Formation of the Sub-oxide Sc4Au2O1-x and the Drastically Negative 27Al NMR Shift in Sc2Al.

During attempts to synthesize Sc4AuAl in the cubic Gd4RhIn-type structure, the solid solution Sc2Au0.5Al0.5 in the PbCl2-type structure formed instead. Subsequently, the solid solution Sc2Au1-xAlx was investigated with respect to its existence range along with the structure types formed for different compositions with x = 0, 0.25, 0.5, 0.75, and 1. According to X-ray powder diffraction studies, Sc2Al and nominal Sc2Au0.25Al0.75 crystallized in the hexagonal Ni2In-type structure (P63/mmc), while Sc2Au0.5Al0.5, Sc2Au0.75Al0.25, and Sc2Au were found to crystallize in the orthorhombic PbCl2-type structure (Pnma). The crystal structures of Sc2Au and Sc2Au0.59(1)Al0.41(1) were refined from single-crystal data (Sc2Au: a = 648.0(1), b = 467.2(1), c = 835.2(2) pm, wR2 = 0.0382, 535 F2 values, 25 variables; Sc2Au0.59(1)Al0.41(1): a = 632.48(5), b = 472.16(3), c = 848.67(6) pm, wR2 = 0.0484, 540 F2 values, 21 variables). Contamination with air during the synthesis of Sc2Au led to the discovery of a compound adopting the cubic W4Co2C-type structure (stuffed cubic Ti2Ni type). Using Sc2O3 as a defined oxygen source led to samples with high amounts of Sc4Au2O1-x. All intermetallic compounds exhibited Pauli paramagnetic behavior in the investigated temperature range of 2.1 to 300 K, and no superconductivity was observed at low temperatures and low fields. Sc2Au and Sc2Al were investigated by 27Al and 45Sc solid-state NMR investigations. For Sc2Al, one signal was found in the 27Al NMR spectra in line with the crystal structure; however, an extremely negative resonance shift of δ = -673 ppm was observed. In both compounds, two Sc resonances were observed, in line with the proposed crystal structure. Finally, it was observed that the stability of Sc2Au in air is limited. This was investigated via thermal analysis and (temperature-dependent) powder X-ray diffraction. DFT calculations helped in assessing charge analysis, electronic properties, and chemical bonding.

Physical and Magnetocaloric Properties of TbPdAl2 and the Ferromagnetic Solid Solution Tb1-xLuxPdAl2 (x = 0.1-0.9).

TbPdAl2 and the solid solution Tb1-xLuxPdAl2 (x = 0.1-1) have been synthesized via arc-melting techniques using the elements as starting materials and crystallize in the orthorhombic MgCuAl2-type structure (Cmcm). As expected, the unit cell volumes decrease due to the lanthanide contraction from the Tb to Lu compounds, thus showing Vegard behavior because of the differences of the ionic radii of the trivalent rare-earth cations. TbPdAl2 orders ferromagnetically below TC = 85.5(5) K and shows partial magnetic saturation already at low fields. The magnetic phase transition has been additionally investigated by heat capacity measurements, showing a broadened λ anomaly at THC = 83.2(1) K. The electrical resistivity is almost linear above TC, indicating dominant electron-phonon interactions. Below the ordering temperature, electron-spin wave scattering with a ρ ∼ T2 behavior is evident. In the solid solution Tb1-xLuxPdAl2 (x = 0-1), the ferromagnetic Curie temperatures decrease in a linear fashion with increasing Lu content. Investigations of the magnetocaloric properties of TbPdAl2 obtained a maximum magnetic entropy change of -ΔSMmax = 1.2, 2.2, 3.1, and 3.6 J kg-1 K-1 for the field changes of ΔH = 10, 20, 50, and 70 kOe, respectively. The rather low values are caused by entropy losses due to hysteresis behavior. The relative cooling power for TbPdAl2, therefore, also exhibits comparably low values of 32, 81, 142, and 178 J kg-1.

Intermetallic RE6T5Al7 Phases (RE = Sc, Y, Ce-Nd, Sm, Gd-Lu; T = Ru, Ir): Diversity in their Magnetic, Magnetocaloric, and Critical Properties.

Over 20 new compounds of the RE6T5Al7 series (RE = Sc, Y, Ce-Nd, Sm, Gd-Lu; T = Ru, Ir; Yb6Ir5Ga7 type structure; superstructure of MgZn2; fully ordered Nb6.4Ir4Al7.6 type) have been synthesized. They crystallize in the hexagonal crystal system with space group P63/mcm. Their lattice parameters are in the ranges of a = 935-963 and c = 851-874 pm for the RE6Ru5Al7 and a = 913-966 and c = 825-865 pm for the RE6Ir5Al7 series. Four structures (Ho6Ru5Al7, Yb6Ru4.68(1)Al7.32(1), Sc6Ir5Al7, and Ho6Ir4.53(1)Al7.47(1)) have been refined from single-crystal data, indicating fully ordered structures for Ho6Ru5Al7 and Sc6Ir5Al7; however, T/Al mixing was observed on one crystallographic site for Ho6Ir4.53(1)Al7.47(1) and on two sites for Yb6Ru4.68(1)Al7.32(1). The Sc-, Y-, and Lu-containing compounds exhibit Pauli paramagnetism in line with a filled d band for Ru and Ir. Ce6Ru5Al7 exhibits mixed-valent behavior, while Yb6Ru5Al7 is solely trivalent. The other compounds exhibit paramagnetism and ferromagnetic phase transitions up to temperatures of TC = 83.4(1) K for Gd6Ru5Al7. In addition to the basic magnetic characterizations and studies of the electrical resistivity and heat capacity, the magnetocaloric properties of Gd6Ru5Al7, Tb6Ru5Al7, and Dy6Ru5Al7 have been investigated, revealing magnetic entropy changes of -ΔSMmax = 6.2(1), 7.7(1), and 5.4(1) J kg-1 K-1 (0 → 5 T) and relative cooling powers RCP = 242, 207, and 135 J kg-1, respectively. For a deeper insight into the second-order magnetic phase transitions, the critical behavior was investigated according to the scaling hypothesis. The critical behavior of Gd6Ru5Al7 is in accordance with the mean-field theory; the critical exponents of Tb6Ru5Al7 and Dy6Ru5Al7, however, deviate strongly from this universality class. For comparison of the structural and magnetic properties, the thus far unstudied members of the RERuAl series (RE = Sc, Y, Sm, Gd-Lu; MgZn2 type) have additionally been prepared and characterized.

Correlations of Crystal and Electronic Structure via NMR and X-ray Photoelectron Spectroscopies in the RETMAl2 (RE = Sc, Y, La-Nd, Sm, Gd-Tm, Lu; TM = Ni, Pd, Pt) Series.

A total of 35 intermetallic aluminum compounds have been synthesized from the elements via arc melting and characterized by powder X-ray diffraction. A total of 15 of them have been previously reported; however, detailed property investigations were missing. Compounds of the RETMAl2 (rare earth metal RE = Sc, Y, La-Nd, Sm, Gd-Tm, Lu) series with transition metal TM = Ni, Pd, and Pt crystallize isostructurally in the orthorhombic MgCuAl2 type structure ( Cmcm, oC16, fc2). Single-crystal X-ray diffraction investigations were conducted on YNiAl2, LaNiAl2, YPdAl2, ScPtAl2, and YPtAl2. The TM and Al atoms form a [TMAl2]δ- polyanion, the RE atoms reside in cavities within the framework. While the Sc, Y, La, and Lu compounds exhibit Pauli-paramagnetic behavior, consistent with all atoms being closed shell, the other RETMAl2 compounds show paramagnetism along with magnetic ordering at low temperatures, in line with an open-shell trivalent oxidation state for the RE atoms. Solid-state 27Al NMR investigations were carried out on the Pauli-paramagnetic samples, all showing only a single central transition, in line with one crystallographic site for the respective atoms. The observed quadrupolar coupling constants and electric-field-gradient asymmetry parameters were found to be in good agreement with the density-functional-theory-calculated values. Isotropic resonance shifts are dominated by the Fermi-contact interactions with s-conduction electron densities at the Fermi edge (Knight shifts). The bonding characteristics mirror the electronic density of states and crystal chemistry of the family of intermetallic compounds under consideration. Both the Knight shifts and quadrupolar coupling constants can be predicted based on element-specific increments.

Abrupt Europium Valence Change in Eu2Pt6Al15 around 45 K.

Eu2Pt6Al15 has been prepared from the elements via arc-melting and subsequent temperature treatment; the structure was refined from single crystal X-ray diffraction data. The compound crystallizes in an orthorhombic (3 + 1)D commensurately modulated structure (Sc2Pt6Al15 type) with space group Cmcm(α,0,0)0 s0 (α = 2/3). Full ordering of the Pt and Al atoms within the [Pt6Al15]δ- polyanion was observed. Magnetic measurements revealed an anomaly in the susceptibility data at T = 41.6(1) K, which was also observed as λ-type anomaly in heat capacity measurements ( T = 40.7(1) K). Temperature dependent powder X-ray diffraction experiments indicated a drastic shortening of the c axis (-18 pm, -1.1%) around 45 K, while the a axis nearly remains the same (-1 pm, -0.2%). Measurements of the electrical resistivity verified the anomaly, indicating a clear change in the electronic structure of the material. The observed anomalies in the physical measurements can be explained by a temperature driven first order valence change from Eu2+ at higher temperatures (>55 K) to Eu3+ at low temperatures. This valence change was proven by temperature dependent 151Eu Mössbauer spectroscopic investigations. Isostructural Eu2Pt6Ga15 was prepared in comparison, and it shows divalent Eu atoms down to 2.5 K along with antiferromagnetic ordering at TN = 13.1(1) K.