  Analytical description of domain structure morphology and phase diagrams of
ferroelectric nanoparticles is developed in the framework of Landau Ginzburg
Devonshire approach. To model realistic conditions of incomplete screening of
spontaneous polarization at the particle surface, it was considered covered by
an ultra-thin layer of screening charge with finite screening length. The phase
diagrams, calculated for spherical Sn2P2S6 nanoparticles in coordinates
"temperature vs surface screening length" by finite element modeling,
demonstrate the emergence of polydomain region at the tricritical point and its
broadening with increasing the screening length for the particle radius over a
critical value. Metastable and stable labyrinthine domain structures exist in
Sn2P2S6 nanoparticles with radius 8 and 10 nm and more, similarly to the case
of CuInP2S6 nanoparticles considered previously. We derived simple analytical
expressions for the boundaries between paraelectric, single-domain and
poly-domain ferroelectric phases, tricritical point and the necessary condition
for the appearance of labyrinthine domains, and demonstrated their high
accuracy in comparison with finite element modeling results. Analytical
description shows that phase diagrams and domain morphologies, which are
qualitatively similar to the ones calculated in this work, can be expected in
other ferroelectric nanoparticles covered by the screening charges, being
rather different for the ferroelectrics with the first and second order
ferroelectric-paraelectric transitions respectively.
