RESUMO
Motivated by the long-standing unresolved enigma of the relaxor ferroelectric ground state, we performed a high-resolution heat capacity and polarization study of the field-induced phase transition in the relaxor ferroelectric single crystal Pb(Mg(1/3)Nb(2/3))O3 (PMN) oriented along the [110] direction. We show that the discontinuous evolution of polarization as a function of the electric field or temperature is a consequence of a true first order transition from a glassy to ferroelectric state, which is accompanied by an excess heat capacity anomaly and released latent heat. We also find that in a zero field there is no ferroelectric phase transition in bulk PMN at any temperature, indicating that the nonergodic dipolar glass phase persists down to the lowest temperatures.
RESUMO
Recently there has been considerable interest in the displacive ferroelectric phase transition near T = 28 K in O-18 isotopic strontium titanate. Special efforts have been made to combine the quantum criticality exponents α = -2 (2D) or -3 (3D), δ = 3, and γ = 2 with the thermodynamic inequalities of Rushbrooke, Griffiths, Widom et al, which become exact equalities under the hypothesis of scaling. In particular, these have led others to the inference that γ = 2.0 and ß = 1.2 in SrTiO(3). First we show that this is mathematically incorrect and explain why (quantum criticality is exact only at T = 0, whereas the thermodynamic (in)equalities are valid everywhere except T = 0). Second, we show that the inferred values strongly violate a new equality, γ-2ß = ν(4-d-2η)>0, we derive from hyperscaling. Third, we show that the existing soft mode frequency data ω(T) from Takesada et al (2006 Phys. Rev. Lett. at press) yield above T(c) (from the Lyddane-Sachs-Teller relationship) γ = 1.0. Fourth, we remeasure ß from the polarization P(T) and find ß = 0.50 ± 0.02. Fifth, we remeasure the electric susceptibility and find that it perfectly satisfies the Salje-Wruck-Thomas equation, which requires γ = 1.0. The important conclusions are: (a) O-18 SrTiO(3) near T(c) is mean-field; (b) the thermodynamic scaling equalities of Rushbrooke, Griffiths et al are mathematically incompatible with quantum criticality theory; (c) a new hyperscaling relationship makes ß = 1.2 and ß>γ/2 impossible.
