The macroscopic symmetry of Pb(Mg_1/3Nb_2/3)_1−xTi_xO₃in the morphotropic phase boundary region (x= 0.25–0.5)

Abstract

The macroscopic symmetry of Pb(Mg1/3Nb2/3)1−xTixO3 (PMN–PT) crystals with x = 0.25–0.5 has been studied by optical microscopy. Precise data on the temperature dependence of the birefringence and optical extinction directions have been obtained. Two different low-symmetry phases separating rhombohedral and tetragonal phases have been observed in the compositional range x = 0.3–0.47. The optical extinction in the range x = 0.3–0.37 is consistent with Cm space group symmetry, while for x = 0.37–0.47 Pm symmetry is appropriate. Gradual rotation of the optical indicatrix has been found at temperatures just below Tc in crystals with x = 0.27–0.3. A refined phase diagram is presented.

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

“…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] …”

“…According to the X-ray diffraction the high temperature symmetry is cubic with the Pm3m space group [9][10][11][12][13][14][15][16][17][18]. As we already mentioned for such symmetry the first order Raman activity is forbidden.…”

“…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.…”

Macroscopic lamellar heterophase pattern inPb(Mg1/3Nb2/3)O3

“…Coexistence of different phases is rather frequent phenomenon in case of first-order phase transformations and a planar macroscopic phase front can be stabilized for example with the aid of temperature or concentration gradient. 26 Macroscopic phase boundary between different ferroelectric phases has been previously observed in PMN-xPT crystal with a linear gradient of Ti concentration 27 . However, in that work, the phase boundary seems to simply delimit Ti-poorer regions from Ti-richer regions, without any obvious relation to the crystallographic orientation of the sample.…”

Micro-Raman study of the microheterogeneity in the MA-MC phase transition in 0.67PbMg1/3Nb2/3O3-0.33PbTiO3 single crystal

“…This is in good agreement with previous reports, in which only the M A phase was observed at low temperature. 27 Such phase transition is expected to relate to the nanoinhomogeneous, i.e., PORs or CORs, in the crystal. It has been indicated that the M C -M A phase transition under external electric field arises from the polarization switching mechanism.…”