CaBe2Ge2 type phosphides REIr2P2 (RE = La–Nd, Sm) and arsenides REIr2As2 (RE = La–Nd): synthesis, structure, and solid state NMR spectroscopy

Abstract

The phosphides REIr₂P₂ (RE = La–Nd, Sm) and arsenides REIr₂As₂ (RE = La–Nd) were synthesized by a ceramic route via precursor compounds REIr₂ with phosphorus and arsenic, respectively. Well-shaped single crystals were obtained from lead and bismuth fluxes, respectively. The nine pnictides crystallize with the tetragonal CaBe₂Ge₂ type structure, space group P4/nmm. The structures of CeIr₂P₂, SmIr₂P₂, LaIr₂As₂ and CeIr₂As₂ were refined from single crystal X-ray diffractometer data. The structures are composed of three-dimensional [Ir₂P₂] and [Ir₂As₂] 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 CeIr₂P₂. The rare-earth and phosphorus local environments in LaIr₂P₂ are characterized further by ¹³⁹La and ³¹P single and double resonance solid state nuclear magnetic resonance (NMR) spectroscopy. Strong ³¹P 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 ³¹P 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 ³¹P–³¹P magnetic dipole–dipole interactions, which can be measured in a site-resolved fashion in this compound using static ³¹P spin echo decay spectroscopy. The highly symmetric La environment in LaIr₂P₂ is characterized by a sharp ¹³⁹La 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 ³¹P as the regular site, according to ¹³⁹La{³¹P} 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 LaIr₂P_1.90.