REPd5Al2 compounds with RE = Ce–Gd as well as Y and Lu have been previously synthesized. Although some compounds with the small lanthanides also exist, the compounds with intermediate-sized rare-earth elements (RE = Tb–Yb) had not been prepared. We report on the missing members of the REPd5Al2 (RE = Tb–Yb) series as well as on the new REPt5Al2 (RE = Y, Gd–Tm, Lu) series, which we have synthesized and structurally as well as magnetically characterized. All members crystallize isostructurally in the ZrNi2Al5 type with an anti-arrangement of the T = Pd/Pt and Al atoms. YPd5Al2 and LuPd5Al2, as well as the respective platinum homologs, YPt5Al2 and LuPt5Al2, have been characterized also by 27Al magic-angle spinning nuclear magnetic resonance spectroscopy. Consistent with the XRD analysis, the spectra indicate the existence of only one distinct Al site in the structure.
Crystalline NpPd5Al2, discovered by Aoki and co-workers in 2007 , was grown from a Pb flux as the first neptunium-based superconductor with TC = 4.9 K. Griveau et al. reproduced the compound by arc-melting the pure elements and investigated it in great detail . UPd5Al2 was reported in the same year to exhibit Curie-Weiss behavior but with a nonmagnetic ground state , while PuPd5Al2 shows antiferromagnetic ordering at TN = 5.6 K . The first rare-earth compound, CePd5Al2, was reported to be a pressure-induced superconductor with a critical temperature of TC = 0.57 K at a pressure of 10.8 GPa [5, 6]. All of the mentioned compounds crystallize in the tetragonal ZrNi2Al5-type structure, with an anti-arrangement with respect to the T = Pd and Al positions. This type exhibits a high anisotropy of the crystal structure as well as of the band structure and the magnetic properties of the compounds. Intensive studies have been carried out regarding the physical properties of CePd5Al2 and its magnetic structure, showing highly anisotropic magnetic susceptibility along  with respect to , as well as a quasi two-dimensional band structure . In addition, the magnetic behavior of REPd5Al2 with RE = Y, Pr, Nd, Sm and Gd has been described . For reference, LuPd5Al2 was prepared and reported as well . Since YPd5Al2 and LuPd5Al2 exist, the rare-earth elements in between Gd and Lu should also crystallize with the 1-5-2 composition in the same structure type. The latter half of the rare-earth elements, however, had not been synthesized and investigated yet. Herein we report the synthesis and magnetic characterization of the missing rare-earth members with RE = Tb–Yb in the RET5Al2 (T = Pd) series as well as the isostructural RET5Al2 (T = Pt) series for the rare-earth elements RE = Y, Gd–Tm and Lu. We also discuss the crystal chemistry of these series by means of symmetry relations. In addition to the magnetic measurements, 27Al NMR investigations have also been performed in order to confirm the crystal structure of the diamagnetic representatives, since single-crystal X-ray crystallographic investigations have faced problems caused by the high anisotropy and by stacking faults along the crystallographic c axis.
2 Results and discussion
2.1 Crystal structures
In the standardized body-centered tetragonal unit cell, the rare-earth cations occupy the Wyckoff site 2b (4/mmm; 0, 0, 1/2) and are surrounded by 12 T atoms in the shape of a slightly distorted cuboctahedron, a structural motif found in, e.g., Cu3Au (Pm3̅m, Fig. 1, left). In RET5Al2, these cuboctahedra are condensed via four of their square faces to form infinite layers in the ab plane. The [RET3] slabs are intercalated by layers of Al atoms, which are also 12-fold coordinated by four Al and eight Pd atoms, again in the shape of a cuboctahedron (Fig. 1, right). While the Al layers form an A, A, … stacking in the crystal structure, the [RET3] layers are stacked in an A, B, A, … sequence along the crystallographic c axis (Fig. 2). Since the whole range of REPd5Al2 compounds (with the exception of La) exists, and also the RET3 series is complete with all rare-earth metals (RE = Sc, Y, La–Nd, Sm–Lu, [9, 10]), one might think that due to the common structural feature an existence of a RET3 phase also favors a 1-5-2 phase. Starting from the above mentioned concept, one can consider other RET3 binaries crystallizing in the Cu3Au-type structure and try to extend the structural regime of the 1-5-2-type structures. For T = Pt, also compounds with the whole range of rare-earth elements exist, with the exception of europium [9, 11–17]. We succeeded in the synthesis of the REPt5Al2 series (RE = Y, Gd–Tm, Lu), where again all known members crystallize in the tetragonal ZrNi2Al5-type structure (I4/mmm, Z = 2). The lattice parameters range from a = 416–409 and c = 1488–1462 pm for the palladium compounds and from a = 409–405 and c = 1501–1488 pm for the platinum compounds, respectively (Table 1). In Fig. 3, the lattice parameters and the unit cell volumes of the REPd5Al2 series are shown and in Fig. 4 for REPt5Al2. All compounds show decreasing lattice parameters and unit cell volumina as expected due to the lanthanide contraction. For the large rare-earth elements (RE = La–Sm) and platinum, however, the expected 1-5-2-type structure is not formed, but a new structure type with a complex, most likely modulated, structure has been found (Renner et al., in preparation).
|REPd5Al2||a (pm)||c (pm)||V (nm3)||REPt5Al2||a (pm)||c (pm)||V (nm3)|
The layered structure of the compounds causes significant problems when searching for single-crystal specimens for X-ray structure analysis. Although the samples contained numerous specimens that have the typical optical appearance of single crystals, the Laue photographs showed low quality. Long time annealing or synthesis under different conditions, e.g., in a high-frequency furnace, did not change the quality of the bulk sample or the single crystal specimens. Inoue et al. used single crystals of CePd5Al2 grown by the Czochralski method for the investigations of direction-dependent property measurements . Lacking the facilities of growing single crystals by this method, we were limited to the investigation of the arc-melted samples. All samples reported here exhibit peak broadening in the Guinier X-ray diffraction patterns as well as diffuse intensities for the investigated single-crystal specimens. The synthesized samples, however, are phase-pure as seen by powder X-ray diffraction and SEM/EDX (energy dispersive X-ray) measurements.
2.2 Structural considerations
The presented 1-5-2 compounds show structural features which also exist in the binary 1-3 compounds. We have investigated a potential correlation of the RET3 and RET5Al2 structures with the help of group-subgroup relationships between the Cu3Au- and ZrNi2Al5-prototypic structures. Both structure types exhibit layers of face-sharing [RET12] cuboctahedra (Fig. 1, left), whereas in ZrNi2Al5 the Al atoms are also coordinated in cuboctahedral fashion (Fig. 1, right). A different coloring of the atoms in Cu3Au along with an enlargement of the unit cell might lead to the ZrNi2Al5 type, as a superstructure of Cu3Au.
Cu3Au and RET3 (T = Pd, Pt) crystallize in the cubic space group Pm3̅m (Z = 1) with a∼ 400 pm. RET5Al2 (ZrNi2Al5 type) crystallizes with the tetragonal space group I4/mmm (Z = 2) with a∼ 400 and c∼ 1450 pm. Both Cu3Au and ZrNi2Al5 can be derived from a fcc cell (Cu, Fm3̅m); a direct relationship, however, does not exist. Figure 5 shows the group-subgroup relationship between the two structures, derived from the crystal structure of copper. Since there is no direct way to transform the cubic space group Pm3̅m (Cu3Au) into the tetragonal space group I4/mmm (ZrNi2Al5), the crystal structures cannot be related to each other from a group-subgroup point of view. While ZrNi2Al5 is one example for the ordered structure crystallizing in I4/mmm, a Zr2NiAl5 sample was found to crystallize in the primitive Cu3Au type with space group Pm3̅m , however, still with a statistical distribution. Therefore, both branches in the symmetry tree are accessible, RET5Al2 favoring the tetragonal one. Two Bärnighausen trees  for fcc superstructures from Cu to ZrAl3 have already been published [20, 21]. The transition from CuAu to ZrNi2Al5 or ZrAl3 is based on the removal of different mirror planes. This removal leads to two different superstructures with the same space group type but with different coloring of the atoms in the unit cell on different Wyckoff sites. A similar transition can be found when going from U3Si2 (P4/mbm) to HT-IrIn3 (P42/mnm) or Zr3Al2 (P42/mnm) by two different klassengleiche transitions of index 2 . The Bärnighausen formalism, showing the relationship between the Cu- and ZrNi2Al5-type structures, also yields additional interesting information. The translationengleiche (t) transition of index 3 from Cu to In is responsible for the formation of trillings, while the two klassengleiche (k) transitions are accountable for the formation of anti-phase boundaries. These two features might be the reason for the difficulties observed when growing large single crystals of these materials.
2.3 27Al MAS-NMR investigations
For structure validation, the diamagnetic representatives YT5Al2 and LuT5Al2 (T = Pd, Pt) were studied by 27Al magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. All samples were measured at 11.74 and 9.4 T; additional data at 7.05 T was acquired for the yttrium compounds. The experimental data and their line shape simulations are summarized in Figs. 6 and 7, and the interaction parameters extracted from them are listed in Table 2. All recorded spectra exhibited only one main signal, as expected from the proposed prototypic structure with only one Al site. Due to the interaction of the conduction electrons with the local moments of the nucleus, a strong resonance shift, known as Knight shift, is observed. These Knight shifts are obtained from the positions of the central MAS NMR peaks, which originate from the +1/2 ↔ –1/2 Zeeman transitions. The palladium compounds show significantly larger Knight shifts than the platinum compounds. The values measured are comparable to those reported in the literature for various transition metal trialuminides [23–25] and for the Zintl compound Ca14AlP11 , but they are significantly smaller than those determined for metallic aluminium , Mg-Al solid solutions  or Cu1–xAl2 intermetallics .
|δiso (ppm)||CQ (MHz)||ηQ||FWHM (Hz)||G/L||νrot (kHz)||p1 (μs)||ϕ (deg.)||d1 (s)|
From Fig. 7, it is evident that both palladium compounds also exhibit severe line broadening effects, suggesting significant local disorder. Sample annealing did not change the appearance of the spectra. To examine the origin of these line broadening effects, the full width at half maximum (FWHM) was carefully studied as a function of applied magnetic field strength. While Knight shift distribution effects would predict the line widths (in Hz) to increase linearly with field strength, second-order quadrupolar broadening effects would result in an inverse field strength dependence. Table 2 reveals that these field dependent line widths do not exhibit any clear trend, suggesting the latter to originate from a composite effect of both broadening mechanisms.
In addition to the dominant central Zeeman transitions, the spectra show wide spinning sideband manifolds, which originate from the outer ±1/2 ↔ ±3/2 and ±3/2 ↔ ±5/2 Zeeman transitions, which are anisotropically broadened by first-order nuclear electric quadrupolar perturbations. Since the coordination environment of the Al atoms is not spherical, the quadrupole moment of the 27Al nucleus interacts with the local electrical field gradient, which results in the specific form of the NMR signal and the resulting spinning sideband pattern. From the intensity distributions of these patterns, the quadrupolar coupling constants CQ and the electric field gradient asymmetry parameters ηQ were obtained via simulations using the Dmfit program . Because of resonance offset effects, matching the experimental spinning sideband profiles within a ±200.000 Hz frequency range of the central resonance was emphasized during the fitting process; at higher resonance offsets, bandwidth limitations cause inevitable attenuation of experimental peak intensities.
2.4 Magnetic properties
Magnetic ordering in the palladium compounds has been reported for CePd5Al2 from susceptibility measurements (TN1 = 4.1 K; TN2 = 2.9 K), and features of ordering have been found for NdPd5Al2 (TN = 1.2 K), SmPd5Al2 (TN = 1.7 K) and GdPd5Al2 (TN = 6 K) using CP measurements, while for PrPd5Al2 no long-range magnetic ordering phenomenon was found down to 0.3 K . Due to the low amount of rare-earth cations in the structure and their large interatomic distances (e.g., d(Ce–Ce) = 416 pm ), only few compounds exhibit magnetic ordering phenomena above 2.5 K. We have investigated the REPd5Al2 (RE = Tb–Yb) and REPt5Al2 (RE = Y, Gd–Tm) series using a quantum design physical property measurement system (PPMS) without a 3He option, thus being able to measure the samples down to only 2.5 K. For the palladium series, TbPd5Al2 and DyPd5Al2 exhibit antiferromagnetic ordering at TN = 9.8(1) and TN = 3.7(1) K, respectively. The smaller rare-earth elements, however, exhibit no magnetic ordering. Figure 8 (top) shows the temperature dependence of the magnetic and inverse magnetic susceptibility (χ and χ–1 data) of DyPd5Al2 measured at 10 kOe (1 kOe = 7.96 × 104 A m–1). A fit of the χ–1 data between 50 and 305 K using the Curie–Weiss law resulted in an effective magnetic moment of μeff = 10.56(1) μB per dysprosium atom and a Weiss constant of θp = –17.8(5) K. The effective magnetic moment agrees well with the theoretical value of 10.65 μB for a free Dy3+ ion. The negative value of the Weiss constant is giving rise to an antiferromagnetic long-range interaction. The saturation magnetization (μsm = 4.84 μB) observed in the magnetization isotherms is drastically smaller than the expected saturation magnetization [(Dy3+) = 10 μB] according to gJ × J. One reason might be that the magnetization isotherm was recorded at 3 K, which is close to the ordering temperature of TN = 3.7(1) K.
Within the platinum series, no sample exhibits magnetic ordering in the observed temperature range. However, all χ–1 data could be fitted using the Curie–Weiss law χ = C/(T–θ) in the temperature range of 50–300 K. Table 3 summarizes the magnetic data. All compounds exhibit effective magnetic moments μeff close to the expected theoretical moments. YPt5Al2 exhibits diamagnetic behavior, no evidence for superconductivity being found down to 2 K using an applied field of H = 50 Oe. The zero-field-cooled (ZFC) measurement (H = 10 kOe) exhibits a diamagnetic susceptibility of χDia ∼ 2.6 × 10–4 emu mol–1 (Fig. 9). In Fig. 10, the ZFC measurement (H = 10 kOe) from 3–305 K, the ZFC/FC investigations (H = 100 Oe) and the magnetization isotherms (recorded at 5, 25, 50 and 75 K) for TbPt5Al2 are shown. As expected, the magnetization isotherms at 50 and 75 K show a linear increase of the magnetization. At 25 K a slight and at 5 K a pronounced curvature of the magnetization can be seen with saturation at high fields. The isotherms appear to be typical Brillouin functions with field-induced saturation of the paramagnet at low temperatures and high fields. However, the saturation magnetization (μsm = 6.61 μB) is significantly smaller than the expected saturation magnetization [(Tb3+) = 9 μB] according to gJ × J.
|Compound||μeff (μB)||θ (K)||Compound||μeff (μB)||θ (K)|
|–||–||–||YPt5Al2||Diamagnetic – no SC found|
In the field of transition metal-rich rare-earth aluminides, the range of REPd5Al2 compounds crystallizing in the ZrNi2Al5-type structure could be extended to include the elements RE = Tb–Yb. The compounds with RE = Y, Ce–Gd and Lu had been prepared previously. In addition, the platinum analogues REPt5Al2 with RE = Y, Gd–Tm and Lu have now been prepared and investigated. TbPd5Al2 and DyPd5Al2 exhibit magnetic ordering (Tb: TN = 9.8(1) K, Dy: TN = 3.7(1) K), while the other compounds, including the Pt series, do not show any ordering phenomena down to 3 K. A fit of the paramagnetic region reveals the full magnetic moment of the respective rare-earth element. Crystallization of the compounds in the arc furnace causes significant problems, and also annealing of the samples does not improve the quality of the single crystals significantly. This is most likely due to the large c/a ratio of about 3.6–3.7 along with the possible formation of trilling and anti-phase boundaries. With the help of a Bärnighausen formalism, the group-subgroup relationship between Cu and the ZrNi2Al5-type structure was elucidated. Since single-crystal diffraction was not possible, powder X-ray diffraction was used to identify the structure type and refine the lattice parameters. The unit cells of both series shrink linearly following the lanthanide contraction. 27Al MAS-NMR experiments were conducted on YT5Al2 and LuT5Al2 (T = Pd, Pt). The presence of only one signal is consistent with one unique crystallographic Al site in the crystal structure.
4 Experimental section
The RET5Al2 compounds were synthesized from the elements, using rare-earth metal ingots (ChemPur, Smart Elements; all with stated purities higher than 99%), platinum powder (Degussa, 99.9%) palladium pieces (Allgussa, 99.9%) and aluminium turnings (Koch Chemicals, 99.99%). Pieces of the rare-earth ingot were first arc-melted  under an argon pressure of 800 mbar in a water-cooled copper hearth. The platinum powder was pressed into pellets with a diameter of 6 mm. The starting materials were arc-melted under argon at 800 mbar. The obtained button was remelted and turned over several times to increase the homogeneity. Fragments of the crushed buttons were sealed in quartz tubes under vacuum and annealed at 800 °C for 7 days. The samples show metallic luster and a pronounced alignment of the crystallites within the button. They are stable in air over months.
4.2 Powder X-ray diffraction
The polycrystalline samples were characterized by Guinier patterns (imaging plate detector, Fujifilm BAS-1800 scanner) with CuKα1 radiation using α-quartz (a = 491.30, c = 540.46 pm, Riedel-de-Häen) as an internal standard. Correct indexing of the diffraction lines was ensured through intensity calculations . The lattice parameters were obtained through least-squares fits with standard deviations smaller than ±0.1 pm for the a axis and ±0.4 pm for the c axis .
4.3 EDX data
Semiquantitative EDX analyses on all bulk samples were carried out on a Leica 420i scanning electron microscope. The polycrystalline pieces from the arc-melted buttons or from the annealed pieces were embedded in a methylmethacrylate matrix and polished with diamond and SiO2 emulsions of different particle sizes. The experimentally observed compositions were close to the weighed ones, and phase pure samples within the limitations of the instrument were observed after annealing. No impurity elements heavier than sodium (detection limit of the instrument) were observed.
4.4 Magnetic properties
Polycrystalline pieces of the annealed ingots were packed in kapton foil and attached to the sample holder rod of a vibrating sample magnetometer unit for measuring the magnetization M(T,H) in a quantum design PPMS. The samples were investigated in the temperature range of 2.5–305 K and with magnetic flux densities up to 80 kOe (1 kOe = 7.96 × 104 A m–1).
4.5 27Al MAS-NMR spectroscopy
The 27Al MAS-NMR spectra were recorded at 130.287, 104.230 and 78.722 MHz on Bruker DSX 500, Bruker AVANCE 400 and Bruker AVANCE III 300 spectrometers using magic-angle spinning (MAS) conditions. The samples synthesized by arc-melting were ground to a fine powder and mixed with an equivalent amount of SiO2 to reduce the density and the electrical conductivity of the sample. The diluted samples were loaded into a cylindrical ZrO2 rotor with a diameter of 2.5 mm. Single-pulse experiments were conducted with a 1 molar Al(NO3)3 solution in H2O set to 0 ppm as a reference. The NMR spectra were recorded using the Bruker Topspin software , and the analysis was performed with the help of the Dmfit software .
We would like to thank Dr. C. Schwickert for discussions about the magnetic properties and Dr. R.-D. Hoffmann for helpful discussions about the group-subgroup relationships.
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