Synthesis, crystal and electronic structure, physical properties and ¹²¹Sb and ¹⁵¹Eu Mössbauer spectroscopy of the Eu₁₄AlPn₁₁ series (Pn = As, Sb)

Abstract

The pnictides Eu₁₄AlAs₁₁ and Eu₁₄AlSb₁₁ were synthesized from the elements in sealed niobium ampoules. They crystallize in the tetragonal crystal system (Eu₁₄AlAs₁₁: a = 1627.6(2), c = 2180.0(4) pm; Eu₁₄AlSb₁₁: a = 1725.6(2), c = 2289.7(7) pm) with space group I4₁/acd, isostructural to Ca₁₄AlSb₁₁ and can be described as Zintl phases. The Al atoms are surrounded by four pnictogen atoms, forming [AlPn₄]⁹⁻ tetrahedra. Additionally isolated Pn³⁻ anions and linear Pn₃⁷⁻ trimers can be found in the crystal structure. The compounds can be described according to (Eu²⁺)₁₄(AlPn⁹⁻)(Pn³⁻)₄(Pn₃⁷⁻). Eu₁₄AlAs₁₁ and Eu₁₄AlSb₁₁ both exhibit an antiferromagnetic transition at T_N = 10.5(1) K and 12.5(1) K, respectively. For the antimonide, the magnetic transition has been confirmed by additional heat capacity measurements. Resistivity investigations indicate that Eu₁₄AlSb₁₁ is a semiconductor with a band gap of E_g = 0.28(5) eV close to room temperature. According to the Zintl formalism, the Eu atoms are divalent, which has been confirmed by magnetic susceptibility and additional ¹⁵¹Eu Mössbauer spectroscopic studies. The measurements conducted at 6 K, below the magnetic ordering temperature, show a full hyperfine field splitting with complex spectra underlining the recorded magnetic data. Furthermore, ¹²¹Sb Mössbauer spectroscopic studies have been conducted to study the different antimonide entities in the title compounds.

---

1. Introduction

Zintl phases can be positioned between intermetallic compounds and insulating valence compounds from an electronic perspective.¹ Usually they contain an electropositive alkali or alkaline-earth metal, which formally reduces the more electronegative element, which can be a triele, tetrele or a pnictogen for example. A more precise definition classifies Zintl compounds as materials in which both ionic and covalent bonding exists. Besides the ‘classical’ binary Zintl phases numerous ternary compounds, meanwhile also containing transition metals, have been reported.² Usually Zintl compounds exhibit semiconducting properties, although also metallic compounds have been reported. The semiconducting properties make Zintl compounds an important class of materials for e.g. thermoelectric applications.³ Yb₁₄MnSb₁₁ ^4,5 or Yb₉Mn_4.2Sb₉ ⁶ are prominent examples for antimony containing high zT thermoelectric materials. Recently we have reported on the first compound in the ternary system Eu–Al–Sb, Eu₅Al₂Sb₆,⁷ which crystallizes isotypically to Sr₅Al₂Sb₆.^8,9 Further investigations lead to the discovery of another prominent composition amongst antimonide Zintl phases: Eu₁₄AlSb₁₁, which crystallizes in the Ca₁₄AlSb₁₁ type structure.¹⁰ Of this 14–1–11 composition numerous compounds have been reported: A₁₄AlSb₁₁ (A = Ca,^10,11 Sr,¹¹ Ba¹¹) Ca₁₄MnX₁₁ (X = P,¹² As,¹³ Sb,¹³ Bi¹⁴), Sr₁₄MnX₁₁ (X = As,¹³ Sb,¹³ Bi¹⁴), Ba₁₄MnX₁₁ (X = As,¹³ Sb,¹³ Bi¹⁴), Ba₁₄InP₁₁,¹⁵ Eu₁₄MnX₁₁ (X = P,¹⁶ As,¹⁶ Sb,¹⁷ Bi¹⁸), Eu₁₄InX₁₁ (X = Sb,¹⁸ Bi¹⁸), Yb₁₄MnX₁₁ (X = Sb,¹⁹ Bi¹⁹) and finally Yb₁₄MSb₁₁ (M = Al,²⁰ Zn^20,21).

Here we report on Eu₁₄AlSb₁₁ and the isostructural arsenide Eu₁₄AlAs₁₁, so far unknown elemental combinations of the Ca₁₄AlSb₁₁ type structure. We have investigated the crystal structure, physical properties (magnetism and electrical resistivity), along with ¹²¹Sb and ¹⁵¹Eu Mössbauer spectroscopic measurements of the bulk material. Theoretical calculations have been used to elucidate the electronic and magnetic structures of Eu₁₄AlSb₁₁ and finally Bader charges were calculated in comparison with the strontium compound Sr₁₄AlSb₁₁.

2. Experimental

2.1. Synthesis

Starting materials for the syntheses of Eu₁₄AlAs₁₁ and Eu₁₄AlSb₁₁ were europium ingots (Alfa Aesar, 99.9%), aluminium turnings (Koch Chemicals, 99.99%), as well as arsenic (Klepin, 99.9%) and antimony pieces (VWR International, 99.9%). The europium ingots were stored under argon; impurities on the surface were removed prior to the synthesis. The argon was purified over molecular sieves, silica gel and a titanium sponge (873 K). The elements were weighed in the ideal atomic ratio and arc-welded²² in niobium tubes in a dried argon atmosphere of about 800 mbar. The metal tubes were subsequently placed in the water-cooled sample chamber of a high-frequency furnace (Hüttinger Elektronik, Freiburg, type TIG 1.5/300) under flowing argon.²³ The temperature was controlled by a Sensor Therm Methis MS09 pyrometer with an accuracy of ±50 K. The samples were rapidly heated to 1273 K, after 10 min the temperature was reduced to 1123 K and the samples were dwelled for two hours. After the annealing step, the samples were cooled to room temperature within two hours. The samples were obtained as brittle polycrystalline pieces with metallic lustre and could easily be separated from the crucible material. No reaction with the container material was observed. The compounds showed moderate sensitivity to moisture and were therefore stored under dry argon atmosphere. The corresponding bismuth compound could not be obtained under the given synthetic conditions (1273 K); the main components were Eu₁₆Bi₁₁ (Ca₁₆Sb₁₁ type)²⁴ and Eu₄Bi₃ (anti-Th₃P₄ type).²⁵ Reactions carried out at lower temperatures (943 K) yielded Eu₁₆Bi₁₁ as major product.

2.2. EDX data

Semiquantitative EDX analysis of the single crystal that has been investigated on the diffractometer was carried out by use of a Zeiss EVO® MA10 scanning electron microscope in variable pressure mode with EuF₃, Al₂O₃ and Sb as standards. The experimentally observed compositions (Eu₁₄AlSb₁₁: 49(2) at% Eu, 8(2) at% Al, 43(2) at% Sb) were close to the theoretical (53.9 at% Eu, 3.8 at% Al, 42.3 at% Sb) and refined ones. The irregularly shaped crystals contained no impurity elements heavier than sodium (detection limit of the instrument). The deviations observed are related to the conchoidal fractures.

2.3. Powder X-Ray diffraction

The polycrystalline samples were characterized by Guinier powder patterns using CuK_α1 radiation and α-quartz (a = 491.30 and c = 540.46 pm) as an internal standard. The Guinier camera was equipped with an imaging plate technique (Fujifilm, BAS-READER 1800). Least-squares refinements led to the lattice parameters listed in Table 1. To ensure correct indexing, the experimental patterns were compared to calculated ones using the refined atomic positions (vide infra) obtained from the single crystal X-ray diffraction.²⁶

Table 1 Lattice parameters of the M₁₄AlPn₁₁ (M = Ca, Sr, Ba, Eu, Yb; Pn = As, Sb; Ca₁₄AlSb₁₁ type, space group I4₁/acd, Z = 8) members

Compound	a (pm)	c (pm)	V (nm^3)	Lit.
a This work, ^P powder data, ^SC single crystal data.
Eu14AlAs11^P	1627.6(2)	2180.0(4)	5.7750
Ca14AlSb11^P	1667.6(5)	2242.3(7)	6.2356	10
Ca14AlSb11^P	1667.2(6)	2243(1)	6.7905	11
Sr14AlSb11^P	1749.3(4)	2348.0(8)	7.1850	11
Ba14AlSb11^P	1829.3(2)	2422.2(9)	8.1055	11
Eu14AlSb11^P	1725.6(3)	2289.7(7)	6.8183
Eu14AlSb11^SC	1726.49(4)	2292.24(6)	6.8326
Yb14AlSb11^P	1656.1(2)	2210.2(3)	6.0635	20

---

2.4. Single crystal X-ray diffraction

Single crystals of Eu₁₄AlSb₁₁ were isolated from the crushed sample. The crystal fragments were glued to thin quartz fibres using beeswax and investigated by Laue photographs on a Buerger camera (white Mo radiation, Fujifilm imaging plate, BAS-1800) in order to check their quality for intensity data collection. The intensity data set was collected at room temperature using a Stoe IPDS II (graphite-monochromatized MoK_α radiation; oscillation mode) diffractometer. Numerical absorption correction was applied to the data set. Details about the data collection and the crystallographic parameters are summarized in Tables 2–4.

Table 2 Crystallographic data and details of the room temperature single crystal structure refinement of Eu₁₄AlSb₁₁ (Ca₁₄AlSb₁₁ type, space group I4₁/acd, Z = 8)

Lattice parameter/pm	a = 1726.49(4)
	c = 2292.24(6)
Volume, V/nm^3	6.8326
Molar Mass/g mol^−1	3493.7
Calc. density, Dx/g cm^−3	6.79
Abs. coeff. μ/mm^−1	33.8
F(000)/e	11 648
Crystal size/μm^3	20 × 30 × 35
Transmission ratio (min/max)	0.019/0.045
Detector distance/mm	80
Exposure time/min	10
Integr. parameters A/B/EMS	13.0/−1.0/0.030
θ range/°	2.4–32.1
Range in hkl	−25, +21; ±25; ±34
Total no. reflections	57 201
Independent reflections/Rint	2971/0.1598
Reflections with I ≥ 3σ(I)/Rσ	1826/0.0487
Data/parameters	2971/62
Goodness-of-fit on F^2	1.16
R 1/wR2 for I ≥ 3σ(I)	0.0345/0.0571
R 1/wR2 for all data	0.0805/0.0695
Extinction coefficient	40(20)
Largest diff. peak/hole/e Å^−3	+2.74/−2.94

---

Table 3 Atomic coordinates and isotropic displacement parameters U_eq (pm²) Eu₁₄AlSb₁₁ (Ca₁₄AlSb₁₁ type, space group I4₁/acd, Z = 8). Standard deviations are given in parentheses

	Wyckoff site	x	y	z	U eq
Eu1	32g	0.04391(3)	0.07244(3)	0.17137(2)	216(1)
Eu2	32g	0.02100(3)	0.37660(4)	0.00263(3)	242(1)
Eu3	16e	0.35607(5)	0	1/4	215(2)
Eu4	32g	0.34128(4)	0.06830(4)	0.09267(2)	252(2)
Al	8a	0	1/4	3/8	209(1)
Sb1	16f	0.13305(4)	0.38305(4)	1/8	200(2)
Sb2	32g	0.36509(5)	0.25319(4)	0.06209(3)	212(2)
Sb3	32g	0.13138(5)	0.02507(4)	0.04697(3)	210(2)
Sb4	8b	0	1/4	1/8	237(3)

---

Table 4 Interatomic distances (in pm) in the structure of Eu₁₄AlSb₁₁ (Ca₁₄AlSb₁₁ type, space group I4₁/acd, Z = 8). Standard deviations are smaller or equal to ±0.1 pm. All distances <400 pm are listed

Eu1	1	Sb3	330.6	Eu2	1	Sb2	325.7	Eu3	2	Sb3	323.7
	1	Sb1	332.5		1	Sb3	329.2		2	Sb2	344.6
	1	Sb3	332.9		1	Sb3	339.0		2	Sb1	351.0
	1	Sb4	333.2		1	Sb1	340.9		2	Eu4	380.3
	1	Sb2	336.0		1	Sb4	357.4		2	Eu1	391.5
	1	Sb2	340.7		1	Al	367.0		2	Eu2	396.5
	1	Eu4	370.8		1	Sb2	380.4
	1	Eu1	389.5		1	Eu4	391.2
	1	Eu3	391.5		1	Eu3	396.5
Eu4	1	Sb2	329.4	Al	4	Sb2	274.0	Sb1	1	Sb4	324.9
	1	Sb1	331.3		4	Eu2	367.0		2	Eu4	331.3
	1	Sb3	332.0						2	Eu1	332.5
	1	Sb3	334.5						2	Eu2	340.9
	1	Sb2	355.4						2	Eu3	351.0
	1	Eu1	370.8
	1	Eu3	380.3
	1	Sb3	384.5
	1	Eu2	391.2
Sb2	1	Al	274.0	Sb3	1	Eu3	323.7	Sb4	2	Sb1	324.9
	1	Eu2	325.7		1	Eu2	329.2		4	Eu1	333.2
	1	Eu4	329.4		1	Eu1	330.6		4	Eu2	357.4
	1	Eu1	336.0		1	Eu4	332.0
	1	Eu1	340.7		1	Eu1	332.9
	1	Eu3	344.6		1	Eu4	334.5
	1	Eu4	355.4		1	Eu2	339.0
	1	Eu2	380.4		1	Eu4	384.5

---

CCDC 1867337 contains the supplementary crystallographic data for this paper.

2.5. Physical property measurements

The powdered samples of Eu₁₄AlAs₁₁ and Eu₁₄AlSb₁₁ were packed in polyethylene (PE) capsules and attached to the sample holder rod of a Vibrating Sample Magnetometer unit (VSM) for measuring the magnetization M(T,H) in a Quantum Design Physical Property Measurement System (PPMS). The sample was investigated in the temperature range of 2.5–300 K with magnetic fields up to 80 kOe. For the heat capacity measurement of Eu₁₄AlSb₁₁, one piece of the sample was fixed to a pre-calibrated heat capacity puck using Apiezon N grease and investigated in the temperature range of 2.0 to 75 K. A cold pressed tablet of Eu₁₄AlSb₁₁ (Ø 4.5 mm, Archimedes density 93(1)%) was annealed at 773 K for 4 weeks. The sintered material was used for the resistivity experiments. The measurements were carried out in the AC transport (ACT) mode²⁷ 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 0.1 K up to 25 K and one data point every 1 K up to 300 K. The AC frequency was set to 17 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.

2.6. Mössbauer spectroscopy

The 21.53 keV transition of ¹⁵¹Eu with an activity of 130 MBq (2% of the total activity of a ¹⁵¹Sm:EuF₃ source) and a Ba^121mSnO₃ source were used for the ¹⁵¹Eu and ¹²¹Sb 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 K, 78 K or RT, while the source was kept at room temperature. The temperature was controlled by a resistance thermometer (∼0.5 K accuracy). The samples were enclosed in small PVC containers, the required sample mass was calculated based on the work by Long et al.²⁸ Fitting of the spectrum was performed with the WinNormos program package.²⁹ The obtained fitting parameters are listed in Table 5.

Table 5 Fitting parameters of the ¹²¹Sb and ¹⁵¹Eu Mössbauer spectroscopic measurements of Eu₁₁AlAs₁₁ and Eu₁₄AlSb₁₁ at RT, 78 K and 6 K. δ = isomer shift, ΔE_Q = electric quadrupole splitting, Γ = experimental line width, B_HF = hyperfine field

Compound	δ/mm s^−1	ΔEQ/mm s^−1	Γ/mm s^−1	B HF
a Fixed during fitting procedure due to strong correlations. b Isomeric shift of Eu^3+ in Eu2O3. c Hyperfine field increment of 0.1 T. d Four signals used.
Eu 14 Al ^121 Sb 11 , 78 K
#1	−9.6(1)	−10.5(7)	3.08^a	—
#2	−8.2(1)	+6.4(5)	3.08^a	—
#3	−8.1(1)	0^a	3.08^a	—
^151 Eu 14 AlAs 11 , 6 K
#1	−10.2(2)	−2.9(7)	2.7^a	17.7(6)
#2	−10.5(2)	−1.2(8)	2.7^a	19.1(6)
#3	−9.1(4)	+0.3(9)	2.7^a	17.5(6)
#4	−10.8(1)	+4.1(4)	2.7^a	17.6(3)
#5 (2%)	+1.05^b	—	2.7^a	—
^151 Eu 14 AlAs 11 , 6 K – hyperfine field distribution fit
#1	−10.5(1)	+0.9(7)	2.7^a	20(7)^c
#2 (2%)	+1.05^b	—	2.7^a	—
^151 Eu 14 AlAs 11 , RT
#1	−9.5(1)	—	2.7^a	—
#2	−10.5(1)	—	2.7^a	—
#3	−12.8(1)	—	2.7^a	—
#4	−11.8(1)	—	2.7^a	—
#5 (2%)	+1.05^b	—	2.7^a	—
^151 Eu 14 AlAs 11 , RT – isomeric shift distribution fit: Δδ = 1.0(1) mm s ^−1
#1	−11(1)^d	—	2.7^a	—
#2	+1.05^b	—	2.7^a	—
^151 Eu 14 AlSb 11 , 6 K
#1	−10.7(1)	−1.1(2)	2.7^a	18.1(1)
#2	−8.6(1)	+6.6(1)	2.7^a	20.0(1)
#3	−12.3(1)	−2.1(2)	2.7^a	11.2(1)
#4	−11.2(1)	−0.5(2)	2.7^a	21.2(1)
#5 (2%)	+1.05^b	—	2.7^a	—
^151 Eu 14 AlSb 11 , 6 K – hyperfine field distribution fit
#1	−10.7(1)	−0.5(1)	2.7^a	21(8)^c
#2 (2%)	+1.05^b	—	2.7^a	—
^151 Eu 14 AlSb 11 , RT
#1	−9.87(2)	—	2.7^a	—
#2	−10.88(2)	—	2.7^a	—
#3	−12.99(4)	—	2.7^a	—
#4	−12.04(2)	—	2.7^a	—
#5 (2%)	+1.05^b	—	2.7^a	—
^151 Eu 14 AlSb 11 , RT – isomeric shift distribution fit: Δδ = 1.0(1) mm s ^−1
#1	−11(1)^d	—	2.7^a	—
#2	+1.05^b	—	2.7^a	—

---

2.7. Theoretical methodology

Electronic structure calculations of Eu₁₄AlSb₁₁ and Sr₁₄AlSb₁₁ were performed using the projector augmented wave method (PAW) of Blöchl^30,31 coded in the Vienna ab initio simulation package (VASP).^32,33 All VASP calculations employed the generalized gradient approximation (GGA) with exchange and correlation treated by Perdew–Burke–Ernzerhof (PBE).³⁴ To describe the electron correlation associated with Eu 4f states, an on-site Hubbard repulsion U parameter³⁵ was applied with an effective U_eff = 6 eV on the Eu atom. The cutoff energy for the plane wave calculations was set to 500 eV and the Brillouin zone integration was carried out using 3 × 3 × 2 Γ-centered k-point meshes. The Bader charge analysis was based on VASP calculations with subsequent calculations with the Bader program developed by the Henkelman group.^36–38

3. Results and discussion

3.1. Crystal chemistry

Eu₁₄AlAs₁₁ and Eu₁₄AlSb₁₁ crystallize in the tetragonal crystal system with space group I4₁/acd and lattice parameters of a = 1627.6(2) and c = 2180.0(4) pm for the arsenide and a = 1725.6(2) and c = 2289.7(7) pm for the antimonide. Both compounds are isostructural to Ca₁₄AlSb₁₁, which forms the respective prototype.¹⁰ The Zintl phase can be described using ionic constituents according to (Eu²⁺)₁₄(AlPn₄⁹⁻)(Pn³⁻)₄(Pn₃⁷⁻) with Pn₃⁷⁻ being a linear tripnictide anion. An extended view as central projection of the unit cell of Eu₁₄AlPn₁₁ is shown in Fig. 1 (top and bottom). In the unit cell, isolated Pn³⁻ and tetrahedral [AlPn₄]⁹⁻ anions (Fig. 2, top) are found besides the aforementioned Pn₃⁷⁻ tripnictide anions (Fig. 2, bottom). The structure of Eu₁₄AlSb₁₁ has been refined from single crystal X-ray diffraction data and will subsequently be used for the structural discussion. The interatomic Al–Sb distances within the tetrahedral [AlSb₄]⁹⁻ units are 274 pm and therefore slightly longer than the sum of the covalent radii (125 + 141 = 266 pm (ref. 39)), yet in good agreement with the distances found in Ca₁₄AlSb₁₁ (272 pm).¹⁰ The distances within the linear Sb₃⁷⁻ triantimonide anion are 325 pm, which is significantly longer than the sum of the covalent radii (2 × 141 = 282 pm (ref. 39)) of the element and also longer compared to antimony–antimony single bonds observed in other Zintl phases e.g., dimorphic BaSb₂ (CaSb₂ type: 291 pm;⁴⁰ BaSb₂ type: 291–296 pm (ref. 41)) or EuSb₂ (CaSb₂ type: 293 pm (ref. 42)). The observed distance, however, is again comparable to the one found in Ca₁₄AlSb₁₁ (320 pm)¹⁰ or Sr₁₄AlSb₁₁ (330 pm).¹¹ In contrast to e.g. Sr₅Al₂Sb₆ ⁴³ or Eu₅Al₂Sb₆,⁷ no higher dimensional antimonidoaluminate polyanion can be found. The four crystallographically independent Eu cations are found in between the anions and exhibit coordination numbers of nine (Eu1, Eu2 and Eu4) and twelve (Eu3) (Fig. 3) with Eu–Al distances of 367 pm and Eu–Sb distances of 324–357 and 380–385 pm. The interatomic Eu–Sb distances are close to the sum of the covalent radii (Eu + Sb = 326 pm (ref. 39)), suggesting bonding contributions between the Eu and Sb atoms, however, the Eu–Al distances are significantly longer compared to Eu + Al = 310 pm. The Eu–Eu distances of 371–401 pm are in line to slightly longer compared to the sum of the covalent radii of Eu + Eu = 370 pm.

Fig. 1 Extended view of the unit cell of Eu₁₄AlPn₁₁ along [100] (top) and [010] (bottom). The bonding within the Pn₃⁷⁻ tripnictide and the tetrahedral coordination environments of the [AlPn₄]⁹⁻ surrounding the Al atoms are highlighted. Eu atoms are depicted in blue, Al atoms as white circles and Pn atoms in orange.

Fig. 2 Anions in the Zintl phase Eu₁₄AlSb₁₁: (top) tetrahedral [AlSb₄]⁹⁻ unit; (bottom) linear Sb₃⁷⁻ triantimonide entity. The interatomic distances are given in pm. Al atoms are depicted as white circles and Sb atoms in orange.

Fig. 3 Coordination environments surrounding the four crystallographically independent Eu atoms in Eu₁₄AlSb₁₁. The Wyckoff sites and local site symmetries are given. Eu atoms are depicted in blue, Al atoms as white circles and Sb atoms in orange.

3.2. Magnetic measurements

High-field magnetic susceptibility measurements of Eu₁₄AlAs₁₁ and Eu₁₄AlSb₁₁ were conducted in zero-field cooled (ZFC) mode at 10 kOe (Fig. 4 and 5, top panel) and indicate antiferromagnetic ordering at low temperatures and paramagnetic behaviour above the Neél-temperature. A Curie–Weiss fit of the reciprocal magnetic susceptibility in the paramagnetic region (75–300 K) was used to determine the effective magnetic moment and the paramagnetic Curie temperatures (Eu₁₄AlAs₁₁: μ_eff = 7.71(1) μ_B/Eu, θ_P = +18.3(1) K; Eu₁₄AlSb₁₁: μ_eff = 7.93(1) μ_B/Eu, θ_P = −1.9(1) K). The magnetic moments are in line with the calculated magnetic moment of a free Eu²⁺ ion (μ_calc = 7.94 μ_B), indicating that all europium atoms are divalent. To confirm the magnetic ground state zero-field-cooled/field-cooled measurements (ZFC/FC) were conducted at an external field of 100 Oe. Fig. 4 and 5 (middle panel) show that the antiferromagnetic transition already observed in the high-field measurements can be seen again at T_N = 10.5(1) and T_N = 12.5(1) K for the arsenide and the respective antimonide. A second anomaly causing a bifurcation between the ZFC and FC curve can be observed at ∼7 K. Subsequently, heat capacity measurements (depicted in red in Fig. 5, middle panel) were conducted for Eu₁₄AlSb₁₁. They confirm the antiferromagnetic transition to be intrinsic and in line with the susceptibility measurements (T = 11.8(1) K). The observed bifurcation has to be attributed to traces of impurities since no peak signal is observed around 7 K in the specific heat. Finally, magnetization isotherms for both compounds were recorded (Fig. 4 and 5, bottom panel). The isotherms at 50 and 100 K show a linear field dependency in line with paramagnetic behaviour. For the arsenide, the 20 and 10 K isotherms show a pronounced curvature, since they are close to the magnetic transition. The 3 K isotherm finally exhibits an S-shaped character, which is a so-called meta-magnetic step (antiparallel to parallel spin realignment). The critical field was determined to be H_crit = 8.2(1) kOe. The isotherm at 10 K for Eu₁₄AlSb₁₁ also shows a curved trace with a tendency for saturation at high fields. The 3 K isotherm finally exhibits again an S-shaped character, underlining the antiferromagnetic ground state (H_crit = 17.0(1) kOe). The saturation magnetization reaches μ_sat = 5.8(1) μ_B per Eu²⁺ atom for Eu₁₄AlAs₁₁ and μ_sat = 6.6(1) μ_B per Eu²⁺ atom for Eu₁₄AlSb₁₁ at 3 K and 80 kOe. This is, in both cases, only slightly lower than the expected saturation magnetization of 7 μ_B according to g_J × J.

Fig. 4 Magnetic properties of Eu₁₄AlAs₁₁: (top) temperature dependence of the magnetic susceptibility (χ and χ⁻¹ data) measured at 10 kOe; (middle) magnetic susceptibility in zero-field-cooled/field-cooled (ZFC/FC) mode, measured at 100 Oe in the low-temperature range; (bottom) magnetization isotherms recorded at 3, 10, 20, 50 and 100 K.

Fig. 5 Magnetic properties of Eu₁₄AlSb₁₁: (top) temperature dependence of the magnetic susceptibility (χ and χ⁻¹ data) measured at 10 kOe; (middle) magnetic susceptibility in zero-field-cooled/field-cooled (ZFC/FC) mode, measured at 100 Oe in the low-temperature range, in red the heat capacity measurement in zero-field between 2–75 K is shown; (bottom) magnetization isotherms recorded at 3, 10, 50 and 100 K.

3.3. Electrical resistivity

The temperature dependence of the electrical resistivity of Eu₁₄AlSb₁₁ is depicted in Fig. 6. The measured resistivity increases with decreasing temperature, therefore exhibits the typical behaviour of a semiconductor. From the high temperature region of this measurement, two band gaps were extracted using the relation ρ ∝ e^E_g/2kT for semiconductors. In Fig. 6 (inset) the Arrhenius plot is depicted. The data between 250 and 300 K yielded a band gap of E_g = 0.28(5) eV (red) whereas the fit between 150–250 K resulted in E_g = 0.13(5) eV (green).

Fig. 6 Temperature dependence of the electrical resistivity of Eu₁₄AlSb₁₁. The inset depicts the Arrhenius plot used to determine the band gap.

3.4. ¹²¹Sb and ¹⁵¹Eu Mössbauer spectroscopy

To further proof the presence of Eu²⁺ in Eu₁₄AlAs₁₁ and Eu₁₄AlSb₁₁, ¹⁵¹Eu Mössbauer spectra were recorded at 6 K and RT. The obtained fitting parameters are listed in Table 5. We want to note, that fitting the obtained spectra with only one signal is not possible in all cases. Since the Eu₁₄AlPn₁₁ compounds exhibit four crystallographic Eu sites (Fig. 3), in principle four signals with respective ratios of 2 : 2 : 1 : 2 could be used for fitting the Mössbauer data. A similar fit using three signals has already been performed for Eu₅Al₂Sb₆.⁷

The calculated Bader charges (vide infra) for the Eu atoms suggest a distribution of isomeric shifts, both at RT and 6 K. The RT spectra of Eu₁₄AlAs₁₁ and Eu₁₄AlSb₁₁ can be fitted using four signals (Fig. 7, left). The refined isomeric shifts range from δ = −9.5 to −13 mm s⁻¹, in line with Eu²⁺ cations. Due to strong correlations, the line width was fixed to typical values of Γ = 2.7 mm s⁻¹.^7,44–49 Alternatively, the experimental spectra can be reproduced using an isomeric shift distribution (Fig. 7, right), yielding an average shift of δ = −11(1) mm s⁻¹ along with Δδ = 1.0(1) mm s⁻¹ for the arsenide and δ = −10.7(3) mm s⁻¹ (Δδ = 1.0(1) mm s⁻¹) for the antimonide. The Eu³⁺ impurities (<3%) that were observed in the 6 K spectra were added with a fixed percental contribution to the RT spectra.

Fig. 7 Experimental (data points) and simulated (continuous lines) ¹⁵¹Eu Mössbauer spectra of Eu₁₄AlAs₁₁ and Eu₁₄AlSb₁₁ at room temperature. The red line corresponds to the sum of the different signals used for fitting. In the left panels, the fits using five signals are shown, they are labelled according to Table 5; in the right panels, the fits utilizing isomeric shift distributions are depicted. The purple signals are Eu³⁺ impurities.

The 6 K spectra were recorded below the respective Néel temperatures, hence, a full hyperfine field splitting was observed. Again, the ¹⁵¹Eu Mössbauer spectra of Eu₁₄AlAs₁₁ and Eu₁₄AlSb₁₁ were fitted with two possibilities. Fig. 8 (left) shows the fits using four signals for the respective Eu sites along with a fifth signal for the Eu³⁺ impurity, most likely originating from surface oxidation. The main signals of the arsenide show isomer shifts of δ = −10.2(2), −10.5(2), −9.1(4) and −10.8(1) mm s⁻¹, the impurity signal is found at δ = +1.05 mm s⁻¹. For the antimonide isomer shifts of δ = −10.7(1), −8.6(1), −12.3(1) and −11.2(1) mm s⁻¹ are found, the impurity signal is located at δ = +1.05 mm s⁻¹. As a consequence of the non-cubic site symmetry of the europium atoms, the signals in both compounds show quadrupole splitting. For the antimonide, Eu1 and Eu4 exhibit similar (distorted) coordination environments (both 32g, 1); with ΔE_Q = −1.1(2) mm s⁻¹ and ΔE_Q = −2.1(2) mm s⁻¹. Eu3 shows a rather symmetric coordination environment (16e, .2.) with an electric quadrupole splitting of ΔE_Q = −0.5(2) mm s⁻¹. Eu2 finally shows a highly distorted coordination environment (32g, 1), in line with a rather large quadrupole splitting ΔE_Q = +6.6(1) mm s⁻¹. In addition, significant magnetic hyperfine field splittings (B_hf = 18.1(1), 11.2(1), 21.2(1) and 20.0(1) T) were observed. The hyperfine fields are in line with other semiconducting and intermetallic europium compounds.^7,44–49 The areas of the four signals were again fixed with a 1 : 2 : 2 : 2 ratio, corresponding to the respective site multiplicities of the crystal structure (16e and 3× 32g). On the right hand side of Fig. 8, fits of the ¹⁵¹Eu Mössbauer spectra of Eu₁₄AlAs₁₁ and Eu₁₄AlSb₁₁ at 6 K, utilizing hyperfine field distributions, are depicted. An averaged isomeric shift of δ = −10.5(1) mm s⁻¹ for Eu₁₄AlAs₁₁ and of δ = −10.7(1) mm s⁻¹ for Eu₁₄AlSb₁₁ was refined. The obtained field distribution is illustrated in the histogram on the very right.

Fig. 8 Experimental (data points) and simulated (continuous lines) ¹⁵¹Eu Mössbauer spectra of Eu₁₄AlAs₁₁ and Eu₁₄AlSb₁₁ at 6 K. The red line corresponds to the sum of the different signals used for fitting. In the left panels, the fits using five signals are shown, they are labelled according to Table 5; in the middle panels, the fits utilizing hyperfine field distributions are depicted. On the right, the hyperfine field distributions are shown. The purple signals are Eu³⁺ impurities.

The ¹²¹Sb Mössbauer spectra of Eu₁₄AlSb₁₁ was measured at 78 K and is depicted in Fig. 9 along with the respective fits. The crystal structure contains four crystallographic antimonide sites (2× 32g, 16f and 8b) of which two sites (Sb1 and Sb4) are forming the Sb₃⁷⁻ anion (16f and 8b), while the other two sites (Sb2 and Sb3) are Sb³⁻ anions. These sites differ, however, in their crystal chemical environment. Sb3 is an isolated Sb³⁻ anion, while Sb2 contributes to the [AlSb₄]⁹⁻ unit. Therefore, the spectrum was fitted using three signals, one for each Sb³⁻ anion and one signal for the Sb₃⁷⁻ (≡Sb^2.33−) anion. The area ratio of the three signals was fixed to 4 : 4 : 3 (Sb2 : Sb3 : (Sb1/4) ≡ Sb³⁻ : Sb³⁻ : Sb^2.33−). While for the ‘Sb^2.33−’ anions an isomer shift of δ = −9.6(1) mm s⁻¹ was found, the Sb³⁻ signals exhibit isomer shifts of δ = −8.2(1) mm s⁻¹ and −8.1(1) mm s⁻¹ (Table 5). For the latter ones, the observed isomer shifts are in line with other antimonides containing formally Sb³⁻ anions, e.g. the binary alkali metal antimonides Li₃Sb,⁵⁰ A₃Sb (A = Na, K, Rb),⁵¹ the III–V semiconductors AlSb, GaSb and InSb,⁵² the MT₂Sb₂ (M = Eu, Yb; T = Mn, Zn) series,⁵³ the antimonide oxides RETSbO (RE = La–Sm, Gd; T = Mn, Zn)⁵⁴ or the half-Heusler phases YbTSb (T = Ni, Pd, Pt, Cu, Ag, Au)⁵⁵ and ScTSb (T = Ni, Pd, Pt)⁵⁶ and M₅Al₂Sb₆ (M = Sr, Eu)⁷ all with isomer shifts around δ ∼−8 mm s⁻¹. Antimonide dumbbells and linear triantimonide units in contrast are rather rare with respect to ¹²¹Sb Mössbauer spectroscopic investigations. With respect to the dumbbells, the investigations of α-Zn₁₃Sb₁₀ ⁵⁷ and M₅Al₂Sb₆ (M = Sr, Eu)⁷ can be found in the literature with reported isomer shifts of δ ∼–9 mm s⁻¹. For linear triantimonides so far no distinct Mössbauer spectroscopic investigations have been reported. The ¹²¹Sb Mössbauer spectra of Eu₁₄MnSb₁₁ and Yb₁₄MnSb₁₁ were fitted with two signals, one for the sample and one for an Sb₂O₃ impurity.⁵⁸

Fig. 9 Experimental (data points) and simulated (continuous lines) ¹²¹Sb Mössbauer spectrum of Eu₁₄AlSb₁₁ at 78 K. The red line corresponds to the sum of the different signals used for fitting. The fit using three signals is shown, they are labelled according to Table 5.

3.5. First principle calculations

To investigate the magnetic interactions in Eu₁₄AlSb₁₁, Eu–Eu pairs for interatomic distances less than 5.0 Å are considered, and nine spin exchange interactions were evaluated by performing mapping analysis of ten magnetic models. The nine spin exchange paths include Eu1–Eu2 (J₁₂), Eu1–Eu3 (J₁₃), Eu1–Eu4 (J₁₄), Eu2–Eu3 (J₂₃), Eu2–Eu4 (J₂₄), Eu3–Eu4 (J₃₄), Eu1–Eu1 (J₁₁), Eu2–Eu2 (J₂₂) and Eu4–Eu4 (J₄₄). The magnetic models include one ferromagnetic (FM) model and nine ferrimagnetic (Fi1–Fi9) models (see Table 6). The relative energies, per formula unit (f. u.), of the ten magnetic models determined from our VASP calculations are summarized in Table 7. The energies of these models can also be approximated in terms of the spin Hamiltonian,
in which, J_ij is the spin exchange parameter for the interaction between the spin site i and j. By applying this energy expression for spin dimers with N unpaired spins per spin site and mapping the relative energy of the ten models determined by VASP calculations onto the corresponding relative energies determined from the above exchange energies, we obtained the values of J_ij listed in Table 7. The magnitudes of these parameters are similar as those for Eu₅Al₂Sb₆,⁷ consistent with the near zero Weiss constant (θ_p = −1.9(1) K) and low ordering temperature (T_N = 12.5(1) K). Eu3–Eu4 and Eu1–Eu1 interactions are ferromagnetic while the rest (seven) are antiferromagnetic (AFM) with four of them stronger than the two FM ones. Therefore, antiferromagnetic interactions dominate, which is consistent with the negative Weiss constant and the AFM transition observed in the magnetic measurements. The two FM interactions may account for the behaviour of the isotherms at 10 K and 3 K. Because of the complexity of the crystal structure, it is not easy to predict the magnetic ground state.

Table 6 The spin arrangement of the ten magnetic structure models. + indicates spin up, − indicates spin down, ± indicates that neighboring Eu atoms have opposite spins

Models	Arrangement of the spins
	Eu1	Eu2	Eu3	Eu4
Fi1	+	−	+	+
Fi2	+	+	−	−
Fi3	+	−	−	+
Fi4	−	+	+	+
Fi5	+	±	+	+
Fi6	+	−	+	−
Fi7	±	+	+	+
Fi8	+	+	+	±
Fi9	+	+	−	+
FM	+	+	+	+

---

Table 7 Total energies and spin exchange energy expressions for the ten magnetic structure models of Eu₁₄AlSb₁₁. N_Eu = 7

Models	Energy^a (meV per f. u.)	Spin exchange energy expression (meV)
a Total energy relative to model FM from VASP calculation.
Fi1	−52.69
Fi2	−51.84
Fi3	−40.38
Fi4	−30.34
Fi5	−26.91
Fi6	−18.86
Fi7	−14.00
Fi8	−13.90
Fi9	−5.94
FM	0.00
J (meV)		J 12 = −0.053, J13 = −0.089, J14 = −0.021, J23 = −0.078, J24 = −0.100, J34 = +0.053, J11 = +0.012, J22 = −0.006, J44 = −0.028

---

The spin-polarized density of states (DOS) curve for the FM state was plotted in Fig. 10. There is a small band gap (∼0.07 eV for FM and ∼0.3 eV for the most stable magnetic model, Fi1) at the Fermi level, consistent with the large electrical resistivity. The majority spin states of Eu-f are all occupied, while its minority spin states are all unoccupied, indicating a high spin 4f⁷ (Eu²⁺) configuration. The calculated Bader effective charges for Eu₁₄AlSb₁₁ and Sr₁₄AlSb₁₁ are listed in Table 8. In both, Eu₁₄AlSb₁₁ and Sr₁₄AlSb₁₁, Eu, Sr and Al atoms have positive charges while Sb atoms have negative charges. The charges on Eu atoms are all between +1 and +2, which indicates that Eu³⁺ can be ruled out, in accordance with the ¹⁵¹Eu Mössbauer result that solely found Eu²⁺. However, the fact that the effective charges on Eu atoms are less than +2 indicates the presence of some partial covalent bonding interactions between Eu and Sb.

Fig. 10 Total and partial spin polarized density of states (DOS) of Eu₁₄AlSb₁₁ for ferromagnetic model. The Fermi level is denoted by the dashed line.

Table 8 Bader charges in Eu₁₄AlSb₁₁ and Sr₁₄AlSb₁₁

Eu14AlSb11	Atom	Eu1	Eu2	Eu3	Eu4	Al	Sb1	Sb2	Sb3	Sb4
	Charge	+1.16	+1.20	+1.13	+1.17	+1.35	−1.52	−1.70	−1.70	−1.11
Sr14AlSb11	Atom	Sr1	Sr2	Sr3	Sr4	Al	Sb1	Sb2	Sb3	Sb4
	Charge	+1.30	+1.33	+1.28	+1.32	+1.15	−1.69	−1.80	−1.91	−1.28

---

4. Conclusions

The Zintl phases Eu₁₄AlAs₁₁ and Eu₁₄AlSb₁₁ crystallize isostructural to Ca₁₄AlSb₁₁ and were synthesized from the elements in niobium ampoules in an induction furnace and subsequently structurally characterized by powder and single crystal X-ray diffraction. In the tetragonal crystal structure, the Al atoms are tetrahedrally surrounded by Pn atoms forming isolated [AlPn₄]⁹⁻ entities. Additionally, linear Pn₃⁷⁻ triantimonide and spherical Pn³⁻ anions are found. The four crystallographically independent Eu sites are located in between the different anionic species. In each compound, one magnetic ordering phenomena at T_N = 10.5(1) K (Eu₁₄AlAs₁₁) and T_N = 12.5(1) K (Eu₁₄AlSb₁₁) was observed in the susceptibility measurements and confirmed by heat capacity measurements in the case of the antimonide. ¹⁵¹Eu Mössbauer spectroscopic studies confirmed the divalent oxidation state of all europium atoms. Measurements at 6 K, therefore below T_N, showed a hyperfine field splitting for all Eu sites due to magnetic ordering. The calculated densities of states using the ferromagnetic model indicate the presence of a band gap in accordance with the semiconducting behaviour found by electrical resistivity measurements. Based on the spin exchange parameters calculated using mapping analysis, the spin exchange interactions in Eu₁₄AlSb₁₁ are similar to Eu₅Al₂Sb₅ and much smaller than other magnetically ordered rare-earth compounds. In addition, antiferromagnetic interactions dominate in Eu₁₄AlSb₁₁ with some weak ferromagnetic interactions. Bader charge analysis confirms the divalent oxidation state of Eu, but also indicates significant covalent interactions involving the Eu atoms.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank Dipl.-Ing. Jutta Kösters for the single crystal intensity data collection. Boniface P. T. Fokwa and Yuemei Zhang thank San Diego Supercomputer Center for providing computing resources.

References

1. S. M. Kauzlarich, Chemistry, Structure, and Bonding of Zintl Phases and Ions, VCH Publishers, 1996.
2. O. Janka and S. M. Kauzlarich, in Encyclopedia of Inorganic and Bioinorganic Chemistry, John Wiley & Sons, Ltd, 2013.
3. G. J. Snyder and E. S. Toberer, Nat. Mater., 2008, 7, 105–114.
4. S. R. Brown, S. M. Kauzlarich, F. Gascoin and G. J. Snyder, Chem. Mater., 2006, 18, 1873–1877.
5. J.-A. Paik, B. Brandon, T. Caillat, R. Ewell and J. P. Fleurial, Proceedings of Nuclear and Emerging Technologies for Space 2011, 2011, paper 3695, 1–10.
6. S. K. Bux, A. Zevalkink, O. Janka, D. Uhl, S. Kauzlarich, J. G. Snyder and J.-P. Fleurial, J. Mater. Chem. A, 2014, 2, 215–220.
7. M. Radzieowski, T. Block, T. Fickenscher, Y. Zhang, B. P. T. Fokwa and O. Janka, Mater. Chem. Front., 2017, 1, 1563–1572.
8. G. Cordier and M. Steher, Z. Naturforsch., B: J. Chem. Sci., 1988, 43, 463–466.
9. A. Zevalkink, Y. Takagiwa, K. Kitahara, K. Kimura and G. J. Snyder, Dalton Trans., 2014, 43, 4720–4725.
10. G. Cordier, H. Schäfer and M. Stelter, Z. Anorg. Allg. Chem., 1984, 519, 183–188.
11. S. L. Brock, L. J. Weston, M. M. Olmstead and S. M. Kauzlarich, J. Solid State Chem., 1993, 107, 513–523.
12. H. Kim and S. M. Kauzlarich, J. Solid State Chem., 2005, 178, 1935–1939.
13. A. Rehr, T. Y. Kuromoto, S. M. Kauzlarich, J. Delcastillo and D. J. Webb, Chem. Mater., 1994, 6, 93–99.
14. T. Y. Kuromoto, S. M. Kauzlarich and D. J. Webb, Chem. Mater., 1992, 4, 435–440.
15. W. Carrillo-Cabrera, M. Somer, K. Peters and H. G. von Schnering, Chem. Ber., 1996, 129, 1015–1023.
16. A. C. Payne, M. M. Olmstead, S. M. Kauzlarich and D. J. Webb, Chem. Mater., 2001, 13, 1398–1406.
17. A. Rehr and S. M. Kauzlarich, J. Alloys Compd., 1994, 207, 424–426.
18. J. Y. Chan, M. E. Wang, A. Rehr, S. M. Kauzlarich and D. J. Webb, Chem. Mater., 1997, 9, 2131–2138.
19. J. Y. Chan, M. M. Olmstead, S. M. Kauzlarich and D. J. Webb, Chem. Mater., 1998, 10, 3583–3588.
20. I. R. Fisher, S. L. Bud'ko, C. Song, P. C. Canfield, T. C. Ozawa and S. M. Kauzlarich, Phys. Rev. Lett., 2000, 85, 1120–1123.
21. A. P. Holm, T. C. Ozawa, S. M. Kauzlarich, S. A. Morton, G. Dan Waddill and J. G. Tobin, J. Solid State Chem., 2005, 178, 262–269.
22. R. Pöttgen, T. Gulden and A. Simon, GIT Labor-Fachz., 1999, 43, 133–136.
23. D. Kußmann, R.-D. Hoffmann and R. Pöttgen, Z. Anorg. Allg. Chem., 1998, 624, 1727–1735.
24. E. A. Leon-Escamilla, W.-M. Hurng, E. S. Peterson and J. D. Corbett, Inorg. Chem., 1997, 36, 703–710.
25. R. J. Gambino, J. Less-Common Met., 1967, 12, 344–352.
26. K. Yvon, W. Jeitschko and E. Parthé, J. Appl. Crystallogr., 1977, 10, 73–74.
27. L. J. van der Pauw, Philips Res. Rep., 1958, 13, 1–9.
28. G. J. Long, T. E. Cranshaw and G. Longworth, Mössbauer Effect Reference and Data Journal, 1983, vol. 6, pp. 42–49.
29. R. A. Brand, WinNormos for Igor6, Version for Igor 6.2 or above: 22.02.2017, Universität Duisburg, Duisburg (Germany), 2017.
30. P. E. Blöchl, Phys. Rev. B: Condens. Matter Mater. Phys., 1994, 50, 17953–17979.
31. G. Kresse and D. Joubert, Phys. Rev. B: Condens. Matter Mater. Phys., 1999, 59, 1758–1775.
32. G. Kresse and J. Furthmüller, Phys. Rev. B: Condens. Matter Mater. Phys., 1996, 54, 11169–11186.
33. G. Kresse and J. Furthmüller, Comput. Mater. Sci., 1996, 6, 15–50.
34. J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865–3868.
35. S. L. Dudarev, G. A. Botton, S. Y. Savrasov, C. J. Humphreys and A. P. Sutton, Phys. Rev. B: Condens. Matter Mater. Phys., 1998, 57, 1505–1509.
36. E. Sanville, S. D. Kenny, R. Smith and G. Henkelman, J. Comput. Chem., 2007, 28, 899–908.
37. G. Henkelman, A. Arnaldsson and H. Jónsson, Comput. Mater. Sci., 2006, 36, 354–360.
38. W. Tang, E. Sanville and G. Henkelman, J. Phys.: Condens. Matter, 2009, 21, 084204.
39. J. Emsley, The Elements, Clarendon Press, Oxford University Press, Oxford, New York, 1998.
40. B. Eisenmann, C. Gieck and U. Rößler, Z. Kristallogr. – New Cryst. Struct., 2001, 216, 36.
41. F. Emmerling, N. Längin, F. Pickhard, M. Wendorff and C. Röhr, Z. Naturforsch., B: J. Chem. Sci., 2004, 59, 7–16.
42. F. Hulliger and R. Schmelczer, J. Solid State Chem., 1978, 26, 389–396.
43. G. Cordier and M. Stelter, Z. Naturforsch., 1988, 43, 463–466.
44. R. Müllmann, B. D. Mosel, H. Eckert, G. Kotzyba and R. Pöttgen, J. Solid State Chem., 1998, 137, 174–180.
45. D. Kußmann, R. Pöttgen, U. C. Rodewald, C. Rosenhahn, B. D. Mosel, G. Kotzyba and B. Künnen, Z. Naturforsch., B: J. Chem. Sci., 1999, 54, 1155–1164.
46. S. Seidel, O. Niehaus, S. F. Matar, O. Janka, B. Gerke, U. C. Rodewald and R. Pöttgen, Z. Naturforsch., B: J. Chem. Sci., 2014, 69, 1105–1118.
47. J.-P. Schmiegel, T. Block, B. Gerke, T. Fickenscher, R. S. Touzani, B. P. T. Fokwa and O. Janka, Inorg. Chem., 2016, 55, 9057–9064.
48. I. Schellenberg, M. Eul and R. Pöttgen, Monatsh. Chem., 2011, 142, 875–880.
49. M. Radzieowski, F. Stegemann, T. Block, J. Stahl, D. Johrendt and O. Janka, J. Am. Chem. Soc., 2018, 140, 8950–8957.
50. P. E. Lippens, J.-C. Jumas and J. Olivier-Fourcade, Hyperfine Interact., 2004, 156, 327–333.
51. K. Kitadai, M. Takahashi and M. Takeda, J. Radioanal. Nucl. Chem., 2003, 255, 311–314.
52. P. E. Lippens, Solid State Commun., 2000, 113, 399–403.
53. I. Schellenberg, M. Eul, W. Hermes and R. Pöttgen, Z. Anorg. Allg. Chem., 2010, 636, 85–93.
54. I. Schellenberg, T. Nilges and R. Pöttgen, Z. Naturforsch., B: J. Chem. Sci., 2008, 63, 834–840.
55. M. Ratikanta, R. Pöttgen, R.-D. Hoffmann, T. Fickenscher, M. Eschen, H. Trill and B. D. Mosel, Z. Naturforsch., B: J. Chem. Sci., 2002, 57, 1215–1223.
56. T. Harmening, H. Eckert and R. Pöttgen, Solid State Sci., 2009, 11, 900–906.
57. R. P. Hermann, F. Grandjean, T.-C. Chen, D. E. Brown, C. E. Johnson, G. J. Snyder and G. J. Long, Inorg. Chem., 2007, 46, 767–770.
58. R. P. Hermann, F. Grandjean, D. Kafle, D. E. Brown, C. E. Johnson, S. M. Kauzlarich and G. J. Long, Inorg. Chem., 2007, 46, 10736–10740.

---