Anelastic loss mechanisms associated with phase transitions in BaCeO 3 have been investigated at relatively high frequency ϳ1 MHz and low stress by resonant ultrasound spectroscopy ͑RUS͒, and at relatively low frequency ϳ1 Hz and high stress by dynamic mechanical analysis ͑DMA͒. Changes in the elastic moduli and dissipation behavior clearly indicate phase transitions due to octahedral tilting: Pnma ↔ Imma ↔ R3c ↔ Pm3m structures at 551 K, 670 K, and 1168 K, and strain analysis shows that they are tricritical, first-order, and second-order phase transitions, respectively. Structures with intermediate tilt states ͑R3c and Imma structures͒ show substantial anelastic softening and dissipation associated with the mobility of twin walls under applied stress. The Pnma structure shows elastic stiffening which may be due to the simultaneous operation of two discrete order parameters with different symmetries. In contrast with studies of other perovskites, BaCeO 3 shows strong dissipation at both DMA and RUS frequencies in the stability field of the Pnma structure. This is evidence that ferroelastic twin walls might become mobile in Pnma perovskites and suggests that shearing of the octahedra may be a significant factor.

The current paper gives an overview of the newly obtained thermal expansion coefficients of skutterudites as well as those so far available in literature. Thermal expansion was determined for CoSb 3 , Pt 4 Sn 4.4 Sb 7.6 , for As-and Ge-based skutterudites as well as for various high-ZT skutterudites ͑microand nanostructured͒ with didymium ͑DD͒ and mischmetal ͑Mm͒ as filler atoms in frameworks of ͑Fe 1−x Co x ͒ 4 Sb 12 and ͑Fe 1−x Ni x ͒ 4 Sb 12 , and for double and triple-filled skutterudites such as Ca 0.07 Ba 0.23 Co 3.95 Ni 0.05 Sb 12 and Sr 0.025 Ba 0.075 Yb 0.1 Co 4 Sb 12 . For low temperatures, a capacitance dilatometer was used ͑4-300 K͒, whereas for temperatures 300Ͻ T Ͻ 750 K, a dynamic mechanical analyzer was employed. For a set of Ge-, P-, and Sb-based skutterudites, lattice parameters of single crystals, measured at three different temperatures, were used to derive the thermal expansion coefficient. The semiclassical model of Mukherjee ͓Phys. Rev. Lett. 76, 1876 ͑1996͔͒ has been successfully used to quantitatively describe the thermal expansion coefficient in terms of Einstein and Debye temperatures, which compare well with the corresponding results from specific heat, electrical resistivity, or temperature dependent x-ray measurements.

Many molecular glass forming liquids show a shift of the glass transition Tg to lower temperatures when the liquid is confined into mesoporous host matrices. Two contrary explanations for this effect are given in literature: First, confinement induced acceleration of the dynamics of the molecules leads to an effective downshift of Tg increasing with decreasing pore size. Secondly, due to thermal mismatch between the liquid and the surrounding host matrix, negative pressure develops inside the pores with decreasing temperature, which also shifts Tg to lower temperatures. Here we present novel dynamic mechanical analysis measurements of the glass forming liquid salol in Vycor and Gelsil with pore sizes of d = 2.6, 5.0 and 7.5 nm. The dynamic complex elastic susceptibility data can be consistently described with the assumption of two relaxation processes inside the pores: A surface induced slowed down relaxation due to interaction with rough pore interfaces and a second relaxation within the core of the pores. This core relaxation time is reduced with decreasing pore size d, leading to a downshift of Tg ∝ 1/d in perfect agreement with recent DSC measurements. Thermal expansion measurements of empty and salol filled mesoporous samples revealed that the contribution of negative pressure to the downshift of Tg is small (< 30%) and the main effect is due to the suppression of dynamically correlated regions of size ξ when the pore size d approaches ξ.

Advantage is taken of the wealth of experimental data relating to the evolution with temperature of spin states of Co(3+) in LaCoO₃ in order to undertake a detailed investigation of the mechanisms by which changes in electronic structure can influence strain, and elastic and anelastic relaxations in perovskites. The macroscopic strain accompanying changes in the spin state in LaCoO₃ is predominantly a volume strain arising simply from the change in effective ionic radius of the Co(3+) ions. This acts to renormalize the octahedral tilting transition temperature in a manner that is easily understood in terms of coupling between the tilt and spin order parameters. Results from resonant ultrasound spectroscopy at high frequencies (0.1-1.5 MHz) reveal stiffening of the shear modulus which scales qualitatively with a spin order parameter defined in terms of changing Co-O bond lengths. From this finding, in combination with results from dynamic mechanical analysis at low frequencies (0.1-50 Hz) and data from the literature, four distinctive anelastic relaxation mechanisms are identified. The relaxation times of these are displayed on an anelasticity map and are tentatively related to spin-spin relaxation, spin-lattice relaxation, migration of twin walls and migration of magnetic polarons. The effective activation energy for the freezing of twin wall motion below ~590 K at low frequencies was found to be 182 ± 21 kJ mol(-1) (1.9 ± 0.2 eV) which is attributed to pinning by pairs of oxygen vacancies, though the local mechanisms appear to have a spread of relaxation times. It seems inevitable that twin walls due to octahedral tilting must have quite different characteristics from the matrix in terms of local spin configurations of Co(3+). A hysteresis in the elastic properties at high temperatures further emphasizes the importance of oxygen content in controlling the properties of LaCoO₃.

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