Dielectric, calorimetric and elastic anomalies associated with the first order I4/mcm \leftrightarrow Pbcm phase transition in (Ca, Sr)TiO₃perovskites

Abstract

Conduction calorimetry has been used to determine with high precision the latent heat and variation in heat capacity which accompany the first order [Formula: see text] phase transition in perovskites with compositions (Ca(1-x)Sr(x))TiO(3), x = 0.65, 0.68, 0.74 (CST65, CST68, CST74). In CST65 (CST68), the latent heat is dissipated/absorbed over a temperature interval of ∼11 K (∼6 K), which is centred on ∼292 K (∼258 K) during cooling and ∼302 K (∼270 K) during heating. The magnitude of the latent heat diminishes with increasing SrTiO(3) content and was not detected in CST74. Integration of the latent heat and excess heat capacity yields small excess entropies, which are consistent with the structural changes being displacive rather than order-disorder in origin. Resonant ultrasound spectroscopy measurements on the same CST65 sample as used for dielectric and calorimetric measurements through the same temperature intervals have allowed quantitative correlations to be made with the bulk modulus, shear modulus and acoustic dissipation parameter, Q(-1). The dielectric anomaly and changes in Q(-1) can be understood as being linear combinations of the properties of the separate I4/mcm and Pbcm phases in proportion to their volume fractions across the two-phase field. A change of only ∼0.5-1 GPa has been detected in the bulk modulus but the shear modulus softens by ∼5-8 GPa as the transition interval is approached from above and below. This shear mode softening presumably reflects clustering and/or phonon softening in both the I4/mcm and Pbcm structures. This pattern of structure-property relations could be typical of first order transitions in perovskites where there is no group/subgroup relationship between the high and low symmetry phases.

Thermochemistry of (CaxSr1‐x)TiO3, (BaxSr1‐x)TiO3, and (BaxCa1‐x)TiO3 Perovskite Solid Solutions

“…40,41 The transition enthalpies from tetragonal to orthorhombic symmetry are within the range of 0.03 kJ/mol-0.06 kJ/mol. 40 The (Sr,Ba)TiO 3 system 36,42-45 has excellent dielectric properties, especially in thin films. 46 Ab initio calculations determined the interaction parameter of the cubic phase of the Sr x Ba 1-x TiO 3 solid solution to be 7.2 kJ/mol.…”

“…Phase transition studies of perovskite (Ca,Sr)TiO 3 , indicate that at room temperature the symmetry changes from orthorhombic to tetragonal to cubic with increasing strontium content, although structures and phase boundaries are still poorly defined. Calorimetric measurements was performed on several compositions in this system, some of them indicated rounded specific heat anomalies with transitions extending over a wide range of temperature . The transition enthalpies from tetragonal to orthorhombic symmetry are within the range of 0.03 kJ/mol–0.06 kJ/mol …”

“…Calorimetric measurements was performed on several compositions in this system, some of them indicated rounded specific heat anomalies with transitions extending over a wide range of temperature . The transition enthalpies from tetragonal to orthorhombic symmetry are within the range of 0.03 kJ/mol–0.06 kJ/mol …”

Strain relaxation mechanisms of elastic softening and twin wall freezing associated with structural phase transitions in (Ca,Sr)TiO3perovskites

“…There is only a small dip in G at the I 4/mcm ↔ Pbcm transition. Data at temperatures below 315 K have been reproduced from Manchado et al (2009), with appropriate correction and scaling to account for porosity.…”

Magnetoelastic coupling and multiferroic ferroelastic/magnetic phase transitions in the perovskite KMnF3

“…is that the ∼0.1to 1-MHz frequency range and low-stress conditions of RUS provide a particularly sensitive window into the mobility of ferroelastic twin walls. [55][56][57][58][59][60][61][62][63] The second objective of the present study was, therefore, to investigate at RUS frequencies the remarkable mobility of domain boundary movement that has been observed at low frequencies (∼0.1-50 Hz) in the tetragonal phase (phase II) of both pure and Ca-doped samples. 35,37,64,65…”