Electron microscopy studies of potassium sodium niobate ceramics.

Using electron microscopy, K0.5Na0.5NbO3 (KNN) ceramics sintered at 1030 degrees C for 8 h and 1100 degrees C for 2 and 24 h was studied. The scanning electron microscopy and X-ray spectrometry revealed that the materials consisted of a matrix phase in which the (Na+K)/Nb ratio corresponded closely to the nominal composition and a small amount of Nb-rich secondary phase. A bimodal microstructure of cube-shaped grains was revealed in the fracture and thermally-etched surfaces of the KNN. In the ceramics sintered at 1100 degrees C, the larger grains (up to 30 mum across), contained angular trapped pores. The transmission electron microscopy analysis revealed that the crystal planes of the grains bordering the intragranular pore faces were of the {100} family with respect to the simple perovskite cell. Ferroelectric domains were observed in the grains of this material.     

Crystal structure of sodium potassium zinc diphosphate NaKZnP2O7

Single crystals of NaKZnP₂O₇ were grown, and their crystal structure was determined by X-ray diffraction (space group P2₁/n, a = 12.585(5) Å, b = 7.277(5) Å, c = 7.428(5) Å, β = 90.00(5)°, Z = 4, 1916 F(hkl), R ₁ = 0.0461). The structure contains a 3D tetrahedral zinc phosphate framework with a system of intersecting channels running along the b and c axes. The sodium and potassium atoms are coordinated to six and seven oxygen atoms, respectively, and reside inside these channels; the potassium cations, which are larger than the sodium cations, are located at channel intersections.     

Crystal lattice dynamics and phase transitions mechanism in potassium hydro (deutrium) sulphate crystals

The phase transition in KHSO₄ crystals of varying deutrium content and different Cu²⁺ concentration is studied. The specific heat at constant pressure,C _p, and DTA measurements are investigated. The measurements are carried out in the vicinity of phase transitionT _c =468 K. Multiple peaks are observed in the temperature dependence ofC _p and a noticeable change due to Cu²⁺ dopant. This behaviour can be attributed to the fact of order disappearance in partial melting of proton in sublattice.

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Zusammenfassung Es wurde die Phasenumwandlung in KHSO₄ Kristallen mit unterschiedlichem Deuteriumgehalt und unterschiedlicher Cu²⁺-Konzentration studiert. Die isobare spezifische WÄrmeC _p und DTA-Messungen wurden untersucht. Die Messungen wurden in der NÄhe des PhasenumwandlungspunktesT _c =468 K durchgeführt. In TemperaturabhÄngigkeit vonC _p wurden Mehrfachpeaks sowie eine bemerkbare Änderung in AbhÄngigkeit vom Cu²⁺-Gehalt beobachtet. Dieses Verhalten wurde erklÄrt.

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On leave Alexandria University, Faculty of Science, Physics Department, Alexandria EGYPT     

Isomerisation and phase transitions in small sodium clusters

Using a distance-dependent tight-binding hamiltonian, we have studied the influence of the temperature on the geometries of small alkali clusters (Na₄, Na₈, and Na₂₀). We have applied a Monte-Carlo thermodynamical method which consists in performing canonical samplings for various temperatures, these samplings being reexpressed in the microcanonical ensemble. This method provides thermodynamical values such as the entropy and the specific heat. Their behaviour shows one phase transition in the case of Na₄ and Na₈, and two phase transitions for Na₂₀. As concerns Na₄ and Na₈, the transition occurs at 200 K, between a solid-like phase and a phase for which the geometry of these clusters oscillates between numerous shapes. In the case of Na₂₀, the two observed phase transitions can be described as a melting of the surface atoms (at 200 K) preliminarily to the fluctuation of an inner icosahedron seed (at 300 K).     

Crystal structure of potassium 2-thiobarbiturate

The crystal and molecular structure of potassium thiobarbiturate C₄H₃KN₂O₂S (C₄H₄N₂O₂S-2-thiobarbituric acid, H₂TBA) is determined. Crystallographic data for KHTBA are as follows: a = 11.2317(17) Å, b = 3.8687(6) Å, c = 14.557(2) Å, β = 97.448(4)°, V = 627.18(17) ų, space group P2/c, Z = 4. Each potassium ion is linked with four oxygen atoms and two S atoms forming a distorted octahedron. N-H…O and C-H…S hydrogen bonds form a branched three-dimensional network. The structure is also stabilized by the π-π interaction of heterocyclic HTBA^? ions.     

Cluster molecular orbitals and an orbital symmetry view of solid phase transitions in perovskites

The molecular orbitals, normalization constants and energies of the M₈(O_h), M₄(T_d) and M₆(O_h) clusters are derived and tabulated through the d-atomic orbitals. A vector method, adapted to computer application, is devised to compute s, p and d overlap between variously oriented orbitals at atoms that do not have co-directional local axes. Mixing of σ, π and δ orbitals to give the same irreducible representation is also included. As illustrations, the orbitals of Sr₈, La₈, TiO₆ and AlO₆ clusters are computed by the Mulliken—Wolfsberg and Helmholz approximations. During solid phase transitions in the perovskite structures of SrTiO₃ and LaAlO₃, the TiO₆ octahedron rotates about the C₄ axis whereas the AlO₆ octahedron rotates about the C₃ axis. This difference is explained qualitatively in terms of the relative symmetries of the cluster HOMOs and LUMOs using the second-order Jahn—Teller effect. Allusions are made to the application of this cluster symmetry approach to other systems.     

Phase transitions and band structure in metallic perovskites (carbides and nitrides)

Properties of Mn₃XN and Mn₃XC types of compounds are analyzed on the basis of a model for the electronic band structure, which consists of a large conduction band overlapping a narrow band that results from the strong hybridization between the p orbitals of the metalloid and some of the d orbitals of the manganese. The crystal field at the sites of manganese is assumed to be strong. The structure of the narrow band is calculated in the tight-binding model. The Fermi energy lies very close to a sharp singularity in the density of states. In the cubic and Pauli paramagnetic phase, such a singularity has a sixfold degeneracy. The magnetic and structural instabilities, which appear when the temperature is decreased, are explained by the removal of that degeneracy by a shear strain and the formation of small magnetic moments. The phase transitions can be studied in detail by expanding the variation of the free energy with respect to the shear strains and the magnetic moments. The coefficients of the expansion are calculated as functions of the temperature. The variation of the volume is explained by the existence of coupling terms to the shear strain and to the magnetic moments.     

Phase equilibria and phase structure in the ternary systems sodium or potassium octanoate-octanoic acid-water

Two isotropic solution regions and several liquid crystalline regions occur in the ternary system sodium octanoate-octanoic acid-water at 20°C The solution regions are an aqueous solution and a solution of sodium octanoate and water in liquid octanoic acid. A region displaying one-dimensional lamellar structure is located in the center of the phase diagram. A region along the soap-water axis has a two-dimensional normal hexagonal structure. Another region at high octanoic acid content has a reversed hexagonal structure. Along the soap-fatty acid axis the acid-soap 2NaC₈:1HC₈ in crystalline state is found.X-ray and density findings for the various phases are presented, and structural parameters for the different liquid crystalline phases are estimated.The phase behavior of the potassium soap system is similar to that of the sodium system.The isothermal ternary phase diagram of a soap, the corresponding fatty acid and water provides information about the ionization state of the system, from the unionized fatty acid to the fully ionized soap.     

Crystal structure,phase transitions,and IR spectroscopic investigations of a new sulfate templated by diethylene triammonium

A novel organic sulfate (C₄H₁₆N₃)SO₄?HSO₄ has been prepared and characterized by single-crystal X-ray diffraction, infrared spectroscopy, thermogravimetric analysis and differential scanning calorimetry (DSC). The structure consists of linked HSO₄^? and SO₄^2? anions assembled into clusters. Organic triple-protonated diethylene triammonium cations are interconnected to these clusters via N–H?O hydrogen bonds to create the three dimensional arrangement. The Hirshfeld surface and associated fingerprint plots of the compound were presented to explore the nature of intermolecular interactions and their relative contributions to building the solid-state architecture. TG-DTA and DSC studies showed the presence of two phase transitions. Infrared spectrum is reported and discussed on the basis of group theoretical analysis and on Density Functional Theory calculations.     

Crystal structure of sodium doped lanthanum orthomanganite

Crystals of sodium doped lanthanum orthomanganites were grown for the first time in sodium chloride flux at 900°C. 0.5 to 0.1mm size crystals were isolated. Composition of the oxide, La_0.82Na_0.13MnO_2.93 was arrived at by chemical analysis of the elements present. Single crystal X-ray structural study showed that the lanthanum as well as oxygen site deficient orthomanganites crystallize in R3¯ space group unlike many divalent cation doped lanthanum orthomanganites which crystallize in R3¯.     

The crystal structure and phase transitions of LiBaPO4

Polymorphism in lithium barium phosphate (LiBaPO₄) was investigated by synchrotron X-ray diffraction (sXRD) and high temperature X-ray diffraction (HT-XRD). Two modifications were isolated using different cooling rates from the synthesis temperature to room temperature. The slowly-cooled sample exhibited a monoclinic structure with a Cc space group (denoted as M-LiBaPO₄) while the quenched sample belonged to a trigonal system with a P31c space group (denoted as T-LiBaPO₄). In both structures, LiO₄ and PO₄ tetrahedra are linked alternatively by sharing each corner in the lattice, forming tridymite-type six-membered rings. The voids in the anionic framework are filled by Ba atoms. The monoclinic distortion in M-LiBaPO₄ can be attributed to “polyhedral tilting” which shifts the axial bridging oxygens from the centroid of the PO₄ tetrahedron, breaking the three-fold symmetry in the trigonal structure. The HT-XRD data upon heating from 298 to 1373 K indicate successive structural transitions as follows; M-LiBaPO₄ (Cc) → T-LiBaPO₄ (P31c) → H-LiBaPO₄ (P6₃) → O-LiBaPO₄ (Pmcn). H-LiBaPO₄ and O-LiBaPO₄ denote the phases exhibiting the hexagonal and orthorhombic systems, respectively. The thermal evolution of the crystal structure of LiBaPO₄ is quite similar to that of LiKSO₄. The sequences of space group change in both compounds are nearly identical and only the transition temperatures differ.     

The effect of refluxing on the alkoxide-based sodium potassium niobate sol-gel system: Thermal and spectroscopic studies

A study on the effects of prolonged heating under reflux conditions of up to 70 h on alkoxides of sodium, potassium and niobium dissolved in 2-methoxyethanol for the synthesis of sols of composition Na_0.5K_0.5NbO₃ (NKN) has been carried out using combined thermogravimetric-Fourier transform infrared spectroscopic analyses. Extended refluxing increases the homogeneity of the Na_0.5K_0.5NbO₃ (NKN) system. Spectroscopic analyses on the non-refluxed and 70 h refluxed NKN gels reveal the existence of inorganic hydrated carbonates and bicarbonates, which we propose arise from the hydration and carbonation of the samples on standing in air. The X-ray diffraction patterns of these two types of gels show orthorhombic NKN phase evolutions at higher temperatures.     

Determination of elements in lithium potassium niobate and lithium niobate containing vanadium by ICP-AES

The elements in lithium potassium niobate and lithium niobate containing vanadium (such as aluminum, lithium, niobium, potassium and vanadium) were determined by inductively coupled plasma atomic emission spectrometry after fusion of the sample with ammonium hydrogensulfate. The samples were completely decomposed and high precision results were obtained for the metallic elements. Oxygen has been determined by a helium carrier fusion thermal conductivity method using both a nickel capsule and tin as a melting flux in a graphite crucible.