Sebastian Stein

A 25Mg, 89Y and 115In solid state MAS NMR study of YT2X and Y(T0.5T′0.5)2X (T/T′ = Pd, Ag, Au; X = Mg, In) Heusler phases

Yttrium-transition metal-magnesium (indium) Heusler phases YPd2Mg, YPd2In, YAg2Mg, YAg2In, YAu2Mg, and YAu2In and their quaternary compounds (solid solutions) Y(Pd0.5Ag0.5)2Mg, Y(Pd0.5Ag0.5)2In, Y(Pd0.5Au0.5)2Mg, Y(Pd0.5Au0.5)2In, Y(Ag0.5Au0.5)2Mg and Y(Ag0.5Au0.5)2In were synthesized from the elements in sealed niobium ampoules in a high-frequency furnace or by arc-melting, respectively. All compounds crystallize with the cubic MnCu2Al type structure (Heusler phase), space group Fm3[combining macron]m. The structure of Y(Ag0.39Au0.61)2Mg was refined from single crystal X-ray diffractometer data: a = 689.97(5) pm, wR2 = 0.0619, 52 F2 values, 6 parameters. Magnetic susceptibility measurements show Pauli paramagnetic behavior for all samples. The compounds were investigated by 25Mg, 89Y and 115In solid state MAS NMR spectroscopy. Large positive resonance shifts are observed for all nuclei. A review of the present data in the context of literature data on isotypic Heusler phases with Cd and Sn indicates that the 89Y shifts show a correlation with the electronegativity of the main group atoms (Mg, Cd, In, Sn). The solid solutions Y(Ag1-xTx)2Mg (x = 0.1, 0.25, 0.33, 0.5; T = Pd, Au) clearly show Vegard-like behavior concerning their lattice parameters, and their main group element resonance shifts arising from spin and orbital contributions are close to the interpolated values of the corresponding end-member compounds.

Ternary indides RE3T2In4 (RE=Dy–Tm; T=Pd, Ir)

Abstract The ternary rare earth transition metal-indides RE3T2In4 (RE=Dy–Tm; T=Pd, Ir) were obtained from high-temperature reactions in sealed niobium ampoules. These indides adopt a hexagonal structure of the Lu3Co1.87In4 type (space group P6̅), a ternary ordered superstructure of the aristotype Fe2P. The structures of three different compounds were refined from single-crystal X-ray diffractometer data: a=768.20(6), c=381.97(3) pm, 1441 F2 values, 24 parameters, wR2=0.0338 (Ho3Pd1.90In4); a=774.98(3), c=378.51(2) pm, 577 F2 values, 23 parameters, wR2=0.0742 (Ho3Ir1.69In4.31) and a=780.3(1), c=369.4(1) pm, 573 F2 values, 22 parameters, wR2=0.0403 (Tm3Ir1.51In4.49). Refinements of the occupancies revealed homogeneity ranges in case of the iridium-based crystals resulting from Ir/In mixing. The refined composition of the palladium compound was Ho3Pd1.90In4 resulting from defects on the Wyckoff position 1d, which was already reported for the prototype Lu3Co1.87In4. The geometrical motifs of the RE3T2In4 structures are three different types of tricapped trigonal prisms around the transition metal and indium atoms which are condensed via common edges and triangular faces. Temperature dependent magnetic susceptibility measurements of Dy3Ir2In4 and Tm3Ir2In4 showed Curie-Weiss behavior and the experimental magnetic moments of 10.59(2) μB (Dy3Ir2In4) and 7.40(2) μB (Tm3Ir2In4) confirming stable trivalent RE3+ states. Dy3Ir2In4 and Tm3Ir2In4 order antiferromagnetically with Néel temperatures of TN=13.6(5) and 5.4(5) K, respectively.

Magnetic and magnetocaloric properties of the coloured Heusler phases GdAg2Mg and REAgAuMg (RE=Gd, Tb, Dy)

Abstract The magnetocaloric effect (MCE) of the ferromagnetic compound GdAg2Mg [TC=98.3(5) K] was investigated along with its electrical resistivity and the specific heat capacity. The magnetic entropy changes (–ΔSM) as well as the changes in adiabatic temperature (ΔTad) have been calculated from these data. Furthermore, the magnetic susceptibility of the pseudo-quaternary Heusler phases GdAgAuMg, TbAgAuMg and DyAgAuMg [i.e. RE(Ag0.5Au0.5)2Mg] were measured and compared to the data for the pure silver and gold compounds REAg2Mg and REAu2Mg. The substitution of the transition metal at the crystallographic Wyckoff site 8c influences the magnetic ground state of the trivalent rare earth metals and therefore drastically alters the Curie temperatures. The structure of GdAgAuMg was refined from single crystal X-ray diffraction data, revealing a small deviation from the equiatomic composition leading to the refined formula GdAg0.92(6)Au1.08(6)Mg [space group Fm3̅m, Z=4, a=695.03(10) pm, wR2=0.0883, 55 F2 values, six parameters]. The intermetallic compounds were synthesised in sealed niobium ampoules under high temperature conditions. They have reddish to brassy colour.

Rare earth-copper-magnesium intermetallics: crystal structure of CeCuMg, magnetocaloric effect of GdCuMg and physical properties of the Laves phases RECu4Mg (RE=Sm, Gd, Tb, Tm)

Abstract The intermetallic magnesium compounds CeCuMg and GdCuMg as well as the ternary Laves phases RECu4Mg (RE=Sm, Gd–Tm) were synthesized from the elements by different annealing sequences in high-frequency and muffle furnaces using niobium ampoules as crucibles. All samples were characterized through the lattice parameters using X-ray powder diffraction (Guinier technique). Two structures were refined from single-crystal X-ray diffractometer data: a=764.75(6), c=414.25(4) pm, space group P6̅2m, wR2=0.0389, 338 F2 values, 15 parameters for CeCuMg (ZrNiAl type) and a=723.18(2) pm, space group F4̅3m, wR2=0.0818, 91 F2 values, eight parameters for Gd1.06(3)Cu4Mg0.94(3) (MgCu4Sn type). The Laves phase shows a small homogeneity range (Gd/Mg mixing). An investigation of the magnetocaloric effect (MCE) of ferromagnetic GdCuMg (ZrNiAl type; TC=82 K) revealed a magnetic entropy change of ΔSM=6.5 J kg−1 K−1 and a relative cooling power of RCP=260 J kg−1 for a field change from 0 to 70 kOe, classifying GdCuMg as a moderate magnetocaloric material for the T=80 K region. Of the Laves phases RECu4Mg, SmCu4Mg shows van-Vleck paramagnetism above a Néel temperature of 10.8(5) K, whereas GdCu4Mg and TbCu4Mg undergo antiferromagnetic phase transitions at about 48 and 30 K, respectively. TmCu4Mg shows Curie-Weiss behavior in the entire temperature range. The electrical resistivity of SmCu4Mg and the specific heat capacity of GdCu4Mg were measured for further characterization.

The alkaline earth-palladium-germanides Sr3Pd4Ge4 and BaPdGe

Abstract The germanides Sr3Pd4Ge4 and BaPdGe were obtained from high-temperature reactions in sealed niobium ampoules and their structures have been determined from single-crystal X-ray diffraction data: a=444.2(1), b=438.1(1), c=2472.2(7) pm, space group Immm, U3Ni4Si4 type, wR2=0.0471, 576 unique reflections, 25 parameters for Sr3Pd4Ge4 and a=677.09(8), space group P213, LaIrSi type, wR2=0.0322, 409 unique reflections, nine parameters for BaPdGe. Both germanides have pronounced three-dimensional [Pd4Ge4]δ− and [PdGe]δ− polyanionic networks with Pd–Ge bonding interactions. This is confirmed by the density functional theory (DFT)-based electronic structure investigations, the trends of charge transfer and crystal orbital overlap population (COOP) analyses.

Rows of corner- and face-sharing Mg4 tetrahedra arranged as hexagonal rod packing in the hexagonal Laves phases REMg2 and the rare earth-rich phases RE9CoMg4 (RE=Y, Dy-Tm, Lu)

Abstract Six new rare earth metal-rich intermetallic compounds RE9CoMg4 with RE=Y, Dy, Ho, Er, Tm and Lu were synthesized by induction-melting the elements in sealed niobium ampoules followed by annealing in muffle furnaces. The structures of Y9CoMg4 and Tm8.56CoMg4.44 were refined from single-crystal X-ray diffraction data: P63/mmc, a=965.65(6), c=971.07(5) pm, wR2=0.0599, 614 F2 values, 20 variables for Y9CoMg4 and a=945.20(4), c=953.11(5) pm, wR2=0.0358, 585 F2 values, 21 variables for Tm8.56CoMg4.44 (a small homogeneity range results from Tm/Mg mixing). The RE9CoMg4 phases crystallize with a coloring variant of the aristotype Co2Al5. The striking structural motif is a hexagonal rod packing of rows of corner- and face-sharing tetrahedral Mg4 clusters with Mg–Mg distances ranging from 304 to 317 pm in Y9CoMg4. These rows are similar to the hexagonal Laves phases REMg2. The space between the rows is filled with rows of face-sharing Co@Y6 trigonal prisms (TP) and empty Y6 octahedra (O) in the sequence –TP–O–O–. The many isopointal coloring variants of the aristotype Co2Al5 are briefly discussed.

Equiatomic indides REIrIn (RE=La, Pr, Nd, Er–Yb) – Crystal and electronic structure

Abstract The equiatomic rare earth iridium indides REIrIn (RE=La, Pr, Nd, Er–Yb) were synthesized by reaction of the elements in induction or muffle furnaces and were characterized through X-ray powder patterns. The structures of LaIr0.86In1.14, PrIr0.89In1.11, NdIr0.94In1.06, ErIrIn (all ZrNiAl type, P6̅2m), and YbIrIn (TiNiSi type, Pnma) were refined from single crystal X-ray diffractometer data. Refinements of the occupancy parameters revealed small degrees of solid solutions with indium substitution on the iridium sites. Chemical bonding analyses and electronic structure calculations indicate the dominance of metallic bonding in addition to partial ionic interactions between the cations and polyanions, as well as covalent contributions between the indium and iridium atoms.

GdCuMg with ZrNiAl-type structure – an 82.2 K ferromagnet

Abstract GdCuMg has been synthesized by induction-melting of the elements in a sealed niobium ampoule followed by annealing in a muffle furnace. The sample was studied by powder and single crystal X-ray diffraction: ZrNiAl type, P6̅2m (a=749.2(4), c=403.3(1) pm), wR2=0.0242, 315 F2 values and 15 variables. Temperature dependent magnetic susceptibility measurements have revealed an experimental magnetic moment of 8.54(1) μB per Gd atom. GdCuMg orders ferromagnetically below TC=82.2(5) K and based on the magnetization isotherms it can be classified as a soft ferromagnet.

Rare earth-ruthenium-magnesium intermetallics

Abstract Eight new intermetallic rare earth-ruthenium-magnesium compounds have been synthesized from the elements in sealed niobium ampoules using different annealing sequences in muffle furnaces. The compounds have been characterized by powder and single crystal X-ray diffraction. Sm9.2Ru6Mg17.8 (a=939.6(2), c=1779(1) pm), Gd11Ru6Mg16 (a=951.9(2), c=1756.8(8) pm), and Tb10.5Ru6Mg16.5 (a=942.5(1), c=1758.3(4) pm) crystallize with the tetragonal Nd9.34Ru6Mg17.66 type structure, space group I4/mmm. This structure exhibits a complex condensation pattern of square-prisms and square-antiprisms around the magnesium and ruthenium atoms, respectively. Y2RuMg2 (a=344.0(1), c=2019(1) pm) and Tb2RuMg2 (a=341.43(6), c=2054.2(7) pm) adopt the Er2RuMg2 structure and Tm3Ru2Mg (a=337.72(9), c=1129.8(4) pm) is isotypic with Sc3Ru2Mg. Tm3Ru2Mg2 (a=337.35(9), c=2671(1) pm) and Lu3Ru2Mg2 (a=335.83(5), c=2652.2(5) pm) are the first ternary ordered variants of the Ti3Cu4 type, space group I4/mmm. These five compounds belong to a large family of intermetallics which are completely ordered superstructures of the bcc subcell. The group-subgroup scheme for Lu3Ru2Mg2 is presented. The common structural motif of all three structure types are ruthenium-centered rare earth cubes reminicent of the CsCl type. Magnetic susceptibility measurements of Y2RuMg2 and Lu3Ru2Mg2 samples revealed Pauli paramagnetism of the conduction electrons.