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 (TN = 105 K) in antiferromagnetic TlNiO3 is significantly lower than that observed in YNiO3
(TN = 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.
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