Abstract

A new Ni(III) compound, TlNiO₃ has been prepared under high oxygen pressure. The pressure and temperature domain required to prepare pure TlNiO₃ was found to be very narrow (P ≥ 7.5 GPa, 650 ≤ T ≤ 700 °C). TlNiO₃ crystallizes in the GdFeO₃-type perovskite structure with the following lattice parameters: a = 5.2549(1), b = 5.3677(1) and c = 7.5620(2) Å. The corresponding unit cell volume of 213.3 Å³ is somewhat larger than the value expected from a plot of unit cell volume vs. ionic radius of the A cation for the analogous ANiO₃ series (A = rare earth metal or yttrium). According to the structural analysis, the twelve Tl–O bonds in TlNiO₃ can be classified into three approximate groups (four short, four medium and four long distances), while, in the other ANiO₃ compounds, the A–O bonds are divided into two groups of eight short and four long distances. The large unit cell volume of TlNiO₃ is surely due to the different coordination of the Tl(III) ion in the perovskite lattice. The average Ni–O–Ni bond angle (147.6°) in TlNiO₃ is very similar to that observed in YNiO₃ (147.3°). However, the Néel temperature (T_N = 105 K) in antiferromagnetic TlNiO₃ is significantly lower than that observed in YNiO₃ (T_N = 145 K), which has a comparable Ni–O–Ni angle. The magnetic properties could be explained by the competing effect of the Tl(III)–O bond on the covalency of Ni(III)–O bond.