  In strongly correlated materials, lattice, charge, spin and orbital degrees
of freedom interact with each other, leading to emergent physical properties
such as superconductivity, colossal magnetic resistance and metal-insulator
transition. Quasi-2D square planar nickelates, Rn+1NinO2n+2 (R=rare earth, n=2,
3...), are of significant interest and long sought for cuprate analogue due to
the 3d9 electronic configuration of Ni+, the same as the active ion Cu2+ in the
high-Tc superconducting cuprates. The field has attracted intense attention
since 2019 due to the discovery of superconductivity in thin films of
Nd0.8Sr0.2NiO2, although no superconductivity has been reported in bulk
polycrystalline powders. Herein, we review the synthesis of polycrystalline
powders of quasi-2D square planar nickelates through topotactic reduction of
parent compounds that are synthesized via solid state reaction, precursor
method, high pressure floating zone method and high-pressure flux method. We
emphasize single crystal preparation using the high-pressure floating zone
techniques. We discuss their crystal structure and physical properties
including resistivity, magnetic susceptibility and heat capacity. We highlight
the cuprate-like physics, including charge/spin stripes and large orbital
polarization, identified in single crystals of R4Ni3O8 (R=La and Pr) combining
synchrotron X-ray/neutron single crystal diffraction and density functional
theory calculations. Furthermore, the challenges and possible research
directions of this fast-moving field in the future are briefly discussed.
