The transition between the cubic and tetragonal phase in SrTiO 3 shows an excess specific heat of 0.0035 J g −1 K −1 . Comparison between the temperature evolution of the excess entropy S = (C/T ) dT and the structural order parameter Q shows S ∝ Q 2 within experimental errors (γ = 1.004±0.006). The apparent order parameter exponent β = 0.35±0.02 was confirmed and analysed using a Landau-type expression for the excess Gibbs free energy

by jerky propagation of phase fronts related to the appearance of avalanches. In this paper we describe a full analysis of this avalanche behavior using calorimetric heat flux measurements and acoustic emission measurements. Two different propagation modes, namely smooth front propagation and jerky avalanches, were observed in extremely slow measurements with heating and cooling rates as low as a few 10 Avalanches appear to be more common for heating rates faster than 5 10 -3 K/h whereas smooth front propagation occurs in all calorimetric measurements and (almost) exclusively for slower heating rates. Repeated cooling runs were taken after a waiting time of 1 month (and an intermediate heating run). Correlations between the avalanche sequences of the two cooling runs were found for the strongest avalanche peaks but not for the full sequence of avalanches. The memory effect is hence limited to strong avalanches.3

The transition between cubic and tetragonal phases in KMnF3 has been studied by x-ray diffraction rocking curves and calorimetry. Comparison of the excess entropy with the order parameter Q obtained from spontaneous strain shows that the mean field relationship S Q 2 is obeyed to within experimental error. The data are fitted to a Landau free energy expression G = ½A (T -TC )Q 2 +(1/4) BQ 4 +(1/6)CQ 6 , with A = 2.781 J K-1 mol-1 , B = -57.63 J mol-1 , C = 574.2 J mol-1 , TC = 185.76 K. No significant excess specific heat is found at T >>TC .

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The existence of temporal correlations during the intermittent dynamics of a thermally driven structural phase transition is studied in a Cu-Zn-Al alloy. The sequence of avalanches is observed by means of two techniques: acoustic emission and high sensitivity calorimetry. Both methods reveal the existence of event clustering in a way that is equivalent to the Omori correlations between aftershocks in earthquakes as are commonly used in seismology.

The cubic to tetragonal phase transition in the solid solution K(Mn, Ca)F 3 has been investigated by conduction calorimetry and x-ray diffraction. The behaviour of the excess specific heat, latent heat and spontaneous strain has been explained in terms of a 2-4-6 Landau potential. The coefficients A and C, prefactors of Q 2 and Q 6 in the free energy expansion, are practically constant with composition, but B (the prefactor of Q 4 ) and T C are functions of composition. The tricritical point occurs when the sign of B changes from negative (in pure KMnF 3 ) to positive (in Ca rich samples). However, the variation of the parameters B and T C with dopant concentration x is non-linear. The dependence of T C with composition is explained in terms of an internal stress due to the doping ion of Ca and is compared with the effect of external uniaxial stress on pure KMnF 3 .

The paraelectric–ferroelectric phase transition of a single crystal of triglycine selenate TGSe, whose first- or second-order character is controversial, has been studied using a high sensitivity calorimetric technique. The specific heat of a highly pure TGSe has been measured and no evidence of latent heat has been found. The anomalous part of the specific heat shows Landau tricritical behaviour. Experimental data have been fitted to a 2–6 Landau potential, whose coefficients have also been obtained. A weak uniaxial stress applied along the ferroelectric axis (10 bar) decreases the anomalous part of the specific heat. This effect is different for cooling and heating runs. The equilibrium data are those obtained on heating that fit to a 2–4–6 Landau potential, which indicates that the transition shifts to second order with the effect of uniaxial stress.

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