Abrupt Europium Valence Change in Eu2Pt6Al15 around 45 K

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Abrupt Europium Valence Change in Eu2Pt6Al15 Around 45 K Mathis Radzieowski, Frank Stegemann, Theresa Block, Juliane Stahl, Dirk Johrendt, and Oliver Janka J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05188 • Publication Date (Web): 21 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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Journal of the American Chemical Society

To be submitted to Journal of the American Chemical Society

19.06.18 04:10:01

Mathis Radzieowski, Frank Stegemann, Theresa Block, Juliane Stahl, Dirk Johrendt, Oliver Janka*

Abrupt Europium Valence Change in Eu2Pt6Al15 Around 45 K

Keywords: rare earth intermetallics; valence change; physical properties; crystal structure; temperature dependent X-ray diffraction

*Corresponding author: Oliver Janka, Institut für Anorganische und Analytische Chemie, Universität Münster, Corrensstraße 30, D-48149 Münster, Germany. E-Mail: [email protected] Mathis Radzieowski, Theresa Block, Frank Stegemann, Institut für Anorganische und Analytische Chemie, Universität Münster, Corrensstraße 30, D-48149 Münster, Germany. Juliane Stahl, Dirk Johrendt, Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstrasse 5-13 (Haus D), D-81377 München, Germany.

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Abstract: 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)0s0 (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 drastically 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

151

Eu Mössbauer spectroscopic

investigations. Isostructural Eu2Pt6Ga15 was prepared in comparison, it shows divalent Eu atoms down to 2.5 K along with antiferromagnetic ordering at TN = 13.1(1) K.

1 Introduction The predominant oxidation state of rare earth metal cations is the trivalent one. Some of them, like cerium, praseodymium or terbium exhibit an additional tetravalent oxidation state, as e.g. found in the oxides CeO2 and mixed valent Pr6O11 and Tb4O7. Samarium, europium and ytterbium, in contrast, form also oxides (EuO, YbO) and halides REX2 (RE = Sm, Eu, Yb; X = F– I) with a formal +2 oxidation state.1 In intermetallic compounds, the latter two elements (Eu and Yb) are usually found in the divalent oxidation state. However, the availability of a second oxidation state results in numerous interesting physical phenomena. Mixed valences can be found (Eu3O4,2 Sm3S4,3 Eu3S4,3a or EuPtP4) as well as intermetallic valence fluctuation materials (e.g. CeAl3,5 Yb4Pt9Ga24,6 CeMo2Si2C,7 CeRu1–xNixAl,8 Ce2Rh3Sn5,9 CeCu2Si2,10 EuCu2Si2,10-11 YbCu2Si210). A rather rare feature is a temperature or pressure dependent valence phase transition. Well known transitions of this type are e.g. the ones from metallic α-Ce to γ-Ce,12 where electron localization takes place, accompanied by a drastic volume effect. In the double perovskites Ba2PrRu1–xIrxO6, the praseodymium valence changes temperature dependent from +3 to +4 while the transition metal valence changes from +5 to +4.13 In the samarium 2


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monochalcogenides SmCh (Ch = S, Se, Te) a pressure dependent insulator to metal phase transition is observed,14 in EuPtP two new valence ordered structures were observed around p = 2.5 GPa and up to 6 GPa,15 while the pnictide EuCo2As2 and the aluminide YbAl3 exhibit pressure dependent shifts of the Eu / Yb valence from divalent towards the trivalent state.16 Finally, the intermetallic solid solutions CeNi1–xCoxSn17 and Yb1–xInxCu218 also exhibit at least a partial temperature dependent valence phase transitions of the respective rare earth atoms. EuPd2Si2 was reported to be a unique mixed valence system,19 extensive 151Eu Mössbauer studies confirmed this behavior.20 Eu(Ir1–xPdx)2Si2 with x = 0.75, 0.81 and 0.94 also shows a continuous valence phase transition over a large temperature region.21 In the solid solution Eu(Pd1–xAux)2Si2 with x = 0.15 a more drastic effect was observed. In this system, the valence rapidly changes from Eu2+ to Eu3+ at low temperatures, in line with a first order phase transition.22 Previously, Eu2Pt6Al15 was briefly mentioned to be a mixed-valent compound, however detailed investigations are missing.23 Here, we report in detail on the first order valence phase transition in the commensurate modulated intermetallic compound Eu2Pt6Al15. The transition was investigated by temperature dependent powder X-ray diffraction, magnetic susceptibility, heat capacity and electrical resistivity measurements.

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Eu Mössbauer spectroscopic experiments were used to prove the

valence phase transition in Eu2Pt6Al15 to be of first order along with its reversibility.

2 Experimental Section 2.1 Synthesis Starting materials for the syntheses of Eu2Pt6Al15 and Eu2Pt6Ga15 were ingots of europium (smart elements), platinum sheets (Agosi), aluminum turnings (Koch chemicals) and gallium lumps (Emmerich am Rhein), all with stated purities of >99.9%. The europium ingots were stored under argon; surface impurities were removed mechanically prior to the synthesis. The elements were weighed in the respective atomic ratios and arc-melted24 under a dried argon atmosphere of about 800 mbar. The argon gas was purified over titanium sponge (873 K), molecular sieves and silica gel prior to use. For annealing, the arcmelted buttons were sealed in evacuated silica ampoules and placed in the water-cooled sample chamber of a high-frequency furnace (Hüttinger Elektronik, Freiburg, type TIG 1.5/300 and TIG 5.0/300).25 They were heated slightly below their melting points for 10 min followed by reduction of the power output of the furnace within 20 min to a temperature of approximately 900 K, which was kept for another 4 h

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followed by quenching. The as cast samples are silver, the ground powders are grey. Both compounds are stable in air over months.

2.2 X-Ray diffraction The polycrystalline samples were characterized by Guinier patterns (imaging plate detector, Fujifilm BAS-1800) with Cu-Kα1 radiation and α-quartz (a = 491.30 and c = 540.46 pm) as an internal standard. The lattice parameters (Table S1) were deduced from least-squares fits of the Guinier data.26 Temperature dependent powder X-ray diffraction experiments of Eu2Pt6Al15 were conducted between 10 and 300 K (every 10 K between 300 and 60 K, every 5 K down to 20 K and finally at 10 K) on a Huber G670 Guinier Imaging Plate diffractometer (Co-Kα1 radiation, Ge-111 monochromator, silicon as external standard) with Low Temperature Device 670.4 and closed cycle He cryostat (Lakeshore temperature controller, model 331). The temperature dependent lattice parameters were obtained by fitting the data using the FullProf suite27 and are listed in Table S1. Small single crystals of Eu2Pt6Al15 were selected from the obtained sample. The crystals were glued to thin quartz fibers using beeswax and investigated by Laue photographs in a Buerger camera (white molybdenum radiation; imaging plate technique, Fujifilm, BAS-1800) in order to check their quality. An intensity data set was collected with a Stoe StadiVari four-circle diffractometer (Mo-Kα1 radiation, λ = 71.073 pm; µ-source; Gaussian beam profile; oscillation mode; hybrid-pixel-sensor, Dectris Pilatus 100 K28) with an open Eulerian cradle setup. Numerical absorption correction along with scaling was applied to the data set. Details about the data collection and the crystallographic parameters are summarized in Tables

1

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S2-S5.

Fachinformationszentrum

Further

details

Karlsruhe,

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structure

refinement

Eggenstein-Leopoldshafen

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(Germany),

from E-mail:

[email protected], by quoting the Registry no. CSD–434080.

2.3 SEM / EDX The single crystal which was used for the data collection and the bulk sample were analyzed by semiquantitative EDX analysis using a Zeiss EVO MA10 scanning electron microscope with Eu2O3, Pt and Al2O3 as internal standards. The polycrystalline piece from the annealed arc-melted button was embedded in a methylmethacrylate matrix and polished with SiO2 and diamond emulsions of different particle sizes. No impurity elements heavier than sodium (detection limit of the instrument) were observed. The experimentally determined composition of the single crystal (8±2 at.% Eu, 26±2 at.% Pt, 66±2 at.% Al; averaged from at least three measurements) and bulk sample (7±1 at.% Eu, 27±1 at.% Pt, 66±1 at.% Al; averaged from at least three measurements) were close to the ideal one (8.7 at.% Eu : 26.1 at.% Pt : 65.2 at.% Al).

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2.4 Physical Property Investigations Susceptibility Measurements: Magnetic susceptibility measurements were conducted on a Quantum Design Physical Property Measurement System (PPMS). The powdered samples of Eu2Pt6Al15 and Eu2Pt6Ga15 were loaded in a polyethylene (PE) capsule and attached to the sample holder rod of a Vibrating Sample Magnetometer (VSM) unit for measuring the magnetization M(T). The samples were investigated in the temperature range of 2.5-300 K with external magnetic fields up to 80 kOe. Heat Capacity: For the heat capacity measurements of Eu2Pt6Al15, a piece was fixed to a precalibrated heat capacity puck using Apiezon N grease and investigated in the temperature range of 2-300 K with 2 K steps from 300 to 50 K, 0.2 K steps from 49.8 to 30 K and 1 K steps from 29 to 2 K. Electrical Resistivity: An arc-melted and annealed button of Eu2Pt6Al15 was embedded in a polymethyl-methacrylate (PMMA) matrix and after solidification of the polymer polished on one side until a cross section of at least 3×3 mm2 was visible. The sample was subsequently removed from the polymer by dissolving the matrix in acetone. In a second step, the ingot was embedded again in PMMA in a self-build mold which allows parallel polishing of the second side. The sample was polished until an approximately 1.8 mm thick specimen remained inside the polymer matrix. The disc shaped sample was removed again by dissolving the PMMA matrix. The resistivity measurements were carried out in the AC transport mode29 of the PPMS. The ACT puck was modified by a van-der-Pauw press contact assembly purchased from Wimbush Science & Technology. The probes are spring contacts, gold plated over nickel; the distance between the pins was set to 2 mm. The resistivity was measured between 2-300 K with a data point every 1 K between 2 and 29 K and 50 to 300 K as well as a data point every 0.1 K between 30 and 50 K. A maximum current of 10 mA was used; the AC frequency was set to 29 Hz with a measurement time of 1 s. The recorded data of channel 1 & 2 was converted according to the van-der-Pauw equation given in the Quantum Design Application Note 1076-304.

2.5 151Eu Mössbauer Spectroscopic Investigations The 21.53 keV transition of 151

151

Sm:EuF3 source) was used for the

Eu with an activity of 130 MBq (2% of the total activity of a

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Eu Mössbauer spectroscopic experiments, which were conducted

in the usual transmission geometry. The measurements were performed with a commercial helium-bath cryostat and a liquid nitrogen bath cryostat. The temperature of the absorber was set to 6, 15, 25, 35, 40, 45, 55 and 78 K along with a second 6 K measurement for Eu2Pt6Al15 and to 6 and 78 K for Eu2Pt6Ga15, while the source was kept at room temperature. The temperature was controlled by a resistance thermometer (±0.5 K accuracy). The sample was enclosed in small PVC container, the required sample mass was calculated based on the work by Long et al..30 Fitting of the spectrum was performed with the NORMOS-90 program system.31 The obtained fitting parameters are listed in Table 2.

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2.6 Theoretical Methodology Electronic structure calculations were performed using the Vienna ab initio simulation package (VASP),32 which is based on density functional theory (DFT) and plane wave basis sets. Projectoraugmented waves (PAW)33 were used and contributions of correlation and exchange were treated in the generalized-gradient approximation (GGA) as described by Perdew, Burke and Ernzerhof.34 An energy cut-off of 500 eV and dense k-point samplings ensured well-converged structures and smooth density-ofstates. The Eu-4f states were treated as valence electrons.

3 Results and Discussion 3.1 Structure Refinement During single crystal structure analysis, it rather quickly became evident that Eu2Pt6Al15 crystallizes isostructurally to Sc2Pt6Al15 in a commensurate modulated structure, which is a superstructure of the hexagonal RE0.67Pt2Al5 compounds (P63/mmc; RE = Y, Ce, Gd-Tm35; Sc0.6Fe2Si4.9 type36) and orthorhombic Yb2Pt6Al15 (Cmcm).37 As the refinement of Sc2Pt6Al15 has been described before in great detail,38 the refinement of the title compounds will be handled rather quickly. The measured data was interpreted as orthorhombic C-centered lattice with q = 2/3 a* in superspace group Cmcm(α,0,0)0s0; a trilling was introduced to account for all measured reflections (Figure S1). The Fourier maps for all atoms along x1/x4 in the orthorhombic (3+1)D case are depicted in Figure S2. The electron density maps of Pt, Al1 and Al3 show continuity along x4, consistent with only positional modulation. The Fourier maps for Eu, Al3 and Al4 exhibit gaps along x4, consistent with occupational modulation. These were employed by crenel functions. Positional parameters and coefficients of the positional and occupational modulation functions for the refinement are listed in Table 1. As described in the literature38 a monoclinic approximant (space group P21/m) can be obtained to visualize the crystal structure (Tables S2S5). 3.2 Crystal Chemistry The superstructure contains two building blocks which get stacked along the crystallographic b axis in a …ABA’B’… sequence (Fig. 1, right). The A/A’ layers (y = 1/4 and y = 3/4) consist of honeycomb like arrangement of the Eu1 and Eu2 atoms, the centers are filled by Al triangles (Fig. 1, left). The interatomic Eu–Eu distances are 429 pm, the bonding Al–Al distances (sum covalent radii 250 pm1b) within the triangle range from 263-268 pm. These distances are slightly 6


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shorter than the ones in other compounds containing Al3 triangles such as Sr2Au6Al3 and Eu2Au6Al339 (Sr2Au6Zn3 type;40 both 286 pm), Y2Co3Al9 (Y2Co3Ga9 type; 271-274 pm) and Gd3Ru4Al12 (Gd3Ru4Al12 type; 272 pm). Aluminum triangles are also known in molecular chemistry, e.g. in the radical (tBu3Si)4Al3• (270, 274 and 278 pm).41 From layer A to A’ the Al3 triangles are inverted. The B layers are located in between (y = 0, 1/2, 1) and are formed by Pt and Al atoms. Here, bonding heteroatomic Pt–Al interactions (d < 275 pm; sum covalent radii 254 pm1b) are observed (black, green and light blue lines in Figure 2, top). The Eu1 and Eu2 atoms exhibit coordination environments with CN = 17. These can be described by a five-fold capped hexagonal prism with corrugated six-membered faces (Figure 2, middle and bottom).

Figure 1. Extended crystal structure of the monoclinic approximant of Eu2Pt6Al15. Bonds between the A/A’ and B/B’ layers have been omitted for clarity. Eu atoms are depicted in blue, Pt atoms in black and Al atoms in light blue and white. The A and A’ layers located at y = 1/4 and 3/4 are depicted on the left. The Eu1 and Eu2 atoms form honeycomb layers. The centers of the hexagons are filled by Al3 triangles. Selected interatomic distances are given in pm.

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Figure 2. (top) B/B’ layers in the crystal structure of monoclinic approximant of Eu2Pt6Al15. The different bonding patterns are highlighted by light blue, green and black lines. Coordination environments surrounding the Eu1 (middle) and Eu2 (bottom) atoms. Eu atoms are depicted in blue, Pt atoms in black, Al atoms in white and light blue. Selected interatomic distances are given in pm.

3.3 Temperature Dependent Powder X-ray Diffraction Guinier patterns of the prepared samples were recorded to check for phase purity and to refine the room temperature lattice parameters listed in Table S1. Although the superstructure crystallizes in the monoclinic crystal system with space group P21/m, only the average hexagonal structure (P63/mmc) can be refined from powder X-ray data due to low intensities of the super structure reflections (vide supra). Additionally, temperature dependent powder X-ray patterns of Eu2Pt6Al15 were recorded between 10 and 300 K. The result is depicted in Figure 3. It is clearly evident, that the a and c axes contract from 300 to about 60 K, as expected. Below 60 K, 8


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however, an abrupt contraction of –18 pm (–1.1 %) can be observed for the c axis. When looking back at the structural picture (vide supra), the c axis (in the hexagonal averaged structure) is the stacking direction of the different slabs. In contrast, only a minor effect is seen for the a axis (–1 pm, –0.2 %). This drastic change in the c direction can be only explained by a valence change of the Eu atoms from Eu2+ (112 pm1b) to Eu3+ (98 pm1b) upon delocalization of one electron into the conduction band.

Figure 3. Temperature dependence of the lattice parameters a and c and the unit cell volume V of the averaged hexagonal structure of Eu2Pt6Al15, determined by powder X-ray diffraction.

3.4 Physical Properties Powdered samples of Eu2Pt6Al15 and Eu2Pt6Ga15 were investigated by magnetic susceptibility measurements. The susceptibility and inverse susceptibility data (χ and χ–1 data) of the aluminum compound is depicted in Figure 4 (top). When going from high to low temperatures the susceptibility rises until an anomaly at T = 50.7(1) K is observed. Below this anomaly the susceptibility slowly rises again. This feature is no antiferromagnetic phase transition, as indicated by low field measurements (100 Oe, Fig. S3) and proven by Mössbauer experiments (vide infra). In contrast to literature reports, no antiferromagnetic ordering has been observed below 3 K.23 The derivative dχ/dT (red line in Figure 4, top) shows a clear anomaly at T = 41.7(1) K as well as a second small bump T = 47.8(1) K. When fitting the paramagnetic region (100-300 K) with the modified Curie-Weiss law (green line in Figure 4, top), the paramagnetic Curie temperature is θP = –39.9(1) K, in line with predominant antiferromagnetic interactions in the paramagnetic temperature region. The effective magnetic moment is calculated to µeff = 7.90(1) µB. This indicates that the Eu atoms are in a divalent oxidation state (µcalc(Eu2+) =

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µcalc(Gd3+) = 7.94 µB; see Mössbauer section). A look at the temperature dependence of the effective magnetic moment, calculated per Eu atom according to µeff = ඥ4߯ܶ (Fig. S4), clearly shows that the moment significantly drops below ~50 K down to µeff = 1.67(1) µB at 3 K. This drastically reduced moment can only be explained by the presence of Eu3+ atoms exhibiting only van Vleck paramagnetism.1a,42 This behavior is also evident in the magnetization isotherms measured at 3 and 10 K since they are curved and found below the linear 50 K isotherm (Fig. S5). These results are in line with the observations from the temperature dependent powder X-ray experiments, also indicating the proposed valence change. Heat capacity measurements (Fig. 4, middle) show a clear λ-shaped anomaly at T = 40.7(1) K along with a second anomaly at T = 47.9(1) K. This second anomaly coincides with the second feature visible in the derivative dχ/dT, with the slight increase of the a lattice parameter in the temperature dependent powder X-ray diffraction experiments (vide supra) and with the resistivity data (vide infra), suggesting a possible two-step process of the valence transition. Like in the low-field ZFC/FC measurements (vide supra), also the heat capacity shows no evidence for a magnetic ordering phenomenon below 3 K stated in the literature.23 Finally, the result of the electrical resistivity measurement is depicted in Figure 4 (bottom). The resistivity changes linear upon cooling. At T = 51(1) K a small bump is visible, followed by an abrupt change in the slope of the trace. Since the electrical resistivity is directly connected to the band structure, one can conclude a drastic change to take place in the band structure, caused by the delocalization of an electron when going from Eu2+→Eu3+. Eu2Pt6Ga15 in contrast, shows antiferromagnetic ordering at TN = 13.1(1) K (Fig. S6). The effective magnetic moment was calculated to µeff = 8.07(1) µB, indicating divalent europium atoms. The Curie temperature is θP = –30.7(1) K, in line with antiferromagnetic interactions in the paramagnetic temperature range. ZFC/FC measurements confirm the antiferromagnetic ground state, an additional anomaly is visible at T = 5.6(1) K, possibly caused by trace impurities. Magnetization isotherms at 3 K show that the antiferromagnetic ground state is extremely stable, since no metamagnetic step was observed up to 80 kOe. The saturation magnetization reaches µsat = 2.58(1) µB (3 K, 80 kOe), which is significantly below 7 µB according to gJ × J.

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Figure 4. Magnetic properties of Eu2Pt6Al15. Top: temperature dependence of the magnetic susceptibility (χ and χ–1 data) measured at 10 kOe; modified Curie-Weiss fit is depicted in green, dχ/dT is depicted in red. Middle: Heat capacity measurements along with an inset showing a detailed view on the observed anomalies. Bottom: Electrical resistivity measurements along with an inset showing a detailed view on the observed anomalies.

3.5 151Eu Mössbauer Spectroscopic Investigations To prove the presence of a valence phase transition in Eu2Pt6Al15 from Eu2+ at high temperatures to Eu3+ at low temperatures, 151Eu Mössbauer spectra were recorded at 6, 15, 25, 35, 40, 45, 55 and 78 K followed by a second measurement at 6 K. Selected spectra recorded at 6, 35, 11



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40, 45 and 78 K are depicted in Figure 5; all spectra are shown in Figure S7. The obtained fitting parameters are listed in Table 2. The spectra recorded at 6, 55 and 78 K could be fitted with one signal, the 15, 25, 35, 40 and 45 K spectra using two signals. The 6 K spectrum exhibits one signal with δ = –1.70(2) mm s–1, in line with Eu in the oxidation state +3. Additionally no hyperfine splitting is observed, underlining that the observed anomaly in the magnetism is no antiferromagnetic ordering. The two signals show isomer shifts of δ = –1.85(3) mm s–1 and – 8.02(8) mm s–1 at 35 K, δ = –2.04(4) mm s–1 and –7.87(4) mm s–1 at 40 K, and δ = –2.19(7) mm s–1 and –7.82(2) mm s–1 at 45 K. They can be attributed to both Eu2+ and Eu3+, with relative ratios of 35(1):65(1)%, 49(1):51(1)% and 74(1):26(1)% showing a transition from Eu3+ at low temperatures towards Eu2+ at high temperatures. At 78 K, finally only one signal with an isomer shift of δ = –7.92(3) mm s–1, in line with Eu2+, remains, underlining the results of the CurieWeiss fit of the magnetic data. The second measurement at 6 K again shows solely Eu3+ (δ = – 1.68(3) mm s–1), confirming a reversible valence phase transition. Because of the non-cubic site symmetry of the europium atoms, the signals of all spectra show quadrupole splitting (Table 2). The experimental line widths upon fitting were correlating with the quadrupole splitting and therefore fixed at 2.5 mm s–1, which is a typical value.43 When comparing the obtained Mössbauer data of Eu2Pt6Al15 with the reported data of the solid solution Eu(Pd1–xAux)2Si2 and EuPd2Si2, one can see that the latter exhibits a continuous signal shift from Eu2+ to Eu3+, suggesting interconfiguration fluctuation (ICF).20 Eu(Pd0.85Au0.15)2Si2 in contrast shows a first order valence phase transition around 50 K,22 in line with our observations. Figure 6 finally depicts the temperature dependence of the isomeric shifts of the respective signals and the area ratios. It is clearly visible that in the temperature range from 15-45 K both signals for Eu2+ and Eu3+ appear whereas at 6 K solely Eu3+ and at 55 K and above solely Eu2+ exists. These observations are in line with the solid solution Eu(Pd1–xAux)2Si2.22 The

151

Eu Mössbauer spectra

of Eu2Pt6Ga15 (Figure S8) in contrast exhibit one signal with isomer shifts of δ = –9.64(3) mm s–1 at 6 and δ = –9.99(3) mm s–1 at 78 K, both in line with Eu2+. Since the measurement temperature of the 6 K spectrum is below the Néel temperature of TN = 13.1(1) K, this spectrum additionally shows a hyperfine field splitting of the signal with an extremely high value of Bhf = 34.7(1) T. Typical hyperfine fields range between 20 and 29 T.44 Larger values are still not unknown, as EuFe4Sb12 for example exhibits a hyperfine field of Bhf = 67 T.45

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Figure 5. Experimental (data points) and simulated (continuous lines) 151Eu Mössbauer spectra of Eu2Pt6Al15 at 6, 35, 40, 45, 78 K. The red line corresponds to the sum of the different signals used for fitting; blue lines correspond to Eu3+ signals, green lines to Eu2+ signals.

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Figure 6. Thermal variation of the 151Eu isomer shifts for Eu2Pt6Al15. Filled circles depict the isomer shifts of the Eu2+ (green) and Eu3+ (blue) signals. The filled diamonds represent the refined area ratios of the Mössbauer signals. The solid lines and grey boxes are a guide to the eye.

3.6 Theoretical Methodology DFT electronic structure calculations using the VASP package identify Eu2Pt6Al15 as metallic and predict an antiferromagnetic ground state. Since the experimental magnetic structure is unknown, we compared A-, C-, and G-type antiferromagnetic as well as ferromagnetic (FM) patterns of the Eu-4f magnetic moments and found the C-type (antiparallel coupling of the spins within the magnetic layer, parallel coupling between the layers) as the most stable (A: +0.0226 eV/f.u.; G: +0.0140 eV/f.u.; FM: +0.0218 eV/f.u.). Figure 7 shows the atom-resolved partial density-of-states (DOS). The Al-3s3p states (green filled area in Fig. 7) spread over the whole energy range similar to aluminium metal. The Pt-5d states (blue line in Fig. 7) are likewise broad and contribute equally at the Fermi energy. Most striking is that the Eu-4f α-spin peak is pinned directly at the Fermi level. This indicates coupling of the occupied Eu-4f orbitals with the conduction electrons instead of the usual semicore-like behaviour. Even increasing the 4f on-site repulsion by using the LDA+U method (U = 8 eV) does not remove the Eu-4f α-spins from the Fermi energy, but increases energy of the β-spins only (red peaks in Fig. 7). Thus, the DFT results confirm an electronic structure with possible variable occupation of the Eu-4f states, but cannot describe the experimentally observed valence transitions.

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Figure 7. Atom-resolved partial electronic density-of-states of Eu2Pt6Al15. Eu-4f states are depicted as red, Pt-5d states as blue line. Al-3s3p states are shown as green solid area.

4. Conclusion Eu2Pt6Al15 and Eu2Pt6Ga15 have been synthesized and characterized via single crystal as well as temperature dependent powder X-ray diffraction experiments. Both compounds adopt the (3+1)D modulated Sc2Pt6Al15 type structure. While the Eu atoms in the gallide remain divalent and show antiferromagnetic ordering at TN = 13.1(1) K, the Eu atoms in the aluminum compound in contrast show a temperature induced first order valence phase transition. A possible explanation for this opposed behavior might be the difference in the bonding situation as indicated by the c lattice parameter. The unit cell of the gallide is approximately 10 pm larger at room temperature compared to the aluminum compound, suggesting stronger Pt–Al bonding between the [Pt3Al6] and the [Eu2Al3] slabs. Upon cooling and the accompanying lattice contraction, a ‘chemical pressure’ is building up in Eu2Pt6Al15. For compensation the Eu atoms undergo a temperature driven valence phase transition around 50 K from a divalent (112 pm) into a trivalent (98 pm) state by transferring the valence electron into the conduction band. This valence change is visible in the temperature dependence of the lattice parameters, the magnetic susceptibility, the specific heat as well as the electrical resistivity. The latter shows a drastic drop upon delocalization of one electron into the conduction band. Temperature dependent

151

Eu

Mössbauer spectroscopic investigations were finally used to prove the proposed valence change.

15



Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.#######. - reconstructed diffraction patterns - Fourier maps - Additional magnetic data of Eu2Pt6Al15 (zero-field-cooled/field-cooled data (100 Oe), effective magnetic moment vs. temperature, magnetization isotherms) - Magnetic data of Eu2Pt6Ga15 (zero-field-cooled data (10 kOe), zero-field-cooled/field-cooled data (100 Oe), magnetization isotherms) - Additional Mössbauer data of Eu2Pt6Al15 and Eu2Pt6Ga15 - Lattice parameters of Eu2Pt6Al15 (temperature dependent) and Eu2Pt6Ga15 - Additional crystallographic information (measurement data, interatomic distances, superspace group symmetry)

Author Information Corresponding Authors * [email protected] ORCID Oliver Janka: 0000-0002-9480-3888 Notes The authors declare no competing financial interest. Acknowledgements: We thank Dr. R.-D. Hoffmann and Dipl.-Ing. J. Kösters for the collection of the single crystal diffractometer data.

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Table 1. Atomic positions, Fourier coefficients of the modulation functions (sin, cos) and equivalent isotropic displacement parameters (pm2) of the (3+1)D description of Eu2Pt6Al15, superspace group Cmcm(α,0,0)0s0 with α = 2/3 a*. Atom Eu Pt

Al1

Al2

Al3 Al4

Wyckoff site Wave x 4c 1/2 crenel, x40 = 0.25, occupancy 2/3 8f 0 sin 0 cos 0.0114(4) 8f 0 sin 0 cos 0.002(3) 8f 1/2 sin 0 cos –0.0031(14) 4c 1/2 crenel, x40 = 0.75, occupancy 1/3 8g 0.813(2) crenel, x40 = 0.956(1), occupancy 1/3

y 0.3330(2)

z 1/4

Ueq 278(8)

0.16637(9) –0.00666(15) 0 0.1643(9) –0.0010(11) 0 –0.0055(11) –0.0033(7) 0 0.5350(15)

0.10690(1) 136(2) –0.00005(4) 0 –0.04568(13) 153(17) 0.0001(3) 0 0.13143(15) 142(15) 0.0050(10) 0 1/4 130(30)

0.2315(12)

1/4

150(20)

Table 2. Fitting parameters for the 151Eu Mössbauer spectroscopic measurements of Eu2Pt6Al15 and Eu2Pt6Ga15. δ: isomer shift; Γ: experimental line width; ∆EQ: quadrupole splitting; Bhf: hyperfine field; A: signal area ratio. T /K Ox.-state δ /mm s−1 Γ /mm s−1 ∆EQ /mm s−1 Eu2Pt6Al15 6 +3 –1.71(3) 2.1(2) 2.5* 15 +3 –1.72(3) 2.0(2) 2.5* +2 –9.2(2) 5.9(9) 2.5* 25 +3 –1.75(1) 1.9(1) 2.5* +2 –8.53(7) 4.8(3) 2.5* 35 +3 –1.88(3) 2.5(2) 2.5* +2 –8.04(8) 4.7(4) 2.5* 40 +3 –2.07(4) 3.1(2) 2.5* +2 –7.89(4) 3.7(2) 2.5* 45 +3 –2.32(8) 4.3(3) 2.5* +2 –7.85(2) 3.2(1) 2.5* 55 +2 –7.85(3) 3.2(2) 2.5* 78 +2 –7.93(3) 2.8(2) 2.5* 6 +3 –1.70(3) 2.0(2) 2.5* Eu2Pt6Ga15 6 +2 –9.64(3) –1.1(1) 2.5* 78 +2 –9.99(3) –1.0(4) 2.5* 300 +2 –10.22(3) –1.8(3) 2.5* * fixed during refinement due to correlations with ∆EQ.

20

Bhf /T

A /%

– – – – – – – – – – – – – –

100 82(1) 18(1) 77(1) 23(1) 65(1) 35(1) 51(1) 49(1) 28(1) 72(1) 100 100 100

34.69(9) – –

100 100 100


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TOC Figure

21



Figure 01 235x151mm (300 x 300 DPI)


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Figure 02 125x295mm (300 x 300 DPI)



Figure 03 246x178mm (300 x 300 DPI)


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Figure 04 258x532mm (300 x 300 DPI)



Figure 05 191x398mm (300 x 300 DPI)


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Figure 06 246x168mm (300 x 300 DPI)



Figure 07 218x110mm (300 x 300 DPI)


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Table of Content 67x47mm (300 x 300 DPI)


Abrupt Europium Valence Change in Eu2Pt6Al15 around 45 K

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