RE3Rh2Sn4 (RE = Y, Gd-Tm, Lu) - first stannides with Lu3Co2In4 type structure

The stannides RE3Rh2Sn4 (RE = Y, Gd-Tm, Lu) were synthesized from the elements by arc-melting and subsequent annealing (1220 K for RE = Y, Gd-Tm and 1170 K for RE = Lu) in sealed silica ampoules for 11 days. X-ray powder diffraction studies confirm the hexagonal Lu3Co2In4 type structure, space group P (6) over bar. The structure of Gd3Rh2Sn4 was refined from single crystal X-ray diffractometer data for a twinned crystal: a = 744.04(6), c = 409.23(4) pm, wR2 = 0.0288, 567 F-2 values and 21 variables. The RE3Rh2Sn4 stannides derive from the wellknown equiatomic stannides RERhSn (asymptotic to RE3Rh3Sn3) by Rh/Sn ordering within the RE6 trigonal prisms. The striking structural motif is the trigonal planar tin coordination of the Sn2 atoms with 288 pm Sn2-Sn1 distances. The Sn2 atoms carry substantially more negative charge than the Sn1 atoms. This is underlined by Sn-119 isomer shifts of delta = 1.86(1) mm s(-1) for Sn1 and delta = 2.26(1) mm s(-1) for Sn2 detected in the Mossbauer spectrum of Lu3Rh2Sn4. From atoms in molecules (AIM) analysis of the charge density obtained with calculation based on density functional theory (DFT) for Y3Rh2Sn4, the charge transfer proceeds from yttrium towards more electronegative rhodium. Little departure from neutrality is observed for tin whose itinerant s-like states are little involved with the bonding. The site projected density of states (DOS) and the crystal orbital overlap population (COOP) plots further illustrate these observations and reveal major Y-Rh and Rh-Sn bonding, while Y-Sn bonding is weaker.