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.

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The high-temperature dehydroxylation and structural change of talc, Mg 3 (Si 2 O 5 ) 2 (OH) 2 , has been investigated in detail using infrared (IR) spectroscopy. The data (in the region of 20-12000 cm -1 ) on quenched samples show that absorptions from structurally incorporated OH and OD, as well as NH 4 -like species, have similar temperature dependences in dehydroxylation. The OH species exhibit weak variation in frequency on heating, which is inconsistent with thermally induced weakening of O-H bonds. Dehydroxylation in talc is a complex process that involves proton migrations and formation of new OH species. Additional fundamental OH bands near 3665 and 3745 cm -1 became detectable near 900 °C. On further heating the former disappear near 1200 °C, whereas the latter became undetectable near 1350 °C. The occurrence of CO 2 is observed in samples quenched between 600 and 1250 °C. The phonon spectrum (20-1500 cm -1 ) of the dehydroxylate (obtained by annealing the sample at 1000 °C) gives features signifi cantly different from that of talc, indicating the loss of the original layer structure. The IR data imply that the talc dehydroxylate consists of disordered SiO 2 and enstatite (MgSiO 3 ). Although MgSiO 3 exists dominantly in the form of orthoenstatite, the characteristic bands of clinoenstatite phase are found to coexist in the samples treated at 1000 °C. The IR data from in situ measurements show that protons become mobile at temperatures below the dehydroxylation and an extra OH species near 3500 cm -1 develops on heating. This new species is not quenchable, and it decreases intensity on cooling and disappears at room temperature. The in situ results also indicate external carbon-related substances can diffuse into talc during dehydroxylation.