Cadmium-rich intermetallic phases ARh₂Cd₂₀ – structure, magnetic behavior, ¹⁵¹Eu Mössbauer and ¹¹³Cd solid-state NMR spectroscopy

Abstract

The cadmium-rich intermetallic compounds ARh₂Cd₂₀ (A = Ca, Sr, Y, La-Nd, Sm-Lu) were synthesized from the elements in sealed tantalum tubes. The elements were reacted in an induction furnace and the samples were post-annealed to increase phase purity and crystallinity. The ARh₂Cd₂₀ phases crystallize with the cubic CeCr₂Al₂₀ type structure, space group Fd3̄m. The polycrystalline samples were characterized by X-ray powder diffraction. The structures of SrRh₂Cd₂₀, LaRh₂Cd₂₀, CeRh₂Cd₂₀, TbRh₂Cd₂₀ and DyRh₂Cd₂₀ were refined from X-ray single crystal diffractometer data. Temperature dependent magnetic susceptibility data show diamagnetism for CaRh₂Cd₂₀, SrRh₂Cd₂₀, YRh₂Cd₂₀, LaRh₂Cd₂₀ and YbRh₂Cd₂₀, thus substantiating stable divalent ytterbium in the latter phase. The remaining phases are Curie Weiss paramagnets. EuRh₂Cd₂₀ contains stable divalent europium and orders ferromagnetically at T_C = 13.8 K. Antiferromagnetic ordering was detected for SmRh₂Cd₂₀ (T_N = 4.3 K), GdRh₂Cd₂₀ (T_N = 9.3K), TbRh₂Cd₂₀ (T_N = 6.6 K) and DyRh₂Cd₂₀ (T_N = 3.9 K). TbRh₂Cd₂₀ and DyRh₂Cd₂₀ exhibit metamagnetic transitions at critical fields of 19 respectively 10 kOe. The divalent ground state in EuRh₂Cd₂₀ was also confirmed by a ¹⁵¹Eu Mössbauer spectrum which shows an isomer shift of δ = −10.86(1) mm s⁻¹. Solid-state NMR spectroscopy was performed for the samples YRh₂Cd₂₀, LaRh₂Cd₂₀, and YbRh₂Cd₂₀. The ¹¹³Cd NMR spectra include three distinct Cd sites for each sample in accordance with the crystallographic data. All samples further show negative Knight shifts for ¹⁰³Rh, suggesting high s–d exchange interaction at the Rh site. In the case of ¹³⁹La, a residual quadrupolar coupling was observed despite cubic site symmetry, confirming the existence of local defect sites. ⁸⁹Y and ¹⁷¹Yb NMR spectra were recorded, the latter confirming the divalent nature of Yb in YbRh₂Cd₂₀.

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1. Introduction

The CeCr₂Al₂₀ structure¹ is one of the important types within the family of so-called cage structures. Although its unit cell contains 184 atoms, its structure can easily be described as derivative of the cubic Laves phase structure of MgCu₂. The substructure of the cerium and chromium atoms corresponds to the magnesium respectively copper sites of MgCu₂. Both atom types have aluminum coordination in form of Frank–Kasper polyhedra, i.e., Ce@Al₁₆ and Cr@Al₁₂. The packing of these polyhedra avoids direct Ce–Ce, Ce–Cr and Cr–Cr contacts.

Meanwhile more than 200 representatives of the CeCr₂Al₂₀ type are known² and their crystal chemistry and physical properties have recently been reviewed.³ The striking physical properties of these phases concern superconductivity (e.g., T_C = 1.65 K for Ga_0.2V₂Al₂₀ or T_C = 1.00 K for ScV₂Al₂₀),^4,5 thermoelectric materials⁶ and especially the heavy fermion zincides YbT₂Zn₂₀ (T = Fe, Co, Ru, Rh, Os and Ir). The outstanding compound in this structural family is YbCo₂Zn₂₀ with a γ value as large as 7900 mJ mol⁻¹ K⁻².^7,8

Although a huge number of aluminides, zinc and cadmium phases has been reported, the rare earth-based series are far from been entirely investigated with respect to phase analyses and also the physical property studies are far from being complete. In the present study we complete the series of RERh₂Cd₂₀ compounds. So far only the members with RE = Ce and Pr have been studied;^9–12 however, no lattice parameters were reported, and no precise structure refinements were performed. The characterization relied only on the magnetic properties. CeRh₂Cd₂₀ orders magnetically at 0.3 K.¹² PrRh₂Cd₂₀ saturates above 300 kOe (without a metamagnetic transition). Besides the diffraction experiments (X-ray powder data for all samples and structure refinements for SrRh₂Cd₂₀, LaRh₂Cd₂₀, CeRh₂Cd₂₀, TbRh₂Cd₂₀ and DyRh₂Cd₂₀) we also report on the magnetic properties, ¹⁵¹Eu Mössbauer spectroscopy on EuRh₂Cd₂₀ and a detailed ⁸⁹Y, ¹⁰³Rh, ¹¹³Cd and ¹³⁹La solid-state NMR spectroscopic characterization.

2. Experimental

Synthesis

Starting materials for the synthesis of the ARh₂Cd₂₀ (A = Ca, Sr, Y, La–Nd, Sm–Lu) samples were calcium granules (Alfa Aesar, 99.5%), sublimed strontium ingots (Smart Elements, >99.9%), ingots of the rare earth elements (smart elements, >99.9%), rhodium heating tapes (Heraeus, >99.9%) and cadmium pellets (onyxmet, >99.9999%). The surface impurities of the moisture sensitive rare earth elements were mechanically removed, and the pieces were stored under dry argon (Westfalen, 99.998%, purified by using titanium sponge (T = 873 K), silica gel and molecular sieves) prior to use. The metals were weighed in a ratio of 1 : 2 : 21 and sealed in tantalum ampoules under argon atmosphere (800 mbar) using an arc furnace.¹³ The slight excess of cadmium was used to compensate the loss due to evaporation of cadmium (high vapour pressure of cadmium) and recondensation at the top of the vertically positioned ampoules. The excess cadmium was found as a thin film at the top lids of the tantalum tubes.

The samples were then placed in a water-cooled sample chamber of an induction furnace (Hüttinger Elektronik, Freiburg, Germany, type TIG 1.5/300).¹⁴ First, they were heated to 1000 K for 30 min to react the cadmium and to prevent the ampoule to burst. After that, the samples were heated twice to 1300 K for 90 s and cooled again to 1100 K. This temperature was kept for two hours, and the samples were cooled to room temperature by turning off the furnace.

The ampoules were subsequently sealed in silica tubes under vacuum (as oxidation protection) and heated to 823 K for up to 20 days in tube furnaces. The early rare earth elements formed the desired phases already after short thermal treatments while the samples with the late rare earth elements deserved longer annealing steps. All samples were silvery with a metallic luster and stable in air over month.

EDX data

The CeRh₂Cd₂₀, TbRh₂Cd₂₀ and DyRh₂Cd₂₀ single crystals were studied by EDX analyses using a Zeiss EVO® MA10 scanning electron microscope operated in variable pressure mode. CeO₂, TbF₃, DyF₃, Rh and Cd were used as standards. The experimentally observed compositions (average of three point analyses on each crystal surface) of 5 ± 1 at% Ce: 8 ± 1 at% Rh: 87 ± 1 at% Cd and 4 ± 1 at% RE: 9 ± 1 at% Rh: 87 ± 1 at% Cd for the terbium and dysprosium containing crystals were close to the ideal one (4.3 : 8.6 : 87.0). No impurity elements (especially no tantalum) were found.

X-ray diffraction

The polycrystalline RERh₂Cd₂₀ samples were characterized by powder X-ray diffraction using an Enraf-Nonius FR552 Guinier camera (equipped with an imaging plate detector, coupled with a Fujifilm BAS–1800 readout system; CuKα₁ radiation) and α-quartz (a = 491.30 and c = 540.46 pm) as an internal standard. The lattice parameters (Table 1) were refined by least-squares fits of the experimental 2θ values. Intensity calculations (Lazy Pulverix routine¹⁵) facilitated the correct assignment of the indices. The experimental and simulated powder patterns of the YbRh₂Cd₂₀ sample are shown as an example in Fig. 1. The patterns of the remaining samples are presented in the ESI (Fig. S1–S16†). The calcium and strontium representatives contained small amounts of yet unknown by-products.

Fig. 1 Calculated (top) and experimental (bottom) Guinier powder patterns (CuKα₁ radiation) of the YbRh₂Cd₂₀ sample.

Table 1 Lattice parameters (Guinier powder and single crystal data) of ARh₂Cd₂₀ (A = Ca, Sr, Y, La–Nd, Sm–Lu), CeCr₂Al₂₀ type, space group Fd3̄m. Standard deviations are given in parentheses. The lattice parameter from ref. 12 was taken from a graph

Compound	a (pm)	V (nm^3)	Ref.
a Single crystal data.
CaRh2Cd20	1562.9(2)	3.8176	This work
SrRh2Cd20	1569.1(1)	3.8632	This work
SrRh2Cd20^a	1570.29(9)	3.8720	This work
YRh2Cd20	1555.76(6)	3.7655	This work
LaRh2Cd20	1564.8(1)	3.8316	This work
LaRh2Cd20^a	1565.92(9)	3.8398	This work
CeRh2Cd20	1562.9(1)	3.8176	This work
CeRh2Cd20^a	1564.67(8)	3.8306	This work
CeRh2Cd20	1558	3.7818	12
PrRh2Cd20	1561.43(6)	3.8069	This work
NdRh2Cd20	1561.0(1)	3.8037	This work
SmRh2Cd20	1559.0(1)	3.7891	This work
EuRh2Cd20	1567.0(2)	3.8478	This work
GdRh2Cd20	1557.3(1)	3.7767	This work
TbRh2Cd20^a	1556.13(9)	3.7682	This work
TbRh2Cd20	1556.4(1)	3.7702	This work
DyRh2Cd20	1555.1(1)	3.7608	This work
DyRh2Cd20^a	1555.38(10)	3.7628	This work
HoRh2Cd20	1554.7(1)	3.7579	This work
ErRh2Cd20	1554.35(5)	3.7553	This work
TmRh2Cd20	1553.32(9)	3.7479	This work
YbRh2Cd20	1563.0(2)	3.8184	This work
LuRh2Cd20	1553.4(1)	3.7484	This work

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Additional powder X-ray diffraction (PXRD) patterns of the LaRh₂Cd₂₀ samples for Rietveld refinements were recorded at room temperature on a D8-A25-Advance diffractometer (Bruker, Karlsruhe, Germany) in Bragg–Brentano θ–θ-geometry (goniometer radius 280 mm) with CuKα-radiation (λ = 154.0596 pm). A 12 μm Ni foil working as Kβ filter and a variable divergence slit were mounted at the primary beam side. A LYNXEYE detector with 192 channels was used at the secondary beam side. Experiments were carried out in a 2θ range of 6 to 130° with a step size of 0.013° and a total scan time of 1 h.

Irregularly-shaped single crystals were selected from the carefully crushed SrRh₂Cd₂₀, LaRh₂Cd₂₀, CeRh₂Cd₂₀, TbRh₂Cd₂₀ and DyRh₂Cd₂₀ samples. The crystals were fixed to small glass fibres using beeswax, and their quality for diffractometer data collection was tested by Laue photographs on a Buerger camera with white Mo radiation. Complete data sets were collected at room temperature using a Stoe IPDS-II image plate system (graphite-monochromatized MoK_α radiation; λ = 71.073 pm) in oscillation mode. Numerical absorption corrections were applied to the data sets. Details of the data collections and the structure refinement data are summarized in Table 2.

Table 2 Crystal data and structure refinement parameters for SrRh₂Cd₂₀, LaRh₂Cd₂₀, CeRh₂Cd₂₀, TbRh₂Cd₂₀ and DyRh₂Cd₂₀, CeCr₂Al₂₀-type, space group Fd3̄m, Z = 8, Stoe IPDS II diffractometer

Refined composition	SrRh2Cd20	LaRh2Cd20	CeRh2Cd20	TbRh2Cd20	DyRh2Cd20
Formula weight, g mol^−1	2541.6	2592.9	2594.1	2612.9	2616.5
Lattice parameter, pm (single crystal data)	a = 1570.29(9)	a = 1565.92(9)	a = 1564.67(8)	a = 1556.13(9)	a = 1555.38(10)
Unit cell volume, nm^3	3.8720	3.8398	3.8306	3.7682	3.7628
Calculated density, g cm^−3	8.72	8.97	9.00	9.21	9.24
Crystal size, μm	20 × 55 × 60	20 × 20 × 80	20 × 30 × 210	10 × 30 × 35	20 × 60 × 160
Transmission (min/max)	0.430/0.674	0.455/0.789	0.195/0.664	0.157/0.658	0.131/0.726
Detector distance, mm	70	70	70	70	70
ω range, increment, °	0.0–180, 1.0	0.0–180, 1.0	0–180, 1.0	0–180, 1.0	0–180, 1.0
Exposure time, min	10	20	12	10	5
Integr. param. (A, B, EMS)	14.0/−1.0/0.030	12.7/−0.3/0.012	14.0/−1.0/0.030	12.7/−0.3/0.012	14.0/−1.0/0.030
Abs. coefficient, mm^−1	25.8	25.4	25.6	27.4	27.6
F(000), e	8704	8856	8864	8920	8928
θ range, °	2.25–33.28	2.25–33.39	2.25–33.29	2.27–33.33	2.27–33.38
Range in hkl	±24/−23 to 24/±24	−24 to 22/±24/±24	±23/−24 to 21/±24	−20 to 24/−22 to 21/±24	±23/±23/−24 to 22
Total no. reflections	11 507	11 891	11 635	11 552	11 539
Independent refl./Rint	404/0.0930	403/0.0840	401/0.0420	392/0.0650	391/0.0792
Refl. with I ≥ 3σ(I)/Rσ	319/0.0194	313/0.0161	365/0.0059	318/0.0122	294/0.0174
Data/parameters	404/17	403/17	401/17	392/17	391/17
Goodness-of-fit on F^2	1.05	0.98	1.17	1.00	0.99
R/wR for I > 3σ(I)	0.0192/0.0178	0.0155/0.0160	0.0134/0.0315	0.0143/0.0148	0.0163/0.0304
R/wR for all data	0.0334/0.0196	0.0323/0.0178	0.0186/0.0331	0.0234/0.0161	0.0276/0.0325
Extinction coefficient	1420(40)	330(20)	1390(50)	2490(50)	2040(50)
Largest diff. peak /hole, e Å^−3	1.70/−2.21	1.17/−3.38	1.64/−1.65	0.87/−1.93	2.04/−1.90

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Structure refinements

The five data sets showed F centered cubic lattices and the systematic extinctions were compatible with the diamond space group Fd3̄m. The starting atomic parameters were obtained with the charge-flipping algorithm¹⁶ implemented in Superflip¹⁷ and the structures were refined on F² with the JANA2020 software package^18,19 with anisotropic displacement parameters for all sites. Separate refinements of the occupancy parameters gave no hints for deviations from the ideal compositions. The final difference Fourier synthesis revealed no significant residual peaks. The final atomic coordinates, displacement parameters and interatomic distances (exemplarily for TbRh₂Cd₂₀) are listed in Tables 3 and 4.

Table 3 Atomic coordinates and anisotropic displacement parameters (pm²) for SrRh₂Cd₂₀, LaRh₂Cd₂₀, CeRh₂Cd₂₀, TbRh₂Cd₂₀ and DyRh₂Cd₂₀. The anisotropic displacement factor exponent takes the form: −2π²[(ha*)²U₁₁+…+2hka*b*U₁₂]. U_eq is defined as one third of the trace of the orthogonalized U_ij tensor

Atom	Wyck. site	x	y	z	U11	U22	U33	U12	U13	U23	Ueq
SrRh2Cd20
Sr	8a	1/8	1/8	1/8	94(3)	U11	U11	0	0	0	94(2)
Rh	16d	1/2	1/2	1/2	75(2)	U11	U11	−11(2)	U12	U12	75(1)
Cd1	96g	0.05888(2)	x	0.32648(3)	198(2)	U11	127(2)	−76(2)	−7(1)	U13	174(1)
Cd2	48f	0.48879(3)	1/8	1/8	116(2)	114(2)	U22	0	0	−40(2)	115(1)
Cd3	16c	0	0	0	266(3)	U11	U11	−59(2)	U12	U12	266(2)
LaRh2Cd20
La	8a	1/8	1/8	1/8	72(2)	U11	U11	0	0	0	72(1)
Rh	16d	1/2	1/2	1/2	76(2)	U11	U11	−13(2)	U12	U12	76(1)
Cd1	96g	0.05921(2)	x	0.32552(2)	177(2)	U11	116(2)	−62(1)	−9(1)	U13	157(1)
Cd2	48f	0.48845(3)	1/8	1/8	109(2)	111(1)	U22	0	0	−35(2)	110(1)
Cd3	16c	0	0	0	225(2)	U11	U11	−48(3)	U12	U12	225(1)
CeRh2Cd20
Ce	8a	1/8	1/8	1/8	119(1)	U11	U11	0	0	0	119(1)
Rh	16d	1/2	1/2	1/2	94(1)	U11	U11	−11(1)	U12	U12	94(1)
Cd1	96g	0.05905(1)	x	0.32569(2)	191(1)	U11	141(1)	−55(1)	−10(1)	U13	174(1)
Cd2	48f	0.48862(2)	1/8	1/8	129(1)	129(1)	U22	0	0	−36(1)	129(1)
Cd3	16c	0	0	0	206(1)	U11	U11	−35(1)	U12	U12	206(1)
TbRh2Cd20
Tb	8a	1/8	1/8	1/8	87(1)	U11	U11	0	0	0	87(1)
Rh	16d	1/2	1/2	1/2	76(1)	U11	U11	−13(1)	U12	U12	76(1)
Cd1	96g	0.05972(1)	x	0.32414(1)	175(1)	U11	124(2)	−60(1)	−14(1)	U13	158(1)
Cd2	48f	0.48803(2)	1/8	1/8	109(2)	110(1)	U22	0	0	−37(1)	109(1)
Cd3	16c	0	0	0	199(2)	U11	U11	−26(2)	U12	U12	199(1)
DyRh2Cd20
Dy	8a	1/8	1/8	1/8	93(1)	U11	U11	0	0	0	93(1)
Rh	16d	1/2	1/2	1/2	79(2)	U11	U11	−12(2)	U12	U12	79(1)
Cd1	96g	0.05978(2)	x	0.32400(2)	177(1)	U11	129(2)	−61(1)	−14(1)	U13	161(1)
Cd2	48f	0.48804(3)	1/8	1/8	110(2)	112(1)	U22	0	0	−36(2)	112(1)
Cd3	16c	0	0	0	199(1)	U11	U11	−20(2)	U12	U12	199(1)

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Table 4 Interatomic distances (pm) in the structure of TbRh₂Cd₂₀. All distances of the first coordination spheres are listed. Standard deviations are all equal or less than 0.1 pm

Tb:	4	Cd3	336.9	Cd2:	2	Rh	275.7
	12	Cd1	341.6		2	Cd1	292.7
Rh:	6	Cd2	275.7		4	Cd2	301.4
	6	Cd1	303.6		4	Cd1	307.3
Cd1:	1	Cd1	287.3	Cd3:	12	Cd1	331.1
	1	Cd2	292.7		2	Tb	336.9
	2	Cd1	294.6
	1	Rh	303.6
	2	Cd2	307.3
	2	Cd1	316.0
	2	Cd3	331.1
	1	Tb	341.6

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CCDC 2393492 (SrRh₂Cd₂₀), 2393494 (LaRh₂Cd₂₀), 2393488 (CeRh₂Cd₂₀), 2393482 (TbRh₂Cd₂₀) and 2393487 (DyRh₂Cd₂₀)† contain the supplementary crystallographic data for this paper.

Physical property measurements

The ARh₂Cd₂₀ samples were ground to fine powders and filled into polypropylene capsules or glued to a quartz paddle using a low temperature adhesive (GE Varnish). The capsules were attached to a brass sample holder rod before being investigated using the VSM (vibrating sample magnetometer) option of a Physical Property Measurement System (PPMS2) or a Dynacool Physical Property Measurement System by Quantum Design. The magnetization M(H, T) was measured in the temperature range of 3.5–305 K or 2–305 K using fields up to 80 kOe or 90 kOe (depending on the machine). Fitting and plotting the data was done with OriginPro2024²⁰ and the graphical editing with the program CorelDraw2023.²¹

⁸⁹Y, ¹⁰³Rh, ¹¹³Cd and ¹³⁹La solid-state NMR spectroscopy

Solid-state NMR spectroscopy was performed using a Bruker Avance III (ν₀(¹H) = 300.13 MHz) and Bruker NEO (ν₀(¹H) = 500.39 MHz) spectrometer. Chemical shifts of all nuclei are referenced relative to the ¹H resonance of solid adamantane (δ(¹H) = 1.85 ppm) or TMS in CDCl₃ (5% solution v/v; δ(¹H) = 0.00 ppm) using the unified chemical shift scale.^22,23 Pulse calibration was performed on liquid state standards of saturated solutions of Y(NO₃)₃ with Fe(NO₃)₃ as relaxation assistant in D₂O for ⁸⁹Y (ν₀(⁸⁹Y) = 24.520 MHz, I = 1/2), CdBr₂ in D₂O for ¹¹³Cd (ν₀(¹¹³Cd) = 66.608 MHz, I = 1/2), 0.1 M LaCl₃ in D₂O for ¹³⁹La (ν₀(¹³⁹La) = 70.683 MHz, I = 7/2), and 0.171 M Yb(Cp*)₂(THF)₂ (ν₀(¹⁷¹Yb) = 87.567 MHz, I = 1/2). For ¹⁰³Rh (ν₀(¹⁰³Rh) = 15.945 MHz, I = 1/2) pulses were calibrated on a neat GeCl₄ (ν₀(⁷³Ge) = 17.455 MHz, I = 9/2) standard.²⁴ Experiments were performed using a 4 mm HX magic-angle spinning (MAS) probe for ¹¹³Cd, a 5 mm HX static probe for ⁸⁹Y and ¹⁷¹Yb, as well as a 1.3 mm HFXY MAS probe for ¹³⁹La. Samples were ground to a fine powder and diluted with MgO 1 : 1 v/v. For NMR spectroscopic experiments using the 4 mm HX MAS probe, 4 mm zirconia rotors were packed with the sample being centered by PTFE tape. Samples for the 1.3 mm probe were packed between small amounts of MgO at either end of the rotor to center the sample and ensure a filled rotor. Samples for static NMR spectroscopic experiments using the 5 mm HX probe were packed in 5 mm o.d. quartz tubes between PTFE tape.

Solid-state NMR spectroscopic experiments were conducted using the static Wideline Uniform Rate Smooth Truncation (WURST) Carr–Purcell–Meiboom–Gill (WCPMG) sequence for ⁸⁹Y, ¹⁰³Rh, and ¹⁷¹Yb,²⁵ the WCPMG-MAS sequence for ¹¹³Cd,²⁶ and direct acquisition for ¹³⁹La. Optimal pulse power ν_opt for WURST pulses was calculated as ν_opt = 0.265/(I + 1/2)*sqrt(R) with I being the nuclear spin and R being the sweep rate of the WURST pulse, calculated as R = Ω/τ with sweep width Ω and pulse length τ. Note that low sweep rates R were generally used to avoid pulse artifacts²⁷ and keep the power requirements low for ν_opt, which was particularly crucial for the static ¹⁰³Rh WCPMG NMR spectroscopic experiments. All experimental parameters for the WCPMG and WCPMG-MAS experiments are summarized in Table S1.† The direct excitation ¹³⁹La MAS NMR experiments were performed at a MAS frequency ν_MAS = 62.5 kHz using central transition selective π/8 pulses of 1.25 μs with a recycle delay of 25 ms and 524288 scans. All data was processed with a lab-written Python code and plotted using the Matplotlib package.²⁸ Fitting of the spectra was performed using the ssNake software package²⁹ and shifts are given in the Haeberlen-Mehring-Spiess³⁰ convention. Since contributions from chemical, Knight, and pseudo-contact shifts to the observed shift cannot be distinguished, all further mentions will be made using shift and shift anisotropy (SA).

Mössbauer spectroscopy

The 21.53 keV transition of ¹⁵¹Eu with an activity of 55 MBq (1% of the total activity of a ¹⁵¹Sm:EuF₃ source) was used for the ¹⁵¹Eu Mössbauer spectroscopic experiment on EuRh₂Cd₂₀, which was performed in transmission geometry at 78 K. The source was kept at room temperature. The sample was mixed with α-quartz to ensure an even distribution within the sample holder (thin walled PMMA container with a diameter of 20 mm). Fitting of the data was performed by using the WinNormos for Igor7 program package.³¹ and the graphical editing with the program CorelDRAW2023.²¹

3. Results and discussion

Crystal chemistry

Our phase analytical work in the RE-Rh–Cd systems completes the series of RERh₂Cd₂₀ compounds. Also, the isotypic phases CaRh₂Cd₂₀ and SrRh₂Cd₂₀ were obtained. The lattice parameters are listed in Table 1 and a graphical presentation (Iandelli plot) is given in Fig. 2. So far, only few basic crystallographic data is available on the RERh₂Cd₂₀ phases. CeRh₂Cd₂₀ has partially been substituted by indium and lead and lattice parameters for samples of the solid solutions CeRh₂Cd_20−xIn_x and CeRh₂Cd_20−xPb_x were presented only graphically.¹² The corresponding lattice parameter for CeRh₂Cd₂₀ was estimated from the respective plot.

Fig. 2 Course of the lattice parameter in the series of the cubic cadmium-rich phases RERh₂Cd₂₀.

The plot of the lattice parameter obtained in this work (Fig. 2) shows the expected lanthanide contraction. We observe a smooth decrease of the lattice parameters from the lanthanum to the lutetium compound. The parameter for YRh₂Cd₂₀ fits in between the values for TbRh₂Cd₂₀ and DyRh₂Cd₂₀. This is also the case for several other series of rare earth-transition metal-cadmium intermetallics.³² The lattice parameters of EuRh₂Cd₂₀ and YbRh₂Cd₂₀ show strong positive deviations from the smooth plot. Both compounds show stable divalent ground states, as confirmed by magnetic susceptibility measurements as well as Mössbauer and solid-state NMR spectroscopy (vide infra). Similar valence behavior was also reported for the isotypic series of RETi₂Al₂₀ phases.³³ In agreement with the course of the ionic radii, the cell parameters of CaRh₂Cd₂₀ and SrRh₂Cd₂₀ are comparable to YbRh₂Cd₂₀ and EuRh₂Cd₂₀ with divalent ytterbium and europium, respectively.

As for the LaRh₂Cd₂₀ sample, additional signals were observed in the initial NMR spectroscopic experiments (vide infra) indicating a potential formation of a side product not directly visible in the Guinier X-ray diffraction patterns. Therefore, high resolution powder X-ray diffraction patterns were recorded for Rietveld refinements. Analysis of the first recorded diffraction pattern (Fig. S17a†) clearly indicated that the sample showed the targeted phase as the main product, however, additional reflections were observed, which could be attributed to RhCd_9+δ, a brass related superstructure (Zn_90.6Ir_11.1 type, space group F4̄3m, Pearson code cF452).³⁴ Due to the complex structure and the possibility of disorder alongside the quite similar atomic form factors of Rh and Cd, the structural data was used as published, only the lattice parameter was refined (Table S2†). The first measurement of the sample yielded ∼30 mass% RhCd_9+δ. In order to prove the proposed instability of LaRh₂Cd₂₀, the sample was remeasured after three weeks, indicating an increase of the RhCd_9+δ phase to ∼40 mass% (Fig. S17b†). The property studies were then conducted with a new, phase-pure sample (on the lever of X-ray powder diffraction). The observation of this decomposition reaction for the cadmium-based phases is a new feature in the family of CeCr₂Al₂₀ type compounds. The RE-Al and RE-Zn phase diagrams show binary phases with distinctly different crystal structures in the aluminum- and zinc-rich parts. Thus, the decomposition reaction is unique for the cadmium-based phases.

Of the whole ARh₂Cd₂₀ family, the structures of SrRh₂Cd₂₀, LaRh₂Cd₂₀, CeRh₂Cd₂₀, TbRh₂Cd₂₀ and DyRh₂Cd₂₀ were refined from single crystal X-ray diffractometer data. Herein we exemplarily discuss the structure of TbRh₂Cd₂₀ (Fig. 3). The substructure of the terbium and rhodium atoms has the topology of the cubic Laves phase MgCu₂. This is also the case for binary TbRh₂.³⁵ This substructure is then strongly expanded (the lattice parameter increases from 749.1 pm for TbRh₂ to 1556.4 pm for TbRh₂Cd₂₀) and the terbium and rhodium atoms are solely coordinated by cadmium. The smaller rhodium atoms have coordination number (CN) 12 in form of an icosahedron and the terbium atoms have CN16 in form of a Frank–Kasper^36,37 polyhedron. The TbRh₂Cd₂₀ structure can consequently be described by a packing of these two coordination polyhedra. The cadmium coordination prevents direct Tb–Rh contacts. This crystal chemical motif is somewhat comparable to the rare earth-rich phases La₁₅Rh₅Cd₂,³⁸ Gd₄RhCd,³⁹ Er₁₀RhCd₃ ⁴⁰ and Gd₂₃Rh₇Cd₄ ⁴¹ where the rhodium and cadmium atoms are completely separated from each other. The basic building units in these phases are rhodium centered trigonal rare earth prisms and Cd₄ tetrahedra, a rare structural motif.

Fig. 3 The crystal structure of TbRh₂Cd₂₀. The terbium, rhodium and cadmium atoms are drawn as gray, blue and red circles, respectively. The MgCu₂ related substructure of terbium and rhodium is shown in the middle. Terbium and rhodium are coordinated by cadmium as shown on the right-hand side of the figure with gray and blue shading. The three crystallographically independent cadmium sites are shown on the left-hand side along with their site symmetry.

The Tb–Cd distances in the Frank–Kasper polyhedron of TbRh₂Cd₂₀ are 4 × 337 and 12 × 342 pm. They compare well with the Tb@Cd₁₂ anticuboctahedron in TbCd₃⁴² (317–351 pm Tb–Cd) and the Tb@Cd₁₆ Frank–Kasper polyhedron in TbCd₆ ⁴³ (326–362 pm Tb–Cd). These Tb–Cd distances are all slightly longer than the sum of the covalent radii of 300 pm for Tb + Cd.⁴⁴ This is also the case for the Rh–Cd distances (6 × 276 and 6 × 304 pm) within the icosahedra (the sum of the covalent radii⁴⁴ is 266 pm for Rh + Cd). Similar ranges of Rh–Cd distances were observed in the binary compounds Rh₂Cd₅ ⁴⁵ (274–300 pm) and Rh₂Cd₁₅ ⁴⁶ (280–298 pm).

The TbRh₂Cd₂₀ structure exhibits three crystallographically independent cadmium sites with coordination numbers 14 (Cd3) and 12 (Cd1 and Cd2) in the form of bicapped hexagonal respectively pentagonal prisms (Fig. 3). The various Cd–Cd distances range from 287 to 342 pm, comparable to hcp cadmium (6 × 298 and 6 × 329 pm).⁴⁷ The different coordination numbers for the cadmium atoms influence the displacement parameters. In both TbRh₂Cd₂₀ and DyRh₂Cd₂₀ the Cd3 atoms (CN14) show enhanced displacements, a consequence of the slightly larger cage. This crystal chemical feature was observed for all other single crystal structure refinements for RET₂Al₂₀, RET₂Zn₂₀ and RET₂Cd₂₀ phases.³

Magnetic properties

The whole series of ARh₂Cd₂₀ (A = Ca, Sr, Y, La–Nd, Sm–Lu) phases was magnetically characterized. Only the LuRh₂Cd₂₀ sample was not obtained in X-ray pure form. The magnetic data of the samples with A = Ca, Sr, Y, La, Nd, Eu, Sm, Gd and Yb are shown in Fig. 4–8. The remaining plots are documented within the electronic ESI (Fig. S18–S24†). The experimental results are summarized in Table 5.

Fig. 4 Temperature dependence of the magnetic susceptibilities of CaRh₂Cd₂₀, SrRh₂Cd₂₀, YRh₂Cd₂₀, LaRh₂Cd₂₀ and YbRh₂Cd₂₀ measured at 10 kOe. The zero line is shown as a guide to the eye.

Fig. 5 Magnetic data of NdRh₂Cd₂₀: temperature dependence of magnetic susceptibilities and inverse susceptibilities measured at 10 kOe (top) and magnetization isotherms recorded at 2.5, 10 and 50 K (bottom).

Fig. 6 Magnetic data of EuRh₂Cd₂₀: zero field cooled/field cooled (ZFC/FC) measurement at an applied field of 100 Oe (inset) and inverse susceptibilities measured at 10 kOe (top) and magnetization isotherms recorded at 2, 10, 50 and 100 K (bottom).

Fig. 7 Magnetic data of GdRh₂Cd₂₀: zero field cooled/field cooled (ZFC/FC) measurements at an applied field of 100 Oe (inset), inverse susceptibilities measured at 10 kOe; the red line emphasizes the fit regime (top) and magnetization isotherms recorded at 2.5, 10, 50 and 100 K (bottom).

Fig. 8 Magnetic data of SmRh₂Cd₂₀: temperature dependence of magnetic susceptibilities (inset) and inverse susceptibilities measured at 25 kOe; the red line emphasizes the fit regime (top) and magnetization isotherms recorded at 3, 10 and 50 K (bottom).

Table 5 Magnetic properties of ARh₂Cd₂₀: Néel temperature T_N; Curie temperature T_C, effective magnetic moment μ_eff, theoretical magnetic moment μ_theo, paramagnetic Curie temperature θ_P, experimental saturation magnetization μ_sat, theoretical saturation magnetization g_J × J, critical field B_crit ⁴⁸

Compound	TN (K)	TC (K)	μeff (μB)	μtheo (μB)	θP (K)	μsat (μB per f.u.)	gJ × J	Bcrit (kOe)	Ref.
a At 90 kOe.b At 90 kOe and 500 kOe.c At 90 kOe and 400 kOe.
CaRh2Cd20	χ(300 K) = −6.2(1) × 10^−4 emu mol^−1
SrRh2Cd20	χ(300 K) = −1.7(1) × 10^−3 emu mol^−1
YRh2Cd20	χ(300 K) = −1.4(1) × 10^−3 emu mol^−1
LaRh2Cd20	χ(300 K) = −5.9(1) × 10^−4 emu mol^−1
CeRh2Cd20	—	—	2.48(1)	2.54	0.1(1)	1.22(1)	2.14	—	This work
CeRh2Cd20	0.3	—	2.60–2.70	2.54	−2.0	1.2/2^b	2.14	—	9 and 12
PrRh2Cd20	—	—	3.71(1)	3.58	−1.2(1)	2.11(1)^a	3.2	—	This work
PrRh2Cd20	—	—	—	—	—	2–2.65/2.90–3.19^c	3.2	—	10
NdRh2Cd20	—	—	3.91(1)	3.62	−6.1(1)	2.21(1)	3.27	—	This work
SmRh2Cd20	4.3(1)	—	0.78(1)	0.85	−2.9(1)	0.14(1)	0.71	—	This work
EuRh2Cd20	—	13.8(1)	7.94(1)	7.94	15.7(1)	6.61(1)^a	7	—	This work
GdRh2Cd20	9.3(1)	—	8.25(1)	7.94	−4.5(1)	6.71(1)	7	—	This work
TbRh2Cd20	6.6(1)	—	10.32(1)	9.72	−4.0(1)	8.00(1)	9	19(1)	This work
DyRh2Cd20	3.9(1)	—	10.96(1)	10.65	−3.9(1)	9.09(1)^a	10	10(1)	This work
HoRh2Cd20	—	—	10.73(1)	10.61	−0.5(1)	9.82(1)^a	10	—	This work
ErRh2Cd20	—	—	9.66(1)	9.58	−1.7(1)	8.42(1)	9	—	This work
TmRh2Cd20	—	—	7.70(1)	7.56	−2.5(1)	6.17(1)	7	—	This work
YbRh2Cd20	χ(300 K) = −9.4(1) × 10^−4 emu mol^−1

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CaRh₂Cd₂₀, SrRh₂Cd₂₀, YRh₂Cd₂₀, LaRh₂Cd₂₀ and YbRh₂Cd₂₀ show diamagnetic behavior at 300 K (Fig. 4) with the susceptibility values listed in Table 5. Thus, for all five samples the Pauli-paramagnetic contribution is overcompensated by the core diamagnetism. This furthermore confirms the divalent ytterbium in YbRh₂Cd₂₀ with a stable [Xe]4f¹⁴ configuration, in agreement with the plot of the cell parameters (Fig. 2).

All other compounds show paramagnetic behavior. The experimental magnetic moments (Table 5) fit with the calculated values for the free RE³⁺ ions.⁴⁸ An exception is EuRh₂Cd₂₀ which also exhibits a stable divalent ground state ([Xe]4f⁷ configuration). The compounds RERh₂Cd₂₀ (with RE = Ce–Nd and Ho–Tm) show paramagnetic behavior over the whole temperature region with no sign of magnetic ordering. The experimental data of NdRh₂Cd₂₀ is shown as an example in Fig. 5. The paramagnetic behavior is also supported by the course of the magnetization isotherms. For CeRh₂Cd₂₀ we find good agreement with earlier work by Takayama et al.¹² These values are listed for comparison in Table 5.

EuRh₂Cd₂₀ is the only ferromagnet in the series of RERh₂Cd₂₀ intermetallics. The precise Curie temperature of 13.8(1) K (Fig. 6) was derived from dχ/dT of a field-cooling measurement with an external flux density of 100 Oe. The paramagnetic Curie temperature of EuRh₂Cd₂₀ has a positive value of 15.7(1) K, indicating ferromagnetic interactions in the paramagnetic temperature regime. Fig. 6 (bottom) shows the magnetization isotherms. In the paramagnetic range (50 and 100 K isotherms), the magnetization increases in an almost linear manner as is usual for simple paramagnets. The isotherms in the magnetically ordered regime (2 and 10 K) show a steep increase and especially at 2 K rapid saturation. The experimental saturation magnetization at 2 K and 90 kOe of 6.61μ_B is close to the maximum value of 7μ_B according to g × J.⁴⁸ We observe almost no hysteresis, classifying EuRh₂Cd₂₀ as a typical soft ferromagnet.

The susceptibility data of GdRh₂Cd₂₀ is shown in Fig. 7 as an example for an antiferromagnet with T_N = 9.3(1) K. The 2.5 K magnetization isotherm increases in a linear fashion and tends towards saturation near 90 kOe. Since Gd³⁺ and Eu²⁺ are isoelectronic, the maximal magnetization is also 7μ_B. Also, TbRh₂Cd₂₀ and DyRh₂Cd₂₀ show antiferromagnetic transitions, however, both phases exhibit metamagnetic transitions at critical fields of 19 respectively 10 kOe.

SmRh₂Cd₂₀ shows van-Vleck paramagnetism (Fig. 8), resulting from the small energy difference between the excited J = 7/2 multiplet and the J = 5/2 ground state of the Sm³⁺ ion.⁴⁹ For a free Sm³⁺ ion, this energy is calculated to be ΔE = 1550 K.⁴⁹ The low effective moment of Sm³⁺ can be described by the antiparallel coupling of the L = 5, S = 5/2 Russel Saunders states. Stewart proposed a theory to describe the magnetism of samarium intermetallics. The expression χ(T) = χ₀ + D/(T − θ) considers the Van Vleck transition between multiplet levels, the Heisenberg interactions and the induced conduction-electron polarization.⁵⁰ The experimental data of the inverse susceptibility of SmRh₄B₄ could be described by Hamaker et al.⁵¹ by the following equation (μ_eff is the effective moment, θ_P the Weiss constant, μ_B the Bohr magneton, N_A the Avogadro number and k_B the Bolzmann constant):

The value δ is defined as δ = 7ΔE/20 in units of K and describes the energy difference between the ground and excited states. The first term represents the Curie Weiss contribution of the J = 5/2 ground state, while the second term is the temperature independent Van Vleck correction caused by the accessible J = 7/2 state.⁵¹ This description does not include crystal field splitting and intermediate coupling between the lowest energy states.^52,53

The susceptibility of SmRh₂Cd₂₀ was fitted with the Hamaker equation in the temperature range of 35–300 K (Fig. 8), resulting in an μ_eff = 0.78μ_B Sm per atom, θ = –2.9(1) K and δ = 260 K. This leads to ΔE = 743 K, somewhat smaller than the predicted value of 1550 K. Comparable values have been observed for isotypic SmCo₂Zn₂₀ (ΔE = 412 K), SmRu₂Zn₂₀ (ΔE = 265 K) and SmPd₂Cd₂₀ (ΔE = 1488 K).⁵⁴ Further examples are SmNiAl₄Ge₂ (ΔE = 706 K),⁵⁵ SmOs₄Sb₁₂ (ΔE = 850 K)⁵⁶ and SmRh₄B₄ (ΔE = 1080 K).⁵¹ SmRh₂Cd₂₀ orders antiferromagnetically at T_N = 4.3 K.

Finally, we draw back to the course of the Néel temperatures within the RERh₂Cd₂₀ series. The comparatively large distances between the rare earth atoms indicate Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions. Therefore, the transition temperatures should be proportional to the de Gennes factor (Fig. 9).⁵⁷ The Curie temperature of EuRh₂Cd₂₀ doesn't fit into the linear behavior of the other samples, because only phases with the same ordering type should be compared. DyRh₂Cd₂₀ shows a small deviation from the linear trend, most likely indicating crystal field effects.⁵⁸

Fig. 9 T_N of RERh₂Cd₂₀ (RE = Sm, Gd–Dy) and T_C of EuRh₂Cd₂₀ versus de Gennes factor (g_J− 1)² × J(J + 1). The linear fit was only applied to the Néel temperatures.

Solid-state NMR spectroscopic results

Within the large family of CeCr₂Al₂₀ type phases, the main solid state NMR spectroscopic studies focused on the RET₂Al₂₀ phases.^59–65 Herein we present the first complete study of a series of isotypic cadmium compounds. The solid-state NMR spectroscopic characterization was performed for the samples ARh₂Cd₂₀ with A = Y, La and Yb using ¹¹³Cd WCPMG-MAS NMR, static ⁸⁹Y and ¹⁰³Rh WCPMG NMR, and ¹³⁹La MAS NMR. Fig. 10–14 and Table 6 summarize the results. All spectra presented in Fig. 10 and 11 are additionally shown in Fig. S25 to S31† including partial fits. The ¹¹³Cd WCPMG-MAS NMR spectroscopic experiments show three distinct ¹¹³Cd signals (see Fig. 10) with Knight shifts ranging from 5000 to 0 ppm (equivalent to Knight shifts between 0.0 and 0.5%). Two of the three ¹¹³Cd signals are clearly influenced by SA and the corresponding fit parameters are summarized in Table 6. The ¹¹³Cd resonances without SA exhibit Knight shifts of 3350, 4750 and 4775 ppm for ARh₂Cd₂₀ with A = Y, La and Yb, respectively. These ¹¹³Cd resonances can be assigned to the Wyckoff site 16c based on the crystallographic site symmetry 3̄m. For YRh₂Cd₂₀ in Fig. 10, the two spectrally broader ¹¹³Cd resonances could be fitted with the SA parameters δ = 2213 ppm, η = 0.48 and ζ = −554 ppm as well as δ = 1387 ppm, η = 0.0 and ζ = −1169 ppm. The presence of a ¹¹³Cd SA tensor with axial symmetry, i.e. η = 0.0, is surprising given the X-ray data (Table 3). In this structure, no Cd atom, except for the one at the Wyckoff 16c site, is located on a site with site symmetry of 3-fold or higher, which would imply η = 0.0. However, upon closer examination of the Cd coordination polyhedra (Fig. 3) a possible assignment to the 48f site can be made. Although the site symmetry of 2mm does not enforce any restrictions by itself, the distorted bicapped pentagonal prism might allow for sufficiently close axial symmetry, leading to η = 0. This is in line with the fact that this site shows η = 0.0 in the case of LaRh₂Cd₂₀ and η = 0.41 for YbRh₂Cd₂₀. The remaining ¹¹³Cd site in each spectrum in Fig. 10 can therefore be assigned to the 96g site. We note that the Knight shifts for all ¹¹³Cd sites exhibit a consistent pattern, characterized by a decrease in shift from A = Yb to La to Y. In addition, a distinct trend for reduced anisotropy is discernible, where, A = La exhibits the highest reduced anisotropy, while A = Yb shows the lowest.

Fig. 10 ¹¹³Cd WCPMG-MAS NMR spectra of ARh₂Cd₂₀ with A = Y, La and Yb at a MAS frequency of 12.5 kHz; fits of all spectra are shown in the ESI in Fig. S25–S28.†

Fig. 11 Static ¹⁰³Rh WCPMG NMR spectra of ARh₂Cd₂₀ with A = Y, La and Yb; fits of all spectra are shown in the ESI in Fig. S29–S31.†

Fig. 12 Static ⁸⁹Y WCPMG NMR spectrum of YRh₂Cd₂₀; experimental spectrum (black line) and fit (red dashed line).

Fig. 13 Single-pulse ¹³⁹La MAS NMR spectrum of LaRh₂Cd₂₀ recorded at a MAS frequency of 62.5 kHz; experimental spectrum (black line), sum of the fits (red dashed line) and partial fits (blue and green shadings) are shown; the spectrum is fitted with two overlapping components because side band intensities could not be reproduced with one site.

Fig. 14 Static ¹⁷¹Yb WCPMG NMR spectrum of YbRh₂Cd₂₀; experimental spectrum (black line) and fit (red dashed line).

Table 6 NMR spectroscopic data for ⁸⁹Y, ¹⁰³Rh, ¹¹³Cd, ¹³⁹La and ¹⁷¹Yb for RERh₂Cd₂₀ with RE = Y, La and Yb. Shifts are given including the isotropic shift δ_iso, reduced anisotropy ζ, and asymmetry η (top);³⁰ Parameters for quadrupolar nuclei are given as isotropic shift, quadrupolar coupling constant C_Q and asymmetry η (bottom)

Compound	Nucleus	Wyckoff site	δiso/ppm	ζ/ppm	η
YRh2Cd20	^113Cd	16c	3350	—	—
	^113Cd	96g	2213	−554	0.5
	^113Cd	48f	1387	−1169	0.0
	^89Y	8a	2756	—	—
	^103Rh	16d	−9173	1075	0.0
LaRh2Cd20	^113Cd	16c	4750	—	—
	^113Cd	96g	2247	−778	0.7
	^113Cd	48f	1313	−1263	0.0
	^103Rh	16d	−8937	1210	0.0
YbRh2Cd20	^113Cd	16c	4775	—	—
	^113Cd	96g	2671	−367	0.6
	^113Cd	48f	1600	−974	0.4
	^171Yb	8a	7546	—	—
	^103Rh	16d	−8880	1870	0.0
Compound	Nucleus	Wyckoff site	δiso/ppm	CQ/MHz	η
LaRh2Cd20	^139La	8a	4307	0.60	0.0

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The static ¹⁰³Rh WCPMG NMR spectra are shown in Fig. 11 and can be seen to be influenced by a notable SA when compared to the few organometallic samples reported so far.⁶⁶ While the change in ¹⁰³Rh shift is only around 300 ppm between the samples, a pronounced difference of reduced anisotropy is observed. The reduced anisotropy increases from ζ = 1075 ppm in YRh₂Cd₂₀ to 1210 ppm in LaRh₂Cd₂₀ to 1870 ppm in YbRh₂Cd₂₀. This difference might be attributed to the change in oxidation state of the A atom from +II to +III in the case of A = Yb. It should also be noted that the Knight shift, which typically is of positive sign, shows a negative sign for ¹⁰³Rh in these samples. This phenomenon is typically attributed to an additional term in the Knight shift containing a non-negligeable polarization of core electrons.⁶⁷ This phenomenon has been shown in e.g., platinum metal.⁶⁸

The solid-state NMR spectroscopic measurements of ⁸⁹Y and ¹⁷¹Yb in ARh₂Cd₂₀ with A = Y and Yb show sharp resonances in accordance with the expected Wyckoff site symmetry. Both nuclei with shifts of 2756 and 7546 ppm, respectively, fall in the expected range of intermetallic compounds.^69,70 For ¹³⁹La, instead of a singular resonance due to cubic site symmetry, a quadrupolar pattern can be observed and fitting this resonance with one component did not yield satisfactory results. Instead, a fit that includes two components of equal shift but one constrained to a pure Voigt shape was necessary. This observation can be explained through defects in the structure breaking local symmetry, resulting in a non-zero EFG for ¹³⁹La (I = 7/2).⁷¹ As the line width of the observable NMR resonance for I > 1/2 in the solid state is governed by the magnitude of the quadrupolar coupling constant C_Q = (eQ/h)V_zz with V_zz being the z-component of the EFG tensor, this effect gets significantly amplified by the absolute magnitude of the quadrupolar moment Q for the probed quadrupolar nucleus (Q = 20.6 fm² for ¹³⁹La).⁷¹ In contrast, since ⁸⁹Y and ¹⁷¹Yb are both I = 1/2 nuclei, such a break of local symmetry might not suffice to manifest itself in the SA (magnitude proportional to the applied magnetic field) and therefore is likely only detectable via the more sensitive quadrupolar coupling induced by stronger EFGs.

¹⁵¹Eu Mössbauer spectroscopy

The ¹⁵¹Eu Mössbauer spectrum of EuRh₂Cd₂₀ (78 K data) is shown in Fig. 15 along with a transmission integral fit. We observe a single signal at an isomer shift of δ = −10.86(1) mm s⁻¹ and an experimental line width of Γ = 2.34(3) mm s⁻¹. In agreement with the cubic site symmetry of the europium atoms no quadrupole splitting occurs. The isomer shift clearly proves the stable divalent ground state of europium, in accord with the plot of the lattice parameters and the susceptibility measurement.

Fig. 15 Experimental (data points) and simulated (red line) ¹⁵¹Eu Mössbauer spectrum of EuRh₂Cd₂₀ measured at 78 K.

It is worthwhile to compare the isomer shift of EuRh₂Cd₂₀ with the recently reported series of the aluminides EuT₂Al₂₀ with T = Ti, V, Nb, Ta, Cr, Mo and W.⁷² The latter series shows a small range of the isomer shifts from −8.34 to −8.98 mm s⁻¹. The distinctly more negative isomer shift of EuRh₂Cd₂₀ points to increasing ionic bonding contributions.^73,74

4. Conclusions

The series of ARh₂Cd₂₀ compounds (cubic CeCr₂Al₂₀ type) forms with A = Ca, Sr, Y, La–Nd and Sm–Lu. X-ray diffraction, magnetic susceptibility, Mössbauer and solid-state NMR spectroscopic data point to divalent europium and ytterbium, while the remaining phases show stable trivalent rare earth states. CaRh₂Cd₂₀, SrRh₂Cd₂₀, YRh₂Cd₂₀, LaRh₂Cd₂₀ and YbRh₂Cd₂₀ are diamagnets while the remaining phases are Curie–Weiss paramagnets. EuRh₂Cd₂₀ is a 13.8 K ferromagnet; antiferromagnetic ordering was detected for SmRh₂Cd₂₀ (T_N = 4.3 K), GdRh₂Cd₂₀ (T_N = 9.3K), TbRh₂Cd₂₀ (T_N = 6.6 K) and DyRh₂Cd₂₀ (T_N = 3.9 K). The ¹¹³Cd NMR spectra of the A = Y, La and Yb samples revealed the presence of three distinct Cd sites in accordance with X-ray diffraction, which via local symmetry considerations could be assigned to the specific Wyckoff positions. The solid-state NMR spectra of ¹³⁹La suggest local defects in the vicinity of the 8a site although similar observation could not be found for A = Y or Yb. For ⁸⁹Y and ¹⁷¹Yb, the NMR spectra could be recorded and fitted with pure Voigt line shapes showing no SA. A negative Knight shift was found for ¹⁰³Rh in all measured samples hinting at strong s-d exchange correlation.

Author contributions

Lars Schumacher: synthesis, structure refinements, magnetic characterization. Florian Schreiner: solid state NMR spectroscopic characterization. Aylin Koldemir: ¹⁵¹Eu Mössbauer spectroscopic characterization. Oliver Janka: X-ray powder diffraction, writing. Michael Ryan Hansen: conceptualization, supervision, interpretation of the solid-state NMR spectra, writing. Rainer Pöttgen: conceptualization, supervision, project administration, writing – review & editing.

Data availability

All data of this contribution is available.

The crystal structure refinement data has been deposited at the Cambridge center.

All other data is documented in the main manuscript and in the ESI.†

For any further requests please contact the corresponding authors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank Dipl.-Ing. J. Kösters for the X-ray data collections. This research was funded by Universität Münster and Deutsche Forschungsgemeinschaft (INST 211/1034-1). Instrumentation and technical assistance for this work were provided by the Service Center X-ray Diffraction, with financial support from Saarland University and German Science Foundation (project number INST 256/349-1).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4dt03523b

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