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

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 Fdm. 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.