Phase transitions in the (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>NbOF<Subscript>5</Subscript> oxyfluoride

The thermal and dielectric properties of the (NH₄)₂NbOF₅ oxyfluoride have been investigated. It has been established that the structural phase transitions Cmc2₁ → C2 → Ia observed at the temperatures T ₁ = 258.0 K and T ₂ = 218.9 K exhibit a nonferroelectric nature. The hydrostatic pressure, which stabilizes the initial phase and destabilizes the low-temperature phase, hardly affects the temperature range of stability of the intermediate phase. The model of sequential ordering of the structural elements due to phase transitions has been analyzed using experimental data on the entropies of the phase transitions.     

Crystal structure of catena-DI(2-thiobarbiturato-O,S)aqualead(II)

The crystal structure of catena-di(μ₂-2-thiobarbiturato-O,S)aqualead(II) C₈H₈N₄O₅S₂Pb (C₄H₄N₂O₂S is 2-thiobarbituric acid, H₂TBA) is determined. Crystallographic data for catena-[Pb(H₂O)(μ₂-HTBA-O,S)₂] are as follows: a = 6.5972(1) Å, b = 9.8917(2) Å, c = 10.0893(2) Å, α = 106.702(1)°, β = 93.395(2)°, γ = 107.48(1)°, V = 593.82(2) ų, space group $P\bar 1$ , Z = 2. The Pb²⁺ ion is linked with six monodentate HTBA^? ligands through two O atoms and four S atoms and also connected with a water molecule. Additionally, there is a shortened Pb-S contact (3.622 Å), given which the complex polyhedron represents a distorted square antiprism. Hydrogen bonds N-H…O and O-H…O form a branched three-dimensional network. The structure is also stabilized by the π-π interaction of heterocyclic HTBA^? ions.     

‘Pseudo-proper’ ferroelectric phase transitions in oxyfluoride K3WO3F3

Based on the structural data on the phases of cryolite (ordered perovskite) K₃WO₃F₃, we develop a statistical model, which allows to describe the sequence of phase transitions observed in this compound using a unified approach. According to the model, the crystal possesses two structural subsystems: the K cations located in the octahedral positions and the WO₃F₃ octahedra in positions alternating with K cations. In the symmetric (cubic) phase, each subsystem can be found in one of the eight states. At decreasing temperature, an orientational phase transition in the subsystem of octahedra occurs first, followed by a phase transition to the low-temperature phase, caused by the loss of stability with respect to the ordering in the K cation subsystem. We find that the electric polarization occurs as pseudoproper and discuss the mechanisms of formation of the phase states.     

Crystal structure and some properties of europium(III) <Emphasis Type="Italic">Catena</Emphasis>-{tris(1,3-diethyl-2-thiobarbiturate)}

The [Eu(HDTBA)₃]_n complex (I), HDTBA is 1,3-diethyl-2-thiobarbituric acid (C₈H₁₂N₂O₂S) is synthesized and its structure is determined by X-ray crystallography. The crystals of I are triclinic: a = 11.0205(2) Å, b = 11.8811(3) Å, c = 12.7312(2) Å, α = 100.933(1)°, β = 109.704(1)°, γ = 101.161(1)°, V = 1479.88(5) ų, space group P-1, Z = 2. Each of three independent DETBA^– ions is a bridging μ2-O,O′-coordinated ligand. The coordination polyhedron of Eu(III) is a distorted octahedron. Bridging DETBA^– organize the octahedra into an infinite two-dimensional layer. The structure contains intramolecular hydrogen bonds but intermolecular hydrogen bonds and the π–π interaction are absent. The results of IR spectroscopy and photoluminescence agree with the single crystal X-ray diffraction data. The main product of the thermal decomposition of I at 900°С is oxysulfate Eu₂O₂SO₄.     

Crystal structure of thallium(I) 2-thiobarbiturate

The crystal and molecular structures of thallium(I) thiobarbiturate C₄H₃N₂O₂STl (C₄H₄N₂O₂S is 2-thiobarbituric acid, Н₂ТВА) have been determined. Crystallographic data for Tl(НТВА) are a = 11.2414(7) Å, b = 3.8444(3) Å, с = 14.8381(9) Å, β = 99.452(2)°, V = 649.00(7) ų, space group P2/с, Z = 4. Each of the two independent thallium ions is bonded to four oxygen and two sulfur atoms to form a distorted tetrahedron. N?H…O and C?H…S hydrogen bonds form a branched three-dimensional network. The structure is also stabilized by π?π interaction between heterocyclic НТВА^- ions. The IR spectra of Tl(НТВА) agree with X-ray powder diffraction data. The compound is also stable below 280°C, and Tl₂SO₄ is one of the thermolysis products in an oxidative medium in the region of 500?650°C.     

Structure of tetrakis((1,3-diethyl-2-thiobarbiturato)(butanol-1))dicobalt(II)

The structure of the dimeric complex [Co₂(BuOH)₄(Detba)₄] (I), where НDetba is 1,3-diethyl-2-thiobarbituric acid, and BuOH is butanol-1, is determined (CIF file CCDC 1475273), and its IR spectrum is studied. The crystals are monoclinic: a = 10.7185(5), b = 21.985(1), c = 12.7235(7) Å, β = 92.196(2)°, V = 2996.1(3) ų, space group P2₁/c, Z = 2. The Co²⁺ ions in compound I are joined in pairs by bridging ligands μ₂-Detba^–?O,O'. In addition, each of them is linked through the O atoms to one terminal Detba–ion and two BuOH molecules to form a trigonal bipyramid СоО₅. Hydrogen bonds O–H···O formed by BuOH and Detba^– join the binuclear complexes into an infinite chain. The data of IR spectroscopy are consistent with the results of X-ray diffraction analysis.     

Specific Heat and Thermal Expansion of Triglycine Sulfate–Porous Glass Nanocomposites

The effect of restricted geometry on specific heat capacity and thermal expansion of the triglycine sulfate (TGS)–borosilicate glass composites have been studied first. A decrease in the entropy and temperature of the P2₁ ? P2₁/m phase transition in the TGS component with decreasing the glass matrix pore diameter at the invariable specific heat and thermal expansion coefficient has been observed. The estimates are indicative of the minor effect of internal pressure on the TGS pressure coefficient dT_C/dp in the composites.     

Two salts and the salt cocrystal of ciprofloxacin with thiobarbituric and barbituric acids: The structure and properties

Ciprofloxacin (CfH, C₁₇H₁₈FN₃O₃) crystallizes with 2‐thiobarbituric (H₂tba) and barbituric acid (H₂ba) in the aqueous solution to yield salt CfH₂(Htba)·3H₂O ( 1 ), salt cocrystal CfH₂(Hba)(H₂ba)·3H₂O ( 2 ), and salt CfH₂(Hba)·H₂O ( 3 ). The compounds are structurally characterized by the X‐ray single‐crystal diffraction. The numerous intermolecular hydrogen bonds N–H?O and O–H?O formed by water molecules, Htba^?/Hba^? and CfH₂⁺ ions, and H₂ba molecules stabilize the crystal structures of 1 to 3 . Hydrogen bonds form a 2D plane network in the salts of 1 and 3 and a 3D network in the salt cocrystal of 2 . There are different π‐π interactions in 1 to 3 . The compounds have been characterized by powder X‐ray diffraction, thermogravimetry/differential scanning calorimetry, and Fourier transform infrared spectroscopy. The compounds dehydration ends at 130°C to 150°C, and their oxidative decomposition is observed in the range of 250°C to 275°C.     

Heat capacity, p-T phase diagram, and structure of Rb2KTiOF5

Elpasolite Rb₂KTiOF₅ (space group, Fm $ \bar 3 Elpasolite Rb₂KTiOF₅ (space group, Fm m, Z = 4) was synthesized using a solid-phase reaction method. The temperature dependences of the heat capacity, the unit cell parameters, the structure, the permittivity, the response to an external pressure, and the Raman spectra were studied. A nonferroelectric phase transition was revealed at T ₀ = 215 K; the transition is accompanied by a tetragonal distortion of the unit cell (space group I4/m, Z = 10) and a change in the entropy (ΔS ₀ = Rln8), which is anomalously large for perovskite-like oxyfluorides with atomic cations. The specific features of the mechanism of structure distortion are discussed in combination with the previous experimental data obtained for cryolite (NH₄)₃TiOF₅ and elpasolite Rb₂KGaF₆. Original Russian Text ? V.D. Fokina, I.N. Flerov, M.S. Molokeev, E.I. Pogorel’tsev, E.V. Bogdanov, A.S. Krylov, A.F. Bovina, V.N. Voronov, N.M. Laptash, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 11, pp. 2084–2092.     

Structural Evolution and Effect of the Neighboring Cation on the Photoluminescence of Sr(LiAl3)1−x(SiMg3)xN4:Eu2+ Phosphors

In this study, a series of Sr(LiAl₃)_1?x(SiMg₃)_xN₄:Eu²⁺ (SLA‐SSM) phosphors were synthesized by a solid‐solution process. The emission peak maxima of SLA‐SSM range from 615 nm to 680 nm, which indicates structural differences in these materials. ⁷Li solid‐state NMR spectroscopy was utilized to distinguish between the Li(1)N₄ and Li(2)N₄ tetrahedra in SLA‐SSM. Differences in the coordination environments of the two Sr sites were found which explain the unexpected luminescent properties. Three discernible morphologies were detected by scanning electron microscopy. Temperature‐dependent photoluminescence and decay times were used to understand the diverse environments of europium ions in the two strontium sites Sr1 and Sr2, which also support the NMR analysis. Moreover, X‐ray absorption near‐edge structure studies reveal that the Eu²⁺ concentration in SLA‐SSM is much higher than that in in SrLiAl₃N₄:Eu²⁺ and SrSiMg₃N₄:Eu²⁺ phosphors. Finally, an overall mechanism was proposed to explain the how the change in photoluminescence is controlled by the size of the coordinated cation.