Cubic–tetragonal–orthorhombic phase transition sequence in 0.5PbMg1/3Nb2/3O3–0.5PbTiO3and 0.36PbMg1/3Nb2/3O3–0.64PbTiO3single crystals

X-ray powder diffraction, Raman scattering and dielectric studies of 0.5PbMg 1/3 Nb 2/3 O 3 –0.5PbTiO 3 (0.5PMN–0.5PT) and 0.36PbMg 1/3 Nb 2/3 O 3 –0.64PbTiO 3 (0.36PMN–0.64PT) single crystals were carried out over a wide temperature range. The structural investigations revealed the existence of two structural phase transitions. The first one, from the paraelectric cubic ( ) to the ferroelectric tetragonal ( P 4 mm ) phase, occurs at 505 and 575 K for 0.5PMN–0.5PT and 0.36PMN–0.64PT, respectively. The second one, from the tetragonal to orthorhombic ferroelectric ( Pmm 2) phase, is observed at 292 K for 0.5PMN–0.5PT and at 235 K for 0.36PMN–0.64PT. The Raman investigations confirmed the existence of those phase transitions. The lowest frequency mode behaves like a soft mode. The dielectric investigations showed a sharp maximum of ε'( T ) at T C , characteristic for normal ferroelectrics. Some evidence of the tetragonal–orthorhombic phase transition was detected by the dielectric and pyroelectric studies of only the polarized samples.