Stress-induced phase transitions in nanoscale <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Cu</mml:mi><mml:mi>In</mml:mi><mml:msub><mml:mi mathvariant="normal">P</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">S</mml:mi><mml:mn>6</mml:mn></mml:msub></mml:mrow></mml:math>

Using Landau-Ginsburg-Devonshire approach and available experimental results we reconstruct the thermodynamic potential of layered ferroelectric CuInP$_2$S$_6$ (CIPS), which is expected to be applicable a wide range temperatures applied pressures. The analysis temperature dependences dielectric permittivity lattice constants for different pressures unexpectedly reveals critically important role nonlinear electrostriction in this material. With included calculated pressure phase diagrams spontaneous polarization bulk CIPS. coefficients reconstructed four-well potential, study strain-induced transitions thin epitaxial CIPS films, as well stress-induced nanoparticles, shape varies from prolate needles oblate disks. We reveal strong influence mismatch strain, elastic stress anisotropy on polar properties nanoscale Also, derived analytical expressions, allow control properties. Hence obtained can particular interest strain-engineering nanoferroelectrics.