  The electronic properties of transition-metal oxides with highly correlated
electrons are of central importance in modern condensed matter physics and
chemistry, both for their fundamental scientific interest, and for their
potential for advanced electronic applications. The design of materials with
tailored properties has been, however, restricted by the limited understanding
of their structure-property relationships, which are particularly complex in
the proximity of the regime where localized electrons become gradually mobile.
RENiO3 perovskites, characterized by the presence of spontaneous metal to
insulator transitions, are one of the most widely used model materials for the
investigation of this region in theoretical studies. However, crucial
experimental information needed to validate theoretical predictions is still
lacking due to their challenging high-pressure synthesis, which has prevented
to date the growth of sizable bulk single crystals with RE different than La,
Pr and Nd. Here we report the first successful growth of single crystals with
RE = Nd, Sm, Gd, Dy, Y, Ho, Er and Lu and sizes up to ~75 {\mu}m, grown from
molten salts in temperature gradient under 2000 bar oxygen gas pressure. The
crystals display regular prismatic shapes with flat facets, and their crystal
structures, metal-insulator and antiferromagnetic order transition temperatures
are in excellent agreement with previously reported values obtained from
polycrystalline samples. The availability of such crystals opens access to
measurements that have hitherto been impossible to conduct. This should
contribute to a better understanding of the fascinating properties of materials
with highly correlated electrons, and guide future efforts to engineer
transition metal oxides with tailored functional properties.
