CaBe sub(2)Ge sub(2) type phosphides REIr sub(2)P sub(2) (RE= La-Nd, Sm) and arsenides REIr sub(2)As sub(2) (RE= La-Nd): synthesis, structure, and solid state NMR spectroscopy

The phosphides REIr sub(2)P sub(2) (RE= La-Nd, Sm) and arsenides REIr sub(2)As sub(2) (RE= La-Nd) were synthesized by a ceramic route viaprecursor compounds REIr sub(2) with phosphorus and arsenic, respectively. Well-shaped single crystals were obtained from lead and bismuth fluxes, respectively. The nine pnictides crystallize with the tetragonal CaBe sub(2)Ge sub(2) type structure, space group P4/nmm. The structures of CeIr sub(2)P sub(2), SmIr sub(2)P sub(2), LaIr sub(2)As sub(2) and CeIr sub(2)As sub(2) were refined from single crystal X-ray diffractometer data. The structures are composed of three-dimensional [Ir sub(2)P sub(2)] and [Ir sub(2)As sub(2)] networks in which the rare earth atoms fill cavities of coordination number 16 (8 P + 8 Ir). The phosphorus and arsenic atoms have tetrahedral and square pyramidal iridium coordination. Temperature dependent magnetic susceptibility measurements show intermediate cerium valence for CeIr sub(2)P sub(2). The rare-earth and phosphorus local environments in LaIr sub(2)P sub(2) are characterized further by super(139)La and super(31)P single and double resonance solid state nuclear magnetic resonance (NMR) spectroscopy. Strong super(31)P Knight shifts and extremely short spin-lattice relaxation times indicate that the bonding character of the phosphide species is strongly metallic. The two crystallographically distinct phosphorus sites are well-resolved in the super(31)P magic-angle spinning (MAS) spectrum and also differ significantly with respect to their effective magnetic shielding anisotropies. Unambiguous site assignments are accomplished on the basis of homonuclear super(31)P- super(31) P magnetic dipole-dipole interactions, which can be measured in a site-resolved fashion in this compound using static super(31)P spin echo decay spectroscopy. The highly symmetric La environment in LaIr sub(2)P sub(2) is characterized by a sharp super(139)La MAS-NMR spectrum, revealing rather weak nuclear electric quadrupole coupling. Furthermore, a second local environment is detected, which is characterized by stronger quadrupolar coupling and similar dipolar coupling strength with super(31)P as the regular site, according to super(139)La{ super(31)P} rotational echo double resonance (REDOR) NMR results. On the basis of these data we attribute this site to a La species next to a phosphorus vacancy. From the signal area of this resonance we deduce a composition LaIr sub(2)P sub(1.90).