Neutron and X-ray diffraction study of cubic [111] field cooled Pb(Mg1/3Nb2/3)O3

Neutron and x-ray diffraction techniques have been used to study the competing long and short-range polar order in the relaxor ferroelectric Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$ (PMN) under a [111] applied electric field. Despite reports of a structural transition from a cubic phase to a rhombohedral phase for fields E $>$ 1.7 kV/cm, we find that the bulk unit cell remains cubic (within a sensitivity of 90$^{\circ}$-$\alpha$ =0.03$^{\circ}$)for fields up to 8 kV/cm. Furthermore, we observe a structural transition confined to the near surface volume or `skin' of the crystal where the cubic cell is transformed to a rhombohedral unit cell at T$_{c}$=210 K for E $>$ 4 kV/cm, for which 90$^{\circ}$-$\alpha$=0.08 $\pm$ 0.03$^{\circ}$ below 50 K. While the bulk unit cell remains cubic, a suppression of the diffuse scattering and concomitant enhancement of the Bragg peak intensity is observed below T$_{c}$=210 K, indicating a more ordered structure with increasing electric field yet an absence of a long-range ferroelectric ground state in the bulk. The electric field strength has little effect on the diffuse scattering above T$_{c}$, however below T$_{c}$ the diffuse scattering is reduced in intensity and adopts an asymmetric lineshape in reciprocal space. The absence of hysteresis in our neutron measurements (on the bulk) and the presence of two distinct temperature scales suggests that the ground state of PMN is not a frozen glassy phase as suggested by some theories but is better understood in terms of random fields introduced through the presence of structural disorder. Based on these results, we also suggest that PMN represents an extreme example of the two-length scale problem, and that the presence of a distinct skin maybe necessary for a relaxor ground state.Comment: 12 pages, 9 figure

Direct observation of the formation of polar nanoregions in Pb(Mg1/3_{1/3}Nb2/3_{2/3})O3_3 using neutron pair distribution function analysis

Using neutron pair distribution function (PDF) analysis over the temperature range from 1000 K to 15 K, we demonstrate the existence of local polarization and the formation of medium-range, polar nanoregions (PNRs) with local rhombohedral order in a prototypical relaxor ferroelectric Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$. We estimate the volume fraction of the PNRs as a function of temperature and show that this fraction steadily increases from 0 % to a maximum of $\sim$ 30% as the temperature decreases from 650 K to 15 K. Below T$\sim$200 K the PNRs start to overlap as their volume fraction reaches the percolation threshold. We propose that percolating PNRs and their concomitant overlap play a significant role in the relaxor behavior of Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$.Comment: 4 pages, 3 figure

Easy induction of a ferroelectric fourfold monoclinic domain state from a tetragonal ferroelectric monodomain state in PbZn1/3Nb 2/3O3

We report a combined pyroelectric, dielectic and structural characterization of PZN single crystals in field cooling (FC) run along [001]. The pyroelectric current and the dielectric permittivity show a new characteristic temperature, at T approximate to 370 K, related to a macroscopic competition between two induced ferroelectric tetragonal (T) and monoclinic (M) phases. The phase transition, which starts around TC-R = 385 K, spreads over a large temperature range and some amount of T-phase is still observed at room temperature (RT). The M-T phase mixture, induced below TC-R, is stable after field removal. According to a previous Raman scattering work, a fourfold symmetry of M-domains is proposed; the residual T-phase is stabilized by the coexistence of four symmetry-allowed M-domains arranged around[001]. Our results confirm that in a restricted temperature range (TC-R - 20 K, TC-R + 20 K) the free energy of the relaxor-ferroelectric PZN is almost independent of the direction of polarization and accordingly the material can switch from one polar order to the other

Kinetic study of PZN single crystal polarization under a static electric field

International audienceA kinetic effect is evidenced in lead zinc niobate (PZN). We measured the time dependencies of the polarisation P(t) of a single crystal. At a given temperature and electric field, P(t) is described by the expression P(t) = P∞(T) f(t/τ(E,T)) where τ(E,T) is the incubation time above which the sample is completely polarised. The τ(E,T) was fitted using the exponential law exp[(W+kδT/E)/kT] where the activation energy W is equal to 0.91±0.36 eV and δT is equal to 2.2×108 K V/m in the 300-360 K temperature range. Taking into account this kinetic effect the electric-field-temperature diagram and the existence of an electric field threshold were discussed

Tetragonal and rhombohedral induced polar order from the relaxor state of PbZn1/3Nb2/3O3

International audienceStructural and dielectric characterizations of the relaxor ferroelectric PbZn1/3Nb2/3 (PZN) were carried out on single crystals in the temperature range of the dielectric anomaly (370 K andlt; T andlt; 440 K), under dc electric field applied along [001]- and [111]-directions. Two ferroelectric phases, tetragonal (T) and rhombohedral (R), can be field induced in this temperature range, the symmetry of which depends on the direction of the applied field. [001] and [111] respectively. E-T diagrams were built and interpreted under the consideration of polar nano-regions (PNRs) with a prominent tetragonal component. These PNRs prevent the onset of a monodomain state when the field is applied along [111] and explain the features of PZN for an applied field along [001] large lattice strain, easy onset of the T phase and suppression of the dielectric relaxation for E ≥ 1.5 kV cm-1. The microscopic origin of these PNRs can be discussed in relation with recent nuclear magnetic resonance and structural results reported in other isomorphous relaxors

Experimental study of the spreading, due to substitution defects, of the first order transitions in the mixed system Pb3P2O 8-Pb3V2O8

Experimental results by means of very precise X-ray goniometry on Pb 3P2xV2 (1-x) O8 system are given. They clearly show that there is no smearing of the order of transitions, x varying from 0 to 1, but a spreading of the critical temperature and of the rate of transformation, i.e. the volumic ratio between the high and low temperature phases. The interfaces between the two phases are described.Des résultats expérimentaux sur le système Pb3P2xV 2(1-x) O8, obtenus par goniométrie très précise des rayons X, sont présentés. Ils montrent clairement qu'il n'y a pas estompement de l'ordre des transitions quand x, varie entre 0 et 1 mais étalement des températures critiques et du taux de transformation, c'est-à-dire du rapport volumique des phases haute et basse température. L'interface entre ces deux phases est décrit