We study the E-T phase diagram and dynamical pyroelectric coefficient of an (001)-oriented flux-grown 0.8Pb(Mg1/3Nb2/3)O3–0.2PbTiO3 single crystal. We found that the highest pyroelectric coefficient can be achieved at the point having quasicritical features. These data establish a quasicritical behavior of the field-induced pyroelectric coefficients in relaxors.

We show that the phase diagram of the ͑001͒ PbMg 1/3 Nb 2/3 O 3 -xPbTiO 3 ͑PMN-xPT͒ ͑x = 0.00, 0.06, 0.13, and 0.24͒ and ͑111͒ PMN-0.24PT lead-magnesium niobate mixed with lead titanate possesses a quasivertical line in the E electric field -T temperature plot, which hardly depends on the field. The existence of this line has been confirmed by independent studies of single crystals grown in different laboratories, by measuring the dielectric permittivity, compliances, and optical transmission, also in different laboratories. A thermal hysteresis inherent to first order phase transitions complicates the phase diagram. The piezoelectric coefficients of the ͑001͒ PMN-xPT ͑x ϭ 0.06 and 0.13͒ have two peaks versus temperature, at finite fields. The first peak is due to the quasivertical phase boundary. The second is in the vicinity of a turning point of the T m ͑E͒ temperature of the dielectric permittivity diffuse maximum. We show that the second peak prevails at reasonable fields, and the piezoelectric coefficients have comparatively large values at this peak, even at small x.

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We experimentally show that, in contrast to the data having been collected so far, some single crystals of NaNbO(3) exhibit a dielectric permittivity of several thousand, at low T, and this value is saturated when approaching 0 K on cooling. Other sodium niobate crystals (having larger dielectric losses) present a first-order phase transition to a ferroelectric phase on cooling (at 80-200 K). The width of the thermal hysteresis in these crystals increases when the temperature of the phase transition obtained on heating decreases. The dielectric permittivity at the phase transition obtained on cooling shows a tendency to increase and saturate, when the thermal hysteresis increases. We identify the ground state of the sodium niobate crystal exhibiting the smallest dielectric losses (in the studied set of crystals) as a novel quantum paraelectric state coexisting with a metastable ferroelectric state. In principle, the crystal presenting the state of quantum paraelectricity can be considered as having the largest (among the crystals studied) thermal hysteresis, for which the low boundary is below 0 K.