Combined atomic ordering in the A and B sublattices of perovskite structure

The simultaneous ordering of cations in the A and B sublattices of cubic perovskite structure has been investigated by the group-theoretical method. It is found that 147 ordered phases may exist. Among them, there may be 121 phases with simultaneous cation ordering in the 1(a)- and 1(c) positions of perovskite structure. It is shown that 53 phases are described by improper order parameters related to cation ordering, the formation of 56 phases is related to improper rotations of octahedra, and 15 phases are improper ferroelectrics. Calculated structures of some types of ordered low-symmetry perovskite modifications are presented.     

Conditions for stabilization of the ordered state of cations in complex oxides with perovskite structure

Analytical expressions for the coefficients in the Landau potential for six types of ordering of complex oxides with perovskite structure in terms of the sums of ordering energies in different coordination spheres are obtained in the approximation of effective pair interactions. On the basis of the pair interaction potentials of the form v(r) = ?1/r⁶ + Acos(qr)/(qr)³, the conditions for q and R are established, which lead to stabilization of the ordered phases.     

Tilting structures in perovskites

Thirty five low-symmetry tilting phases of the octahedra in which the atoms located at the initial octahedral position remain equivalent are derived for the perovskite structure at the k₁₀(X), k₁₁(M), and k₁₃(R) points of the Brillouin zone. For each low-symmetry phase, structural data are presented and a relationship between the atomic displacements and the order parameters is deduced. All the low-symmetry phases can be obtained by considering only one or two order parameters.     

Structural studies on AsSe–AgI glasses

The atomic structure of the binary AsSe, ternary (AsSe)₈₀Ag₂₀, (AsSe)₈₅I₁₅ and quaternary (AsSe)₆₅(AgI)₃₅ glasses has been studied with the X-ray and neutron diffraction. The local order was also probed with the extended X-ray absorption fine structure spectroscopy at Ag, As and Se K-edges. All experimental data were modeled simultaneously by the reverse Monte Carlo technique. Analysis of the partial pair distribution functions and their characteristics enabled to retrace the structural changes from binary AsSe to pseudo-binary (AsSe)_100−x(AgI)_x system and to study the influence of Ag and I incorporation on the local structure.     

Combinatorial approach to the simulation of superstructures of the NaCl type

The combinatorial approach is used to simulate superstructures of the NaCl type. An algorithm for searching for possible models of superstructures is developed which includes alternating the occupation of the metal sublattice by chemically different cations in an isolated cubic fragment. The application of this algorithm to small cubic fragments is demonstrated. The models are compared with known complex metal oxides (binary A-B-O and ternary A-B-C-O compounds).     

Crystal chemistry of germanates: Characteristic structural features of Li,Ge-germanates

Crystallochemical classification of eleven compounds from the Li-germanate family is suggested. Depending on the set of the primary building units (PBU) (M-octahedra of the composition [GeO₆] and T-tetrahedra of the composition [GeO₄]) and the type of their “condensation,” these germanates are divided into three crystallochemical groups: framework MT-structures (four phases), condensed MT-structures (two phases), and tetrahedral T-condensed structures (five phases). The structural characteristics of the framework Li,Ge-germanates are considered, i.e., their symmetry, crystallographically independent sets of the primary building units, framework architecture, and the types of chains and layers of the (Ge,O)-radicals.     

Strain of interatomic bonds and crystallochemical aspects of AB 1 2/′ B 1 2/″ O3 oxides with perovskite structure

A method for determining the strain characteristics of interatomic bonds in crystals of ternary oxides AB ₁ ^2/′ B ₁ ^2/″ O₃ with perovskite structure, i.e., AB ₁ ^2/′ B ₁ ^2/″ O₃ (B″ = Nb, Ta, Sb, Re, or Bi) and AB ₁ ^2/′2+ B ₁ ^2/″6+ O₃ (B″ = Mo, W, Re, Os, or U), is developed. A linear relationship is established between the effective lengths of unstrained B-O bonds (l _0BO) and the lengths of unstrained B′-O (l _0B′O) and B″-O (l _0B″O) bonds, which differs from the Vegard rule. The found values of l _0B″O for ternary oxides with perovskite structure turned out to be close to the average interatomic B″-O distances in crystals of polymorphic phases of low-symmetry simple oxides. It is shown that the average length of the unstrained Pb-O bond in PbB ₁ ^2/′ B ₁ ^2/″ O₃ perovskites corresponds to the length of the same bond in binary oxides PbBO₃. For ternary oxides with perovskite structure, a linear correlation between the bond-strain energy and the temperature of their transition to the cubic phase is established. A linear correlation is found between the ratios of the Curie temperatures and the bond-strain energy for lead niobates and tantalates.     

Phase equilibria and crystal chemistry of silver-nickel-tellurium system

Ternary isotherm in the silver-nickel-tellurium system has been investigated using X-ray diffraction and metallographic techniques. At 873 K, only monoclinic Ag₂Te phase with a = 0.815 nm, b = 0.449 nm, c = 0.811 nm, and β = 112°60' is stable in the Ag-Te system. Existence of two phases viz., Ni₃Te₂ and NiTe_1+x has been established. NiTe_1+x phase has a large homogeneity range in the binary and ternary system. solid solubility of nickel in Ag₂Te as well of silver in Ni₃Te₂ is about 5 at.% at the temperature of investigation. The reasons for the large homogeneity range of NiTe_1+x in binary and ternary system has been discussed in detail.     

Intermediate Phases,structural variance and network demixing in chalcogenides: The unusual case of group V sulfides

We review intermediate phases (IPs) in chalcogenide glasses and provide a structural interpretation of these phases. In binary group IV selenides, IPs reside in the 2.40 < r < 2.54 range, and in binary group V selenides they shift to a lower r, in the 2.29 < r < 2.40 range. Here, r represents the mean coordination number of glasses. In ternary alloys containing equal proportions of group IV and V selenides, IPs are wider and encompass ranges of respective binary glasses. These data suggest that the local structural variance contributing to IP widths largely derives from four isostatic local structures of varying connectivity r, two include group V based quasi-tetrahedral (r = 2.29) and pyramidal (r = 2.40) units, and the other two are group IV based corner-sharing (r = 2.40) and edge-sharing (r = 2.67) tetrahedral units. Remarkably, binary group V (P, As) sulfides exhibit IPs that are shifted to even a lower r than their selenide counterparts; a result that we trace to excess S_n chains either partially (As–S) or completely (P–S) demixing from network backbone, in contrast to excess Se_n chains forming part of the backbone in corresponding selenide glasses. In ternary chalcogenides of Ge with the group V elements (As, P), IPs of the sulfides are similar to their selenide counterparts, suggesting that presence of Ge serves to reign in the excess S_n chain fragments back in the backbone as in their selenide counterparts.     

Acentric niobium-rich analogue of Labuntsovite-Fe: Crystal structure and microtwinning

A new mineral of the labuntsovite group from Vuoriyarvi (North Karelia) was studied by the method of X-ray diffraction analysis. The unit-cell parameters are a = 14.450(6) Å, b = 13.910(6) Å, c = 7.836(4) Å, β = 117.42(1)°, V = 1398.1 ų, sp. gr. Cm. The symmetry, unit-cell parameters, splitting of the A position, and ordered distribution of the cations over the B and C positions of the new mineral makes it similar to the zinc-rich analogue of labuntsovite. Unlike the zinc-rich analogue, the D position in the new representative of the labuntsovite group is dominated by Fe atoms when the position occupancy is lower. In addition, the new representative is characterized by high occupancies of the A and B positions and the ordered distribution of the Ti and Nb atoms over two octahedral positions. In both minerals, the A position is split into two positions, with one of the positions in both minerals being occupied by Na and the other position being occupied by K in the new mineral and Sr in the Zn-analogue.     

Fluoride glasses in the ZrF4BaF2YF3AlF3 quaternary system

The quaternary system ZrF₄BaF₂YF₃AlF₃ has been investigated in order to determine the nature of the vitreous domain. Addition of aluminium fluoride to the basic ZrF₄BaF₂YF₃ ternary system results in an increase in the size of the vitreous area and a lowering of the crystallization rate.The change of density, refractive index and T_G with respect to composition has been studied. It has been shown that the substitution of Zr⁴⁺/Al³⁺ involves a change in the cationic distribution rather than in the anionic packing.     

On the local coordination of Fe in Fe2O3-glass and Fe2O3-glass ceramic systems containing Pb, Na and Si

The effect of the composition and casting temperature on the structural role of Fe in a series of binary (Fe₂O₃-PbO and Fe₂O₃-Na₂O) and ternary (Fe₂O₃-PbO-SiO₂) glass and glass ceramic materials is studied by means of X-ray fluorescence (XRF) mapping, X-ray absorption fine structure (XAFS) and Mössbauer spectroscopies. According to the Mössbauer results the majority of Fe exists in the Fe⁺³ state. The XRF maps reveal that Fe-rich islands evolve into the vitreous matrix of ternary samples that contain more than 40 wt% Fe₂O₃. In these samples the XAFS results disclose that 40-43 at.% of the Fe atoms belong to the Fe-rich microcrystalline islands formed by Fe_xO_y oxides. Furthermore, the structural role of Fe⁺³ in the ternary glasses is found to depend on the Fe₂O₃ content. Finally in the binary Fe₂O₃-PbO systems the majority of Fe⁺³ is octahedrally coordinated in the Fe₂O₃ and/or PbFe₁₂O₁₉ crystalline phases.     

Structural Studies on Ni75(SnSi)25 and Ni75(SnBi)25 Alloys

Ternary alloys on the quasi-binary system Ni₃Sn(r) in equilibrium condition. It has been observed that these phases undergo transformations at high temperatures but upto 500 °C room temperature modifications are stable. The two phases do not have any appreciable solid solubility in either of them. The phase Ni₃Si(r) crystallizes in to Cu₃Au (Li₂) structure where as Ni₃Sn(r) is based on c.p.h. structure with a = 5.305 A°, c = 4.254 A°. No new ternary phase has been detected in Ni₃Sn Ni₃Si section. The investigated alloys of the Ni Sn Bi system contain 75 at.% Ni. All the ternary alloys show simultaneous occurrence of three phases, namely Ni₃Sn(r), NiBi and Ni(Sn) in equilibrium state. The phase NiBi has NiAs(B8) type of structure. Due to non-existence of isostructural phases in the two binary systems (Ni Sn and Ni Bi), single solid solution phases are not formed. Widely differing atomic sizes of nickel and bismuth atoms restrict the formation of solid solution of bismuth in nickel in contrast to Ni(Sn) where atomic size factor is favourable.     

Glass formation of Ti–Ni–Sn ternary alloys correlated with TiNi–Ti3Sn pseudo binary eutectics

Glass-forming ability (GFA) of Ti–Ni–Sn ternary alloys was investigated. Applying recent models based on atomic size ratio and efficient packing, the composition favoring the glass formation is predicted. Our experiments indicate that the optimized glass-forming composition is located at Ti₅₆Ni₃₈Sn₆, with the critical thickness of complete glass formation approaching 100 μm for the melt-spun ribbons. The Ti₅₆Ni₃₈Sn₆ metallic glass exhibits a sizable supercooled liquid region (ΔT_x) of about 35 K and a reduced glass transition temperature (T_rg) of 0.52. We demonstrate that the glass formation of the Ti₅₆Ni₃₈Sn₆ alloy correlates with the (L → TiNi + Ti₃Sn) pseudo binary eutectic reaction in Ti–Ni–Sn ternary system, which has an invariant temperature and composition at ～1370 K and ～Ti₅₈Ni₃₄Sn₈, respectively. With respect to Sn-free Ti–Ni binary alloys, the GFA is enhanced for the Ti–Ni–Sn ternary alloys, but the improvement is limited possibly due to changes in the crystalline phases competing with glass formation.     

Calorimetric and phase equilibria studies of the Ni–Sn–Bi system

The phase equilibria at 1273 K were investigated and an isothermal section of the Ni‐Sn‐Bi phase diagram was constructed. The phase boundaries are inclined to the Bi‐rich region that is agreement with phase equilibria at lower temperatures. The molar enthalpies of formation of liquid ternary Ni–Sn–Bi alloys have been determined at 833, 873 and 933 K by direct reaction calorimetry using pellets of 325 mesh powders of pure Ni, Sn and Bi. Measurements were performed with alloys containing from 0.05 to 0.10 mole fractions of nickel, and at ratios of tin and bismuth mole fractions X_Sn/X_Bi = 2.8, 1.2 and 0.43. The experimental calorimetric data were used to calculate a regular solutions parameter of the ternary liquid phase by means of two different thermodynamic data files. It was found that the Ni–Sn–Bi ternary liquid phase could be described as temperature independent ternary regular solution. The assessed values of the ternary interaction parameter are ‐280 J·mol^–1 and 56 000 J·mol^–1, depending on the binary parameters used. Enthalpies of formation of solid binary phases were measured, too. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ionic conductivity of the phases of five types formed in the BaF2-GdF3 system

The ionic conductivity has been measured in ceramic phases of five structure types found in the BaF₂-GdF₃ system: the fluorite type (CaF₂), its trigonal and tetragonal distortions, the tysonite type (LaF₃), and the orthorhombic ??-YF₃ modification. The phases have been obtained by solid-phase synthesis from BaF₂ and GdF₃ mixtures in hermetic nickel containers at 925, 964, and 1067°C for 108?C360 h in a fluorine atmosphere. Their conductivity ?? is compared in correlation with the composition and structure type. The highest conductivity values are found for the tysonite Gd_{1 ? y} Ba _y F_{3 ? y} phase (0.10 ?? y ?? 0.25): (1?2) × 10^?3 S/cm at 683 K. The ordered Ba_0.60Gd_0.40F_2.40 and Ba_0.57Gd_0.43F_2.43 phases with fluorite-derived structures and different degrees of order are characterized by the lowest conductivities: i(1.5?3.5 × 10^?5) S/cm.     

Dielectric properties of binary and ternary mixtures of alcohols: Analysis of H-bonded interaction in complex systems

The heterogeneous H-bonded interactions in mixtures of glass-former liquids, i.e., binary mixtures of ethyl alcohol (EA) with ethylene glycol (EG) and ternary mixtures of equi-molar EA + EG system with glycerol (Gly) have been investigated at 21 concentrations by using precisely measured values of static and high frequency limiting dielectric constants at 25 °C. Concentration dependent observed negative values of excess dielectric constant (ε^E) and effective Kirkwood correlation factor (g^eff) implies the formation of H-bonded complexes between EA and EG molecules which reduces the effective number of dipoles. The EA + EG system forms the strongest complexes corresponding to the molar ratio ∼1:1. The observed positive ε^E values of the (EA + EG) + Gly ternary mixtures confirm the increase in number of effective dipoles contributed to the dielectric constant values due to the formation of the ternary complexes. Critical analysis of the experimental results of the EA + EG and the (EA + EG) + Gly mixtures suggest the suitability of ε^E and g^eff study in order to explain the dipolar ordering mechanism formed during the complexes between the alcohol molecules in their ternary mixtures.     

Characterization of the Hexatic B and Crystal B phases by Optical Microscopy

Recent investigations concerning the structure of the B Phase have unearthed a variety of packing arrangements of the molecular layers for this semctic modification. Two general structures have been discovered; one in which the B Phase has three-dimensional crystalline order (crystal B), and one that is characterized by short-range in-plane positional ordering of the constituent molecules, and long-range bond orientational order, both within and between the layers (heatic B). In this present study n-hexyl 4′-n-pentyloxybiphenyl-4-carboxylated (650BC) has been synthesized and its smectic phases charaterized. Typically, this ester exhibits an A, B,E, phase sequence, with the B Phase being of the hexactic type. The Synthesis, microscopic textures, and miscibility studise ar reported here. The results show that while the two types of B phase appear co-miscibly by conventional optical mehtods, in fact X-ray diffraction studies show that a transition occurs between the two phases in certain binary mixtures. Thus, the hexactic B and crystal B modification are thermodynamically distint phasaes.     

Rapid Synthesis of Polycrystalline CuGa1‐xInxTe2 Compounds

A series of CuGa_1‐xIn_xTe₂ bulk quaternary compounds (x=0.17, 0.33, 0.50, 0.67, 0.83) has been synthesized from the corresponding stoichiometric melts by a rapid cooling procedure. The same procedure was used to obtain the ternary bulk compounds CuGaTe₂ and CuInTe₂ with the aim to compare, according the Vegard law, the match between molar fraction of the indium (x) and the lattice parameters for the aforementioned quaternary compounds. The nature of the crystalline phases, the local structure homogeneity and stoichiometry of these compounds have been investigated by x‐ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The analysis revealed the presence of the chalcopyritic structure for all the samples.     

Theoretical crystal chemistry of M x (TO4)y sulfates and selenates: Topological analysis and classification of suprapolyhedral invariants

A geometric topological analysis of orthotetrahedral phases M _x (TO₄)_y (T = S or Se) is performed for 46 sulfates and 17 selenates with the TOPOS 3.2 software package. The values of coordination sequences {N _k } of T atoms are used as classification parameters of topologically different MTO nets. The crystal structures are analyzed within 12 coordination spheres of T sites and assigned to 26 topological types. It is established that only 7 types are common for the structures of sulfates and selenates, 16 types include only sulfates, and 3 types include only selenates. The average values of the bond lengths are determined: 〈R(S-O)〉 = 1.48(2) Å and 〈R(Se-O)〉 = 1.63(2) Å. The hierarchical ordering of the crystal structure is performed using the concept of a polyhedral microensemble of the structure.