Spin-Glass Behavior and Electronic Structure of LiEu2Si3

ARTICLE pubs.acs.org/JPCC

Spin-Glass Behavior and Electronic Structure of LiEu2Si3 Eduardo Cuervo Reyes, Adam Slabon-Turski, Christian Mensing, and Reinhard Nesper* Laboratory of Inorganic Chemistry, ETH-Zurich, CH-8093 Switzerland ABSTRACT: The electronic structure and properties of LiEu2Si3 are investigated, using ab initio methods and experimental techniques. All europium ions are found in a 8S7/2 configuration. The system is metallic. We found evidence of competing exchange interactions between Eu moments which may result in the onset of a spin-glass behavior below 70 K. The electronic structures of LiEu2Si3 and the isostructural (inexistent) LiSr2Si3 are compared. The ambiguous role of lithium is discussed. The fact that LiEu2Si3 and LiSr2Si3 crystallize in different structure types is justified here, considering their relative thermodynamical stability and the difference between the effective charges of europium and strontium.

’ INTRODUCTION LiEu2Si3 was synthesized half a decade ago for the first time,1 as a result of a systematic search for ternary Zintl phases. Those works were propelled by the idea that new spin-glasses could be obtained if alkaline earth (Æ) metals were totally or partially replaced by rare-earth (R), in silicon-rich Zintl compounds containing delocalized π-electrons. The R-cations would contribute with magnetic centers, and the conductive polyanions could provide a long-range RKKY coupling between them. The sign of this coupling changes with the distance; it would lead to frustration of the magnetic interactions, which is another ingredient in the building-up of a spin-glass system. The commonly encountered triangular prismatic arrangement of divalent cations could also be a source of frustration. The inclusion of lithium, which cannot take Æ positions without distorting the structure, would allow the formation of a diversity of new Zintl anions.2 LiEu2Si3 was indeed found to crystallize in its own structure type, space group C2/m (12), featuring a new silicon polyanion. The latter is a branched planar zigzag chain (shown in gray in Figure 1) containing three different crystallographic positions. They correspond to terminal (Si3), branched (Si2), and unbranched (Si1) points, where according to the Zintl concept3
5 silicon takes 3
, 1
, and 2
charges, respectively. The chains are ecliptically stacked in the b direction at a typical distance of 4.6 Å. The basic crystallographic data are given in Table 1. As was mentioned in ref 1, an Eu3+ ion is very unusual in a Zintl compound, and therefore, with the expected valence counting Li1+[Eu2]4+[Si3]6

1e, there is a deficit of one electron every three silicons. The lack of electrons in the π* states has been observed in many other Zintl compounds with ecliptically stacked planar anions, even when a closed shell configuration would be anticipated.6
9 So, one should expect either an additional enhancement of the net π bonding character or the unusual presence of Eu3+ ions. Due to technical difficulties with calculations involving europium atoms, the discussion of the electronic structure of LiEu2Si3, presented in ref 1, was done replacing europium by strontium in the band structure calculations. r 2011 American Chemical Society

Interestingly, the LiSr2Si3, isostructural to LiEu2Si3, has never been synthesized. In spite of the chemical similarity between Eu2+ and Sr2+ cations, LiSr2Si3 has been found to crystallize in a different structure type, space group Fddd,10 which contains planar Si6 rings. We found it interesting to compare the electronic structures of LiEu2Si3 and of the inexistent LiSr2Si3 (which will be called LiSr2Siq3 in the following). To date, neither measurements of the magnetic properties of LiEu2Si3 nor first-principles studies of its electronic structure have been reported. With this work we answer four open questions: (I) on magnetism and conductivity of LiEu2Si3, (II) on actual electronic structure of LiEu2Si3, (III) how similar are the electronic structures of LiEu2Si3 and LiSr2Siq3 and (IV) why LiEu2Si3 and LiSr2Si3 are found in different structure types. To this end, we measured the magnetic response of single crystals of LiEu2Si3; we made pellets which then we reused for measurements of electrical resistivity and of the heat capacity; and the theoretical electronic structures were calculated using several first-principles codes at the density functional theory (DFT) level.

’ METHODS Synthesis and Preparation. Europium (99.9 wt %) was redistilled twice under high-vacuum conditions and mixed in stoichiometric amounts with lithium (99.99 wt %) (Sigma Aldrich) and silicon lumps (99.9999 wt %) (Alfa Aesar). The mixture was sealed in a niobium tube and heated in a high-frequency furnace at 1000
C for 10 min. After the prereaction, the sample was ground, pressed into a pellet, heated at 1000
C for 24 h, and cooled at a rate of 20
C/h to 850
C for 7 days. The product consists of metallic shining crystals with prismatic shapes which are very sensitive to air and moisture; therefore, the crystals used for structural characterization and measurements of physical properties were handled in an Received: June 22, 2011 Revised: October 31, 2011 Published: November 29, 2011 1158

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Figure 1. Crystal structure of LiEu2Si3.

Table 1. Crystallographic Data of LiEu2Si3 SG: C2/m (12) monoclinic, Z = 4 axes (Å)

a = 15.473

b = 4.597

c = 7.829

— (deg)

α = 90

β = 104.55

γ = 90

(Wy)

x/a

Figure 2. XRD pattern of LiEu2Si3. y/b

z/c

Li1 (4i)

0.213

0

0.271

Eu1 (4i)

0.376

0

0.062

Eu2 (4i)

0.385

0

0.570

Si1 (4i)

0.044

0

0.146

Si2 (4i)

0.046

0

0.648

Si3 (4i)

0.202

0

0.711

most relevant interatomic distances (Å) Si
Si/Li Intrachain Si1
Si2

2.374

Si2
Si3

2.342

Si2
Si2

2.403

Si1
Li1

2.554

Si3
Li1

2.631

Eu1
Eu2

3.940

Si3
Eu1

3.311

Eu1
Li1

3.327

Si3
Eu2

3.234

Eu2
Li1

3.070

Si2
Eu1 Si2
Eu2

3.240 3.202

Eu1
Eu2 Eu1
Eu2

3.940 4.076

Si2
Eu2

3.330

Eu2
Eu2

3.971

Si1
Eu1

3.409

Eu1
Eu2

3.894

Si1
Eu1

3.243

Eu1
Eu2

4.076

Si1
Eu2

3.193

Eu1
Eu1

4.195

Prism Li12Eu12Eu22Si3

Prism Eu12Eu24Si2

Prism Eu14Eu22Si1

argon-filled glovebox (O2 < 1 ppm, H2O < 1 ppm). The sample is nearly phase pure, which was confirmed by X-ray powder diffraction measured in transmission mode with Cu Kα1 radiation on a STOE STADI-P2 diffractometer equipped with a DECTRIS Mythen 1K detector. The sample consists of LiEu2Si3 with a small amount of EuSi as impurity phase, as shown in Figure 2; the observed and the calculated powder patterns are shown in black and red, respectively. The reflection visible around 31
corresponds to the strongest reflection peak of the EuSi diffraction pattern. Magnetic Measurements. The measurements were done with a Superconducting Quantum Interference Device (SQUID) from Quantum Design.11 Our SQUID can operate an applied field up to 5 T and measures moments as low as 10
8 emu (10
11 Am2) over a

temperature range from 1.9 to 400 K. We used single crystals of about 30 mg and performed scans of 3 cm. The magnetic moments reported for each temperature are averaged for three scans, and every point of each scan was measured ten times. The standard deviations of the moments were three or more orders of magnitude smaller than the average values. Specific Heat Measurements. A pellet was made of pressed ground crystals of LiEu2Si3 and annealed at 650
C for 7 days. The measurements were carried out by means of a Physical Properties Measurement System from Quantum Design QD-PPMS 9. In a standard configuration, this system has a resolution of 10 nJ/K at 2 K and can operate over a temperature range of 1.9
400 K. At every temperature, three measurements were performed. To ensure the reproducibility, the whole temperature scan was repeated three times. Resistivity Measurements. Resistivity measurements were performed on the same pellet, by the four-point van der Pauw method, from 75 to 300 K, every 0.5 K. The temperature was varied at a rate of 50
C/h. Unfortunately, our aparatus is not suited for measurements at lower temperatures, and the intents to measure air-sensitive samples in our PPMS system have not been successful. Computational Methods. We used three different DFT implementations: (1) plane wave basis with pseudopotentials (PP) within the CASTEP12 package of Materials Studio (MS), (2) partial waves with numerical radial functions within the DMol3 package of MS, and (3) the tight- binding linear-muffin-tin-orbitals method in the atomic spheres approximation (TB-LMTO-ASA) from Andersen’s group.13
18 (1) We used norm-conserving PP and the exchange-correlation functional from Perdew
Burke
Erzenhof (PBE)19 in the generalized gradient approximation (GGA). The pseudoatomic calculations were done on the configuration Si 3s23p2, Sr 4s24p65s2, Li 1s22s1, and Eu 5s25p64f76s2. For the periodic systems, the following orbitals were added: Si 3d, Sr 4d, Eu 5d, and Eu 6p. The Gaussian smearing was set to 0.05 eV, the plane wave cutoff to 880 eV, and the separation between k-points to less than 0.02 Å
1. Broyden mixing was used in the self-consistent field (SCF) algorithm with a tolerance of 0.5  10
6 eV for the energy per atom, 10
7 eV for the eigenvalues, and 10
8 eV for the Fermi level (EF). Up- and down-spin occupations were considered independent, and each europium atom was given the 8S7/2 configuration as an initial guess. 1159

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Figure 3. Temperature dependency of the electrical resistivity of Eu2LiSi3.

(2) We took the basis set double-numerical with polarization functions DNP 3.5. We used the PBE-GGA exchange correlation functional. The number of k-points was chosen to give a separation of less than 2  10
2 Å
1 between them. The SCF tolerance was set to 10
6 Ha and the orbital smearing to 5  10
4 Ha. Relativistic corrections were taken into account. The calculations were spin-unrestricted with the initial configuration 8S7/2 for europium atoms. (3) The version LMASA-46 was employed. We included relativistic corrections and chose the nonlocal exchange-correlation due to Langreth
Mehl
Hu. We used Broyden mixing in the SCF algorithm with a tolerance of 10
5 Ry for the total energy, 10
5 e for the atomic charges, and 10
6 Ry for EF. We took as basis set: 3s, 3p, and 3d for Si; 2s and 2p for Li; 5s, 5p, and 4d for Sr; and for Eu, 6s, 6p, 5d, and 4f. The k-space integration was done following the tetrahedral method,20,21 on a grid of 16  16  16 points. The muffin-tin radii, after blowing up to fill the Wigner
Seit cell, were R(Eu1) = 4.421, R(Eu2) = 3.987, R(Si1) = 2.538, R(Si2) = 2.528, R(Si3) = 2.529, and R(Li) = 2.652, all given in atomic units. A good space filling with small overlap between the atomic spheres was achieved without including empty spheres.

’ RESULTS AND DISCUSSION Experimental Results. Electrical Resistivity. The electrical resistivity of LiEu2Si3, as a function of temperature, is shown in Figure 3. The material is metallic. Assuming that the linear dependence extends to lower temperatures, the extrapolated value at T = 0 K is F0 ≈ 0.4 μΩ 3 m. The resistivity could have been overestimated due to grain-boundary effects; we expect these undesirable effects to be small after the annealing of the pellet, though. The temperature coefficient (1/F)(ΔF/ΔT) at 200 K is approximately 0.33  10
2 K
1. Our experimental setup is not very accurate; therefore, we prefer not to make any further interpretation of the small deviation from a straight line observed in the present measurements. Magnetic Structure and Properties. In LiEu2Si3, europium atoms are located in two inequivalent crystallographic positions. If all of them are, as in most Zintl compounds, in a f7 configuration (8S7/2), they should contribute with a local magnetic moment of 7.94 μB each. This can be checked with measurements of the

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Figure 4. Susceptibility (χ) vs temperature (T) per mol of Eu; H = 100 Oe (empty circles) and H = 1000 Oe (full circles). In red, zero-field-cooled; in blue, field-cooled. Insets: high-T linear fit (cyan line) for χ
1 vs T (up) and χ vs T
1 (down).

Table 2. Linear Fit for χ vs T
1 as y = Cx + χ0 and for (χ
χ0)
1 vs T as y = C
1(x
θ) value C

8.17

χ0


0.0029

std. error 0.03

R2 0.9998

0.0001

[C
1]
1

8.17

0.03

C
1θ

0.0

0.1

0.9998

magnetic susceptibility of highly pure single crystals. If there is some Eu3+ (nonmagnetic 7F0), the measured effective moment per Eu (μeff) should be smaller. At high temperatures (T much larger than the strongest of the magnetic couplings), the magnetic susceptibility data can be modeled with a Curie
Weiss+Pauli (CWP) function χ¼

C þ χ0 T
θ

ð1Þ

θ is a linear combination of all the couplings, acquiring a clear meaning when there is only one coupling type; C is the Curie constant; and χ0 is a temperature-independent term, which accounts for the combined effect of the Pauli spin-paramagnetism of the conduction electrons and the diamagnetic contribution from the paired electrons and Landau √ currents. μeff, in Bohr magnetons, is obtained as μeff = 2.82 C, provided χ is given in cm3 mol
1 Eu and T in Kelvin. It is very important to verify that we are using data from the high-temperature regime; small deviations from the CWP behavior lead to completely wrong μeff. Consequently, we should not only look at the R2 of the linear regression but also compare the C obtained from the fit χ vs T
1 with the inverse of the slope from χ
1 vs T. Both values must be equal (within the error margin). Figure 4 shows the magnetic susceptibility, measured from 2 to 300 K, of a zero-field-cooled sample, in red, and a field-Cooled sample, in blue. Table 2 and the insets in Figure 4 show the CWP fit of the high-temperature part (from 200 to 300 K). Both fits give identical C, with excellent R2 values. The effective moment per europium is 8.06 ( 0.01 μB, which is very close to the theoretical 1160

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Figure 5. Field dependency of the magnetization of LiEu2Si3 at a temperature of 5 K.

Figure 7. Top: Band structure of LiEu2Si3 excluding the core f-bands (left) and LiSr2Siq3 (right). Bottom: the corresponding DOS with Eu in dashed line and with Sr in full line.

Figure 6. Temperature dependency of the specific heat of LiEu2Si3. The inset shows the Sommerfeld coefficient.

value for Eu-f7. This confirms that we can rule out the presence of Eu3+ in the structure. The small χ0, actually negative, indicates that the diamagnetic contribution dominates over the spin-paramagnetism of the conduction electrons. We obtained θ ≈ 0, which could either mean that there is no magnetic coupling or that the couplings nearly cancel at the molecular field level. Here, the second choice is the right one since the low-temperature behavior is that of a frustrated system, showing history dependence under 70 K, with an antiferromagnetic-like hip between 25 and 50 K, depending on the field and history. The field dependence of the magnetization, at low temperatures, is nonlinear, but no real hysteresis opens (see Figure 5). This magnetic behavior strongly suggest the not unexpected presence of a spin-glass system for such type of material. We are planning to perform more specific magnetic measurements, to confirm the glassy behavior and to have a more complete characterization based on the spin dynamics. Asuming that the spin-freezing temperature corresponds to the cusp in the susceptibility, we obtain for it Tsg ≈ 22 K.

Specific Heat. The temperature dependence of the zero-field specific heat (C) and the Sommerfeld coefficient (γ = C/T) of LiEu2Si3 is shown in Figure 6; C is given in units of the universal gas constant (R) and γ in SI units. The peak around 22 K corresponds to the spin-freeze in agreement with the magnetic measurements (Figure 4). There are also small changes in the slope of γ in the temperature range where the magnetic response becomes field and history dependent (T ≈ 70 K). The rather large values of γ (∼500 mJ/mol K2) and the fact that it is temperature dependent are indications of the non-Fermi liquid contribution to the specific heat, originating from the localized 4f electrons, which are weakly hybridized with the conduction band. Theoretical Results. The electronic structures obtained with CASTEP, DMol3, and LMTO methods are quite similar. Exceptions are the Mulliken charges, which are basis-dependent and as such not expected to be equal. Results presented in the following are taken from either of the three calculations; differentiations will be made when pertinent. First, we compare the band structures of LiEu2Si3 and LiSr2Siq3. We consider this to be relevant since the underoccupation of the π* orbitals could cause the extraction of electrons from the 4f states. This would be unusual for a Zintl compound; however, it is possible for europium atoms, while it is impossible for strontium. 1161

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Table 3. Charge Partitioning and Overlap Population Eu2LiSi3

Sr2LiSiq3

Electron Counting AO M1 s

0.23

0.22

M1 p

0.27

0.00

M1 d

1.13

1.05

M1 f

6.94

M2 s

0.28

0.23

M2 p

0.39

0.00

M2 d

1.29

1.20

M2 f Si1 s

6.93 1.50

1.52

Si1 p

3.03

3.17

Si2 s

1.42

1.41

Si2 p

2.93

2.98

Si3 s

1.50

1.57

Si3 p

3.14

3.42

Li s

0.00

0.22

Mulliken Charge AO/DNP M1

0.43/0.65

M2

0.11/0.46

0.72/1.17 0.57/1.13

Si1


0.53/
0.33


0.69/
0.68

Si2


0.35/
0.32


0.37/
0.62

Si3


0.67/
0.30


0.99/
0.80

1.00/
0.16

0.78/
0.20

Li

Muffin-Tin Charge M1

1.56

1.54

M2 Si1

0.94
0.83

0.88
0.81

Si2


0.65


0.63

Si3


0.89


0.87

Li


0.13


0.11

Mulliken Overlap Population Si1
Si1 2.35 Å

0.75

0.68

Si1
Si2 2.37 Å

0.73

0.67

Si2
Si2 2.40 Å Si2
Si3 2.34 Å

0.67 0.76

0.62 0.72

Figure 7 shows the band structures and the total density of states (DOS). The occupied flat f-bands of LiEu2Si3, appearing around
0.6 eV, have not been drawn to ease the comparison. Differences between LiEu2Si3 and LiSr2Siq3 band structures are negligible. Thus, the use of Sr instead of Eu, for the analysis of the chemical bonding in ref 1, is again justified. Our calculations confirm that all europium atoms keep seven electrons in the 4f shell and the π* states are, indeed, underpopulated. Here, we would like to point out a small difference with the bonding pattern obtained in ref 1. There, the authors found more residual πbonding interaction in the Si2
Si2 bond, which is, counterintuitively, the longest. Here, we found an inverse correlation (with no exception) between bond length and bonding interaction. The differences in the numerical values are small, though, and we think the origin of the discrepancy with ref 1 is that, there, the overlap populations were calculated within a simpler approximation; namely, the extended-H€uckel method. For numerical details, Mulliken charges and overlap populations between silicon atoms are shown in Table 3.

Figure 8. Projected DOS of LiEu2Si3 excluding the f-states. Solid, s; short-dashed, p; dotted, d.

The partial DOS from LMTO are shown in Figure 8, from
11 to 11 eV; core and f states were left out. The six bands between
11 and
4.5 eV consist of silicon 3s states, responsible for the high binding energy sector of the chains (three bonds per formula unit, two formula units per primitive cell). From
4.5 eV to the Fermi level (EF), one finds a combination of mainly Si-3p, Eu-5d, Eu-6s, and Eu-6p and a small contribution of lithium orbitals. The system is predicted to be metallic, with EF crossing two bands with dominant Eu-5d and Si-3p character, both. A significant part of the π* orbitals lays unoccupied above EF. The role played by lithium, and how it shows up in our calculations, is another interesting point to comment on. It has been observed2 that in Zintl phases lithium atoms often occupy terminal anion positions, for this reason earning the term of “zwitter”. In LiEu2Si3, lithium can be seen as part of distorted cationic prisms (Li2Eu4) which enclose terminal Si atoms, analogous to the Eu6 prisms that enclose every Si of the backbone of the chain, but it can also be seen as replacing the missing terminal silicon atoms, so giving a fully branched chain (LiSi3). In Table 3 we show 1162

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Figure 9. DFT polarization function for two magnetic configurations; section in the plane of the Eu atoms along the chain. (a) Two Eu moments up and two down; (b) all Eu moments up.

the Mulliken analysis according to the projection of the charge onto atomic orbitals (AOs), onto double numerical orbitals with polarization functions (DNP), as well as the charge partitioning into muffin-tin spheres (MT). When AOs are used, lithium is seen to take nearly its formal charge (1+), which arises from the extraction of its valence electron from the 2s orbital. When polarization functions are included, and according to the MT partitioning, lithium appears slightly negatively charged. This can be understood from the partially covalent interaction with the silicon chain and the delocalization of the charge along the latter. Thus, the ambiguous role of lithium seems to be traceable by suitable charge partitioning. As it was mentioned before, we considered the system to be spin polarized in all our calculations or, alternatively, put the 4f orbitals in the core. Although DFT is not quite reliable for studying magnetic states, the spin polarization should be taken into account in order not to violate some basic rules. Nonpolarized calculations or spin restricted are not appropriate for systems like LiEu2Si3, which have localized electrons in partially full shells. A nonpolarized calculation forces the SCF loop to put an equal number of spins up and down in every orbital. This is a violation of the first Hund rule; it causes the open shell to move upward in energy and to be crossed by the Fermi level, sometimes leading to unphysical metallic states, migration of electrons from the open shell toward other empty orbitals, etc. Within a spin-polarized calculation, one has to give an initial guess for the spin configuration; the SCF loop may converge to the closest minimum (not necessarily the absolute minimum). We ran several SCF cycles, each one with a different initial spin distribution. In all of them, we gave each europium the S = 7/2 configuration and zero polarization to every other orbital; the spins were allowed to relax. All converged electronic distributions had nearly seven electrons and spin S = 7/2 in the f shell of every Eu atom. The total local moment per europium was slightly larger due to the weak polarization of the d orbitals, which is induced by the local exchange interaction with the f electrons. Figure 9 shows the polarization function η = |Fv
FV|/(Fv + FV) for a section across the europium atoms, parallel to the silicon chain, for two of the many possible spin configurations. Fv (FV) is the spin-up (down) electron density. In configuration (a), the two

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europium atoms to the left have spin-up and the two to the right spin down; in configuration (b), all spins are pointing up. The main contribution to the magnetic polarization is localized in the europium 4f states. These magnetic centers are not isolated, as they communicate through the polarizable delocalized electrons along the chain, the tenuous blue band visible in the picture. The node of η along the a direction, in the upper frame, arises from the continuity of η and the spins of right- and left-side atoms being opposite. The energy per cell in (a) turned out to be about 0.0033 eV (38 K) lower than the energy in (b). The differences between (a) and other alternating-spin configurations are much smaller, in agreement with the glass-like behavior observed in the magnetic measurements. Lastly, we address the question concerning the preference of LiEu2Si3 and LiSr2Si3 for different structure types. We computed their cohesive energies (εcoh) and estimated their formation enthalpies (ΔHf
) from the difference between the εcoh of the compounds and those of the elemental solids. We obtained, for LiEu2Si3 in its own structure type, εcoh = 21.48 eV/formula unit and ΔHf
=
286.34 kJ/mol, while for LiEu2Si3 in the LiSr2Si3 structure type, ΔHf
=
277.22 kJ/mol. On the other hand, LiSr2Si3 is thermodynamically more stable than LiSr2Siq3, with a difference in formation enthalpy of
19.34 kJ/mol. While Li plays a very ambiguous role in LiEu2Si3, its position in LiSr2Si3 is more anionic-like, occupying silicon vacancies so completing the honeycomb planar polyanion. In the LiEu2Si3 structure type, the divalent metal is surrounded by 8Si and 2Li, while in the LiSr2Si3 structure type, it is surrounded by two six-rings (Si6 + Li2Si4). In the LiSr2Si3 structure type, silicon and lithium atoms form, together, a honeycomb sublattice. Sr takes the interstitial positions, between two hexagons (six-rings), which correspond to a hexagonal sublattice. Taking into account that strontium ions generally appear having larger effective charge than europium ions, it is not surprising that the formers prefer more negatively charged environments than the latter. In other words, europium does not give away as many electrons as necessary for lithium to complete the honeycomb Fddd substructure of LiSr2Si3, and therefore, the structure is distorted, acquiring C2/m symmetry and allowing lithium to take a less anionic character.

’ SUMMARY LiEu2Si3 single crystals were used for magnetic measurements. Pellets were made from ground crystals and used for conductivity and specific heat measurements. According to our experiments and calculations, LiEu2Si3 is metallic, with a resistivity of 0.8 μΩ 3 m and a temperature coefficient (1/F)(ΔF/ΔT) of 0.33  10
2 K
1 at 200 K; all europium atoms in the structure have a core 8 S7/2 configuration, resulting in local magnetic moments of 8.06 ( 0.01 μB. Below 70 K the magnetic response becomes field- and history-dependent with a signature of spin-freezing around 22 K. We showed that LiEu2Si3 and the isostructural but nonexistent LiSr2Si3 (C2/m) have similar electronic distributions, excluding the 4f electrons; strontium atoms always lose more of their electrons in comparison to europium in the same crystallographic positions; and the ambiguous role (cationic- and/ or anionic-like) of lithium can be traced by a suitable charge analysis. The occurrence of LiEu2Si3 and LiSr2Si3 in different structure types seems to be related to the ability(difficulty) of strontium(europium) to give away enough electrons for the formation of a common [Li2Si4][Si6] substructure with sufficient positive overlap population for LiSr2Si3 (Fddd). This is confirmed 1163

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by their relative thermodynamical stabilities; i.e., LiEu2Si3 (C2/m) has lower enthalpy than LiEu2Si3 (Fddd) by 9.12 kJ/mol, while LiSr2Si3 (Fddd) has lower enthalpy than LiSr2Si3 (C2/m) by 19.34 kJ/mol.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: +41 (0)44 632 30 69. Fax: +41 (0)44 632 11 49.

’ ACKNOWLEDGMENT This work has been supported by the Swiss National Science Foundation under project no. 2-77937-10. ’ REFERENCES (1) Qinxing, X.; Nesper, R. Z. Anorg. Allg. Chem. 2006, 632, 1743. (2) Nesper, R. Prog. Solid State Chem. 1990, 20, 1. (3) Zintl, E. Angew. Chem. 1939, 52, 1. (4) Klemm, W. Proc. Chem. Soc. London 1959, 329. (5) Nesper, R. Zintl Phases Revisited in Silicon Chemistry; Jutzi, P., Schubert, U., Eds.; Wiley VCH: New York, 2004. (6) Evers, J.; Weiss, A. Solid State Commun. 1975, 17, 41. (7) Wengert, S. Ph. D. Dissertation ETH N
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