Synthesis, crystal structure and superconductivity of LaNiPO

Single crystals of LaNiPO were synthesized by reacting La, P and NiO at 1173 K in a tin flux under argon atmosphere. A phase analysis and crystal structure determination was carried out using X-ray powder and single crystal methods. The quaternary phosphide oxide crystallizes in the tetragonal ZrCuSiAs structure (P4/nmm, a = 404.53(1) pm, c = 810.54(3) pm, Z = 2), which is characterized by layers of edge-sharing (2)(infinity) [La4/4O] and (2)(infinity) [NiP4/4] tetrahedra alternating along [001]. LaNiPO is a Pauli paramagnetic metal at room temperature and becomes a type-II superconductor at T-C = 4.3 K with an upper critical field H-c2 approximate to 320 mT at 1.8 K. Connections between the crystal chemistry and the occurrence of superconductivity in LaNiPO and related ThCr2Si2 type compounds are discussed. (C) 2007 Elsevier Masson SAS. All rights reserved.