Philipp Jacobs

High-resolution neutron diffraction study of CuNCN: New evidence of structure anomalies at low temperature

Copper carbodiimide (CuNCN) is the nitrogen-containing analogue of cupric oxide. Based on high-resolution neutron-diffraction data, CuNCNs lattice parameters are derived as a function of the temperature. In accordance with a recent synchrotron study, a clear trend in the cell parameter a is observed accompanying the changing magnetic behavior. With decreasing temperature, a slowly decreases to a minimum at ~100 K after which it rises again. The same trend-albeit more pronounced-is observed for the c lattice parameter at ~35 K. The herein presented neutron powder-diffraction data also support the conjectured sequence of transitions from the high-temperature one-dimensional resonating valence-bond (RVB) state to a transient two-dimensional RVB state and eventually, at lowest temperatures, into another two-dimensional RVB state, presumably the ground state.

Tetrasulphur Tetranitride: Phase Transition and Crystal Structure at Elevated Temperature

Rietveld X-ray crystal structure investigations on S4N4 give evidence for a new orthorhombic high-temperature phase (Pbcn, a = 883.9(1) pm, b = 875.5(1) pm, c = 725.81(9) pm) very close to the explosion temperature of the solid-state material. The phase transition can be described using Landaus theory, yielding a critical temperature of 397 K and a critical exponent of 0.50. Tetraschwefeltetranitrid: Phasenubergang und Kristallstruktur bei erhohter Temperatur Rietveld-Kristallstrukturuntersuchungen mit Rontgenstrahlen an S4N4 geben eine neue orthorhombische Hochtemperaturphase (Pbcn, a = 883,9(1) pm, b = 875,5(1) pm, c = 725,81(9) pm) sehr nahe an der Explosionstemperatur des Feststoffs zu erkennen. Der Phasenubergang kann mit Hilfe der Landautheorie beschrieben werden; man erhalt eine Umwandlungstemperatur von 397 K und einen kritischen Exponenten von 0,50.

Atomic motions in the layered copper pseudochalcogenide CuNCN indicative of a quantum spin-liquid scenario

We explore the thermodynamic properties of the layered copper(II) carbodiimide CuNCN by heat-capacity measurements and investigate the corresponding thermal atomic motions by means of neutron powder diffraction as well as inelastic neutron scattering. The experiments are complemented by a combination of density-functional calculations, phonon analysis and analytic theory. The existence of a soft flexural mode-bending of the layers, characteristic for the material structure-is established in the phonon spectrum of CuNCN by giving characteristic temperature-dependent contributions to the heat capacity and atomic displacement parameters. The agreement with the neutron data allows us to extract a residual-on top of the lattice-presumably spinon contribution to the heat capacity [Formula: see text], speaking in favor of the spin-liquid picture of the electronic phases of CuNCN.

Neutron powder diffraction and theory-aided structure refinement of rubidium and cesium ureate

Abstract Urea (CN2H4O) is a fundamental biomolecule whose derivatives are abundant throughout chemistry. Among the latter, rubidium ureate (RbCN2H3O) and its cesium analog (CsCN2H3O) have been described only very recently and form the first structurally characterized salts of deprotonated urea. Here, we report on a neutron diffraction study on the aforementioned alkaline-metal ureates, which affords the positions for all hydrogen atoms (including full anisotropic displacement tensors) and thus allows us to gain fundamental insights into the hydrogen-bonding networks in the title compounds. The structure refinements of the experimental neutron data proceeded successfully using starting parameters from ab initio simulations of atomic positions and anisotropic displacement parameters. Such joint experimental-theoretical refinement procedures promise significant practical potential in cases where complex solids (organic, organometallic, framework materials) are studied by powder diffraction.

Ammonothermal Synthesis, Crystal Structure, and Properties of the Ytterbium(II) and Ytterbium(III) Amides and the First Two Rare-Earth-Metal Guanidinates, YbC(NH)3 and Yb(CN3H4)3

We report the oxidation-controlled synthesis of the ytterbium amides Yb(NH2)2 and Yb(NH2)3 and the first rare-earth-metal guanidinates YbC(NH)3 and Yb(CN3H4)3 from liquid ammonia. For Yb(NH2)2, we present experimental atomic displacement parameters from powder X-ray diffraction (PXRD) and density functional theory (DFT)-derived hydrogen positions for the first time. For Yb(NH2)3, the indexing proposal based on PXRD arrives at R3̅, a = 6.2477(2) Å, c = 17.132(1) Å, V = 579.15(4) Å(3), and Z = 6. The oxidation-controlled synthesis was also applied to make the first rare-earth guanidinates, namely, the doubly deprotonated YbC(NH)3 and the singly deprotonated Yb(CN3H4)3. YbC(NH)3 is isostructural with SrC(NH)3, as derived from PXRD (P63/m, a = 5.2596(2) Å, c = 6.6704(2) Å, V = 159.81(1) Å(3), and Z = 2). Yb(CN3H4)3 crystallizes in a structure derived from the [ReO3] type, as studied by powder neutron diffraction (Pn3̅, a = 13.5307(3) Å, V = 2477.22(8) Å(3), and Z = 8 at 10 K). Electrostatic and hydrogen-bonding interactions cooperate to stabilize the structure with wide and empty channels. The IR spectra of the guanidinates are compared with DFT-calculated phonon spectra to identify the vibrational modes. SQUID magnetometry shows that Yb(CN3H4)3 is a paramagnet with isolated Yb(3+) (4f(13)) ions. A CONDON 2.0 fit was used to extract all relevant parameters.

A Rietveld refinement method for angular- and wavelength-dispersive neutron time-of-flight powder diffraction data

This paper introduces a novel approach to Rietveld refinements of two-dimensional angular- and wavelength-dispersive powder diffraction data as measured at time-of-flight neutron diffraction instruments. To do so, the authors’ ansatz for diffraction data obtained from the POWGEN diffractometer has been verified, and furthermore its feasibility and benefit in simulations for the novel POWTEX instrument presently under construction are demonstrated.

Instrumental resolution as a function of scattering angle and wavelength as exemplified for the POWGEN instrument

A fundamental description of the instrument resolution file is elaborated for the angular- and wavelength-dispersive cases of Rietveld refinement, exemplified for the POWGEN instrument. It is shown how to refine the necessary profile function parameters from a dataset measured on a diamond reference sample. The analysis is performed in a two-dimensional refinement space based on the convenient variables d and d ⊥.

Cr2(NCN)3, a ferromagnetic carbodiimide with an unusual two-step magnetic transition

While most transition-metal carbodiimides exhibit antiferromagnetism, chromium(III) carbodiimide, the first transition-metal(III) carbodiimide with a 3d3 configuration, displays ferromagnetic coupling. We report here on a neutron-diffraction study of Cr2(NCN)3 demonstrating that a complete three-dimensional ferromagnetic order is established below T = 114 K. Complementary ac and dc susceptibilities as well as electron paramagnetic resonance (EPR) and heat-capacity measurements allow us to further analyze its magnetic properties, elucidating another magnetic transition around T = 160 K. We describe here a magnetic two-step transition scenario over an unusual wide temperature range, from paramagnetic to a lower-dimensional ferromagnetic ordering, presumably a 2D-ordering within the chromium layers, below TLD = 166(4) K and from this low-dimensional ordering to a complete 3D-ferromagnetic ordering below TC = 114 K.