  Searching for functional square lattices in layered superconductor systems
offers an explicit clue to modify the electron behavior and find exotic
properties. The trigonal SnAs3 structural units in SnAs-based systems are
relatively conformable to distortion, which provides the possibility to achieve
structurally topological transformation and higher superconducting transition
temperatures. In the present work, the functional As square lattice was
realized and activated in Li0.6Sn2As2 and NaSnAs through a topotactic
structural transformation of trigonal SnAs3 to square SnAs4 under pressure,
resulting in a record-high Tc among all synthesized SnAs-based compounds.
Meanwhile, the conductive channel transfers from the out-of-plane pz orbital to
the in-plane px+py orbitals, facilitating electron hopping within the square 2D
lattice and boosting the superconductivity. The reorientation of p-orbital
following a directed local structure transformation provides an effective
strategy to modify layered superconductors.
