Low-temperature ground state structure of PbTiO3

${\mathrm{Mn}}⁴⁺$, ${\mathrm{Cr}}³⁺$, and ${\mathrm{Fe}}³⁺$ electron paramagnetic resonance and $^47,49\mathrm{Ti}$ and $²⁰⁷\mathrm{Pb}$ nuclear magnetic resonance (NMR) measurements are carried out in $\mathrm{PbTi}{\mathrm{O}}₃$ single crystals at temperatures from 300 K down to 20 K. It was found that the tetragonal crystal-field parameter of the ${\mathrm{Mn}}⁴⁺$ and ${\mathrm{Cr}}³⁺$ impurity ions undergoes an unexpected change in its temperature dependence at $T<170$ K: it increases with decreasing temperature to 150--170 K and then strongly decreases with further temperature lowering. Electric-field gradient at Ti site measured from $^47,49\mathrm{Ti}$ NMR shows nonuniform change with decreasing temperature: it sharply increases with decreasing temperature to 150--170 K and is practically temperature independent below these temperatures. Moreover, substantial broadening of NMR linewidth is seen for Ti and Pb nuclei at $T<170$ K due to essential increase of lattice parameters fluctuations. All these facts may indicate proximity of a transition to another phase with barely perceptible crystal-structure modification as reported in the paper by Kobayashi et al. [Phys. Rev. B 28, 3866 (1983)] while, actually, it was not detected in NMR. The experimental data are compared with those obtained from ab initio calculations.