Intermediate-Valence Ytterbium Compound Yb4Ga24Pt9: Synthesis

Article pubs.acs.org/IC

Intermediate-Valence Ytterbium Compound Yb4Ga24Pt9: Synthesis, Crystal Structure, and Physical Properties Olga Sichevych,† Yurii Prots,*,† Yuki Utsumi,†,∥ Lev Akselrud,† Marcus Schmidt,† Ulrich Burkhardt,† Mauro Coduri,‡ Walter Schnelle,† Matej Bobnar,† Yu-Ting Wang,†,⊥ Yu-Han Wu,§ Ku-Ding Tsuei,§ Liu Hao Tjeng,† and Yuri Grin*,† †

Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Straße 40, 01187 Dresden, Germany ESRF−The European Synchrotron, 38043 Grenoble, France § National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, 30076 Hsinchu, Taiwan ‡

S Supporting Information *

ABSTRACT: The title compound was synthesized by a reaction of the elemental educts in a corundum crucible at 1200 °C under an Ar atmosphere. The excess of Ga used in the initial mixture served as a flux for the subsequent crystal growth at 600 °C. The crystal structure of Yb4Ga24Pt9 was determined from single-crystal X-ray diffraction data: new prototype of crystal structure, space group C2/m, Pearson symbol mS74, a = 7.4809(1) Å, b = 12.9546(2) Å, c = 13.2479(2) Å, β = 100.879(1)°, V = 1260.82(6) Å3, RF = 0.039 for 1781 observed reflections and 107 variable parameters. The structure is described as an ABABB stacking of two slabs with trigonal symmetry and compositions Yb4Ga6 (A) and Ga12Pt6 (B). The hard X-ray photoelectron spectrum (HAXPES) of Yb4Ga24Pt9 shows both Yb2+ and Yb3+ contributions as evidence of an intermediate valence state of ytterbium. The evaluated Yb valence of ∼2.5 is in good agreement with the results obtained from the magnetic susceptibility measurements. The compound is a bad metallic conductor.

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INTRODUCTION Intermetallic compounds of ytterbium have attracted steady interest due to the possible instability of its 4f shell, which provides a venue to investigate and tune the interplay of magnetism and heavy fermion behavior, with perhaps also unconventional superconductivity. Although the number of publications involving the investigation of Yb compounds has risen drastically in the last 20 years, the prediction of the possible valence state of the ytterbium is not obvious. A systematic investigation of physical properties allows establishment of the so-called valence field maps revealing the dependence of the electronic state of ytterbium on the chemical composition of the compound in the given system. Such investigations in the homologous systems Yb−Al−Ni1 and Yb−Ga−Ni2 revealed that the electronic state of ytterbium in the ternary system is rather influenced by the content of the transition metal and is less sensitive to the concentration of the main-group element. Usually, for a final interpretation of the results of the physical property measurements, an analysis of the chemical bonding information based on crystallographic data of high quality should be involved. Recently we extended investigations on the electronic state of ytterbium in ternary Yb−X−T compounds by including noble metals as the transition-metal component T and Ga as the main-group component X. Here we report on Yb4Ga24Pt9, a compound with a crystal structure based on © XXXX American Chemical Society

building principles known for the extended family of the intermetallic phases originating from the Y2Ga9Co3 (space group Cmcm).3 In addition to the diversity of the atomic arrangements, the representatives of this family contain usually only one crystallographic position for rare-earth species,3−6 which facilitates essentially the interpretation of the experimental data. Thus, Yb-based compounds of the Y2Ga9Co3 family may be well suited for a study of the electronic state of ytterbium. Several Yb-containing representatives of the family discussed above are known. A group of isostructural compounds Yb2X9T3 (Y2Ga9Co3 structure type) has been reported with X = Al, Ga and T = Co, Rh, Ir.7−9 These studies show an interesting influence of the X component on the valence state of ytterbium. Whereas Yb species in the compounds with aluminum are found in the 4f13 state, those with gallium show an intermediate electronic state. The same behavior is observed for the Kondo-lattice substances YbX9Ni3 (X = Al, Ga, ErAl9Ni3 structure type).10−14 Here, YbAl9Ni3 is an antiferromagnetic heavy fermion material with TN = 3.4 K,10,12,14 while in the Yb 3d5/2 hard X-ray photoelectron spectroscopy (HAXPES) spectra of YbGa9Ni3,13 both Yb2+ and Yb3+ components were observed as evidence of the valence fluctuations. The physical property investigations of Yb2Al15Pt66 Received: June 15, 2017

A

DOI: 10.1021/acs.inorgchem.7b01530 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Atomic Coordinates and Equivalent Displacement Parameters (in Å2) in the Crystal Structure of Yb4Pt9Ga24 atom

site

occupancy

Yb1a Pt1 Pt2 Pt3 Pt4 Ga1a Ga2 Ga3 Ga4 Ga5a Ga6 Ga7 Ga8 Yb2a Ga9aa Ga9ba Ga9ca

8j 2b 4g 4i 8j 4i 4i 4i 4i 8j 8j 8j 8j 4i 8j 8j 8j

1 − 1/2G(x) 1 1 1 1 1 − G(x) 1 1 1 1 − G(x) 1 1 1 G(x) 1/2G(x) 1/2G(x) 1/2G(x)

x 0.7314(1) 1 /2 0 0.6271(1) 0.11781(8) 0.4341(4) 0.3130(4) 0.9465(4) 0.8490(4) 0.1252(3) 0.3540(3) 0.4405(3) 0.8145(3) 0.225(5) 0.623(17) 0.939(16) 0.622(17)

y

z

Ueq

0.16687(6) 0 0.16905(7) 0 0.16961(5) 0 0 0 0 0.1024(2) 0.1676(2) 0.1641(2) 0.1668(2) 0 0.055(10) 0.161(10) 0.275(10)

0.19228(5) 0 0 0.36320(7) 0.36174(4) 0.1839(2) 0.4400(2) 0.3281(2) 0.0542(2) 0.1829(2) 0.0577(2) 0.3278(2) 0.4424(2) 0.190(3) 0.181(9) 0.183(9) 0.178(9)

0.0086(2) 0.0077(3) 0.0074(2) 0.0077(2) 0.0073(2) 0.0098(7) 0.0096(7) 0.0099(7) 0.0091(7) 0.0094(5) 0.0090(5) 0.0100(5) 0.0093(5) Ueq (Yb1) Ueq (Ga1) Ueq (Ga1) Ueq (Ga1)

a The occupancy parameters of Yb1, Ga1, and Ga5 positions were constrained with additional positions (Yb2, Ga9a, Ga9b, Ga9c). The refinement resulted in G(x) = 0.032(3).

(ESRF, Grenoble, France; particle size 300 K. Electrical resistivity in magnetic fields up to 9 T was measured using an ac four-wire method (PPMS, Quantum Design). Hard X-ray Photoelectron Spectroscopy (HAXPES). The experiment was performed at the BL12XU beamline of SPring-8 (Japan) with the photon energy set at hν ≈ 6.5 keV. Clean surfaces were obtained by fracturing the sample in situ in a preparation chamber with a pressure in the low 10−9 mbar range and transferring within a few minutes into the measurement chamber with a pressure in the low 10−10 mbar range. All HAXPES spectra were measured at 85 K using an MB Scientific A1-HE hemispherical electron energy analyzer positioned in the direction of the E vector of the light. The overall energy resolution was set to 160 meV, and the binding energy of the spectra was calibrated by using the Fermi cutoff of an Au film.

Figure 1. Difference Fourier map in the slab A obtained after refinement of the “ideal” structure of Yb4Ga24Pt9. Positions of the majority sites are shown with spheres. Isolines are drawn with steps of 1 e·Å−1.

displacement parameters for all atomic sites, with the exception of Yb2, Ga9a, Ga9b, and Ga9c. The ADPs for the latter were constrained to the respective Ueq value of the majority sites (Table 1). The refinement (R = 0.039) resulted in the occupancy factor of 0.984(3) for the majority orientation of the slab A and 0.016(3) for the minority orientations. Since the occupancy factor of the latter is small, the following structure description of Yb4Ga24Pt9 is based on the completely ordered model. The atomic arrangement in Yb4Ga24Pt9 represents a new structure prototype. (Note: traditionally the formulas of the intermetallic compounds are written using the Pettifor string,27,28 e.g. Yb4Pt9Ga24. Recent quantum chemical studies showed that the effective charges of the transition metals reveal their higher electronegativity in comparison with the most half-metals, see for example refs 6 and 29. This is the reason for the labeling of the compounds in this study.) The crystal structure of Yb4Ga24Pt9 consists of a quasi-two-dimensional (2D) substructure of the ytterbium atoms, which are arranged within almost planar layers in the form of condensed hexagons (honeycomb-like pattern, slab A, composition Yb6Ga4; Figure 2). Each hexagon is centered by a triangle formed by Ga atoms. The slabs of type A are separated by one or two distorted-hexagonal-close-packed slabs B formed by Ga and Pt atoms, similar to the three adjacent layers

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RESULTS AND DISCUSSION The microstructure of the sample with nominal composition Yb7Ga75Pt18 is formed mainly by the target phase Yb4Ga24Pt9 (Figure S1 in the Supporting Information). The EDX spectra showed solely the presence of the constituent elements. The composition of the majority phaseYb11.1(2)Ga65.4(2)Pt23.4(1) agrees excellently with those of Yb10.8Ga64.9Pt24.3 obtained from the crystal structure determination (uncertainties were calculated from three measured points). As an admixture, gallium traces were observed on the grain boundaries of the majority phase. Two endothermic signals at 945 and 955 °C are detected in the DSC heating curve of Yb4Ga24Pt9 (Figure S2 in the Supporting Information). They find their equivalents with a small overcooling in the cooling curve. Such thermal behavior is characteristic for a peritectic formation of the main phase at 945 °C and crossing of the liquidus at 955 °C (peritectic reaction). Analysis of the collected single-crystal data integrated in the monoclinic lattice (a = 7.4809 Å, b = 12.9546 Å, c = 13.2479 Å, β = 100.879°) unambiguously provided 2/m as the Laue symmetry; the extinction symbol C1−1 led to the possible space groups C2, Cm, and C2/m. The structure was solved in the centrosymmetric space group C2/m by an application of direct methods, which yield ytterbium, all platinum, and most gallium positions. The remaining gallium sites were extracted from the difference Fourier maps. The refinement of the structural model with 1Yb, 4Pt, and 8Ga positions converged promptly to R = 0.045. At this step, a difference Fourier map at z ≈ 0.19 (and z ≈ 0.81), designated as slab A in the following discussion, reveals clearly defined maxima in the centers of Ga3 triangles and around Yb1 positions (Figure 1). The relative locations of these maxima reproduce exactly the atomic arrangement of Yb1 atoms and Ga3 triangles within an ordered slab A, but shifted by [0,∼1/3,0]. Thus, the maxima observed on the difference Fourier map were considered rather as a trace of the stacking disorder of slabs A in the crystal structure, and not as a partial disorder within these slabs. This phenomenon is frequently observed in the structures of the Y2Ga9Co3 family, especially for its Sc1.2Si9.8Fe4 branch.6 Thus, for subsequent refinement the occupancies of the additional positions observed in the centers of Ga3 triangles and around Yb1 were constrained with Yb1 and both gallium sites (Ga1 and Ga5), respectively (Table 1). This reduced slightly the residual to R = 0.043 and smoothed significantly the final difference Fourier map (residual peaks −1.63 and 2.71 e Å−3). Finally, the crystal structure was refined with anisotropic

Figure 2. Slabs A and B constituting the crystal structure of Yb4Ga24Pt9 shown in the directions perpendicular and parallel to the slab plane: (top) planar layer A, composition Yb4Ga6; (bottom) distorted hexagonal close-packed slabs B, composition Ga12Pt6. C

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ABA-I and ABBA-I slabs represents 98.4% of the crystal structure; the remaining 1.6% reflects slabs ABA and ABBA in the stacking variants II and III. Whereas the 3-fold axes of the slabs B coincide by their stacking, the 3-fold axes of the A slabs coincide with that of the neighboring slabs B but not with that of the following slab A for any ABA or ABBA sequences (Figure 4). Because the trigonal axes of the single slabs do not coincide, this reduces the symmetry of the resulting structure. The peculiarities of the arrangement of the slabs A and B just discussed cause the diversity of structures of the Y2Ga9Co3 family as well as partial or complete disorder of these structures, observed mainly in slabs A (Table 2). For numerous representatives of the discussed series disordered atomic arrangements were found, usually described by Sc1.2Si9.8Fe4 (space group P63/mmc)47 and Gd1.33Al8Pt3 (R3̅m)62 models. Apparently that disorder reported for the series reflects the relative configuration of slabs A, but not the disordered distribution of the atoms within these slabs. In addition to the representatives of Y2Ga9Co3 structure type (space group Cmcm), the ordered varieties of this family of compounds adopt mainly ErAl9Ni3 (space group R32),71 Er4Al24Pt9 (P1̅),69 Yb2Ga9Pd3 (P6122),16−18 Tb2Ge3Pt9 (C2/m),5 and Yb2Al15Pt6 (Cmcm)6 structure types. Two structure arrangements of this series have the same sequence of the building fragments ABABB, as observed in the reported structure: one ordered Er4Al24Pt9 (space group P1)̅ 69 and one disordered Gd1.33Al8Pt3 (R3̅m).62 The latter can be considered as a mixture of the both ordered varieties: Yb4Ga24Pt9 and Er4Al24Pt9. It is remarkable that for Gd1.33Al8Pt3 finally a disordered model was found,40 whereas in an earlier publication the crystal structure was assigned to the ordered Er4Al24Pt9 type.69 This observation is in favor of the idea that the ordering of slabs A may depend on the preparation route (kinetics of crystallization). In the simplified form the arrangement of the slabs A can be reproduced by set of X3 triangles, which center a condensed hexagon.5,6 To differentiate between the relative orientations of these triangles, caused by the different stacking configurations (Figure 4), an additional designation superscript index was introduced.6 Thus, Er4Al24Pt9 and Yb4Ga24Pt9 are described by the symbols A(a)BA(b)BB and A(a)BA(a)BB, respectively (Figure 5). The relative dislocation of the slabs A in Yb4Ga24Pt9 occurs in the same direction, similar to the case for Y2Ga9Co3 (A(a)BA(−a)B) and Yb2Al15Pt6 (A(a)BBA(−a)BB) structures. From this point of view, the structure of Yb4Ga24Pt9 can be considered as being intermediate between Y2Ga9Co3 and Yb2Al15Pt6. The calculation of the electronic structure for the chemically homologous compound Yb2Al15Pt66 revealed a non-negligible density of states at the Fermi level indicating metallic behavior. Taking into account the structural similarity, metallic resistivity might be expected also for Yb4Ga24Pt9. The measurements in the temperature range between 4 and 300 K (Figure 6) indeed revealed the typical behavior of a bad metallic conductor. At 300 K a resistivity of 165 μΩ cm is attained, and the residual/ resistance ratio is RRR ≈ 10. (Note: according to the X-ray diffraction, chemical analysis, metallography (including EDXS) the sample is single phase (with less than 0.1% impurities). However, this is a polycrystalline material with small grains, and this leads to an RRR value that is less than those for single-crystal specimens. For a polycrystalline sample the observed RRR value is in the usual range for rare-earth intermetallic compounds.) No clear power-law behavior of ρ(T), especially not the Fermi-liquid behavior ρ(T) = ρ0 + AT2, can be discerned at low temperatures; however, a sizable magnetoresistance is observed (Figure 6,

I−Cd−I in the structure type CdI2 (composition Ga12Pt6; Figure 2). The constituent slabs are stacked along [001] in the sequence ABABB (Figure 3). The pseudotrigonal symmetry of the

Figure 3. Crystal structure of Yb4Ga24Pt9: (top) stacking of slabs A and B; (middle) relative arrangement of slabs A; (bottom) coordination of the Yb atom and Ga3 group.

constituent blocks A and B is also reflected in the almost ideal ratio of √3 observed for the corresponding lattice parameters: (b/c)2 = 2.998. All ytterbium atoms in the structure of Yb4Ga24Pt9 are located in the planar slabs A, where they are coordinated by three gallium atoms at d(Yb−Ga) = 3.08 Å and three ytterbium atoms at d(Yb−Yb) = 4.32 Å. The latter value is appreciably larger than the interatomic distance of 3.88 Å found for elemental Yb (fcc, CN = 12).30 To the coordination sphere of ytterbium belong also two corrugated Ga3Pt3 hexagons located in the adjacent slabs B (d(Yb−Ga) = 3.04−3.06 Å and d(Yb−Pt) = 3.51−3.54 Å) and two additional Ga atoms on the pseudotrigonal axis above and below the hexagons (d(Yb−Ga) = 3.25 Å; Figure 3, bottom). Similar to the case for single Yb atoms is the coordination of the triangular Ga groups (formed by one Ga1 and two Ga5 atoms) which are sandwiched between corrugated Ga3Pt3 hexagons. Within the slab A each gallium atom of the Ga3 triangle is in contact with two ytterbium atoms located in the apexes of a large hexagon (cf. Figure 2, top). The crystal structure of Yb4Ga24Pt9 extends logically the Y2Ga9Co3 family of structures consisting of slabs A and B as building fragments. The different sequences and relative arrangements of the building slabs A and B result in the varieties of compositions and overall symmetries of the structures of the discussed series (Table 2 and refs 3−18 and 31−72). There are three possibilities for stacking of two slabs A sandwiching a slab B (ABA; Figure 4, top). The same situation is also observed for the ABBA sequence (Figure 4, bottom). In the crystal structure of Yb4Ga24Pt9 (stacking sequence ABABB), the combination of D

DOI: 10.1021/acs.inorgchem.7b01530 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Ternary and Quaternary Compounds of Y2Ga9Co3 Family compound R2Al9Co3 R2Al9Rh3 R2Al9Pd3b R2Al9Ir3 R2Ga9Co3 R2Ga9Ru3 R2Ga9Rh3 R2Ga9Ir3 R2Ga9Pd3 R2Ge3Pt9 R2Al15Pt6 R1.2Si9.8Fe4c R0.67(Ga,Tt)5Ni2d R0.67Al5Pd2 R2Al15Pt6e−g R2Ga15Pt6h R4Al24Pd9 R4Al24Pt9 R4Ga24Pt9 R1.33Al8Pt3i RAl9Ni3k RGa9Ni3 R0.67(Ga,Tt)6T2l

R

ref

Y2Ga9Co3 (Cmcm), (AB)2a Y, Pr, Nd, Sm, Gd−Yb, Sc, Zr, Hf, U Y, La−Nd, Sm, Gd−Lu, U Y, Gd−Tm Y, La−Nd, Sm, Gd−Lu, U Y, Nd, Sm, Gd−Lu, Sc, Zr, Hf Y, La−Nd, Sm, Gd−Tm Y, Ce, Sm−Yb, U Y, Ce, Sm−Yb, U Yb2Ga9Pd3 (P6122) (AB)6a Yb Tb2Ge3Pt9 (C2/m), (AB)2a Y, Tb−Lu Yb2Al15Pt6 (Cmcm), (ABB)2a Yb Sc1.2Si9.8Fe4 (P63/mmc), (ABB)xa Y, Gd−Lu, Sc, U Y, Sm, Tb, Ho Sm, U Y, La−Lu, Zr, U Y, La−Nd, Sm, Gd−Lu Er4Al24Pt9 (P1̅), ABABBa Gd−Tm Y, Gd−Lu Eu Gd1.33Al8Pt3 (R3̅m), “(ABABB)3”a Ce, Gd ErAl9Ni3 (R32), (ABCB)3a,j Y, Gd, Dy, Er, Yb Yb Y, Sm, Gd, Dy

7, 31−37 4, 8, 38, 39 4, 40 4, 8, 38, 39, 41 3, 7, 35, 36 42−44 8, 9, 38, 39, 45, 46 8, 9, 38, 39, 45, 46 16−18 5 6, 15 47−53 54 55−57 55, 58−63 64−68 69 69 70 40, 59, 62 10, 12−14, 71, 72 11−13 54

a

Structure type (space group), stacking sequence. bFor disordered Tb2Al9Pd3 the formula Tb0.67Al3Pd is used in ref 40. cThe formula R2Al9Fe4 is used in ref 50. dTt = Ge for Y, Sm, Ho; Tt = Si for Tb. eThe formula R1.33Al10Pt4 is used in refs 60 and 63. fThe formula R0.67Al5Pt2 is used in refs 55, 58, 59, 61, and 62. gQuaternary R0.67Al4SiPt2 are reported in ref 62. hThe formula R2−xGa8+yPt4 is used in ref 68, and the formula R1.33Ga10Pt4 is used in ref 64. iThe quaternary composition R1.33Al7SiPt3 is reported in ref 62. jAn additional planar triangular slab C formed by X element (composition X3) is present in ErAl9Ni3 and isotypes. kThe formula YbAl9.23Ni3 is used in ref 14. lT = Ni and Tt = Si for Sm; T = Ni and Tt = Ge for Gd, Dy; T = Co and Tt = Ge for Y; all structures are disordered.

Figure 4. Stacking variants for the sequences ABA (top) and ABBA (bottom). For better visualizations only slabs A are shown.

inset). The resistivity increases by up to 59% in a field μ0H = 9 T. The relatively large resistivity of Yb4Ga24Pt9 is in the range observed for typical transition metal compounds. The magnetic susceptibility χ = M/H of Yb4Ga24Pt9 (Figure 7) is only weakly dependent on temperature. A very weak field

dependence of χ on high fields is due to traces of ferromagnetic impurities (e.g., a few ppm of iron). The upturn at low T is dependent on field and is due to traces of paramagnetic impurity phases containing stable-valence Yb3+ species (e.g., Yb2O3) obeying a Curie law at elevated temperatures. The Curie−Weiss E

DOI: 10.1021/acs.inorgchem.7b01530 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Selected representatives of the Y2Ga9Co3 family: (left) basic arrangements of X3 triangles in the slabs A and the set of the vectors used for labeling of their relative arrangement; (right) stacking of slabs A in the structures of Y2Ga9Co3, Yb2Al15Pt6, Er4Al24Pt9, and Yb4Ga24Pt9. Only the triangular X3 units are shown.

The magnetic susceptibility of Yb4Ga24Pt9 measured up to 1000 K shows a broad maximum at about 900 K. The susceptibility was fitted in the range between 400 and 1000 K using the interconfigurational-fluctuation (ICF) model74 for the intermediate-valence Yb ions. The fit resulted in the spinfluctuation temperature Tsf = 563 K and the energy separation between the ground state configuration (Yb2+, 4f14) and the excited state configuration (Yb3+, 4f13) Eexc = 3250 K. This model gives a temperature-dependent average Yb oxidation number of ∼2.2 at 400 K, which increases with temperature (Figure 7, bottom, insert). However, the ICF model is known to produce a valence of Yb that is too low; thus, the value obtained by HAXPES experiments is more accurate (see below). The presence of Yb2O3 is confirmed by a measurement of the specific heat at low temperatures (Figure S3 in the Supporting Information), revealing a small magnetic transition in Yb2O3 at 2.3 K in agreement with the literature data.75 Systems with intermediate oxidation state may undergo valence fluctuations depending on temperature. The presence or absence of valence fluctuation can be estimated indirectly from the development of the lattice parameters of the investigated phase.76−79 The analysis of the powder X-ray diffraction patterns depending on temperature (Figure S4 in the Supporting Information) revealed no discontinuity of the unit cell parameters (a, b, and c) with temperature, whereas the monoclinic angle remains practically unchanged (Figure S5 in the Supporting Information). Such behavior indicates indirectly that the ratio of Yb2+ to Yb3+ species in Yb4Ga24Pt9 remains nearly constant over the whole investigated temperature range. However, for the final conclusions about the electronic state of ytterbium, a spectroscopic study was necessary. A wide-scan HAXPES spectrum of Yb4Ga24Pt9 (Figure S6 in the Supporting Information) displays only the core-level lines from its three elemental contributions. The valence-band

Figure 6. Electrical resistivity ρ(T) of Yb4Ga24Pt9. Inset: field dependence of ρ(T) at low temperatures.

fit to the 0.1 T data (Figure 7, top, inset) results in Tc = −2.94 K, which is close to the known AFM transition temperature of Yb2O3 at 2.25 K, and in μeff = 0.158 μB, indicating about 0.1% of Yb atoms in the Yb2O3 impurity. Around 300 K, a steplike anomaly is seen which shows a hysteresis between heating and cooling runs (Figure 7, top). Such behavior is often observed in gallium-rich intermetallic compounds and is due to the melting and supercooled freezing of elemental gallium traces on the grain boundaries (cf. Figure S1 in the Supporting Information). In addition, some effects due to superconductivity of strained Ga are seen in low fields. In contrast, the upturn of χ(T) for high T is intrinsic and typical for compounds with intermediate-valence Yb species.73 The level of the susceptibility χ0 around 200 K is +4.0 × 10−3 emu mol−1. The estimated sum of the core increments is χdia ≈ − 0.5 × 10−3 emu mol−1. F

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Figure 7. Magnetic susceptibility χ = M/H of polycrystalline Yb4Ga24Pt9: (top) in magnetic fields μ0H = 3.5 and 7.0 T within the temperature interval 1.8−400 K (inset: magnetic susceptibility in magnetic field μ0H = 0.1 T at low temperatures); (bottom) in magnetic field μ0H = 3.5 T and temperature intervals of 300−1000 K and 1.8−400 K (inset: behavior of the Yb valence at high temperatures).

Figure 8. Valence band spectrum of Yb4Ga24Pt9 measured at T = 85 K (top). Experimental (points) and simulated (lines) Yb 3d spectrum of Yb4Ga24Pt9 evidencing the intermediate valence state of Yb (bottom).

integral-type background85 was subtracted from the Yb 3d spectrum and the Yb2+- and Yb3+-derived plasmon structures were also simulated using the relative intensities and energy positions of the Ga 2p3/2 peak and its plasmon structure observed in the overview spectrum. The Yb valence z was then evaluated from the intensity ratio of Yb2+ and Yb3+ components using z = 2 + I(Yb3+)/[I(Yb3+) + I(Yb2+)]. Here I(Yb2+) and I(Yb3+) denote the integrated intensities of Yb2+ and Yb3+ components, respectively. The evaluated Yb valence of z ≈ 2.5 confirmed that Yb4Ga24Pt9 is indeed an intermediate-valence compound, as indicated by the magnetic susceptibility and low-temperature powder X-ray diffraction data.

spectrum (Figure 8, top) reveals the 4f7/2 and 4f5/2 final-state peaks close the Fermi level and at around 1.5 eV, respectively, originating from the Yb2+ (4f14) component of the ground state. The broad intensity below these two peaks is assigned to the Pt 5d and Ga 4p valence-band states, while the high-intensity structure between 3.5 and 6 eV is attributed to the Ga 4s band, in analogy to the photoemission spectral analysis of the valence band in SrPt4Ge12.80 We note that the HAXPES experimental geometry used here is highly sensitive to states with s-orbital symmetry,81 thereby highlighting the Ga 4s contribution to the valence band. On top of this valence band, we identified features between 6 and 12 eV that are characteristic of the 4f12 multiplet structure as the final state coming from the Yb3+ (4f13) part of the ground state.82 In order to be more quantitative about the Yb valence, we also measured the Yb 3d spectrum of Yb4Ga24Pt9 (Figure 8, bottom). The adequacy of obtaining the Yb valence states from Yb 3d core level spectroscopy has been reported earlier.82−85 The Yb 3d spectrum is split into a 3d5/2 region at 1515−1540 eV and a 3d3/2 region at 1560−1585 eV due to the spin-orbit interaction. Both components are further split into a single peak of the final state originating from the Yb2+ part of the ground state and a multiplet structure coming from the Yb3+ part. The spectrum thus demonstrates very clearly the presence of the Yb2+ and Yb3+ components. A fitting analysis was performed using the full atomic multiplet calculations for the Yb spectral features. An

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CONCLUSIONS The new compound Yb4Ga24Pt9 was synthesized by the reaction of the elemental components. An excess of Ga was used for the single-crystal growth. The reported phase adopts a new prototype crystal structure (space group C2/m) and consists of pseudotrigonal slabs A and B arranged in an ABABB sequence. The overall symmetry of the structure is defined by the relative arrangement of the slabs A. Stacking faults in the structure organization observed in the investigated single-crystal specimen are explained by the three equivalent possibilities for the stacking of two neighboring slabs A. The structure of Yb4Ga24Pt9 belongs to the large series of intermetallic compounds derived from orthorhombic Y2Ga9Co3. The variety of crystal structures and symmetries (from triclinic to hexagonal) represented in this G

DOI: 10.1021/acs.inorgchem.7b01530 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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series is caused by the different sequences of the building slabs A and B and by different relative arrangements of the slabs A. The electrical resistivity of Yb4Pt9Ga24 shows typical behavior for a bad metallic conductor. Magnetic susceptibility measurements and hard X-ray photoelectron spectra indicate an intermediate valence of ∼2.5 for the ytterbium species, being in agreement with the results of the low-temperature powder X-ray diffraction data.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01530. Crystallographic data and interatomic distances, metallography, thermal analysis (DSC), powder X-ray diffraction pattern at room temperature including Rietveld fit, behavior of the lattice parameters at 10−300 K, specific heat of Yb4Ga24Pt9 a function of temperature, and hard Xray photoelectron spectrum (HAXPES) of Yb4Ga24Pt9 measured at T = 85 K (wide scan) (PDF) Accession Codes

CCDC 1548921 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for Yu.P.: [email protected]. *E-mail for Yu.G.: [email protected]. ORCID

Yurii Prots: 0000-0002-7418-9892 Present Addresses ∥

SOLEIL Synchrotron, L’Orme des Merisiers, Saint-Aubin, BP48, 91192 Gif-sur-Yvette, France. ⊥ Department of Electrophysics, National Chiao Tung University, 1001 Ta Hsueh Road, 30010 Hsinchu, Taiwan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Dr. C. Geibel for valuable discussions. REFERENCES

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