Potassium Lanthanum Nitrate System: Phase Equilibria and Thermal and Structural Properties

TlSr2NiO4+has been synthesized and its structure determined using high-resolution neutron powder diffraction data collected at 5 K, 90 K, and room temperature. It crystallizes in an orthorhombic cell, space groupPmmm, lattice parametersa=3.7217 (1),b=7.5158(2), andc=8.8321(3) ?. The compound is isostructural with the orthorhombic form of TlSr2CuOy, with alternating TlO and oxygen-deficient NiO2-xlayers connected through oxygen. Ordering of the oxygen vacancies in the NiO2-xlayer occurs, producing a superstructure with alternating NiO6octahedra and NiO4square planes along theb-direction. A comparison between the nickel and copper environments in the two materials is performed. Magnetic susceptibility data reveal that TlSr2NiO4 +displays Curie–Weiss behavior in the range 80–300 K with a weak antiferromagnetic ordering at 15 K.