G. M. Reisner

Catenated and non-catenated inclusion complexes of trimesic acid

Trimesic acid (benzene-1, 3,5-tri-carboxylic acid; TMA) can in principle form two-dimensional hydrogen-bonded hexagonal networks in which central holes of the network have net diameters of 14 Å. Although such holes would be expected to be natural locations for guest molecules, non-catenated single networks have not been found in any of the crystals containing TMA studied in the last sixteen years. Instead, anhydrous α-TMA, TMA pentaiodide (TMA.I5) and (so-called) γ-TMA have mutually triply-catenated structures in which triplets of networks are interlaced [3,4,5], while the hydrated complexes are based on non-catenated nets of composition TMA.H2O [6]. We have now found conditions under which single networks are preserved without catenation, the cavities being occupied by guests such as n-tetradecane, n-heptanol, n-octanol, n-decanol, octene, cyclooctane and isooctane. The structures of 2TMA. n-tetradecane and 2TMA. isooctane have been solved and refined to R=13.0% and R=11.3%, respectively, disorder of the guest molecules having prevented further refinement of the room-temperature data. Determination of the crystal structures of the other complexes, which are isostructural with 2TMA. n-tetradecane, is now in progress. We are also investigating other potential guests.

Relationship of solubility parameter (x), powder properties and phase formation in the Nd1+xBa2−xCu3O6.5+x2+δ system

Abstract The relationship between the solubility parameter, x, in the solid solutions Nd 1+x Ba 2−x Cu 3 O 6.5+x 2+δ (Nd123SS) and XRD patterns, powder surface area (SA), particle size, morphology and melting points was investigated. An efficient way to determine the value of x and residual BaCuO2 content during Nd123SS powder synthesis is presented. The method is based on calculation of the orthorhombic splitting (OS) factor from the unit cell parameters obtained from XRD data. The final phase in the Nd123SS system is formed through a diffusion controlled reaction between BaCuO2 and Nd123SS and a kinetic model is developed to describe the formation of the Nd123 (x=0) superconducting powder. Finally, high values of Tc (measured by DC magnetization) were found in the powders. The highest Tc of 98.7 K was measured for the x=0 case. This value is the highest ever reported for the NdBaCuO system. The Tc was found to be insensitive to the value of x, ranging between 98.7 and 94 K for x between 0 and 0.25, respectively. This data is contrast to published results on samples synthesized at higher temperatures, where Tc fell to 40 K at x = 0.25.

Pulsed versus dc I-V characteristics of resistive manganites

We report on pulsed and dc I-V characteristics of polycrystalline samples of three charge-ordered manganites, Pr2∕3Ca1∕3MnO3, Pr1∕2Ca1∕2MnO3, and Bi1∕2Sr1∕2MnO3, and of a double perovskite, Sr2MnReO6, in a temperature range where their Ohmic resistivity obeys the Efros-Shklovskii variable range hopping relation [J. Phys. C 8, L49 (1975)]. For all samples, the dc I(V) exhibits at high currents negative differential resistance and hysteresis, which mask a perfectly Ohmic or a moderately non-Ohmic conductivity obtained by pulsed measurements. This demonstrates that the widely used dc I-V measurements are usually misleading.

X-ray and neutron diffraction study of benzoylacetone in the temperature range 8–300 K: comparison with other cis-enol molecules

The crystal structure of benzoylacetone (1-phenyl-1,3-butanedione, C(10)H(10)O(2); P2(1)/c, Z = 4) has been determined at 300, 160 (both Mo Kalpha X-ray diffraction, XRD), 20 (lambda = 1.012 Å neutron diffraction, ND) and 8 K (Ag Kalpha XRD), to which should be added earlier structure determinations at 300 (Mo Kalpha XRD and ND, lambda = 0.983 Å) and 143 K (Mo Kalpha XRD). Cell dimensions have been measured over the temperature range 8-300 K; a first- or second-order phase change does not occur within this range. The atomic displacement parameters have been analyzed using the thermal motion analysis program THMA11. The most marked change in the molecular structure is in the disposition of the methyl group, which has a librational amplitude of approximately 20 degrees at 20 K and is rotationally disordered at 300 K. The lengths of the two C-O bonds in the cis-enol ring do not differ significantly, nor do those of the two C-C bonds, nor do these lengths change between 8 and 300 K. An ND difference synthesis (20 K) shows a single enol hydrogen trough (rather than two half H atoms), approximately centered between the O atoms; analogous results were obtained by XRD (8 K). It is inferred that the enol hydrogen is in a broad, flat-bottomed single-minimum potential well between the O atoms, with a libration amplitude of approximately 0.30 Å at 8 K. These results suggest that at 8 K the cis-enol ring in benzoylacetone has quasi-aromatic character, in agreement with the results of high-level ab initio calculations made for benzoylacetone [Schiøtt et al. (1998). J. Am. Chem. Soc. 120, 12117-12124]. Application [in a related paper by Madsen et al. (1998). J. Am. Chem. Soc. 120, 10040-10045] of multipolar analysis and topological methods to the charge density obtained from the combined lowest temperature X-ray and neutron data provides evidence for an intramolecular hydrogen bond with partly electrostatic and partly covalent character, and large p-delocalization in the cis-enol ring. This is in good agreement with what is expected from the observed bond lengths. Analysis of the total available (through the Cambridge Structural Database, CSD) population of cis-enol ring geometries confirms earlier reports of correlation between the degree of bond localization in the pairs of C-C and C-O bonds, but does not show the dependence of bond localization on d(O.O) that was reported earlier for a more restricted sample. It is suggested that the only reliable method of determining whether the enol hydrogen is found in a single or double potential well is by low-temperature X-ray or (preferably) neutron diffraction.

Electronic transport in MgB2, AlB2 and ZrB2--a comparative study

Abstract We report on the temperature dependence of the resistivity ( ρ ) and the absolute thermopower ( S ) of the polycrystalline title materials and of AlB 2 single crystals. For all samples ρ ( T ) exhibits a Bloch–Gruneisen-like temperature dependence, with large characteristic temperatures θ R (≈ θ D ––the Debye temperature). At high temperatures the thermopower S ( T ) for ZrB 2 (n-type) is almost the mirror image of S ( T ) for MgB 2 (p-type) while S (AlB 2 ) is very small for all temperatures. The density of states distribution N ( E ) around E F seems to play a dominant role in determining S ( T ) of these materials. ln( T ) terms in the low-temperature ρ ( T ) and S ( T ) of ZrB 2 samples bear evidence for weak localization in 2D.

Transport measurements in the 1-2-3 system CLBLCO in both the oxygen-underdoped and -overdoped regions

Abstract “CLBLCO” is a family of 1-2-3 superconductors, represented by the chemical formula (Ca x La 1− x )(Ba c − x La 2− c + x )Cu 3 O y . By annealing in oxygen at various pressures, up to 4300 bar, at a temperature of 300°C and above, we have prepared specimens whose oxygen content covers a wide range, from y =6.4 (a totally “underdoped” insulating phase) to a highly overdoped phase with y ≳7.354. In contrast to YBCO, the specimens remain tetragonal for all values of x , c , and y . Measurements of the superconducting transition temperature, the resistivity, and the thermoelectric power, over a range of y values, 6.84≤ y ≤7.30 are presented, for samples with calcium content from x =0.1 to 0.4, and with c =1.75. We also report some transport data for CLBLCO with nickel impurities. Some significant differences were found between the properties of the underdoped and overdoped regimes. In contrast to YBCO, the thermoelectric power characteristics behave like those of most other high- T c cuprates. We found that a proposed universal relation between the thermoelectric power and the ratio between T c and its maximum value T c,max is not truly universal; it holds in some cases, but fails in others. The temperature-dependence of the resistivity shows a departure from linearity in the overdoped region, which is perhaps the signature of the appearance of a normal-state gap. The temperature at which the non-linearity sets in appears to increase as the doping level falls.

The crystal structures of three isomorphous trimesic acid channel inclusion complexes of polyhalide ions. A second example of triple catenation in the solid state

The three polyhalide complexes of trim esic acid (benzene-1, 3, 5-tricarboxylic acid; TMA) with compositions TMA.0.7H2O. 0.09 HI5(I), TMA. 0.7H2O. 0.103 HBr5(II), TMA. 0.7H2O. 0.167 HIBr2(III) are isomorphous (for example, for I, a = 21.945(7), b = 17.917(6), c = 16.711(6) Å, the space group is 1222, Z = 24; II and III have very similar cell dimensions). Crystal structure analysis (four-circle diffractometer, graphite-monochro-mated MoKa: I, 2273reflexions, R = 12.2% ; III, 1747 reflexions, R = 12.4%) shows that all three have the same TMA fram ework, which consists of two mutually-perpendicular infinite hexagonal planar networks of hydrogen-bonded TMA molecules. The two networks are parallel to (110) and (11̄0) respectively and are mutually triply catenated. The arrangement is such as to leave channels of square cross section along the z-direction, which contain the linear polyhalide ions. I an d II have pentahalide ions incommensurable with the TMA framework; the diffuse scattering pattern from I allows the structure of the I-5 ion to be determined as consisting of strongly interacting . . . I2 - - - I- I2 . . . units and it is inferred that Br-5 has a similar structure. Compound III has IBr-2 ions commensurable with the TMA framework, but some difficulties remain in the interpretration of these results. The TMA framework has additional voids in the region about 1/2, 0, 1/2 and these are occupied by the water molecules (ca. sixteen per unit cell) in disordered fashion. The protons required to balance the charges of the polyhalide ions are presumably attached to some of these water molecules. The materials are characterized as crystalline acid-molecule complexes.

Synthesis of Nd123 superconducting powder via oxalate coprecipitation

Abstract The preparation of pure and fine Nd123 powder was investigated in five ways. Precursor powders were generated by the oxalate coprecipitation. Some of the powders were heated in vacuum (770°C for 20 h at 10–15 Pa), followed by calcination at different temperatures and oxygen partial pressures. The vacuum step was omitted with the remaining powders. Calcination under pure oxygen requires excessively high temperatures ( T >940°C). Under these conditions the powder is badly fused, with grain sizes in the 7–12 μm range. In addition, the orthorhombicity of this powder is low due to the formation of Nd123 solid solution — implying the presence of residual BaCuO 2 . The optimal route to produce the desired powder is in a low partial pressure of 350–1000 ppm oxygen in nitrogen, without a vacuum preheating step. Similar powders can be obtained in 1% O 2 with a vacuum preheating step. At 60 ppm O 2 the reaction between Nd 1+ x Ba 2− x Cu 3 O 6.5+ x /2+δ and BaCuO 2 is incomplete, with about 5% of barium cuprate remaining. Finally, the T c of the powder produced at the optimal conditions was found by DC magnetization to be ∼ 99 K.

The polyiodide salts: Pyridinium pentaiodide;β-naphthylammonium pentaiodide; andN-methyl-γ-picolinium heptaiodide. Structures with channel inclusion features

The crystal structures of three polyiodode salts are reported (pyridinium pentaiodide, monoclinic,P21/m,a=9.221(5),b=12.918(5),c=6.026(4) Å, β=103.60(7)o,Z=2,RF=0.087 for 1187 intensities; β-naphthyl-ammonium pentaiodide, triclinic,173-1,a=10.390(5),b=9.502(5),c=4.462(3) Å, α=99.19(7), β=90.40(7),γ=108.49(8)o,Z=2,RF=0.059 for 1319 intensities;N-methyl-γ-picolinium heptaiodide, monoclinic,C2/c,a=19.315(7),b=12.714(5),c=8.442(4) Å, β=107.26(7)o,Z=4,RF=0.065 for 1336 intensities). All three structures can be described as having channel inclusion features; the cations are contained in channels in polyiodide frameworks based on different arrangements of I2 molecules and I3− anions. This structural type is the converse of the more widespread kind where polyiodide anions are contained in an organic matrix (e.g., cyclodextrin polyiodides).

Fully oxygenated RBa2Cu3−xCoxOy, (R=Y, Eu, Pr, and 0≤x≤1)— from high‐temperature superconductors to high‐resistivity nonmetals

Reported here are the results of measurements of the resistivity (ρ) up to 300 K and of the thermoelectric power (TEP) up to 400 K of ceramic samples of the title materials. We determined also their room‐temperature lattice parameters and oxygen content as functions of Co concentrations. The metal–nonmetal transition in YBa2Cu3−xCoxOy and in EuBa2Cu3−xCoxOy is marked by the onset of the deviation of the maximum absolute TEP from a value calculated from a simple narrow‐band formula. The results indicate that the effective valency of Pr in PrBa2Cu3−xCoxOy varies from ∼3.5 at x=0 to ∼3 for x=0.5. For x=0 this material is close to the metal–nonmetal transition. In the nonmetallic regime the electrical transport is by activated hopping. In certain ranges of Co content the results are consistent with two‐band hopping conductivity with two branches, a low‐temperature and a high‐temperature branch of variable‐range‐hopping (VRH) conductivity. In the VRH regime there is a remarkable correlation between the two res...