Using atomic force microscopy and piezoresponse force microscopy (PFM), as well as complementary dielectric measurements, the ferroelectric domain structure of single crystals of pure and ruthenium (Ru)-doped 0.9Pb(Zn1/3Nb2/3)O-3 (PZN)-0.1PbTiO(3) (PT), and Ru/(Zn+Nb+Ti) similar to 0.002 was analyzed. The coexistence of tetragonal and nontetragonal ferroelectric domains in PZN-PT near the morphotropic phase boundary (MPB) was directly observed via PFM imaging. The incorporation of Ru in the perovskite structure substantially decreases the ferroelectric domain size, thus reducing the polar fraction distributed in the pseudocubic matrix. The polarization and strain hysteresis loops show that Ru doping leads to significant ferroelectric hardening, which is attractive for high-power applications. The local structure and valent state of the incorporated Ru cations were additionally analyzed by electron paramagnetic resonance spectroscopy. Our results demonstrate that doping of tetragonal PZN-PT near MPB is a promising route to design advanced multifunctional single-crystal materials. (C) 2009 American Institute of Physics. {[}doi:10.1063/1.3238487]