Phase transition and thermal decomposition of silver isocyanate (AgNCO)

The thermal behavior of AgNCO (silver isocyanate) has been studied via thermal analysis, optical spectroscopy, X-ray powder diffraction and transmission electron microscopy. Upon quenching the high temperature polymorph (HT-AgNCO) to room temperature, a new modification has been obtained (q-AgNCO). Its crystal structure was solved from X-ray powder diffraction data and refined by the Rietveld method (Pmmn (no. 59), a = 3.579(3) Å, b = 5.777(4) Å, c = 5.807(2) Å, V = 120.08(3) Å³, Z = 2, T = 295 K). The structure consists of chains of Ag⁺ ions bridged by isocyanate units. HT-AgNCO exists between T = 135 °C and the melting/decomposition point and exhibits virtually free rotation of the complex anions. According to preliminary single-crystal studies, HT-AgNCO (C2/m, a = 5.87 Å, b = 3.51 Å, c = 5.81 Å, ß = 105.953°, Z = 2, T = 373 K) is structurally related to α-NaN₃. The crystal structures of both, HT-AgNCO and q-AgNCO have been compared with that of the room temperature modification (RT-AgNCO). The thermal behavior and the ionic conductivity of AgNCO are discussed with respect to the related compounds AgN₃ and KSCN. Decomposition of AgNCO proceeds in distinct steps, as seen from TGA, and results in the formation of nanoparticles of elemental silver and an amorphous polymer consisting of C, N and O, only.     

Real structure of SrSi2O2N2

SrSi₂O₂N₂ is an important host lattice for Eu²⁺ doped phosphors. Its crystal structure (space group P1, a = 7.0802(2) Å, b = 7.2306(2) Å, c = 7.2554(2) Å, α = 88.767(3)°, β = 84.733(2)°, γ = 75.905(2)° and V = 358.73(2) Å³, Z = 4) is isotypic with EuSi₂O₂N₂: highly condensed silicate layers are separated by Sr²⁺. The samples are characterized by pronounced real structure effects owing to pseudosymmetry of partial structures. Polysynthetic twinning with domains of various sizes is ubiquitous and oriented intergrowth of domains with different orientations has also been observed and analysed in detail by means of electron diffraction and high-resolution electron microscopy. These effects also affect the X-ray powder pattern and were taken into account in a Rietveld refinement.     

A new intersecting tunnel structure in the AIMIII[PO3(OH)]2 series for AI = Ag,MIII = In: Analysis of structural relationships

A new indium hydroxyphosphate containing silver, AgIn[PO₃(OH)]₂, has been synthesized using hydrothermal method. It crystallizes in the P2₁/c space group with the cell parameters a = 6.6400(2) Å, b = 14.6269(6) Å, c = 6.6616(4) Å, β = 95.681(5)°, V = 643.82(6) Å³, Z = 4. Its three-dimensional framework, built up of corner-sharing PO₃(OH) tetrahedra and InO₆ octahedra, presents intersecting tunnels running along <111> and [100] directions, in which the Ag⁺ cations are located. The presence of hydroxyl groups has been confirmed from IR spectroscopy studies and hydrogen atoms were located from the single crystal X-ray diffraction study. The structural relationships with the other compounds of general formula A^IM^III[PO₃(OH)]₂ are analyzed.     

The crystal structure of δ-VOPO4 and its relationship to ω-VOPO4

The crystal structure of δ-VOPO₄ was determined from powder X-ray diffraction data in the tetragonal space group P4₂/mbc (No. 135) with a = 9.0547(7) Å and c = 8.6080(8) Å. The structure is found to be closely related to that of ω-VOPO₄, thus disproving two traditional structure hypotheses commonly found in the literature. The structural relationship between the two phases is discussed in the light of a recently observed fast phase transition from ω- to δ-VOPO₄.     

A new potassium rare earth oxyborate K2La2(BO3)2O

A series of potassium rare earth oxyborates, K₂RE₂(BO₃)₂O (RE = La, Nd, Sm and Eu), have been synthesized. Single crystal of the first member of the series, K₂La₂(BO₃)₂O, has been grown by the flux method. Its structure, determined by single crystal X-ray diffraction, shows that it belongs to the monoclinic system, space group P2₁/c with unit cell parameters of a = 11.422(2) Å, b = 6.6803(13) Å, c = 10.813(2) Å, β = 17.23(3)° and Z = 4. Optical transmission spectrum shows that the K₂La₂(BO₃)₂O crystal is highly transparent from 215 nm to 2750 nm.     

Synthesis,growth, and characterization of a new NLO material 3-(2,3-dimethoxyphenyl)-1-(pyridin-2-yl)prop-2-en-1-one

3-(2,3-Dimethoxyphenyl)-1-(pyridin-2-yl)prop-2-en-1-one (DMPP) a potential second harmonic generating (SHG) has been synthesized and grown as a single crystal by the slow evaporation technique at ambient temperature. The structure determination of the grown crystal was done by single crystal X-ray diffraction study. DMPP crystallizes with orthorhombic system with cell parameters a = 20.3106(8) Å, b = 4.9574(2) Å, c = 13.4863(5) Å, α = 90°, β = 90°, γ = 90° and space group Pca2₁. The crystals were characterized by FT-IR, thermal analysis, UV–vis–NIR spectroscopy and SHG measurements. Various functional groups present in DMPP were ascertained by FTIR analysis. DMPP is thermally stable up to 80 °C and optically transparent in the visible region. The crystal exhibits SHG efficiency comparable to that of KDP.     

Crystal structure and ionic conductivity of AgCr2(PO4)(P2O7)

Single crystals of a new phosphate AgCr₂(PO₄)(P₂O₇) have been prepared by the flux method and its structural and the infrared spectrum have been investigated. This compound crystallizes in the monoclinic system with the space group C2/c and the parameters are, a = 11.493 (3) Å, b = 8.486 (3) Å, c = 8.791 (2) Å, β = 114.56 (2)°, V = 779.8 (3) ųand Z = 4. Its structure consists of CrO₆ octahedra sharing corners with P₂O₇ units to form undulating chains extending infinitely along the [110] direction. These chains are connected by the phosphate tetrahedra giving rise to a 3D framework with six-sided tunnels parallel to the [101] direction, where the Ag⁺ ions are located. The infrared spectrum of this compound was interpreted on the basis of P₂O₇^4? and PO₄^3? vibrations. The appearance of ν_sP–O–P in the spectrum suggests a bent P–O–P bridge for the P₂O₇^4? ions in the compound, which is in agreement with the X-ray data. The electrical measurements allow us to obtain the activation energy of (1.36 eV) and the conductivity measurements suggest that the charge carriers through the structure are the silver captions.     

Li6B18(Li2O)x – A boride with a porous framework of B6 octahedra

The synthesis and crystal structure of the red transparent lithium boride Li₆B₁₈(Li₂O)_x (0 < x ≤ 1) is reported. The lattice constants are a = 8.21708(17) Å and c = 4.15893(16) Å for x = 0.26 (powder data), a = 8.223(4) Å and c = 4.160(2) Å for x = 0.7 (single crystal data), a = 8.21179(16) Å and c = 4.14485(13) Å for x = 0.9 (powder data). The compound crystallizes in the space group P6/mmm (no. 191). The crystal structure consists of B₆ octahedra forming a 3-dimensional network with large open channels. This compound has remarkable topological similarities with hexagonal tungsten bronzes and zeolites and is only formed, when a template is present during the synthesis.     

Structure elucidation of BaSi2O2N2 – A host lattice for rare-earth doped luminescent materials in phosphor-converted (pc)-LEDs

BaSi₂O₂N₂ is a promising host lattice for rare-earth doped luminescent materials in phosphor-converted (pc)-LEDs. Applying a combined approach, its orthorhombic average structure (space group Cmcm (no. 63), a = 14.3902(3) Å, b = 5.3433(1) Å, c = 4.83256(7) Å and V = 371.58(2) Å³, Z = 4) has been elucidated by electron diffraction and structure solution from X-ray and neutron powder diffraction data with subsequent Rietveld refinement (wRp = 0.0491 for X-ray data). The structure contains layers of highly condensed SiON₃ tetrahedra with O terminally bound to Si. The Ba²⁺ ions are situated between the layers and are surrounded by a cuboid of O atoms capped by two N atoms. In the structure, there is only one Ba site and one Si site, respectively, which is in accordance with a single sharp ²⁹Si NMR signal observed at ?52.8 ppm typical for SiON₃ tetrahedra in MSi₂O₂N₂ type oxonitridosilicates. Lattice energy calculations support the results of the structure determination.     

Ce(H2O)(PO4)3/2(H3O)1/2(H2O)1/2, a second entry in the structural chemistry of cerium(IV) phosphates

A cerium(IV) phosphate has been prepared using precipitation methods and its structure has been solved by single crystal X-ray diffraction (R₁ = 0.0292 for 3092 reflections with I>2σ(I) and wR₂ = 0.0540). Ce(H₂O)(PO₄)_3/2(H₃O)_1/2(H₂O)_1/2 crystallises in the monoclinic space group C2/c (a = 15.7058(17) Å, b = 9.6261(9) Å, c = 10.1632(4) Å, ß = 121.623(7)°, and V = 1308.4 (2) Å³). Its structure is based on a negatively charged 3D framework, made of cerium atoms connected by PO₄ tetrahedra. There are two types of PO₄ units; one shares only corners with the cerium coordination polyhedra while the other one shares edges and corners. This structure also includes hydronium cations, to balance the framework charge, and water molecules. One special feature of this 3D framework is the formation of interconnected tunnels which extend along the c axis and contain the hydronium cations and the water molecules. This open framework and the presence of cationic species in the tunnels are in perfect agreement with the previously reported ion exchange properties.     

Crystal structure of single crystals of nonlinear optical l-histidinium trichloroacetate

Single crystals of a new histidinium salt: l-histidinium trichloroacetate {abbreviated as LHTCA; [(C₃N₂H₄) CH₂CH (NH₃) (CO₂)]⁺ CCl₃COO⁻} were grown by slow evaporation of an aqueous solution at room temperature. The compound crystallizes in a non-centrosymmetric space group P2₁ of monoclinic system with cell parameters a = 5.4505(18) Å, b = 25.769(8) Å, c = 9.210(2) Å and β = 99.98(2)° with Z = 4. The structure has been refined to an R-value of 0.05 for 2539 observed reflections using three-dimensional X-ray diffraction data. The vibrational structure of the compound confirms the presence of various functional groups in the molecule. The UV–Vis–NIR spectrum shows a good transparency in the whole of the region from ultraviolet to near IR. The Kurtz–Perry powder SHG measurement confirms the frequency doubling of the crystal and its powder SHG efficiency was measured as d_eff = 0.33 d_eff (KDP).     

The crystal structure of Rb2SeO4 at high temperature

The crystal structure of Rb₂SeO₄ in its high-temperature phase is reported for the first time. Powder diffraction data collected at T = 898 K show that it is hexagonal (a = 6.3428(1) Å, c = 8.5445(1) Å, V = 297.71(1) Å³, space group P6₃/mmc (194), Z = 2) and is isostructural to Tl₂SeO₄, thus belonging to the α-K₂SO₄ structure type. DSC measurements indicate that the phase transition occurs at T = 818 K.     

Quaternary Gd4Ni2Sb1±xSi2±x and Gd4Ni2Bi1±xSi2±x: Crystal structure,homogeneity regions and magnetic behavior

A new quaternary Gd₄Ni₂Sb_1.07(1)Si_1.93(1) phase was synthesized by arc-melting and its structure was determined through single crystal X-ray diffraction techniques. It crystallizes in an orthorhombic unit cell (the Pnma space group) with a = 11.1735(9), b = 4.2054(2) and c = 16.711(1) Å and represents a new structure type. The isostructural Gd₄Ni₂BiSi₂ phase was obtained and characterized using the powder X-ray diffraction techniques: Pnma space group, a = 11.2715(2), b = 4.2046(1) and c = 16.7421(3) Å. By the means of electron microprobe analysis, Sb/Si and Bi/Si solid solutions were proven to exist for corresponding phases, and their general formulas can be given as Gd₄Ni₂Sb_1±xSi_2±x and Gd₄Ni₂Bi_1±xSi_2±x. Gd₄Ni₂Sb_1.07Si_1.93 and Gd₄Ni₂BiSi₂ order ferromagnetically at 93 K and 46 K, respectively.     

The crystal structure of potassium ammonium hexamolybdotellurate with telluric acid K5NH4[TeMo6O24].Te(OH)6.6H2O

An inorganic compound formulated as K₅NH₄[TeMo₆O₂₄].Te(OH)₆.6H₂O (1) has been isolated by conventional solution method and structurally characterized by single-crystal X-ray diffraction methods, scanning electron microscopy (SEM), IR, UV–vis spectra, and cyclic voltammetry measurements. This compound crystallizes in the monoclinic system, space group C2/c with unit a = 18.6841(1) Å, b = 10.0513(1) Å, c = 21.1065(1) Å, β = 116.495(1)°, V = 3547.49(4) ų, Z = 4, R = 0.033 and wR (F²) = 0.087 for 3432 unique observed reflexions [I > 2σ(I)]. The crystal structure of (1) is built up from an Anderson clusters connected through hydrogen-bonding interactions into a three-dimensional supramolecular network.     

Jahn–Teller phase transition in Cu0.50TiO(PO4): Powder structural characterization of the β-variety and thermal study

The thermal study of Cu_0.50TiO(PO₄), by X-ray diffraction and DSC, shows a phase transition α → β with a hysteresis (∼600 °C during heating; ∼300 °C during cooling). Single crystals have been obtained for the α-phase but the β-phase can only be stabilised at room temperature as a powder mixture with α. Structural characterization of the β-variety has been done with diffraction data (X-ray Cu Kα₁ and neutrons) using a powder rich in β-phase (α(20%) + β(80%)). A monoclinic cell (a = 7.1134(7) Å; b = 7.7282(7) Å; c = 7.3028(7) Å; β=119.30(1)°; V = 350.1(1) Å³) has been found for β-phase, space group P2₁/c. An “ab initio” structure determination has been done, and the Rietveld refinement leads to cR_wp = 0.150 and R_B = 0.041. The results from the X-ray data were confirmed by refinements from neutron data.Similarly to the α-phase, the structure of β-Cu_0.50TiO(PO₄) can be described as a TiOPO₄ framework constituted of chains of tilted corner-sharing [TiO₆] octahedra running parallel to the c axis and cross linked by [PO₄] tetrahedra. Ti atoms are displaced from the centres of the octahedral units, leading to long (2.27 Å) and short (1.73 Å) Ti–O(1) bonds. The [CuO₆] octahedra exhibit a typical Jahn–Teller distorted coordination with four short equatorial Cu–O bonds (2 × 1.93 Å and 2 × 2.06 Å), and two longer apical Cu–O bonds (2 × 2.33 Å). The two longer Cu–O bonds are almost parallel to the b axis.The transition from the α to the β-phase is characterized by a “rocking” of the Jahn–Teller elongation from the (a,c) plane to the b direction accompanied by a relatively strong expansion of the cell volume.     

Nonlinear optical crystal BiAlGa2(BO3)4

The new nonlinear optical crystals BiAlGa₂(BO₃)₄ have been grown by spontaneous crystallization with molten flux based on a Bi₂O₃–B₂O₃ solvent. From single crystal X-ray diffraction measurement, BiAlGa₂(BO₃)₄ has been found to crystallize in the trigonal huntite structure type, space group R32, with cell dimensions a = 9.4433(9) and c = 7.4130(10) Å. The diffuse reflectance spectrum on a powder sample indicated that the short-wavelength absorption edge of BiAlGa₂(BO₃)₄ extends to approximately 271 nm. Second-harmonic generation (SHG) on powder samples has been measured using Kurtz and Perry technique, which indicated that BiAlGa₂(BO₃)₄ is a phase-matchable material, and its SHG coefficient is measured to be four times as large as that of KDP.     

Synthesis and characterization of a novel three-dimensional open-framework: Metal diphosphonate,Zn[HO3PCH2(C6H4)CH2PO3H]

A novel zinc diphosphonate, Zn[HO₃PCH₂(C₆H₄)CH₂PO₃H] (1) was synthesized from tetraethyl para-xylylenediphosphonate, Et₂O₃PCH₂C₆H₄CH₂PO₃Et₂, and Zn (AcO)₂·2H₂O under solvothermal conditions. The structure of compound 1 was determined by single-crystal X-ray diffraction, which reveals that the structure crystallizes in the monoclinic space group C2/c (No. 15), with a = 22.4844(19) Å, b = 6.4361(5) Å, c = 8.1194(7) Å, β = 102.595(2)°, V = 1146.70(16) Å³, T = 298(2) K, Z = 8. The novel three-dimensional (3D) construction is simply built up from linear inorganic chains of corner-sharing four-rings of tetrahedral [ZnO₄] and [PO₃C] which connected adjacent chains by the organophosphorus ligand para-xylylenediphosphonate. The framework has 10 Å × 4 Å (containing the van der Waals radii of atoms) channels running along the b-axis.     

Preparation and crystal structure of members of the solid solution phase Ba5Ru2−xAl1+x−yCuyO11 with x=0.378, y=0.085 and x=0.5, y=0

Single crystals of the new phase Ba₅Ru_2−xAl_1+x−yCu_yO₁₁ (x=0.378, y=0.085) have been grown from a powder mixture of BaCO₃, RuO₂ and CuO in an alumina crucible. The new compound crystallizes isostructurally to Ba₅Ir₂AlO₁₁. The crystal structure was determined by X-ray single-crystal diffraction technique and refined to a composition of Ba₅Ru_1.622(8)Al_1.29(1)Cu_0.085(6)O₁₁ (orthorhombic, Pnma (No. 62), a=18.615(4) Å, b=5.771(1) Å, c=11.098(2) Å, Z=4, R₁=0.048, wR₂=0.075). The composition of the new compound obtained from crystal structure refinement is in good agreement with the result of electron probe microanalysis using wavelength-dispersive X-ray spectroscopy. Octahedra [RuO₆] are connected via faces forming pairs. The central positions of the octahedra pairs are statistically occupied by Ru and Al atoms. These octahedra pairs are interconnected to one-dimensional chains extending along [010] via tetrahedra [Al_1−yCu_yO₄]. Isotypic Ba₅Ru_1.5Al_1.5O₁₁ is a further member of the solid solution with the lattice parameters a=18.6654(5) Å, b=5.7736(1) Å, c=11.0693(3) Å according to Rietveld refinement on a microcrystalline sample.     

Synthesis and structure of the novel anion [AsMo6O15(OH)3(SO4)3]6–, first example of a reduced molybdo-sulphate cluster

A novel anion derived from the Anderson structure has been obtained in aqueous acid solution by condensation of three pairs of edge-sharing molybdenum octahedra around a central arsenite. This association forms a hexanuclear ring of Mo(V) with three external sulphate groups bridging the binuclear molybdenum units. This compound crystallizes in the presence of alkaline ions and its orthorhombic structure was solved by single crystal X-ray diffraction [Cmca, a=24.3189(3) Å, b=12.4454(1) Å, c=26.9211(3) Å, V=8 147,9 ų] from 4 277 unique reflections [I≥2 σ(I)], R₁=0.0471 and _wR₂=0.1429. The solid is a three-dimensional network built up from cluster anions strongly connected to alkaline ions via ionic links. This soluble compound, presenting labile peripheral sulphates, is predisposed for exchanges to build new anions. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SASpolyoxometalate / sulphate / reduced cluster / arsenite / molybdenum     

Structural characterization,molecular dynamics,dielectric and spectroscopic properties of tetrakis(pyrazolium) bis(μ2-bromo-tetrabromobismuthate(III)) dihydrate, [C3N2H5]4[Bi2Br10]·2H2O

The structure of [C₃N₂H₅]₄[Bi₂Br₁₀]·2H₂O, (PBB) was determined by single crystal X-ray diffraction at 100 K. It crystallizes in the monoclinic space group C2/m, with a = 12.992(4) Å, b = 16.326(5) Å, c = 8.255(3) Å, β = 108.56°(3), V = 1659.9(9) Å³ and Z = 2. The structure consists of discrete binuclear [Bi₂Br₁₀]⁴⁻ anions, ordered pyrazolium cations and water molecules. The crystal packing is governed by strong N–H⋯O and weak O–H⋯Br hydrogen bonds. A sequence of structural phase transitions in PBB was established on the basis of differential scanning calorimetry and dilatometric studies. Two reversible first-order phase transitions were found: (I → II) at 381/371 K (on heating/cooling) and (II → III) at 348/338 K. Dielectric response near both phase transitions is characteristic of crystals with the “plastic-like” phases. Over the phase III a low frequency dielectric relaxator is disclosed. The possible molecular motions in the PBB compound are characterized by the ¹H NMR studies. The infrared spectra of polycrystalline compound in the temperature range 300–380 K are reported for the region 4000–400 cm⁻¹. The observed spectral changes through the structural phase transition III → II are attributed to an onset of motion both of the pyrazolium cations and water molecules.