Temperature induced valence phase transition in intermediate-valent YbPd2Al3

Synthesis, magnetic and NMR spectroscopic properties of the MAl5Pt3 series (M = Ca, Y, La-Nd, Sm-Er)

Following recent investigation in the ternary system Sr-Al-Pt led to the discovery of SrAl5Pt3 which crystallizes in the orthorhombic YNi5Si3 type (Pnma) structure. Interestingly, only two more aluminum representatives, CeAl5Pt3 and EuAl5Pt3, have been reported to adopt this structure type. Therefore, we decided to investigate the existence range of compounds adopting the YNi5Si3 type structure. Besides the already known Sr, Ce and Eu members, the series could be extended to Ca, Y and La-Nd as well as Sm-Er. All compounds were synthesized from the elements and characterized by powder X-ray diffraction. While for CaAl5Pt3 and LaAl5Pt3 also the respective M2Al16Pt9 members were observed, the other compounds could be obtained either as X-ray pure materials or with small amounts of Al3Pt2 as a side phase. The structure of ErAl5Pt3 could be refined from single crystal data, verifying that also the small rare-earth elements adopt the YNi5Si3 type structure. Selected members of the series were furthermore characterized by magnetization and susceptibility measurements. Since YAl5Pt3 could be obtained as a phase pure material and exhibits no paramagnetic behaviour it was investigated by 27Al MAS NMR investigations. Also, XPS measurements were conducted on this compound to gain an insight into the charge distribution. Finally, quantum-chemical calculations supported the NMR measurements and gave an insight into the chemical bonding and the charge distribution.

On the Ytterbium Valence and the Physical Properties in Selected Intermetallic Phases

The present review summarizes important aspects of the crystal chemistry of ytterbium-based intermetallic compounds along with a selection of their outstanding physical properties. These originate in many cases from the ytterbium valence. Different valence states are possible here, divalent (4f14), intermediate-valent, or trivalent (4f13) ytterbium, resulting in simple diamagnetic, Pauli or Curie-Weiss paramagnetic, or valence fluctuating behavior. Especially, some of the Yb3+ intermetallics have gained deep interest due to their Kondo or heavy Fermion ground states. We have summarized their property investigations using magnetic and transport measurements, specific heat data, NMR, ESR, and Mössbauer spectroscopy, elastic and inelastic neutron scattering, and XAS data as well as detailed thermoelectric measurements.

Crystallizing covalent organic frameworks from metal organic framework through chemical induced-phase engineering

The ordered porous frameworks like MOFs and COFs are generally constructed using the monomers through distinctive metal-coordinated and covalent linkages. Meanwhile, the inter-structural transition between each class of these porous materials is an under-explored research area. However, such altered frameworks are expected to have exciting features compared to their pristine versions. Herein, we have demonstrated a chemical-induction phase-engineering strategy to transform a two-dimensional conjugated Cu-based SA-MOF (Cu-Tp) into 2D-COFs (Cu-TpCOFs). The structural phase transition offered in-situ pore size engineering from 1.1 nm to 1.5-2.0 nm. Moreover, the Cu-TpCOFs showed uniform and low percentage-doped (~ 1-1.5%) metal distribution and improved crystallinity, porosity, and stability compared to the parent Cu-Tp MOF. The construction of a framework from another framework with new linkages opens interesting opportunities for phase-engineering.

Eu4Al13Pt9 – a coloring variant of the Ho4Ir13Ge9 type structure

Investigations in the ternary system Eu–Al–Pt led to the discovery of Eu4Al13Pt9, a new representative exhibiting a coloring variant of the Ho4Ir13Ge9 type structure. The orthorhombic structure was refined based on single crystal X-ray diffraction data (Pmmn, Wyckoff sequence e 9 b 3 a 5, a = 415.38(1), b = 1149.73(2), c = 1994.73(5) pm, wR2 = 0.0622, 1901 F 2 values, 88 variables) and full atomic ordering was observed for all atoms. The structure features a complex [Al13Pt9] δ– network with the Eu atoms occupying hexagonal prismatic cavities. The bonding situation of this new platinide was investigated via quantum-chemical calculations. According to Density Functional Theory (DFT) the title compound has to be described as a polar intermetallic material with a covalently bonded [Al13Pt9] δ– polyanion showing strong Pt–Al alongside weak Al–Al and Pt–Pt bonding and Eu cations in the cavities.

Extending the knowledge on the quaternary rare earth nickel aluminum germanides of the RENiAl4Ge2 series (RE=Y, Sm, Gd–Tm, Lu) – structural, magnetic and NMR-spectroscopic investigations

The quaternary rare earth nickel aluminum germanide series RENiAl4Ge2 (RE = Y, Sm, Gd–Tm, Lu) has been extended by several members. The compounds were synthesized from the elements by arc-melting, and single crystals of YNiAl4Ge2, GdNiAl4Ge2, and LuNiAl4Ge2 were grown from an aluminum flux. All members crystallize isostructurally in the rhombohedral SmNiAl4Ge2-type structure (R3̅m, Z = 3). The compounds can be described as a stacking of RE δ+ and [NiAl4Ge2] δ− slabs with an ABC stacking sequence, or alternatively as stacking of CsCl and CdI2 building blocks. The results of the magnetic measurements indicate that all rare earth atoms are in a trivalent oxidation state. Of the RENiAl4Ge2 series, the members with RE = Sm, Gd–Dy exhibit antiferromagnetic ordering with a maximum Néel temperature of T N = 16.4(1) K observed for GdNiAl4Ge2. 27Al NMR spectroscopic investigations yielded spectra with two distinct signals, in line with the crystal structure, however, significantly different resonance frequencies of δ iso ms(YNiAl4Ge2) = 77(1) and 482(1) ppm as well as δ iso ms(LuNiAl4Ge2) = 90(1) and 467(1) ppm were observed. These indicate significantly different s-electron densities at the two crystallographically different Al atoms, in line with the results from DFT calculations. The Bader charge analysis confirms that the present compounds must be considered as germanides, as expected from the relative electronegativities of the constituent elements, while the low charges on Al and Y indicate significant covalent bonding.