Structural, dielectric and Raman scattering studies of (1−x)PbMg_1/3Nb_2/3O₃–xPbTiO₃single crystals (0 ≤ x ≤ 0.38)

Role of the TiO6 octahedra on the ferroelectric and piezoelectric behaviour of the poled PbMg1/3Nb2/3O3–xPbTiO3 (PMN–PT) single crystal and textured ceramic

“…The main phase transition observed at 134°C and at 140°C for the FC poled and unpoled single crystals, respectively can be attributed (according to the diffraction data [11,13,17]) to the transition from the rhombohedral symmetry to the cubic paraelectric one. Similar, the main transitions detected near 160°C in the case of RT poled/unpoled texture ceramic might be assigned to the transition from the tetragonal symmetry to the cubic one.…”

“…Low wavenumber region (100 -250 cm -1 ) chiefly involves cationic network and the lattice modes while a 250 to 800 cm -1 spectral range reveals the bending and stretching modes of the covalent-bonded octahedron [3,33]. Figure 7 are typical for relaxor ferroelectrics [3,17,18,[33][34][35][36][37]. They consist of a number of broad bands observed in the whole measured temperature range, even within the paraelectric cubic phase with the Pm3m space group.…”

“…Note that for such cubic symmetry the first order Raman activity is not allowed [38,39]. This phenomenon has already been connected with the short range intrinsic disorder and the presence of the polar nanoregions (PNR) where the ions are displaced from their high mean symmetry positions giving rise to the local electric dipoles [3,17,18,[35][36][37]40]. Since the Raman spectroscopy is sensitive to the atomic dynamics (through the wavenumbers) and the local electric charge transfer (through the intensities), the local electric defects strongly disturb the spectra, especially intensities [41].…”

“…In the case of the PMN-29%PT the Noheda' phase diagram [11] does not show such phase transition. However, recently some articles appears in which additional phase transition was observed in the solid solutions with similar compositions: 27%PT and 29%PT [13,16,17] and interpreted as structural phase transitions from the monoclinic (Cm or Pm space group) to the rhombohedral symmetry. Moreover, even presence of structural phase transitions between two different monoclinic symmetry and/or the coexistence of the phases (two monoclinic or rhombohedral-monoclinic) were suggested [13,17] …”

“…In general, these materials posses already ferroelectric behaviour and undergo the monoclinic (Pm) -> tetragonal (P4mm) -> cubic (Pm3m) structural phase transitions [11,12]. However, many authors [11,[13][14][15][16][17] suggested that the symmetry observed in the vicinity of the MPB is more complex. Namely, the monoclinic symmetry can coexist with a secondary minority phases: tetragonal or rhombohedral, and the presence of the orthorhombic phase was also postulated.…”