Leonore Wiehl

Resistivity studies under hydrostatic pressure on a low-resistance variant of the quasi-two-dimensional organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br: Search for intrinsic scattering contributions

Resistivity measurements have been performed on a low (LR)- and high (HR)-resistance variant of the kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br superconductor. While the HR sample was synthesized following the standard procedure, the LR crystal is a result of a somewhat modified synthesis route. According to their residual resistivities and residual resistivity ratios, the LR crystal is of distinctly superior quality. He-gas pressure was used to study the effect of hydrostatic pressure on the different transport regimes for both variants. The main results of these comparative investigations are (i) a significant part of the inelastic-scattering contribution, which causes the anomalous rho(T) maximum in standard HR crystals around 90 K, is sample dependent, i.e. extrinsic in nature, (ii) the abrupt change in rho(T) at T* approx. 40 K from a strongly temperature-dependent behavior at T > T* to an only weakly T-dependent rho(T) at T < T* is unaffected by this scattering contribution and thus marks an independent property, most likely a second-order phase transition, (iii) both variants reveal a rho(T) proportional to AT^2 dependence at low temperatures, i.e. for T_c < T < T_0, although with strongly sample-dependent coefficients A and upper bounds for the T^2 behavior measured by T_0. The latter result is inconsistent with the T^2 dependence originating from coherent Fermi-liquid excitations.

Structural compression and vibrational properties of Bi12SiO20 sillenite from experiment and theory

The crystal structure of the bismuth silicon oxide Bi(12)SiO(20) was determined by single-crystal x-ray diffraction at ambient conditions and at high pressure. Single-crystal intensity data between 0.0001 and 16.8(3) GPa were collected in house with Mo Kα radiation and with synchrotron radiation (λ = 0.45 Å) at HASYLAB (D3), while lattice parameters were measured up to 23.0(3) GPa. The large cavities which exist in the crystal structure and host the lone electron pairs of the Bi(3 + ) ions are considerably compressed at high pressure. The crystal structure, however, remains stable and the lone electron pair is stereochemically active up to at least 16.8 GPa. A larger compression in the direction of the lone electron pairs by shear deformation was not observed. Raman spectra of Bi(12)SiO(20) were measured on powder samples during pressure decrease from 39.1(1) GPa down to ambient pressure and on single crystals during pressure increase up to 12.50(3) GPa. Density functional perturbation theory was used to compute Raman frequencies and intensities at ambient pressure and to investigate pressure-induced changes up to 50 GPa.

Persistence of the stereochemical activity of the Bi3+ lone electron pair in Bi2Ga4O9 up to 50 GPa and crystal structure of the high-pressure phase

The crystal structure of the high-pressure phase of bismuth gallium oxide, Bi(2)Ga(4)O(9), was determined up to 30.5 (5) GPa from in situ single-crystal in-house and synchrotron X-ray diffraction. Structures were refined at ambient conditions and at pressures of 3.3 (2), 6.2 (3), 8.9 (1) and 14.9 (3) GPa for the low-pressure phase, and at 21.4 (5) and 30.5 (5) GPa for the high-pressure phase. The mode-Grüneisen parameters for the Raman modes of the low-pressure structure and the changes of the modes induced by the phase transition were obtained from Raman spectroscopic measurements. Complementary quantum-mechanical calculations based on density-functional theory were performed between 0 and 50 GPa. The phase transition is driven by a large spontaneous displacement of one O atom from a fully constrained position. The density-functional theory (DFT) model confirmed the persistence of the stereochemical activity of the lone electron pair up to at least 50 GPa in accordance with the crystal structure of the high-pressure phase. While the stereochemical activity of the lone electron pair of Bi(3+) is reduced at increasing pressure, a symmetrization of the bismuth coordination was not observed in this pressure range. This shows an unexpected stability of the localization of the lone electron pair and of its stereochemical activity at high pressure.

High-pressure phase transition of Bi2Fe4O9.

The high-pressure behaviour of Bi2Fe4O9 was analysed by in situ powder and single-crystal x-ray diffraction and Raman spectroscopy. Pressures up to 34.3(8) GPa were generated using the diamond anvil cell technique. A reversible phase transition is observed at approximately 6.89(6) GPa and the high-pressure structure is stable up to 26.3(1) GPa. At higher pressures the onset of amorphization is observed. The crystal structures were refined from single-crystal data at ambient pressure and pressures of 4.49(2), 6.46(2), 7.26(2) and 9.4(1) GPa. The high-pressure structure is isotypic to the high-pressure structure of Bi2Ga4O9. The lower phase transition pressure of Bi2Fe4O9 with respect to that of Bi2Ga4O9 (16 GPa) confirms the previously proposed strong influence of cation substitution on the high-pressure stability and the misfit of Ga3+ and Fe3+ in tetrahedral coordination at high pressure. A fit of a second-order Birch–Murnaghan equation of state to the p–V data results in K0 = 74(3) GPa for the low-pressure phase and K0 = 79(2) GPa for the high-pressure phase. The mode Grüneisen parameters were obtained from Raman-spectroscopic measurements.

Influence of grain size, surface energy, and deviatoric stress on the pressure-induced phase transition of ZnO and AlN

The high pressure behavior of aluminum nitride (AlN) and zinc oxide (ZnO) nanocrystals was studied up to 30 GPa using second harmonic generation. ZnO (10 nm) crystals transform to the high pressure B1 phase at 16.6 GPa, ≈5 GPa higher than the corresponding value for a bulk sample. The transition of AlN (20 nm) and AlN (100 nm) occurred at 14 and 21.5 GPa at lower values than for bulk samples. Under non-hydrostatic pressure conditions, the transition pressures of ZnO (10 nm) and AlN (100 nm) decrease to 12.5 and 18 GPa, respectively. We determined the surface energy ( and ) of the B1 polymorphs. We show that the main reason for the size-dependent decrease of the transition pressure of AlN nanocrystals is due to the higher surface energy of the B4 phase relative to the surface energy of the B1 phase. We predict that it is possible to quench or synthesize pure B1 AlN to ambient conditions if the grain size is less than 8.5 nm.

Structural and magnetic properties of betaine adducts with transition metals: I. ((CH3)3NCH2COO)3MnMCl4 with M = Mn2+, Co2+, Zn2+

Large single crystals with diameters up to 20 mm of betaine adducts with 3d metals, namely 3bMnCl2MCl2 with M = Mn2+ (BMM), Co2+ (BMC) and Zn2+ (BMZ), were grown from aqueous solution by slow evaporation of the solvent. The isomorphous crystal structures, space group , are built up from carboxylate-bridged octahedral chains and isolated MCl4 tetrahedra. The magnetic susceptibilities of these low-dimensional spin-systems were determined by superconducting quantum interference device (SQUID) measurements and are interpreted with an antiferromagnetic Heisenberg-chain model. We studied by experiment and density functional theory (DFT) calculations how the replacement of half of the Mn content (S = 5/2) in 3b2MnCl2 by metals with different spin (Co, S = 3/2; Zn, S = 0) modifies the structural and magnetic properties. The results show that, predominantly, Mn occupies the octahedral sites in the chains and is responsible for the magnetic interaction, whereas the other metal (Co or Zn) is found on the isolated tetrahedral sites.

Magneto-structural correlations in a new oxalato-bridged Cu(II) alternating-exchange spin-chain compound

A new oxalato-bridged copper(II) compound, Cu(ox)(pyOH)H2O (ox = oxalate, pyOH = 3-hydroxypyridine), has been synthesized and characterized by x-ray diffraction and magnetic susceptibility measurements. The Cu(II) ions are bridged by oxalate molecules with two different arrangements alternating along a chain parallel to the b-axis. To the best of our knowledge, this is the first example of a metal–oxalate chain with such a combination of oxalate bridges. Due to the specific structural properties, the magnetic susceptibility was analysed in the framework of an alternating-exchange spin-chain. From a least-squares fit, an antiferromagnetic coupling constant J1 = (442 ± 5) K and an alternation parameter α = 0.13 ± 0.06 were derived, which classify Cu(ox)(pyOH)H2O as a strongly dimerized spin-chain compound. It is argued that the strength of the magnetic coupling is mainly determined by the displacement of the Cu(II) ions out of the basal plane of the local Cu environment.

Crystal structure of poly-[(3-hydroyxpyridine)oxalatocopper(II)] monohydrate, Cu(NC5H4OH)(COO)2 · H2O

Abstract C7H7CuNO6, monoclinic, P121/c1 (no. 14), a = 12.9147(8) Å, b = 9.1615(6)Å, c = 8.2474(5)Å, β = 98.490(5)°, V = 965.1Å3, Z = 4, Rgt(F) = 0.031, wRref(F2) = 0.079, T = 293 K.

Crystal structures of catena-(tris(μ-betaine-O,O′)manganese(II)) tetrachlorozincate and catena-(tris(μ-betaine-O,O′)manganese(II)) tetrachlorocobaltate, [{(CH3)3NCH2COO}3Mn][MCl4] (M= Zn, Co)

Abstract C15H33Cl4MnN3O6Zn, trigonal, P3̅ (no. 147), a = 12.7751(5) Å, c = 9.0835(2) Å, V = 1283.9 Å3, Z = 2, Rgt(F) = 0.032, wRref(F2) = 0.083, T = 293 K. C15H33Cl4CoMnN3O6, trigonal, P3̅ (no. 147), a = 12.7832(7) Å, c = 9.0769(4) Å, V = 1284.6 Å3, Z = 2, Rgt(F) = 0.028, wRref(F2) = 0.076, T = 293 K.