Synthesis, structure, and physical properties of la_3-xm_xni₂o_7-δ (m = ca²⁺, sr²⁺, ba²⁺; 0 < x ≤ 0.075)

The synthesis, structure, and physical properties of La_3-xM_xNi₂O_7-δ, with M = Ca²⁺, Sr²⁺, or Ba²⁺, and 0 < x ≤ 0.075, were investigated. These compounds were prepared by a precursor method with tetramethyl ammonium hydroxide and were characterized by room temperature and high-temperature powder X-ray diffraction, TGA, electrical resistivity, and magnetic susceptibility measurements. The substituted compounds form with orthorhombic symmetry in space group Fmmm, similar to the as-prepared parent compound, La₃Ni₂O_6.92. As the amount of divalent alkaline earth metal substitution increases, the c cell parameter does not significantly change for Ca and increases significantly for Sr and Ba substitution, while the a and b cell parameters remain nearly unchanged for all cases. The observed trend in the c parameter is due to the increasing Ni³⁺ ion concentration for the case of Ca²⁺ substitution, while for Sr²⁺ and Ba²⁺ substitutions the effective larger size of the divalent cations is dominant. Significant oxygen deficiencies are noted in all of the as-prepared samples. However, upon high-pressure oxygen annealing, stoichiometric oxygen contents can be achieved. The room temperature resistivity of the as-prepared substituted compounds decreases relative to La₃Ni₂O_6.92, and at x = 0.075 a semiconductor to metal transition is observed for all M. The high-pressure oxygen annealed samples for all compositions show metallic behavior from room temperature down to 20 K. The magnetic susceptibility is nearly temperature independent in the temperature range 100-300 K, and paramagnetic behavior is observed below 100 K.