Dielectric, ultraviolet/visible, and Raman spectroscopic investigations of the phase transition sequence in 0.71Pb(Mg<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow/><mml:mrow><mml:mn>1</mml:mn><mml:mo>/</mml:mo><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>Nb<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow/><mml:mrow><mml:mn>2</mml:mn><mml:mo>/</mml:mo><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>)O<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow/><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>-0.29PbTiO<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow/><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>crystals

The structural changes of 0.71Pb(Mg${}_{1/3}$Nb${}_{2/3}$)O${}_{3}$-0.29PbTiO${}_{3}$ (PMN-29PT) are studied by dielectric and optical absorption and hard-mode Raman spectroscopy as a function of temperature. In addition to the known phase transitions of this material at 100, 110 and 124 ${}^{\ifmmode^\circ\else\textdegree\fi{}}$C two new transitions at 42 and 60 ${}^{\ifmmode^\circ\else\textdegree\fi{}}$C are observed in unpoled [111]-oriented PMN-29PT single crystals. The phase transitions are associated with changes of the dielectric and optical properties arising from small changes of the inherent local symmetry. The experimental results are analyzed theoretically and a model for the temperature-dependent rotation of the spontaneous polarization is proposed. The formation of two new low-temperature phases, at least on a local scale, reflect an underlying polarization rotation path being much more complex than currently believed.