Temperature-driven polarization rotation and triclinic phase at the morphotropic phase boundary of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>Pb</mml:mi> <mml:mrow> <mml:mo>(</mml:mo> <mml:mrow> <mml:mi mathvariant="

Information about the crystal structures in the range of morphotropic phase boundary of ferroelectric perovskite solid solutions is important for understanding their intricate properties, which result in wide opportunities for practical applications. However, for the $(1\ensuremath{-}x)\mathrm{Pb}(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}){\mathrm{O}}_{3}--x\mathrm{PbTi}{\mathrm{O}}_{3}$ solid solution system, this information is contradictory. Different composition-temperature phase diagrams have been reported for this system in literature based on the investigations of single crystals and ceramics using various experimental techniques. In this work, we apply polarized light microscopy (PLM), x-ray diffraction (XRD) and dielectric spectroscopy to study the crystal structure and phase transitions in the $0.68\mathrm{Pb}(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}){\mathrm{O}}_{3}--0.32\mathrm{PbTi}{\mathrm{O}}_{3}$ single crystal. We confirm the monoclinic ${\mathrm{M}}_{\mathrm{B}}$ symmetry (space group $\mathit{Cm}$) of the room-temperature phase. According to PLM, it transforms with increasing temperature into a triclinic (Tr) phase rather than the previously reported monoclinic ${\mathrm{M}}_{\mathrm{C}}$ or tetragonal phase. XRD data are consistent with the presence of Tr phase. The Tr phase transforms to the monoclinic ${\mathrm{M}}_{\mathrm{C}}$ ($Pm$) phase and then to the cubic phase. Ergodic relaxor behavior is observed above the Curie temperature. The unit cell in the ${\mathrm{M}}_{\mathrm{C}}$ phase is pseudotetragonal with the lattice parameters $a=b&lt;c$ and a small monoclinic angle \ensuremath{\beta}. In the ${\mathrm{M}}_{\mathrm{B}}$ phase, the direction of spontaneous polarization is temperature independent and close to the $\ensuremath{\langle}111\ensuremath{\rangle}$ pseudocubic direction. In the Tr and ${\mathrm{M}}_{\mathrm{C}}$ phases, it changes with temperature so that near the Curie point it is close to the [001] axis. No significant anomalies in the dielectric properties or changes in the domain structure are observed at the ${\mathrm{M}}_{\mathrm{B}}\ensuremath{\rightarrow}\mathrm{Tr}$ and $\mathrm{Tr}\ensuremath{\rightarrow}{\mathrm{M}}_{\mathrm{C}}$ phase transitions. The domain structure changes dramatically when the temperature varies within the Tr phase, causing a sharp change in birefringence.