Crystal Structure , Spectroscopic Investigations , and Physical Properties of the Ternary Intermetallic RE Pt 2 Al 3 ( RE = Y , Dy – Tm ) and RE 2 Pt 3 Al 4 Representatives ( RE = Tm , Lu )

The REPt2Al3 compounds of the late rare-earth metals (RE = Y, Dy–Tm) were found to crystallize isostructural. Single-crystal X-ray investigations of YPt2Al3 revealed an orthorhombic unit cell (a = 1080.73(6), b = 1871.96(9), c = 413.04(2) pm, wR2 = 0.0780, 942 F2 values, 46 variables) with space group Cmmm (oC48; q2pji2hedb). A comparison with the Pearson database indicated that YPt2Al3 forms a new structure type, in which the Pt and Al atoms form a [Pt2Al3] polyanion and the Y atoms reside in the cavities within the framework. Via a group-subgroup scheme, the relationship between the PrNi2Al3-type structure and the new YPt2Al3-type structure was illustrated. The compounds with RE = Dy–Tm were characterized by powder X-ray diffraction experiments. While YPt2Al3 is a Pauli-paramagnet, the other REPt2Al3 (RE = Dy–Tm) compounds exhibit paramagnetic behavior, which is in line with the rare-earth atoms being in the trivalent oxidation state. DyPt2Al3 and TmPt2Al3 exhibit ferromagnetic ordering at TC = 10.8(1) and 4.7(1) K and HoPt2Al3 antiferromagnetic ordering at TN = 5.5(1) K, respectively. Attempts to synthesize the isostructural lutetium compound resulted in the formation of Lu2Pt3Al4 (Ce2Ir3Sb4-type, Pnma, a = 1343.4(2), b = 416.41(8), c = 1141.1(2) pm), which could also be realized with thulium. The structure was refined from single-crystal data (wR2 = 0.0940, 1605 F2 values, 56 variables). Again, a polyanion with bonding Pt–Al interactions was found, and the two distinct Lu atoms were residing in the cavities of the [Pt3Al4] framework. X-ray photoelectron spectroscopy (XPS) measurements were conducted to examine the electron transfer from the rare-earth atoms onto the polyanionic framework.


Introduction
In the field of intermetallic compounds [1,2], some structure types are found with an impressive number of entries listed in the Pearson database [3].Amongst them are the binary Laves phases of the MgCu 2 -type (Fd3m) [4] and MgZn 2 -type (P6 3 /mmc) [5] structures (together with more than 5500 entries), the cubic Cu 3 Au-type (Pm3m, >1950 entries) structures [6], and the hexagonal CaCu 5 -type (P6/mmm, >1650 entries) structures [7].For ternary intermetallic compounds, the tetragonal body-centered ThCr 2 Si 2 -type (I4/mmm, >3250 entries) [8], the orthorhombic TiNiSi-type (Pnma, >1550 entries), and the hexagonal ZrNiAl-type (P62m, >1450 entries) [9] representatives show a broad variety of compounds with numerous, different elemental combinations.The structures and physical properties of the equiatomic RETX (RE = rare-earth element, T = transition metal, X = element of group [12][13][14][15] representatives have been recently summarized in a series of review articles [10][11][12][13]. Derived from the binary CaCu 5 -type structure, two prototypic ternary representatives with different chemical compositions have been reported: the CeCo 3 B 2 - [14] and the PrNi 2 Al 3 -type [15] structures.From a crystal chemical point of view, YNi 2 Al 3 is also worth mentioning [16], because this compound can be considered to be an i3-superstructure of the PrNi 2 Al 3 -type structure.Recently, an i7-superstructure of PrNi 2 Al 3 has also been reported, which was also found for ErPd 2 Al 3 [17].Our interests in the compounds of the REPt 2 Al 3 series originate from the fact that only CePt 2 Al 3 (PrNi 2 Al 3 -type) has been reported previously [18].Therefore, we synthesized and characterized the missing members of the REPt 2 Al 3 series with the late, small rare-earth elements.From a basic research point of view, investigations of the magnetic ground state of the open f -shell rare-earth atoms are also of great interest.

Synthesis
The starting materials for the synthesis of the REPt 2 Al 3 and RE 2 Pt 3 Al 4 samples were pieces of the sublimed rare-earth elements (Y, Dy-Tm, and Lu from Smart Elements), platinum sheets (Agosi), and aluminum turnings (Koch Chemicals), all with stated purities better than 99.9%.For the REPt 2 Al 3 compounds (RE = Y, Dy-Tm), the elements were weighed in the ideal 1:2:3 atomic ratio and arc-melted [19] in a water-cooled copper hearth under 800 mbar of argon pressure.The argon gas was purified with a titanium sponge (873 K), molecular sieves, and silica gel.Re-melting of the obtained buttons from each site several times enhanced the homogeneity.The as-cast buttons of the yttrium compound were crushed, and the fragments were sealed in quartz ampoules, placed in the water-cooled sample chamber of a high-frequency furnace (Typ TIG 5/300, Hüttinger Elektronik, Freiburg, Germany) [20], and heated until a softening of the piece was observed.The power was subsequently reduced by 10%, and the sample was kept at this temperature for 120 min before being cooled to room temperature.The other samples were annealed in muffle furnaces.They were heated to 1223 K and then kept at this temperature for 14 days, followed by slow cooling until they reached 573 K. Afterwards, the furnace was switched off.These different annealing procedures led to X-ray pure samples suitable for physical properties measurements.For the RE 2 Pt 3 Al 4 compounds (RE = Tm, Lu), the elements were weighed in the ideal 2:3:4 atomic ratio and arc-melted as described above.Again, an annealing step in a high-frequency furnace was subsequently conducted.The specimens are stable in air over weeks and show metallic luster; the ground samples are grey.

X-ray Image Plate Data and Data Collections
The polycrystalline samples were characterized at room temperature by powder X-ray diffraction on a Guinier camera (equipped with an image plate system, Fujifilm, Nakanuma, Japan, BAS-1800,) using Cu Kα 1 radiation and α-quartz (a = 491.30,c = 540.46pm, Riedel-de-Haën, Seelze, Germany) as an internal standard.The lattice parameters (Table 1) were obtained from a least-squares fit.Proper indexing of the diffraction lines was ensured by an intensity calculation [21].
Irregularly shaped crystal fragments of the YPt 2 Al 3 and Lu 2 Pt 3 Al 4 compounds were obtained from the annealed crushed buttons.The crystals were glued to quartz fibers using beeswax, and their quality was checked by Laue photographs on a Buerger camera (white molybdenum radiation, image plate technique, Fujifilm, Nakanuma, Japan, BAS-1800) for intensity data collection.The datasets were collected on a Stoe StadiVari four-circle diffractometer (Mo-K α radiation (λ = 71.073pm); µ-source; oscillation mode; hybrid-pixel-sensor, Dectris Pilatus 100 K [22]) with an open Eulerian cradle setup.Numerical absorption correction along with scaling was applied to the datasets.All relevant crystallographic data, deposition, and details of the data collection and evaluation are listed in Tables 2-8.Further details of the crystal structure investigation may be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: +49-7247-808-666; E-Mail: crysdata@fiz-karlsruhe.de, http://www.fiz-karlsruhe.de/request_for_deposited_data.html)by quoting the depository numbers CSD-434174 (YPt 2 Al 3 ) and CSD-434175 (Lu 2 Pt 3 Al 4 ).

Energy Dispersive X-ray Spectroscopy (EDX) Data
The crystals measured on the diffractometer were analyzed semi-quantitatively using a Zeiss EVO MA10 scanning electron microscope with YF 3 , TmF 3 , LuF 3 , Pt, and Al 2 O 3 as standards.No impurity elements heavier than sodium (the detection limit of the instrument) were observed.

Magnetic Properties Measurements
Fragments of the annealed buttons of the X-ray pure REPt 2 Al 3 phases were attached to the sample holder rod of a vibrating sample magnetometer (VSM) unit using Kapton foil for measuring the magnetization M(T, H) in a Quantum Design physical property measurement system (PPMS).The samples were investigated in the temperature range of 2.5-300 K with external magnetic fields up to 80 kOe.The magnetic data are summarized in Table 9.

X-ray Photoelectron Spectroscopy (XPS)
XPS was performed using an ESCALAB 250 Xi instrument (Thermo Fisher, East Grinsted, UK) with mono-chromatized Al Kα (hν = 1486.6eV) radiation.All samples were cleaned by Ar + sputtering (MAGCIS ion gun, 36 keV) for 60 s to remove adventitious carbon.High-resolution spectra were measured with pass energies of 10 eV (Pt 4f, Al 2s, Al 2p, and C 1s) and 20 eV (Y 3d and Pr 3d).Peak deconvolution was performed using a Gaussian-Lorentzian peak shape by the software Avantage (Thermo Fisher).All spectra were referenced to remaining adventitious carbon at 284.8 eV.Because of the overlap of the Pt 4f and Al 2p signals, Al 2s was used for Al quantification.The obtained data are summarized in Table 10.

Results and Discussion
During attempts to synthesize aluminum intermetallics with the composition REPt 2 Al 3 , well-resolved X-ray powder patterns for the small rare-earth elements RE = Y, Dy-Tm were observed.For the thulium compound, additional reflections showed up in the unannealed sample, which were initially interpreted as impurities.Subsequently, single crystals from the yttrium sample were isolated and structurally investigated (vide infra).The large and early rare-earth elements (RE = La-Nd, Sm, Gd, and Tb) do not form the same structure type.Investigations on the structures formed by these elements are still ongoing.Attempts to synthesize LuPt 2 Al 3 also yielded a diffraction pattern different from the slightly larger rare-earth elements Dy-Tm.As cast specimen, TmPt 2 Al 3 and LuPt 2 Al 3 were subsequently investigated by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDX).The impurity phase in TmPt 2 Al 3 and the main phase in nominal LuPt 2 Al 3 were found to be Tm 2 Pt 3 Al 4 and Lu 2 Pt 3 Al 4 , respectively.Finally, samples with these compositions were prepared, and single crystals from Lu 2 Pt 3 Al 4 were isolated and investigated.

Structure Refinements
A careful analysis of the obtained intensity dataset of YPt 2 Al 3 revealed an orthorhombic C-centered lattice.The centrosymmetric group Cmmm was found to be correct during structure refinement.A systematic check of the Pearson database [3], using Pearson code oC48 and Wyckoff sequence q 2 pji 2 hedb, gave no matches; hence, YPt 2 Al 3 must be considered a new structure type.The starting atomic parameters were obtained using SuperFlip [23], implemented in Jana2006 [24,25].The structure was refined on F 2 with anisotropic displacement parameters for all atoms.As a check for the correct composition and site assignment, the occupancy parameters were refined in a separate series of least-squares cycles.All sites were fully occupied within three standard deviations.No significant residual peaks were evident in the final difference Fourier syntheses.At the end, the positional parameters were transformed to the setting required for the group-subgroup scheme discussed below.Figure 1 depicts the X-ray powder diffraction pattern of YPt 2 Al 3 along with the calculated pattern obtained using the positional information from the single-crystal structure refinement.
Lu 2 Pt 3 Al 4 was also found to crystallize in the orthorhombic crystal system with space group Pnma.A comparison with the Pearson database [3], using Pearson code oP36 and Wyckoff sequence c 9 , indicated isotypism with Ce 2 Ir 3 Sb 4 [26,27].The structure was refined on F 2 with anisotropic displacement parameters for all atoms.As a check for the correct composition and site assignment, the occupancy parameters were refined in a separate series of least-squares cycles.All sites were fully occupied within three standard deviations.No significant residual peaks were evident in the final difference Fourier syntheses.In the powder X-ray diffraction experiments, trace amounts of TmPtAl or LuPtAl (TiNiSi-type) were evident.Thermal treatment was not able to remove these impurities.The details of the structure refinement, final positional parameters, and interatomic distances are listed in Tables 2-8.The isostructural aluminum compounds of the REPt 2 Al 3 series (RE = Y, Dy-Tm) crystallize in the orthorhombic crystal system, space group Cmmm, Pearson code oC48 and Wyckoff sequence q 2 pji 2 hedb.The lattice parameters (Figure 2) and unit cell volumes (Table 1) decrease from the dysprosium to the thulium compound, as expected, from the lanthanide contraction.The lattice parameters of the yttrium compound are in the same range, explainable by the similar ionic radii (Y 3+ : 90 pm; Dy 3+ : 91 pm; Ho 3+ : 90 pm [28]).
As YPt 2 Al 3 was investigated by single-crystal X-ray diffraction experiments, its crystal structure will be used for the structural discussion.A view of the crystal structure along the crystallographic c axis is depicted in Figure 3.The crystal structure features a polyanionic [Pt 2 Al 3 ] δ-network and shows full Pt/Al ordering.The heteroatomic Pt-Al distance range from 253 to 261 pm indicates substantial Pt-Al bonding, because these distances are in the range of the sum of the covalent radii for Pt+Al of 129 + 125 = 254 pm [29].The polyanionic networks of YPtAl (TiNiSi-type) [30] and Y 4 Pt 9 Al 24 (Y 4 Pt 9 Al 24 -type) [31] show similar distances of 257-269 and 246-274 pm, respectively.Additionally, homoatomic Al-Al distances ranging from 274 to 280 pm, and Pt-Pt distances of 301 pm can be found.The latter distances are slightly longer compared to what is found in elemental Pt (Cu-type, 284 pm) [32], while the aluminum distances are in line with elemental Al (Cu-type, 286 pm) [33].Three crystallographically distinct Y 3+ cations can be found in the cavities of the polyanion.They exhibit 18-fold coordination environments in the shape of six-fold-capped hexagonal prisms (Figure 4).A view of the unit cell along the c axis readily reminds us of the ternary CaCu 5 -type derivatives PrNi 2 Al 3 [15], YNi 2 Al 3 [16], DyNi 4 Si [34], CeCo 3 B 2 [14], and the recently found i7 superstructure of PrNi 2 Al 3 [17].Recoloring in intermetallics is found quite frequently, often accompanied by distortions and puckering within the respective structures [35].These structural effects between different structure types can be investigated by so-called group-subgroup relations.The structures of PrNi 2 Al 3 and YPt 2 Al 3 are related by such a group-subgroup scheme, which is presented in the Bärnighausen formalism [36][37][38][39] in Figure 5.In the first step, an isomorphic symmetry reduction of index 4 takes place, which causes a doubling of the a and b axis, along with a splitting of the Pr (1a to 1a and 3f ), Ni (2c to 2c and 6l), and Al (3g to 6k and 6m) sites.In the second step a translationengleiche transition of index 3 takes place, reducing the hexagonal symmetry from space group P6/mmm to orthorhombic Cmmm.Again, a splitting of the crystallographic position occurs along with the introduction of additional degrees of freedom regarding the crystallographic positions.This enables a distortion of the polyanion and a recoloring of the crystallographic sites.The Y1 atoms finally occupy the 2d rather than the 2a site as suggested by the group-subgroup scheme.Hence, they are shifted by 1/2 z compared to the original position.The same shift is also observed in YNi 2 Al 3 [16,35] and i7-PrNi 2 Al 3 [17].Refinement as orthorhombic trilling, as suggested by the translationengleiche symmetry reduction of index 3, is not necessary because the orthorhombic crystal system was found directly by the indexing routine.

Magnetic Properties
Magnetic susceptibility data has been obtained for the X-ray pure REPt 2 Al 3 samples with RE = Y, Dy-Tm.The basic magnetic parameters that have been derived from these measurements are listed in Table 9.The temperature dependence of the magnetic susceptibility of the yttrium compound is depicted in Figure 9. YPt 2 Al 3 is a Pauli-paramagnetic material with a room temperature susceptibility of χ = 1.85(1) × 10 -4 emu mol -1 .The weak upturn at lower temperature arises from small amounts of paramagnetic impurities.The present data clearly proves the absence of local moments on all constituent atoms.Thus, the magnetic properties of the remaining phases arise solely from the rare-earth elements.The magnetic properties of DyPt 2 Al 3 , HoPt 2 Al 3 , ErPt 2 Al 3 , and TmPt 2 Al 3 have been depicted in Figures 10-13.The top panels always depict the susceptibility and inverse susceptibility data (χ and χ -1 ).The effective magnetic moments have been obtained from fitting the χ -1 data using the Curie-Weiss law between 50 and 300 K.They were calculated from the Curie constant [40,41].All rare-earth atoms are in the trivalent oxidation state; the effective magnetic moments compare well within the calculated moments, as stated in Table 9.The calculated moments are tabulated [40,41] or can be calculated according to µ calc = g J(J + 1) with g = 1 + J(J+1)+S(S+1)−L(L−1) 2J(J+1) [40,41].Because a positive Weiss constant of θ P is observed for the antiferromagnetically ordered compounds, the ordering phenomena could be a so-called Type-A antiferromagnetic ground state.In this ordered state, the intra-plane coupling is ferromagnetic while inter-plane coupling is antiferromagnetic [42].From the zero-field-cooled/field-cooled (ZFC/FC) measurements depicted in the middle panels, it is evident that DyPt 2 Al 3 and TmPt 2 Al 3 exhibit ferromagnetic ordering at Curie temperatures of T C = 10.8(1) and 4.7(1) K due to the plateau-like susceptibility at low temperatures.ErPt 2 Al 3 exhibits no magnetic ordering down to 2.5 K, while HoPt 2 Al 3 finally orders antiferromagnetically at T N = 5.5(1) K, characterized by decreasing susceptibility below the Néel temperature.The Curie temperatures were obtained from the derivatives dχ/dT of the field-cooled curves (depicted in red) by determination of the temperature at the minimum in the derivative curve.The bottom panels finally display the magnetization isotherms measured at 3, 10, and 50 K.The 3 K isotherms of DyPt 2 Al 3 and TmPt 2 Al 3 show a fast increase at low fields, in line with the ferromagnetic ground state.The 3 K isotherm of HoPt 2 Al 3 displays a slightly delayed increase, suggesting a spin-reorientation, in line with a weak antiferromagnetic ground state.The 3 K isotherm of DyPt 2 Al 3 displays small 'wiggles', suggesting trace impurities, which are hardly noticeable in the ZFC/FC measurements.In the 3 K isotherm of HoPt 2 Al 3 , a small bifurcation is visible, also suggesting trace impurities, visible around 3 K in the ZFC/FC measurements.The isotherms at 50 K are all linear, in line with paramagnetic materials.The saturation magnetizations determined at 3 K and 80 kOe are all below the calculated values according to g J × J (Table 9).The extracted values are, in all cases, lower than the expected moments, suggesting that the applied external field is not strong enough to achieve full parallel spin ordering.Table 9. Magnetic properties of the YPt 2 Al 3 -type compounds.T N , Néel temperature; T C , Curie temperature; µ eff , effective magnetic moment; µ calc , calculated magnetic moment; θ p , paramagnetic Curie temperature; µ sat , saturation moment; and saturation according to g J × J.The experimental saturation magnetizations were obtained at 3 K and 80 kOe.

X-ray Photoelectron Spectroscopy
The reported compounds were described by rare-earth cations located in the cavities of a polyanion.Hence, the rare-earth atoms transfer electron density to the framework.This is in line with the effective magnetic moments of the rare-earth cations, proving them to be formally in a trivalent oxidation state.When looking at the electronegativities χ of the constituting elements of the REPt 2 Al 3 series, it is evident that platinum is by far the most electronegative element.According to the Pauling scale, the values are as follows: χ(Y) = 1.22,χ(Dy) = 1.22,χ(Ho) = 1.23, χ(Er) = 1.24, χ(Tm) = 1.25, χ(Pt) = 2.28, and χ(Al) = 1.61 [30].Because all reported compounds are of a metallic nature, a distinct ionic platinide character as found in A 2 Pt (A = K [43], Rb [43], Cs [43,44]) is highly unlikely, especially when considering the three-dimensional framework with strong covalent bonding character formed by Pt and Al.Therefore, XPS measurements were performed to investigate exemplarily the platinide character of YPt 2 Al 3 along with the reference substances YPt 5 Al 2 (anti-ZrNi 2 Al 5 -type [45]), YPtAl (TiNiSi-type [31]), and elemental Pt.
The obtained binding energies are listed in Table 10. Figure 14  ).This can be explained by a higher electron density at the Pt atoms, in line with an electron transfer from the less electronegative Y and Al atoms.The existing literature [46] shows shifts of the Pt 4f 7/2 signal towards higher binding energies for the binary phases PtAl and PtAl 2 (PtAl: 71.6, PtAl 2 : 72.1 eV), which can be explained by the bond formation between Pt and Al.In the ternary compounds, the additional electron transfer from the rare-earth atoms causes the lower binding energies and the 'platinide' character.While YPtAl and YPt 2 Al 3 exhibit extensive Pt-Al bonding within the polyanion, only few heteroatomic Pt-Al bonds are observed in Pt-rich YPt 5 Al 2 .Consequently, the spectra of YPt 5 Al 2 show the smallest shift in comparison with elemental Pt.In YPtAl, an equal ratio of Pt and Al can be found in contrast with YPt 2 Al 3 .In the latter compound, additional homoatomic bonding takes place; therefore, YPt 2 Al 3 shows a smaller shift in the Pt 4f 7/2 binding energies than YPtAl.As expected, Y is acting as electron donor, and therefore, the Y 3d 5/2 signal is shifted by approximately 1 eV to higher binding energies (c.f.Table 10).However, all samples show a minor Y 3d 5/2 component, that appears around 155.5 eV, in line with possible contaminations by traces of elemental yttrium.

Figure 2 .
Figure 2. Plot of the unit cell parameters of the REPt 2 Al 3 phases as a function of the rare-earth element.

Figure 3 .
Figure 3.The crystal structure of YPt 2 Al 3 .Yttrium, platinum, and aluminum atoms are drawn as green/blue, black-filled, and open circles, respectively.The polyanionic [Pt 2 Al 3 ] δ-network is highlighted.

Figure 4 .
Figure 4. Coordination polyhedra surrounding the three crystallographically independent yttrium sites in YPt 2 Al 3 .Yttrium, platinum, and aluminum atoms are drawn as green/blue, black-filled, and open circles, respectively.The local site symmetries are given.

Figure 5 .
Figure5.Group-subgroup scheme in the Bärnighausen formalism[36][37][38][39] for the structures of PrNi 2 Al 3 and YPt 2 Al 3 .The index for the isomorphic (i) and translationengleiche (t) symmetry reduction, the unit cell transformation, and the evolution of the atomic parameters are given.

3. 3 .Tm 2
Crystal Chemistry of Tm 2 Pt 3 Al 4 and Lu 2 Pt 3 Al 4 Pt 3 Al 4 and Lu 2 Pt 3 Al 4 crystallize in the orthorhombic crystal system with space group Pnma (oP36, c 9 ) in the Ce 2 Ir 3 Sb 4 -type structure [26,27].In the following paragraph, Lu 2 Pt 3 Al 4 will be used for the structure description.As in the REPt 2 Al 3 series, the platinum and aluminum atoms form a network.

Figure 6
depicts the extended unit cell along [010], and the polyanionic [Pt 3 Al 4 ] δ-network and the two different lutetium sites are highlighted.The heteroatomic Pt-Al distances span a larger range (246-269 pm) compared to YPt 2 Al 3 ; however, Pt-Al bonding is still present.In contrast to YPt 2 Al 3 , only additional Al-Al bonds can be found ranging from 278 to 300 pm.In the polyanion, no Pt-Pt bonds below 400 pm are found.The Al atoms form corrugated layers consisting of rectangles and hexagons in the boat conformation (Figure 7, top) that are capped by the Pt atoms (Figure 7, bottom).The lutetium cations occupy two distinct crystallographic sites and are again found in the cavities of the polyanion.Lu1 is surrounded by 16 atoms in a four-fold-capped hexagonal prismatic environment (Lu1@[Al 6 Pt 6 +Al 4 ]; Figure8, top), while Lu2 has a three-fold-capped pentagonal prismatic coordination sphere (Lu2@[Al 6 Pt 4 +Al 2 Pt]; Figure8, bottom).The Lu-Pt distances range from 299 to 310 pm, and the Lu-Al distances range from 327 to 347 pm.The Lu-Pt distances are in line with LuPtAl; the Lu-Al contacts are significantly longer (Lu-Pt: 302-327 pm; Lu-Al: 284-301 pm)[30].

Figure 6 .
Figure 6.Extended crystal structure of Lu 2 Pt 3 Al 4 along [010].Lutetium, platinum, and aluminum atoms are drawn as green/blue, black-filled, and open circles, respectively.The polyanionic [Pt 3 Al 4 ] δ- network and the two different coordination environments for the lutetium atoms are highlighted.

Figure 7 .
Figure 7.The Al arrangement in the crystal structure of Lu 2 Pt 3 Al 4 (top).The Pt atoms capping the layers are depicted in the bottom image.Platinum and aluminum atoms are drawn as black-filled and open circles, respectively.The Pt-Al bonds in the polyanionic [Pt 3 Al 4 ] δ-network are highlighted.

Figure 8 .
Figure 8. Coordination polyhedra surrounding the two crystallographically independent lutetium sites in Lu 2 Pt 3 Al 4 .Lutetium, platinum, and aluminum atoms are drawn as green/blue, black-filled, and open circles, respectively.The local site symmetries are given.

Figure 9 .
Figure 9. Temperature dependence of the magnetic susceptibility (data) of YPt 2 Al 3 measured at 10 kOe.

Figure 14 .
Figure 14.Fitted X-ray photoemission spectrum of Pt 4f in YPt 2 Al 3 .The experimental data is shown as black squares, the Pt 4f components are depicted in green, the Al 2p lines in blue, and the envelope function in red.The background is depicted as a dashed line.
Attempts to synthesize the CaCu 5 -type related compounds REPt 2 Al 3 with the late rare-earth elements Dy-Tm and Y led to the discovery of a new structure type, which was refined from single-crystal data obtained for YPt 2 Al 3 .The structure crystallizes in the orthorhombic space group Cmmm and can be derived from CaCu 5 by distortion and recoloring of the framework.Attempts to synthesize LuPt 2 Al 3 led to the discovery of Lu 2 Pt 3 Al 4 (Ce 2 Ir 3 Sb 4 -type), which was also refined from single-crystal data.The REPt 2 Al 3 compounds could be obtained in phase pure form for property investigations.While YPt 2 Al 3 is Pauli-paramagnetic, DyPt 2 Al 3 to TmPt 2 Al 3 , in contrast, show paramagnetism in line with formal RE 3+ cations, along with magnetic ordering for RE = Dy, Ho, and Tm at low temperatures.Via XPS investigations, the binding energies of the constituent elements were investigated and compared with the electronegativities.In comparison with reference substances, the expected charge transfer onto the Pt atoms within the polyanionic [Pt 2 Al 3 ] δ-network could be proven.

Table 3 .
Atom positions and equivalent isotropic displacement parameters (pm 2 ) for YPt 2 Al 3 .U eq is defined as one-third of the trace of the orthogonalized U ij tensor.

Table 4 .
Atom positions and equivalent isotropic displacement parameters (pm 2 ) for Lu 2 Pt 3 Al 4 .U eq is defined as one-third of the trace of the orthogonalized U ij tensor.y = 1/4 all 4c.

Table 7 .
Interatomic distances (pm) for YPt 2 Al 3 .All distances of the first coordination spheres are listed.All standard uncertainties were less than 0.2 pm.

Table 8 .
Interatomic distances (pm) for Lu 2 Pt 3 Al 4 .All distances of the first coordination spheres are listed.All standard uncertainties were less than 0.2 pm.
3-Type Structure: Crystal Chemistry and Group-Subgroup Relations

Table 10 .
Fitted binding energies (in eV) determined by XPS of YPt 2 Al 3 , YPt 5 Al 2 , YPtAl, PrPtAl, and Pt and data from the literature.The determined uncertainty of binding energies in this work is ±0.1 eV.