High pressure synthesis and crystal structure of a new Ni(III) perovskite:TlNiO3

A new Ni(III) compound, TlNiO3 has been prepared under high oxygen pressure . The pressure and temperature domain required to prepare pure TlNiO3 was f ound to be very narrow (P greater than or equal to 7.5 GPa, 650 less than o r equal to T less than or equal to 700 degreesC). TlNiO3 crystallizes in th e GdFeO3-type perovskite structure with the following lattice parameters: a = 5.2549(1), b = 5.3677(1) and c = 7.5620(2) Angstrom. The corresponding u nit cell volume of 213.3 Angstrom (3) is somewhat larger than the value exp ected from a plot of unit cell volume vs. ionic radius of the A cation for the analogous ANiO(3) series (A = rare earth metal or yttrium). According t o the structural analysis, the twelve Tl-O bonds in TlNiO3 can be classifie d into three approximate groups (four short, four medium and four long dist ances), while, in the other ANiO(3) compounds, the A-O bonds are divided in to two groups of eight short and four long distances. The large unit cell v olume 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 degree s) in TlNiO3 is very similar to that observed in YNiO3 (147.3 degrees). How ever, the Neel temperature (T-N = 105 K) in antiferromagnetic TlNiO3 is sig nificantly lower than that observed in YNiO3 (T-N = 145 K), which has a com parable Ni-O-Ni angle. The magnetic properties could be explained by the co mpeting effect of the Tl(III)-O bond on the covalency of Ni(III)-O bond.