  Undoped and Mn-doped ZnS nanocrystals were produced by the microwave-assisted
solvothermal method and characterized by X-ray diffraction, photoluminescence
spectroscopy and scanning electron microscopy with energy-dispersive X-ray
spectroscopy. All samples have the cubic zinc blende structure with the lattice
parameter in the range of $a$ = 5.406-5.411 \r{A}, and the average size of
crystallites is in the range of 6-9 nm. These nanoparticles agglomerate and
form large grains with an average size of up to 180 nm. The photoluminescence
of the undoped ZnS sample shows a broad emission band located at 530 nm,
attributed to the defects at the surface of nanoparticles. In all Mn-doped
samples, the emission peak at 598 nm was observed assigned to the
characteristic forbidden transition between excited ($^4$T$_1$) and ground
($^6$A$_1$) levels of Mn$^{2+}$. Synchrotron radiation X-ray absorption
spectroscopy at the Zn and Mn K-edges combined with reverse Monte Carlo (RMC)
simulations based on the evolutionary algorithm confirms that manganese ions
substitute zinc ions. However, the difference in the ion sizes
($R$(Mn$^{2+}$(IV)) = 0.66 \r{A} and $R$(Zn$^{2+}$(IV)) = 0.60 \r{A}) is
responsible for the larger interatomic distances Mn-S (2.40(2) \r{A}) compared
to Zn-S (2.33(2) \r{A}). The static structural relaxations in ZnS:Mn
nanoparticles are responsible for the large values of the mean-square
displacements factors for Zn, S and Mn atoms obtained by RMC simulations.
