Low-temperature X-ray Studies Research Articles

Low-temperature X-ray studies of TlInS2, TlGaS2, and TlGaSe2 single crystals

The results of X-ray diffraction studies of the unit-cell parameters and thermal-expansion coefficients of TlInS2, TlGaS2, and TlGaSe2 crystals in the temperature range 100–300 K are described. It is shown that the unit-cell parameters of all the studied crystals gradually increase with increasing temperature. The temperature dependences of these parameters exhibit anomalies in the form of bends and kinks at temperatures corresponding to phase transitions in the crystals. The thermal-expansion coefficients along the [001] crystallographic direction of the crystals under study are determined. It is found that their values slightly change with increasing temperature.

Structural, theoretical and spectroscopic studies of the dichloride hexahydrate cube [Cl2(H2O)6]2−

The structure of the dichloride hexahydrate cube, [Cl(2)(H(2)O)(6)](2-), as a salt with the tris(diisopropylamino)cyclopropenium cation, [C(3)(N(i)Pr(2))(3)](+), has been determined by low-temperature X-ray and neutron-diffraction studies. H atoms not involved in O-H···Cl bonding are disordered over two 0.5 occupancy sites around the O(6) ring. Calculations of the dianionic cube in the gas phase show remarkably good agreement with the solid-state structures with the exception of short O-H bond distances around the O(6) ring that suggests the involvement of a dynamic process. The cluster was also characterised by single-crystal infrared spectroscopy, and vibrational wavenumbers were found to be in good agreement with hydrogen bonding distances. Dibromide and difluoride hexahydrates were also studied theoretically, and O···O distances were found to decrease in the order difluoride > dichloride > dibromide > (H(2)O)(6) and as O···O···O angles increased towards an almost planar ring in (H(2)O)(6). NMR spectra of a chloroform solution of the hydrated salt at -25 °C is consistent with cluster formation.

ChemInform Abstract: Crystal and Electronic Structure of TlxV6S8: Relation Between Thallium Abundance and the Thallium Environment.

Abstract Low-temperature single-crystal X-ray studies of Tl x V 6 S 8 ( x = 0.78 and 0.033) and analysis of the anisotropic displacement parameters U ii reveal that the Tl atoms are statically disordered within the hexagonal channels, even at 100 K. In Tl 0.78 V 6 S 8 the average environment of the Tl atoms may be described by a 3 + 6 coordination whereas in Tl 0.033 V 6 S 8 the Tl atoms are in a trigonal antiprismatic environment. Core level photoelectron spectroscopy was used to study the changes in the electronic structure which arise from variation of the Tl concentration. The photoemission data reveal that the Tl atoms are not ionic in either compound. The Tl atom in the Tl-poor sample exhibits a lower charge density than that in the Tl-rich compound.

Magnetic properties and low temperature X-ray studies of the weak ferromagnetic monoclinic and trigonal chromium tellurides Cr 5Te 8

Monoclinic and trigonal Cr 5Te 8 show a transition into the ferromagnetic state with a Curie temperature T c, which sensitively depends on the actual composition. Monoclinic samples exhibit a lower T c despite their higher Cr content. This observation is explained on the basis of less effective ferromagnetic superexchange in the monoclinic compounds and the larger number of Cr atoms being antiferromagnetically coupled. Magnetization experiments performed at 5 K demonstrate that the compounds saturate already at rather low magnetic fields. In addition, small values are estimated for the coercitive field H c as well as for the remanence magnetization both being typical for weak ferromagnetic materials. The values for the saturation magnetization amount to about 72% and 65% for monoclinic and trigonal Cr 5Te 8, respectively. These low values can partially be explained on the basis of antiferromagnetically coupled Cr(III) d 3 centers. An additional spin canting is assumed to fully account for the reduced saturation moments. Above 300 K the susceptibilities follow a Curie–Weiss law with large positive values for the Weiss constant and magnetic moments in accordance with a Cr 3+ 3 d 3 spin configuration. Low temperature X-ray investigations reveal unusual thermal expansion of the a as well as b lattice parameters of the monoclinic and of the a lattice parameter of the trigonal sample, respectively. Near T c the slope of the thermal expansions changes significantly.

Synthesis, solid-state structure, and bonding analysis of the beryllocenes [Be(C5Me4H)2], [Be(C5Me5)2], and [Be(C5Me5)(C5Me4H)

The beryllocenes [Be(C(5)Me(4)H)(2)] (1), [Be(C(5)Me(5))(2)] (2), and [Be(C(5)Me(5))(C(5)Me(4)H)] (3) have been prepared from BeCl(2) and the appropriate KCp' reagent in toluene/diethyl ether solvent mixtures. The synthesis of 1 is facile (20 degrees C, overnight), but generation of decamethylberyllocene 2 demands high temperatures (ca. 115 degrees C) and extended reaction times (3-4 days). The mixed-ring beryllocene 3 is obtained when the known [(eta(5)-C(5)Me(5))BeCl] is allowed to react with K[C(5)Me(4)H], once more under somewhat forcing conditions (115 degrees C, 36 h). The structures of the three metallocenes have been determined by low-temperature X-ray studies. Both 1 and 3 present eta5/eta1 geometries of the slip-sandwich type, whereas 2 exhibits an almost regular, ferrocene-like, sandwich structure. In the mixed-ring compound 3, C(5)Me(5) is centrally bound to beryllium and the eta(1)-C(5)Me(4)H ring bonds to the metal through the unique CH carbon atom. This is also the binding mode of the eta(1)-ring of 1. To analyze the nature of the bonding in these molecules, theoretical calculations at different levels of theory have been performed on compounds 2 and 3, and a comparison with the bonding in [Be(C(5)H(5))(2)] has been made. As for the latter molecule, energy differences between the eta5/eta5 and the eta5/eta1 structures of 2 are very small, being of the order of a few kcal mol(-1). Constrained space orbital variations (CSOV) calculations show that the covalent character in the bonding is larger for [Be(C(5)Me(5))(2)] than for [Be(C(5)H(5))(2)] due to larger charge delocalization and to increased polarizability of the C(5)Me(5) fragment.

High field and low temperature X-ray study on phase segregation for Nd 0.5Sr 0.5MnO 3 powder and single crystal

It has been found that the coexistence of high-temperature ferromagnetic (FM) and low-temperature antiferromagnetic (AFM) phases was observed below the charge-order transition temperature T co for a Nd 0.5Sr 0.5MnO 3 powder sample. In order to investigate details of the phase segregation, the X-ray diffraction was carried out for the Nd 0.5Sr 0.5MnO 3 single crystal in magnetic fields up to 5 T . It was found that the phase segregation is strongly depends on the wave length of X-ray. These results suggest that the phase segregation of FM and AFM mainly occurs near the surface within a few μm in depth.

The synthesis and low-temperature X-ray study of bis[ N, N, N′, N′-tetrakis-(2-hydroxyethyl)ethylenediamine]copper(II) perchlorate

The crystal structure of the product [Cu(H 3L)ClO 4] 2 synthesized from an aqueous solution of L= N, N, N′, N′-tetrakis(2-hydroxyethyl)ethylenediamine (THEEN, C 10H 4N 2O 4) and an alkaline aqueous solution of (CuClO 4) 2 has been determined by X-ray diffraction methods. The copper complex of THEEN forms a dimeric species comprising two (CuH 3L) + monomers bonded by the formation of two metaloxygenmetal bridges. This dinuclear species lies across a crystallographic inversion center. The coordination sphere about the copper(II) ion is a highly distorted octahedron, with a distorted square planar configuration of two CuN bonds of 2.021 and 2.038 Å and two CuO bonds of 1.940 and 1.959 Å, respectively. Two longer axial CuO bonds of 2.674 and 2.332 Å complete the distorted octahedral geometry.

Dimerization Effect on the Physical Properties in New One-Dimensional Organic Conductors: (ChTM-TTP)2AuBr2, (ChTM-TTP)2GaCl4, and (ChTM-TTP)ReO4

Abstract Crystal structure analyses, measurements of transport and magnetic properties, and low-temperature X-ray studies of new organic conductors: (ChTM-TTP)2AuBr2, (ChTM-TTP)2GaCl4, and (ChTM-TTP)ReO4, where ChTM-TTP is 2-[4,5-(1,2-cyclohexylenedithio)-1,3-dithiol-2-ylidene]-5-[4,5-bis (methylthio)-1,3-dithiol-2-ylidene]-1,3,4,6-tetrathiapentalene, have been carried out. In these compounds, the donors form quasi-one-dimensional columns with dimerization induced by the steric hindrance of the 1,2-cyclohexylene unit. The crystal structures and the energy band structures of both AuBr2- and GaCl4- salts are similar to those of the Bechgaard salts. The electrical resistivity and the thermoelectric power show metallic behavior above 100 K for the AuBr2- salt and above 150 K for the GaCl4- salt. The results of the ESR spin susceptibility, the static magnetic susceptibility, and the low-temperature X-ray studies indicate that the ground states of both salts are antiferromagnetic insulators. The Néel temperatures are about 38 K for the AuBr2- salt and about 32 K for the GaCl4- salt. These salts are the first materials showing an antiferromagnetic state among the TTP series conductors. In (ChTM-TTP)2X (X = Au(CN)2, AuBr2, and GaCl4), the strength of the dimerization is related to the charge localization temperature. For the ReO4- salt, the donor to anion ratio is 1 : 1 in spite of the tetrahedral anion being similar to the GaCl4- anion. Although the interstack interactions are not small, the degree of charge transfer of the donor molecule is +1; thus this salt has an energy gap at the Fermi level, in agreement with the semiconducting transport behavior.

Low temperature X-ray study of β-A xV 2O 5

The vanadium oxide bronzes β(β′)-A x V 2O 5 of A=Na ( x=0.33), A=Sr ( x=0.33) and A=Cu ( x=0.65) having one-dimensional chain and ladder along the b-axis were studied by X-ray diffraction measurements. We observed the satellite reflections of q=1/2 b ∗ due to A-site cation ordering below 260 K for A=Na and room temperature for A=Sr. And then, we observed the satellite reflections of q=1/6 b ∗ below the metal–insulator transitions of 135 K for A=Na and 165 K for A=Sr. Moreover, we found the reflection accompanying the symmetry breaking below 255 K for A=Cu, corresponding to the slight anomaly in the resistivity of the metallic behavior. The superstructure analyses indicate the characteristic movement of vanadium atoms in the ladder, suggesting the charge disproportionation below 135 K for A=Na and 255 K for A=Cu.

The crystal structures of phenacite-type Li2(MoO4), and scheelite-type LiY(MoO4)2 and LiNd(MoO4)2

Abstract The crystal structures of flux-grown Li2(MoO4), LiY(MoO4)2 and LiNd(MoO4)2 were solved and refined from single-crystal intensity data collected with a four-circle diffractometer (MoK αX-radiation, CCD area detector, room temperature). Li2(MoO4) is isostructural with phenacite, Be2(SiO4) and has space group R3̅, a = 14.330(2), c = 9.584(2) Å, Z = 18 (R1 = 1.6%). The structure consists of a three-dimensional network of corner-linked, slightly distorted LiO4 and fairly regular MoO4 tetrahedra. Average Li–O and Mo–O distances are 1.965 Å (Li1) and 1.967 Å (Li2), and 1.764 Å (Mo). The atomic arrangement is characterised by a narrow open channel along the three-fold axis. Comparisons are drawn with other phenacite-type compounds. Low-temperature single-crystal X-ray studies of Li2(MoO4) in the range from 103 to 293 K gave no evidence of a phase transition, thus contradicting recent literature reports. Both LiY(MoO4)2 and LiNd(MoO4)2 crystallise in the scheelite-type structure, with space group I41/a and Z = 2 (R1 = 1.7 and 2.7 %, re-spectively). The first has a = 5.148(1), c = 11.173(2) Å, V = 296.11(10) Å3, the latter a = 5.243(1), c = 11.440(2) Å, V = 314.47(10) Å3. Both Li and Y/Nd are completely disordered on a jointly occupied site. Mean (Y,Li)– and (Nd,Li)–O distances are 2.40 and 2.47 Å, respectively. In both compounds the unique MoO4 tetrahedron has four identical Mo–O bonds with lengths of 1.779 Å. The stoichiometry of LiRE(MoO4)2 (RE = rare-earth element) compounds is discussed and the relation to structure types of other MRE(XO4)2 (M = alkali metal, X = Mo, W) compounds is briefly addressed.

Chains and channels in polynuclear copper(II) complexes with 2,3-bis(2-pyridyl)pyrazine (dpp) as bridging ligand; syntheses, crystal structures and magnetic properties

The preparation and crystal structures for three Cu(II) polynuclear, chain complexes with 2,3-bis(2-pyridyl)pyrazine (dpp) as bridging ligand are reported, [Cu(dpp)(H 2O) 2] n (NO 3) 2 n ·2 n/3H 2O ( 1), [Cu(dpp)(H 2O) 2] n (CF 3SO 3) 2 n ( 2), and [Cu(dpp)(H 2O) 2] n (BF 4) 2 n ·2 nH 2O ( 3). For the latter compound the crystal structure at four different temperatures have been studied. Variable-temperature magnetic susceptibility data and ESR measurements of 1– 3 and of the related copper(II) chain [Cu(dpp)(H 2O) 2] n (ClO 4) 2 n ·2 nH 2O ( 4) (whose structure was previously reported) have been performed. Compounds 1 and 2 crystallize in the same trigonal space group, R 3 ̄ c; 3 is orthorhombic, space group Pbca. Complexes 1 and 2 are built of linear dpp-bridged chains which extend along threefold screw axes. The copper atom has in each case an elongated octahedral geometry with pyrazine nitrogen atoms in axial positions. The prominent feature of the crystal packing is the supramolecular arrangement of six chains around a threefold inversion axis, creating channels housing the counter ions, and in the case of 1, also crystal water. In 3 the chain is zig–zag shaped and extends along a twofold screw axis. Counter ions and crystal water are situated in channels formed between four symmetry related chains. At room temperature (r.t.) the X-ray results show a copper ion with a compressed octahedral coordination geometry, pyrazine and pyridyl nitrogen atoms binding in equatorial and axial positions, respectively. The low temperature X-ray studies of 3 show significant changes in the copper coordination geometry, strongly suggesting that the apparent compressed geometry at r.t. is due to a dynamic Jahn–Teller distortion. The Cu⋯Cu separations across the bridging dpp at r.t. are, 7.133(1) ( 1), 7.228(1) ( 2), 7.005(1) ( 3) and 7.002(2) Å ( 4). X-band ESR spectra of 1 and 2 exhibit the pattern of Cu(II) in elongated geometry ( g ∣∣> g ⊥>2.0), whereas those of 3 and 4 exhibit inverse ( g ⊥> g ∣∣>2.0) patterns with a shoulder in the perpendicular signal. Complexes 1– 4 exhibit a Curie law behaviour with very weak intrachain antiferromagnetic coupling, the relevant magnetic parameters being J=−0.27 cm −1, g=2.11 for 1, J=−0.17 cm −1, g=2.09 for 2, J=−1.38 cm −1, g=2.15 for 3, and J=−1.36 cm −1, g=2.14 for 4 (the Hamiltonian being H ̂ =− JS A S B).

New Patulin Derivatives

Two new derivatives of patulin (1), the dimer (2) arising from cycloaddition at the 3,4-double bond, and 3,4-dihydroxypatulin acetonide (4), from attempted epoxidation with dimethyldioxirane, have been characterized. The structure of (2) was established by a single-crystal low-temperature X-ray study, while that of (4) was deduced from spectroscopic studies.

Low Temperature X-Ray and ESR study of one-dimensional compound (TTM-TTP)I 3

(TTM-TTP)I 3 which has been considered as the typical one-dimensional half-filled band system, has been critically restudied by structural and magnetic measurements. X-ray diffuse scattering study reveals the formation of 2 k F CDW superstructure with the wave vector q 1=(0, 0, 0.5) below 160 K of the metal-insulator transition temperature. Another structural modulation with q 2= (0.5, 0, 0.5) is observed below 90–100 K. Modulated structure analysis using the q 2 satellite reflections and the Bragg reflections shows the large displacement in I 3 anion. Anomalies in χ, line width and g-value of ESR measurements which correspond to the structural anomaly, are also reported.

Low temperature X-ray study of purely one-dimensional electronic system Me 4X(CPDT-TCNQ) 2(X=AS, N)

Isostructural anion radical salts Me 2X(CPDT-TCNQ) 2(X=N,P,As) have one-dimensional electronic structure, and show several type of metal-insulator transitions depending on X. The result of the polarized reflectance spectra shows that the compounds are probably purely one-dimensional electronic system. The results of X-ray photographs and crystal structure analyses indicate that the origins of the transitions are 4k F CDW accompanying a cation ordering in the Me 2As, Me 4P salts and by 2k F CDW in the Me 4N salt.

Low-temperature diffuse X-ray studies of charge-density waves coexisting with spin-density waves in the organic conductors (TMTSF) 2PF 6 and (TMTSF) 2AsF 6

The behaviors of 2 k F and 4 k F charge-density waves (CDWs) coexisting with spin-density waves (SDWs) in the title compounds were studied by diffuse X-ray scattering down to 1.6 K. It was found in the PF 6 compound that CDWs disappear below 3–4 K while they are present in the temperature range between 4 and 12 K, the SDW onset temperature. This behavior is possibly related to the subphase structure of SDW. In the AsF 6 compound, the amplitude of CDW is very small, which is related to the difference between the heat capacities of these two compounds observed previously.

Reaction of dimethylselenourea and selenourea with dibromine to produce selenourea–dibromine, the ‘T’-shaped 1:1 molecular adduct N,N-dimethyl-2-selenourea–dibromine, its solvent of crystallisation-containing analogue and the unusual ionic compound 5[(H2N)(Me2N)CBr]+[SeBr6]2−[Se2Br9]−2[Br3]−. A low temperature crystallographic reinvestigation of N-methylbenzothiazole-2-selone–dibromine

The reactions of the selenoamides N,N-dimethyl-2-selenourea, dmsu, selenourea, su and N-methylbenzothiazole-2-selone, mbts, with dibromine has been studied. In all reactions the bulk product formed is the 1∶1 T-shaped addition compound, selenoamide·Br2. The crystal structure of dmsu·Br2 1 has been determined and compared to its solvated analogue, dmsu·Br2·CH2Cl2. Despite the different crystal packing in these two adducts, an asymmetry in d(Se–Br) is exhibited by both, ruling out the possibility that differences in Se–Br bond lengths are attributed solely to crystal packing forces. Both structures are essentially zwitterionic, a negative charge resides on the SeBr2 moiety and the positive charge is supported by the two nitrogen atoms. The recently reported mbts·Br2, previously described as a carbene interacting with SeBr2, was reinvestigated. Considering findings from a low temperature X-ray study and the 13C NMR spectrum, this compound is in fact also best described as zwitterionic, analogous to 1 and previously described structures. An interesting minor product from the reaction of two equivalents of dibromine with dmsu has also been characterised crystallographically. This complicated ionic structure of formula 5[(H2N)(Me2N)CBr]+[SeBr6]2–[Se2Br9]–2[Br3]– illustrates the susceptibility of certain selenoamides to carbon–selenium bond cleavage when a dihalogen that is more strongly oxidising than I2 or IBr is treated with them in a stoichiometric ratio greater than 1∶1. This curious (and disordered) structure represents the first report of the anion [Se2Br9]–.

The imbalanced kekule structure contributions in the ring in 1,3,5-trisubstituted benzene derivatives: Low temperature X-ray study on 1,3,5-trimethoxybenzene, and ab initio calculations on 1,3,5-triformylo - benzene and trimethoxy-benzene and the related di-substituted systems

A low temperature X-ray structural determination for 1,3,5-trimethoxybenzene reveals π-electron localisation in the ring. ab initio calculations for 1,3,5-trimethoxybenzene and 1,3,5-triformylobenzene show that these nonsymmetrical substituents induce opposite imbalance of Kekule structures in the ring. An empirical rule is proposed - that cis CC-bonds in the ring show increased double bond character (compared to the trans bond) when the substituent is of X-Y kind (with a single XY bond), and less double bond character if the substituent is of the X=Y type (with a double XY bond). Additional calculations for cis and trans conformers of 1,4-dimethoxybenzene differ dramatically in the degree of π-electron localisation, supporting the idea that the dominating factor of these distortion is a through-space effect.

New organic superconductor, λ-(BETS)2GaCl4 and metal-insulator transition of BETS conductor with magnetic anions (BETS=bis(ethlenedithio)tetraselenafulvalene)

Abstract Crystal structures and electronic properties of BETS compounds with tetrahedral anions, MX4 (BETS=bis(ethylenedithio)tetraselenafulvalene; MGa, Fe, In; XCl, Br) were examined. The salts can be classified into two types (κ, λ). All the compounds with so-called κ-type molecular arrangements retained metallic behavior down to 4 K. λ-(BETS)2GaCl4 demonstrated a a superconducting transition around 8 K. Whereas λ-(BETS)2FeCl4 with the isomorphic λ-type structure exhibited a sharp metal-insulator transition at 8 K. Low-temperature X-ray studies showed that the structural difference between λ-GaCl4 and λ-FeCl4 salts is very small. The structure analysis of κ-(BETS)2FeCl4 at 18 K indicated that the FeCl4 anion has approximately C3v symmetry. The ESR measurements revealed the high-spin state of Fe3+ in λ-FeCl4 salt and and low-spin state in κ-FeCl4 salt. ESR intensities of κ- and λ-FeCl4 salts suggested antiferromagnetic interaction between Fe3+ ions.

Low-temperature X-ray study of benzoid-quinoid tautomerism in crystals of 2-(N,N-diphenylhydrazinoethylidene)-3(2H)-benzo[b]thiophenone

An X-ray investigation of 2-(N,N-diphenylhydrazinoethylidene)-3(2H)-benzo[b]thiophenone was performed at room and low (−80 °C) temperatures. It was established that the structure of this compound in the crystalline state at R-20 °C is intermediate between the ketoenehydrazine and hydroxyhydrazone forms, while at −80 °C the molecular structure is completely transformed to the hydroxyhydrazone form. This confirms the dynamic nature of hydrogen atom disordering in the molecule at ∼20 °C.

Lanthanocene (Ln = Pr III, Yb III) chlorides involving tetramethyldisiloxane-interlinked cyclopentadienyl ligands

Two new lanthanocenophanes [{O(Me 2SiC 5H 4) 2}Pr IIICl · THF] x( 2; x = 1 or 2) and [{O(Me 2SiC 5H 4) 2}Yb III Cl] 2 3, were made from LnCl 3 (Ln = Pr or Yb) and K 2[O(Me 2SiC 5H 4) 2] in THF. The 1H NMR spectra of THF- d 8 solutions of paramagnetic 2 (at + 35 to −25°C) and 3 (at < 10°C) reflect an asymmetrical disposition of the siloxy linkage, while solutions of 2 in CD 2Cl 2 (at 20°C) and 3 in THF- d 8 (at > 10°C), respectively, display somewhat less complex 1H NMR spectra consistent with a virtually symmetrical position of the Me 2SiOSiMe 2 unit with respect to the cent-Ln- cent′ plane (cent = C 5H 4Si centroid). A low-temperature (−100°C) single-crystal X-ray study of 3 has confirmed the presence of centrosymmetric, Cl-bridged dimers in which the molecular configuration of the mononuclear {O(Me 2SiC 5H 4) 2}Yb III Cl 2 units correspond to that of the previously described complex [{O(Me 2SiC 5H 4) 2Ti IVCl 2] 1. The high-resolution mass spectrum of 3 displays several signals from dinuclear fragments, the B/E-linked scans of which suggest, inter alia, intramolecular reorientation of at least one O(Me 2SiC 5H 4) 2 ligand from the chelating into a metal-bridging position.