivities up to 25 mScm-1.[1] Recently lithium phosphides have been introduced as a new class of lithium ion conductors with ionic conductivities of 1.1 mScm-1 for Li14SiP6 [2] and 3 mScm-1 for Li9AlP4. [3] Due to the increased negative charge of the phosphide anion compared to conventionally used chalcogenide anions, the lithium content and therefore the charge carrier concentration is significantly higher. Since aliovalent doping can lead in sulfides to an ionic conductivity an order of magnitude higher than the parent undoped compounds, we aim here for the stepwise aliovalent substitution of the four-valent tetrel-ion in lithium phosphido-tetrelates with three-valent aluminum, to further increase the lithium concentration as well as the volume of the unit cell.[5][6] In this work we show the effect of the substitution of germanium by aluminum on the ionic conductivity in the solid solution Li8+x Al x Ge1-x P4 (0≤x≤1). β-Li8GeP4,[4] Li9AlP4 and the solid solution Li8+x Al x Ge1-x P4 crystallize in the cubic space group P-43n with the phosphorus atoms building a distorted fcc-lattice. The solid solution strictly follows Vegard’s law showing a linear dependency of the lattice parameter on the substitution grade over the whole investigated range (figure 1). Ge and Al atoms occupy 1/8 of the tetrahedral voids. Remaining tetrahedral voids and some octahedral voids are partially occupied by lithium. The cubic structure makes the system a convenient model for investigations of substitution effects, as symmetry restrictions prohibit extent distortion. Lithium ions are mobile and migrate between face sharing octahedral and tetrahedral positions as well as edge sharing tetrahedral voids. Impedance spectroscopy measurements were conducted in a custom-made cell setup for powder samples, where powders are pelletized in situ and contacted by steel electrodes. High pressure is applied over six screws, tightened with defined torque. The temperature was controlled by a specially designed heating block connected to a heating circulator, which allows to do all measurements in argon atmosphere. The ionic conductivity in Li8+x Al x Ge1-x P4 increases with the amount x of aluminum by nearly one order of magnitude. This increase does not proceed linearly, revealing a more complex correlation (figure 1). Figure 1: Left: Vegard plot for the solid solution Li8+x Al x Ge1-x P4 (0≤x≤1) created on powder X-ray diffraction data. The black line acts as a guide to the eye, emphasizing the linear dependency of the lattice parameter a on the grade of substitution x. The upper left corner shows the crystal structure of Li8+x Al x Ge1-x P4. Right: Temperature dependency of the ionic conductivity for different compositions of Li8+x Al x Ge1-x P4.

Alloys from the region of existence of the solid solution TbCo4.5SixLi0.5-x were synthesized by arc melting. Quantitative and qualitative composition of alloys and powders of electrode materials was determined by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The Tb/Co/Si ratio in the samples was confirmed by X-ray fluorescence spectroscopy. The change in cell parameters within the solid solution existence was established by the results of X-ray powder diffraction (TbCo4.5SixLi0.5-x, x = 0.1–0.4: a = 4.9518(5) – 4.9324(3), c = 3.9727(4) – 3.9746(3) Å). The crystal structure of the solid solution was determined by the Rietveld method (CaCu5 structure type, space group P6/mmm). Cobalt atoms are partially replaced by silicon and lithium only in 2c position. The ability of alloys to reversibly absorb hydrogen was studied by the method of electrochemical hydrogenation. Under experimental conditions the amount of deintercalated hydrogen was about 0.19 H/f.u. The change in cell parameters after hydrogenation (volume increases from 83.74(1) to 85.54(6) Å3) and the stability of the electrode in the electrolyte solution was further confirmed by X-ray phase analysis. Measurements of the electrical resistivity of the samples indicated a decrease of resistivity value with a slight increase in the amount of alkali metal in samples.

Phosphide‐based compounds are promising materials for solid electrolytes. In recent times, a multiplicity of compounds featuring isolated MP4 (M = Si,Ge,Sn,Al,Ga) tetrahedra as structural building units in different arrangements with superionic lithium conductivity have been discovered. ω‐Li9AlP4, ω‐Li9GaP4, and ω‐Li9InP4 are presented as new high‐temperature modifications with superionic lithium conductivity reaching 4.5 mS cm−1 at room temperature. Impedance spectroscopy and static temperature‐dependent 7Li NMR experiments reveal conductivity values in the range of 0.2 to 4.5 mS cm−1 at room temperature and low activation energies for the title compounds. X‐ray and neutron diffraction methods disclose that the phosphorus atoms form a cubic‐close packing. The triel element and Li atoms are located in tetrahedral voids, further Li atoms partially fill the octahedral voids. Temperature‐dependent neutron diffraction shows for Li9AlP4 a phase transition at 573 K that influences the occupation of voids with Li and significantly affects the Li‐ion mobility. The evaluation of nuclear scattering densities by the maximum‐entropy approach and application of the one‐particle‐potential formalism reveal 3D lithium diffusion with a low activation energy preferentially on paths of adjacent tetrahedral and octahedral voids. The investigation of different polymorphs suggests that the equilibrated filling of tetrahedral and octahedral voids is a crucial parameter for the enhancement of superionic lithium conductivity.

Crystal structures in the system ZnInS

In this work, the formation of a self-reinforced HEA–WC powder material during the mechanical alloying (MA) of a mixture of elemental powders of the W-Fe-Co-Ni system in a planetary ball mill in a gasoline environment was investigated. X-ray diffraction (XRD), microstructural, and energy dispersive spectral (EDS) analysis revealed that after 20 hours of MA of the powders in a carbon-containing medium, a powder material based on a high-entropy alloy was formed. HEA based powder material contains two solid solutions with BCC and FCC crystal structure, as well as a carbide WC phase formed "in-situ" owing to the large negative value of the mixing enthalpy between carbon and tungsten. The “in-situ” formation of WC particles contributes to the enhancement of interfacial bonding with the metal matrix of the HEA. All phase components of the HEA–WC powder material are in the nanostructured state. The particles of the powder material have a close to spherical shape and a bimodal particle size distribution. The powder material consists of fine particles with a size of 1–10 μm and their agglomerates with a size up to 100 μm. The microstructure of the particles consists of the main gray phase of the BCC solid solution, dark spaces of the FCC solid solution and fine WC particles with a size of 0.1 μm to 1 μm evenly distributed in the HEA matrix. High-entropy alloys with the addition of carbon are promising in terms of practical application, because carbon, having a small atomic radius, dissolves as an interstitial element in the crystal lattice of the substitutional solid solutions of the HEA principal elements and, as a result, significantly affects their structure and phase composition, and, therefore, will contribute to the improvement of the mechanical properties of the self-reinforced powder material due to the effects of both solid-solution strengthening by interstitial atoms and the precipitation strengthening by the carbide phase particles.

Single crystals of the new compounds Sc(3)Al(3)O(5)C(2) and ScAl(2)ONC were obtained by reacting Sc(2)O(3) and C in an Al-melt at 1550 degrees C. Their crystal structures continue the row of transition metal oxide carbides with an ordered distribution of anions and cations with ScAlOC as the first representative. In the structure of Sc(3)Al(3)O(5)C(2) (P6(3)/mmc, Z = 2, a = 3.2399(8) A, c = 31.501(11) A, 193 refl., 23 param., R(1)(F) = 0.024, wR(2)(I) = 0.058) the anions form a closest packing with five layers of oxygen separated by two layers of carbon atoms. Sc is placed in octahedral voids and Al in tetrahedral voids thus forming layers of AlOC(3) tetrahedra and ScC(6)- and ScO(6)-octahedra, respectively. Surprisingly the layers of ScO(6) octahedra are connected by an additional layer of undistorted trigonal bipyramids AlO(5). The structure of ScAl(2)ONC (space group R3m, Z = 3, a = 3.2135(8) A, c = 44.636(1) A, 187 refl., 21 param., R(1)(F) = 0.023, wR(2)(F(2)) = 0.043) can directly be derived from the binary nitrides AlN (wurtzite-type) and ScN (rocksalt-type). The anions form a closest packing with alternating double layers of C and O separated by an additional layer of N. Again, Al and Sc occupy tetrahedral and octahedral voids, respectively. All compositions were confirmed by energy dispersive X-ray spectroscopy (EDXS) measurements on single crystals. According to band structure calculations Sc(3)Al(3)O(5)C(2) is electron precise with a band gap of 0.3 eV. Calculations of charges and charge densities reveal that the mainly ionic bonding contains significant covalent contributions, too. As expected Sc and C show higher covalent shares than Al and O. The different coordinations of O, Al, and Sc are clearly represented in the corresponding p and d states.

Optimization radiation shielding properties of aluminum-based spinel minerals through the crystal tetrahedral and octahedral voids and their ions composition

Ag8I2(CrO4)3 has been synthesized by solid state reaction, starting from stoichiometric mixtures of Ag2O, AgI and Cr2O3, at elevated oxygen pressures. The compound crystallizes in the hexagonal space group P63/m, with the unit cell dimensions a = 9.4474(4) Å, c = 10.2672(4) Å, γ = 120°, V = 793.61(6) Å3, and Z = 6. The crystal structure was solved by direct methods and refined, basing on single crystal diffraction data (815 independent reflections, R1 = 2.45 %). The structure is fully ordered. The CrO42– ions are arranged in the mode of an hcp packing. Such a building principle is providing channels of face sharing octahedral voids. In the case of the title compound, there are three of such channels in the unit cell. Two of them accommodate two iodine atoms and one silver each, with iodine occupying the octahedral voids, and silver centering the triangular faces connecting the octahedra. Thus, for silver a trigonal bipyramidal coordination by three oxygen and two iodine ions result. In the third column of face sharing octahedra, silver is in the centre of the octahedra. The remaining silver atoms are located in the tetrahedral voids, between the CrO42– ions. According to the results of impedance measurements, Ag8I2(CrO4)3 is a silver ion conductor. The compound shows an increase in the ionic conductivity in the temperature range from 25 to 175 °C, and has a silver ion conductivity of 6.5 × 10–4 Ω–1·cm–1 at 30 °C. The activation energy for silver ion conduction is 0.21 eV, in the temperature range from 25 to 50°.

Microstructure, crystal structure and electrical properties of Cu 0.1Ni 0.8Co 0.2Mn 1.9O 4 ceramics obtained at different sintering conditions

The ferroelectric-antiferroelectric phase transition in a ceramic solid solution Li0.12Na0.88Ta0.2Nb0.8O3 at 350°C was studied using Raman spectroscopy. A considerable broadening of the lines referred to the translational vibrations of the cations in octahedral and cuboctahedral voids and to the vibrations of the oxygen framework, along with a decrease to zero of the intensity of the line corresponding to the bridge stretching mode of the oxygen atoms from the octahedral anion BO6, was found to take place as the temperature of the solid solution approaches the transition point from below. It is shown that, during the transition, the solid solution loses its ferroelectric properties, probably owing to the preferential increase in the anharmonicity of the vibrations of the cations in the octahedral voids.

The title compound Zn(NH3)4(ClO4)2 crystallizes in the space group F4̄3m with a = 10.240(1) Å. The crystal structure consists of tetrahedral Zn(NH3)4 cations and two nonequivalent ClO4 anions with crystallographic Td symmetry. The complex ions constitute an arrangement which is known from the Zintl phase MgAgAs. The Zn(NH3)4 cations are ccp packed with perchlorate anions in octahedral and tetrahedral voids. Whereas the ClO4 ions centered at tetrahedral holes do not interact with the other lattice components, the perchlorate ions in the octahedral voids are connected with the ammine ligands by a hydrogen bonded three-dimensional network involving all their N, H, and O atoms. The repeating unit of this network is a N4O4(μ-H)12 cube with N-H = 1.19(2) Å and O···H = 1.98(2)Å . Raman and IR spectra were recorded between 150 and 4000 cm-1. All the expected internal modes of the complex ions could be detected and assigned. The crystallographically different ClO4 anions have nearly the same vibrational spectra, only a slight splitting of two IR modes is observed

The dependence of the lattice parameter on dopant concentration in Ce1-xMxO2 (M = Sn and Ti) solid solutions is not linear. A change towards a steeper slope is observed around x ∼ 0.35, though the fluorite structure (space group Fm3m) is preserved up to x = 0.5. This phenomenon has not been observed for Ce1-xZrxO2 solid solutions showing a perfectly linear decrease of the lattice parameter up to x = 0.5. In order to understand this behavior, the oxidation state of the metal ions, the disorder in the oxygen substructure and the nature of metal-oxygen bonds have been analyzed by XPS, (119)Sn Mössbauer spectroscopy and X-ray absorption spectroscopy. It is observed that the first Sn-O coordination shell in Ce1-xSnxO2 is more compact and less flexible than that of Ce-O. The Sn coordination remains symmetric with eight equivalent, shorter Sn-O bonds, while Ce-O coordination gradually splits into a range of eight non-equivalent bonds compensating for the difference in the ionic radii of Ce(4+) and Sn(4+). Thus, a long-range effect of Sn doping is hardly extended throughout the lattice in Ce1-xSnxO2. In contrast, for Ce1-xZrxO2 solid solutions, both Ce and Zr have similar local coordination creating similar rearrangement of the oxygen substructure and showing a linear lattice parameter decrease up to 50% Zr substitution. We suggest that the localized effect of Sn substitution due to its higher electronegativity may be responsible for the deviation from Vegard's law in Ce1-xSnxO2 solid solutions.