Crystal Growth in Magnetic Fields (I) Crystallization of Me (NH4)2(SO4)2 · 6 H2O (Me = Zn,Cu, Ni,Fe) from Aqueous Solutions in Moderate Magnetic Fields

There are several reports in the literature concerning the study of the influence of magnetic fields on nucleation and crystal growth. Because of the partially opposite findings, the question whether the application of magnetic fields especially for mass crystallization processes may be of advantage has not been cleared yet. Therefore, the displacement rates of [110] and [001] single crystal faces of four Tutton's salts Me(NH₄)₂(SO₄)₂ · 6 H₂O (Me = Zn, Cu, Ni, Fe) from supersaturated aqueous solutions in a transversal direct magnetic field (B ≦ 1.4 T) has been measured and compared. Within the detection limit no reproducible effect was observed with Me = Zn, Cu, and Fe. A small growth acceleration occured in case of Ni(NH₄)₂(SO₄)₂ · 6 H₂O. The results which will be completed by measurements at higher magnetic flux densities, support the viewpoint that the application of magnetic fields on crystallization of diamagnetic substances from solutions offers no real advantage.     

Superconducting Transition of (TMTSF)2CLO4 in Magnetic Fields

Abstract

Superconducting transition of (TMTSF)₂ClO₄ was studied by conductivity measurements along the most conductive a-axis in magnetic fields applied along three different principal crystallographic axes. The GL coherence lengths at OK along the a-, b-, and c*-axes, ?_a(0)≈ 600A, ?(0) ≈ 540A, and ?_c*(0) ≈ 60A were obtained from the measurements of the temperature dependence of the upper critical field H_C2 near the transition temperature. The anisotropy is discussed in terms of the dimensionality arising from the crystal structure and of the conductivity anisotropy in the normal state.     

Preparation and studies of new crystals in the K3H(SO4)2-(NH4)3H(SO4)2-H2O system

To elucidate the effect of isomorphic substitution on the kinetics of phase transitions, single crystals of (K _x (NH₄)_1?x ) _m H _n (SO₄)_{(m + n)/2} · yH₂O solid solutions are grown from the K₃H(SO₄)₂-(NH₄)₃H(SO₄)₂-H₂O system, whose end members are known to undergo superprotonic phase transitions of fundamentally different kinetics. The chemical composition of the single crystals grown is determined by energy dispersive X-ray microanalysis. The thermal and optical behavior of (K,NH₄)₉H₇(SO₄)₈ · H₂O single crystals is studied in the temperature range 295–420 K and the crystal structure at 295 K is determined. A comparison of the results of the studies with data for crystal K₉H₇(SO₄)₈ · H₂O published earlier shows that the substitution of ammonium for potassium atoms lowers the temperature of the structural phase transition by 8 K.     

Ultrafast Optical Response in Quasi-One Dimensional Halogen-Bridged Mixed-Valence Complexes [Pt(en)2][PtBr2(en)2](ClO4)4 and [Pd(en)2][PdCl2(en)2](ClO4)4

Abstract

Ultrafast optical response in two quasi-one-dimensional halogen-bridged mixed-valence complexs [Pt(en)₂][PtBr₂(en)₂](ClO₄)₄ (en=ethylenediamine) (abbreviated as PtBr) and [Pt(en)₂][PtBr₂(en)₂](ClO₄)₄ (as PdCl) has been investigated by femtosecond absorption spectroscopy at room temperature by pump-probe spectroscopy. The photo-induced absorption around 1.3 eV and the bleaching from 1.5 eV to 2.7 eV were observed in PtBr. Both consist of a fast-decay component due to STEs and a slow-decay component due probably to polaron pairs. The former decays exponentially with the time constant of 1.4 ± 0.2 ps. The latter decays as erf(t ^β) with β = -0.22 ± 0.02, indicating the geminate recombination of an electron polaron and a hole polaron after moving freely along the chain. The deviation of β from the ideal random-walk model (β = -0.5) is explained by introducing the effect of potential barrier between the polarons hindering the recombination. A pump-probe absorption spectrum of PdCl is obtained from the reflection spectrum by the Kramers-Kronig relations. The time dependence of the transient photoinduced absorption around 1.7 eV and the bleaching from 1.9 eV to 2.5 eV were calculated to be described with three components. They correspond to free excitons with lifetime of about 800 fs, self-trapped excitons with lifetime of about 3 ps, and polaron pairs which hardly relax within 100 ps.     

Phase transitions in Cs5(HSO4)2(H2PO4)3 crystal

The symmetry (sp. gr.I $\bar 4$ 3d) and lattice parameters have been determined for the first time for Cs₅(H₂SO₄)₂(H₂PO₄)₃ crystals in the temperature range from 172 to 390 K. The thermal and optical properties of crystals, as well as their conductivity, have been investigated at elevated temperatures. It is shown that a crystal heated to T = 365 K undergoes a phase transition with symmetry lowering to the tetragonal phase (with the parameters a = 4.965(1) Å and c = 5.016(1) Å), while at T ≈ 390 K a phase transition to the cubic phase is presumably observed. With a decrease in temperature, a phase transition without a change in symmetry occurs at T = 240 K.     

X-Ray Radiography Investigations of Solid Solutions with Limited Miscibility in K2SO4-(NH4)2SO4-H2O System

The measurements of crystal lattice parameters (a, b, c) were conducted and the volumes (v) and densities (∂) of solid solutions of (K, NH₄)₂SO₄ were calculated. It was established that in the system K₂SO₄-(NH₄)₂SO₄-H₂O at 25 °C mixed crystals with limited miscibility were formed. The gap in the continuity of solid solutions occurred in a very narrow range of concentrations of (NH₄)₂SO₄ in mixed crystals, namely from 69.5 to 72.5%. In order to get the confirmation of the above, the detailed equilibrium investigations nearby the peritonic point (the concentration of K₂SO₄ −3.32%, the concentration of (NH₄)₂SO₄ −38.66%) were carried out.     

Electron irradiation‐induced effects on optical spectra of (NH4)2ZnCl4: x Sr2+ single crystals

The effect of electron‐beam irradiation with different doses on optical constants of (NH₄)₂ZnCl₄: x Sr²⁺ crystals with x=0.000, 0.020, 0.039, 0.087 or 0.144 wt% has been studied. The optical transmission in the energy range 3.4‐6.4 eV was measured hence the absorption coefficient was computed as a frequency function. The absorption coefficient was also calculated as a function of electron‐beam dose. Irradiation with e‐beam did not affect the allowed indirect type of transition responsible for interband transitions of (NH₄)₂ZnCl₄: x Sr²⁺ crystals. Values of the optical energy gap E_g and optical moment E_p for electronic interband transition of unexposed and (NH₄)₂ZnCl₄: x Sr²⁺ crystals after e‐beam exposure were deduced. The area under the absorption band at 5.30 eV was used to evaluate the effect of e‐irradiation on optical parameters of samples with x=0.00, 0.020 or 0.039. A shift in the position and a nonmonotonic change in the intensity of this band with increasing e‐beam dose was observed. Changes in the E_g value were used to evaluate the effect of e‐beam exposure dose on (NH₄)₂ZnCl₄: x Sr²⁺ samples with x=0.087 or 0.144. The obtained results were compared with those obtained for the same crystals after irradiation with different γ‐doses.     

Magnetic interactions in frustrated Mn3Fe4(VO4)6

Magnetic interactions in a site-disordered multicomponent vanadate Mn₃Fe₄(VO₄)₆ are studied using DC magnetization and multifrequency Electron Paramagnetic Resonance (EPR). The static magnetic susceptibility χ shows antiferromagnetic interactions between Fe³⁺ and Mn²⁺ spins with a Curie–Weiss temperature Θ = ?165(5) K. EPR measurements indicate a strong dependence of χ on the frequency and temperature. The EPR spectra due to iron and manganese ions are observed in the X-band. It is mostly manganese ions that are observed at 80 GHz while two kinds of magnetic centers are identified at frequencies above 160 GHz. The observed shifts of the resonance lines for Fe³⁺ ions at low frequencies differ from those at high frequencies. The observed features may be due to different magnetic sublattices which modify the magnetic ground state, while competing magnetic interactions may lead to magnetic frustration. It appears that the very high magnetic fields employed in our high-frequency EPR measurements may affect the spin-flop transitions anticipated below Neel temperature T_N.     

Preparation of (NH4)2SO4 monocrystals

The preparation of (NH₄)₂SO₄ crystals has been studied, and the difficulties, especially the formation of numerous parasitical crystals and the irregular growth are discussed from a theoretical point of view and with regard to the performed investigations. Further on the habit of crystals and an acceptable procedure for the growing of homogeneous regular crystals are given. Likewise the effect of temperature, of P(H) of the crystallizing solution, the choice of seed crystals and the admixture of glycerine are reported.     

Alkali-metal hydrogen selenate-phosphates M 2 H 3(SeO4)(PO4) (M = Rb or K) and M 4H5(SeO4)3(PO4)(M = K or Na)

Crystalline hydrogen selenate-phosphates M ₂H₃(SeO₄)(PO₄) [M = Rb (I) or K (II)] and M ₄H₅(SeO₄)₃(PO₄) [M = K (III) or Na (IV)] were obtained by reactions of Rb, K, and Na carbonates with mixtures of selenic and phosphoric acid solutions. The X-ray structure study of single crystals revealed that I and II are isostructural (sp. gr. Pn). In these structures, SeO₄ and H₃PO₄ tetrahedra are linked by hydrogen bonds to form corrugated layers. Structures III and IV (sp. gr. $P\bar 1$ ) have similar arrangements of non-hydrogen atoms but different hydrogen-bond systems. In III = K₄(HSeO₄)₂{H[H(Se,P)O₄]₂}, the HSeO₄ groups branch from the infinite anionic {H[H(Se,P)O₄]₂} chains. In IV = Na₄[H(SeO₄)₂]{H[H_1.5(Se, P)O₄]₂}, the anionic {H[H_1.5(Se,P)O₄]₂} chains are crosslinked by hydrogen bonds formed by the [H(SeO₄)₂] dimers.     

Spectroscopic and Transport Properties of (NH4)2CuCl4 · 2 H2O Single Crystal

Electron paramagnetic resonance (EPR), Optical absorption and Transport properties of (NH₄)₂CuCl₄ 2 H₂O single crystals have been studied. An anisotropic g tensor was observed with gl = 2.241 and g∥ = 2.081 by EPR method. The spin orbit coupling constant is found to be 540 cm⁻¹. The optical absorption in UV region are characterized by charge transfer band, in the visible and near infrared region at 13,333, 4,480, 4,336, and 3,998 cm⁻¹ attributed to the transitions between the (d-d) stark energy levels of the copper (II) ion in an extended octahedral crystal field. Dc electrical conductivity measurements with temperature reveal an anisotropic characteristic of a two-dimensional layered structure, and exhibits two first order structural phase transitions at about 383 K and 413 K. These transitions are attributed to loss of the two water inolecules of hydration and free rotation of NH⁺₄ ion from a state of torsional oscillation.     

X-Ray Diffraction Studies of 4-n-Decyl(oxy)phenyl-4'-n'-butyl(oxy)benzoate and 4-n-Decyl(oxy)phenyl-4'-n'-hexyl(oxy)benzoate

Liquid crystal aromatic esters of compositions C₁₀H₂₁OC₆H₄C(O)OC₆H₄OC₄H₉ (1) and C₁₀H₂₁OC₆H₄C(O)OC₆H₄OC₆H₁₃ (2) have been investigated by X-ray structural analysis. On temperature rise, these compounds undergo phase transitions crystal-smectic (S_A)-nematic-isotropic (1) and crystal-smectic (S_C)-smectic(S_A)-nematic-isotropic (2) at temperatures 64.5, 80.7, 89.1°C and 62.0, 77.0, 82.5, 88.5°C, respectively. Crystal packing of the compounds consists of alternating loosely packed aliphatic and closely packed aromatic regions. The number of types of weak directional interactions in aromatic regions in both crystal packings was found to be equal to a number of phase transitions in crystal-mesophase-isotropic systems.     

Synthesis and crystal structures of nitratocobaltates Na2[Co(NO3)4], K2[Co(NO3)4], and Ag[Co(NO3)3] and potassium nitratonickelate K2[Ni(NO3)4]

The cobalt(II) and nickel(II) nitrate complexes with an island structure (Na₂[Co(NO₃)₄] (I) and K₂[Co(NO₃)₄] (II)] and a chain structure [Ag[Co(NO₃)₃] (III) and K₂[Ni(NO₃)₄] (IV)] are synthesized and investigated using X-ray diffraction. In the anionic complex [Co(NO₃)₄]^2? of the crystal structure of compound I, the Co coordination polyhedron is a twisted tetragonal prism formed by the O atoms of four asymmetric bidentate nitrate groups. In the anion [Co(NO₃)₄]^2? of the crystal structure of compound II, one of the four NO₃ groups is monodentate and the other NO₃ groups are bidentate (the coordination number of the cobalt atom is equal to seven, and the cobalt coordination polyhedron is a monocapped trigonal prism). The crystal structures of compounds III and IV contain infinite chains of the compositions [Co(NO₃)₂(NO₃)_2/2]^? and [Ni(NO₃)₃(NO₃)_2/2]^2?, respectively. In the crystal structure of compound III, seven oxygen atoms of one monodentate and three bidentate nitrate groups form a dodecahedron with an unoccupied vertex of the A type around the Co atom. In the crystal structure of compound IV, the octahedral polyhedron of the Ni atom is formed by five nitrate groups, one of which is terminal bidentate. The data on the structure of Co(II) coordination polyhedra in the known nitratocobaltates are generalized.     

Bimetallic hydrogen sulfates K4 M II[H(SO4)2]2(H2O)2 (M II = Mn or Zn)

Hydrogen sulfate hydrates K₄{M ^II[H(SO₄)₂]₂(H₂O)₂}, where M ^II = Mn or Zn, are synthesized, and their single-crystal structures are determined by X-ray diffraction. The structural units of the orthorhombic crystals (space group Pccn) are potassium and M ^II cations, SO ₄ ^2? and HSO ₄ ^? anions, and water molecules. Strong (2.52 Å) and moderate-in-strength (2.71–2.75 Å) hydrogen bonds link the anions and water molecules into hexamers. The M ^II cations, which have the octahedral environment (Mn-O, 2.14–2.19 Å and Zn-O, 2.07–2.11 Å), link the hexamers into flat layers. The structures of bimetallic hydrogen chalcogenate hydrates with different compositions are compared.     

Theoretical phase diagrams for [N(CD3)4]2ZnCl4 crystal

A theoretical phase diagram of [N(CD₃)₄]₂ZnCl₄ crystal is constructed in the plane of two thermodynamic-potential coefficients. The orientation of the temperature (T) and pressure (P) axes is chosen in the diagram. The first diagram is used as a basis to construct the second theoretical T-P phase diagram, expanded to the region of weak negative pressures for nondeuterated [N(CH₃)₄]₂ZnCl₄ crystal. It is suggested that this region can be observed for deuterated [N(CD₃)₄]₂ZnCl₄ crystal.     

New (TMTSF)2X Derivatives: A Change in the Selenium Network Dimensionality Derived From the Molecular and Crystal Structures of (TMTSF)2(Fso3) [T=298K, 123K] and (TMTSF)2(Bro4) T=298K

Abstract

We report the first crystallographic analysis, as a function of temperature, of a TMTSF derivative. Both (TMTSF)₂(FSO₃) and (TMTSF)₂(BrO₄) are isostructural (triclinic, with space group PI) with superconducting (TMTSF)₂(ClO₄). (TMTSF)₂(FSO₃) undergoes a metal-to-insulator transition at 86-90K as observed by microwave conductivity, D.C. conductivity, and magnetic susceptibility. The crystal structure contains 2-dimensional sheets of short Se-Se contacts in the molecular stacking direction and perpendicular to the stacking direction. The temperature dependent variations in these contact distances appear to be of special importance in determining the conduction properties of these materials, and are observed to change in a surprising manner when (TMTSF)₂(FSO₃) is cooled (298 → 123K). The homoatomic Se separations within each TMTSF molecule appear to increase slightly, but not significantly. At the same time the entire 2-dimensional sheet of intermolecular (intra- and interstack) Se-Se contacts between TMTSF molecules contract quite anisotropically, which results in an increase in “dimensionality” of the Se-Se network. Hence, an increase in electrical conduction, in the absence of insulating phenomena, over the temperature range 298 → 123K is not surprising. The intermolecular Se-Se contact distances in (TMTSF)₂(BrO₄) are significantly longer than in (TMTSF)₂(FSO₃) which suggests that the room temperature electrical conductivity of the (BrO₄)^? salt may be diminished compared to the (FSO₃)^? analogue.     

The Characteristics of the Mixed Crystals of the K2SO4-(NH4)2SO4-H2O System at 298 K

Equilibrium studies of the K₂SO₄-(NH₄)₂SO₄-H₂O system at 298 K have been conducted. The isotherm of solubility and the curve of distribution have been mathematically described. The parameters of the crystal lattice and the enthalpy of crystallization of the [K_x(NH₄)_1-x]₂SO₄ solid solutions as a function of their composition have been presented.     

Crystalloluminescence and Mechanoluminescence Spectra of Ba(ClO3)2 · H2O and 2 K2SO4 · Na2SO4 Crystals

Crystalloluminescence and mechanoluminescence of Ba(ClO₃)₂ · H₂O and 2 K₂SO₄ · Na₂SO₄ crystals are investigated. The crystalloluminescence spectra are almost similar to the photoluminescence spectra; however, they differ completely from the mechanoluminescence spectra. The crystalloluminescence excitations may take place due to the various processes: (i) the recombination of ions, (ii) from amorphous to crystalline transition, (iii) from the phase change during the crystallization, and (iv) from the dielectric breakdown by the electric field produced due to the creation of microfracture during the crystal growth. It is concluded that the crystalloluminescence excitation in Ba(ClO₃)₂ · H₂O and 2K₂SO₄ · Na₂SO₄ crystals may be chemical in origin.     

Size effects in [N(CH3)4]2Zn0.58Cu0.42Cl4 crystals

The temperature dependences of birefringence in thin [N(CH₃)₄]₂Zn_0.58Cu_0.42Cl₄ crystals in the thickness range 20 × 10^?6 ≤ d ≤ 500 ? 10^?6 m have been investigated. An increase in the temperatures of the parent phase-incommensurate phase and incommensurate phase-ferroelectric phase transitions has been found. The reasons for the shift in the phase transition temperatures with a decrease in the thickness of [N(CH₃)₄]₂Zn_0.58Cu_0.42Cl₄ crystal and the size effect in crystals with an incommensurate superstructure are discussed.     

Structural and spectroscopic study of Mg13.4(OH)6(HVO4)2(H0.2VO4)6

Single‐crystals of the polar compound magnesium hydrogen vanadate(V), Mg_13.4(OH)₆(HVO₄)₂(H_0.2VO₄)₆, were synthesized hydrothermally. It represents the first hydrogen vanadate(V) among inorganic compounds. Its structure was determined by single‐crystal X‐ray diffraction [space group P 6₃mc, a = 12.9096(2), c = 5.0755(1) Å, V = 732.55(2) ų, Z = 1]. The crystal structure of Mg_13.4(OH)₆(HVO₄)₂(H_0.2VO₄)₆ consists of well separated, vacancy‐interrupted chains of face sharing Mg2O₆ octahedra, with short Mg2—Mg2 distances of 2.537(1) Å, embedded in a porous magnesium vanadate 3D framework having the topology of the zeolite cancrinite. All three hydrogen positions in the structure were confirmed by FTIR spectroscopy. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)