  We report on the B-site substitution of Pt in the system LaNi1-xPtxO3. The
system can only be synthesized for x <= 0.50, with LaNiO3 (x = 0.00) and the
stoichiometric double perovskite La2NiPtO6 (x = 0.50) as the end members.
Higher Pt-contents (x > 0.50) are unachievable due to the preference of Pt to
either be in oxidation state +IV in octahedral coordination. Upon introducing
Pt into LaNiO3, a phase transformation from rhombohedral (R-3c) to monoclinic
(P21/n) symmetry is observed for 0.075 <= x <= 0.125, where all monoclinic
samples are B-site ordered, and Pt show a strong preference for the Pt-site.
Powder X-ray diffraction analysis reveal disorder of the Pt-distribution in
several of the samples with a non-equimolar Ni/Pt ratio (0.20 <= x <= 0.40),
which point toward cluster formation with domains of high and low Pt-content
within each sample. La2NiPtO6 further show an antiferromagnetic transition at
approx. 40 K. A similar transition is observed for all monoclinic samples (x >=
0.20), however, the transition becomes weaker for lower x. This is explained in
light of the structural disorder, i.e. by the coexistence of antiferromagnetic
domains with long range order and paramagnetic domains dominated by short range
antiferromagnetic interactions.
