Neutron diffraction study of superconductingLa3Ni2B112N3−δ

We have studied structural properties of La 3 Ni 2 B 2 N 3− δ samples with distinctly different values of the superconducting transition temperature by means of powder neutron diffractometry and specific heat measurements. The refinement of lattice site occupations reveals full occupations for all sites in the La 3 Ni 2 B 11 2 N 3 structure with space group I 4/ mmm except for nitrogen site N(2). For samples with T c = 13.0 K and 13.7 K we obtain for the N(2) site distinctly different occupation factors of 0.90 and 0.93, respectively. The latter confirms a direct relation between the nitrogen stoichiometry and the superconducting transition temperature. Based on the analysis of temperature dependent lattice parameters, atomic displacement factors, lattice heat capacity data and ab initio phonon density of states calculations we discuss the thermal expansion and vibrational properties of superconducting La 3 Ni 2 B 2 N 3− δ .