The behavior of the Pm3m-R3c phase transition in LaAlO 3 ͑T C = 813 K from differential scanning calorimetry measurements͒ has been studied using temperature-dependent measurements of the crystal structure, dielectric relaxation, specific heat, birefringence, and the frequencies of the two soft modes ͑via Raman spectroscopy͒. While all these experiments show behavior near T C consistent with a second-order Landau transition, there is extensive evidence for additional anomalous behavior below 730 K. Below this temperature, the two soft mode frequencies are not proportional to each other, the spontaneous strain is not proportional to the square of the AlO 6 rotation angle, and anomalies are seen in the birefringence. Twin domains, which are mobile above 730 K, are frozen below 730 K. These anomalies are consistent with biquadratic coupling between the primary order parameter of the transition ͑AlO 6 rotation͒ and a second process. From the dielectric results, which indicate a smooth but rapid increase in conductivity in the temperature range 500-800 K, we propose that this second process is hopping of intrinsic oxygen vacancies. These vacancies are essentially static below 730 K and dynamically disordered above 730 K. The interaction between static vacancies and the displacive phase transition is unfavorable. A similar anomaly may be observed in other aluminate perovskites undergoing the same transition.
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.
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