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

The cadmium-rich intermetallic compounds ARh(2)Cd(20) (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 ARh(2)Cd(20) phases crystallize with the cubic CeCr2Al20 type structure, space group Fd (3) over barm. 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 TC = 13.8 K. Antiferromagnetic ordering was detected for SmRh2Cd20 (TN = 4.3 K), GdRh2Cd20 (TN = 9.3K), TbRh2Cd20 (TN = 6.6 K) and DyRh2Cd20 (TN = 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 Eu-151 Mossbauer spectrum which shows an isomer shift of delta = -10.86(1) mm s(-1). Solid-state NMR spectroscopy was performed for the samples YRh2Cd20, LaRh2Cd20, and YbRh2Cd20. The Cd-113 NMR spectra include three distinct Cd sites for each sample in accordance with the crystallographic data. All samples further show negative Knight shifts for Rh-103, suggesting high s-d exchange interaction at the Rh site. In the case of La-139, a residual quadrupolar coupling was observed despite cubic site symmetry, confirming the existence of local defect sites. Y-89 and Yb-171 NMR spectra were recorded, the latter confirming the divalent nature of Yb in YbRh2Cd20.