M. C. Gallardo

The cubic-tetragonal phase transition in strontium titanate: excess specific heat measurements and evidence for a near-tricritical, mean field type transition mechanism

The transition between the cubic and tetragonal phase in shows an excess specific heat of . Comparison between the temperature evolution of the excess entropy and the structural order parameter Q shows within experimental errors . The apparent order parameter exponent was confirmed and analysed using a Landau-type expression for the excess Gibbs free energy with A = 0.70 J , B = 31.22 J , C = 42.17 J , K and K. The closeness to the tricritical point is seen by B < C; all thermodynamic data between 85 K and could be described selfconsistently using this approach although small deviations cannot be excluded in a temperature interval of less than 1 K around and a small tail of excess entropy at .

Experimental device for measuring the influence of a uniaxial stress on specific heat: Application to the strontium titanate ferroelastic crystal

A conduction calorimeter where the sample is pressed between two identical heat fluxmeters, each having 50 chromel–constantan thermocouples, is described. The fluxmeters were built to obtain linear heat conduction and with enough rigidity to apply a controlled uniaxial stress on the sample. The method of measurement, together with the fact that the sensors themselves apply the stress to the sample, allow us to obtain absolute values of the specific heat. The specific heat of strontium titanate ferroelastic crystal is measured without stress and under an applied uniaxial stress on the face (110). The measurements were carried out while cooling the sample at a constant rate of 0.06 K/h. The data obtained at intervals of 0.02 K are compared with those obtained with an unannealed and an annealed sample, respectively.

Dynamic heat flux experiments in Cu67.64Zn16.71Al15.65: Separating the time scales of fast and ultra-slow kinetic processes in martensitic transformations

Crackling noise and avalanches during the martensite phase transformation of Cu67.64Zn16.71Al15.65 were investigated. Heat flux measurements with extremely slow heating rates of 0.005 Kh−1 allowed sufficient separation between the continuous background and the avalanche jerks. The jerk enthalpy is below 3% of the total transformation enthalpy. The crackling noise follows power law behavior with an energy exponent near ɛ = 1.8. The jerks are almost uncorrelated with approximately a Poisson distribution of the waiting times between jerks. Quantitative analysis showed a scaling behavior with p(wt) ∼ wt(γ−1)exp(−wt/τ)n with γ = 0.7 and n ≈ 1.

The structural phase transition in under uniaxial stress in the [110] direction: a calorimetric study

The excess specific heat of the structural phase transition of has been measured on single crystals under uniaxial stress along the crystallographic [110] direction and for extremely slow cooling rates. The corresponding excess entropy scales as the square of the thermodynamic order parameter. The order parameter exponent is for stress-free samples and increases systematically with increasing stress. At a stress of 5 bar the exponent is and the phase transition follows mean-field behaviour.

Avalanche correlations in the martensitic transition of a Cu–Zn–Al shape memory alloy: analysis of acoustic emission and calorimetry

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 influence of an electric field on the latent heat of the ferroelectric phase transition in KDP

The specific heat, heat flux (DTA trace) and dielectric constant of KDP ferroelectric crystal have been measured simultaneously for various electric fields with a conduction calorimeter. The specific heat presents a strong anomaly, but these simultaneous measurements allow us to evaluate the latent heat accurately. The latent heat decreases with field, and the value of the critical electric field—that where the latent heat disappears—is estimated to be (0.44 ± 0.03) kV cm−1. Incidentally, we have measured simultaneously the dielectric permittivity, which suggests that latent heat is developed as domains are growing.

Low-temperature calorimetric study of SrTiO3

The specific heat of SrTiO3 shows an anomaly at 105 K due to a structural transition from the tetragonal to cubic phase. The shape of the anomalous specific heat, ΔCp(T), measured previously using quasi-static conduction calorimetry is confirmed and compared with results from rapid AC calorimetry (scanning rate ~30 K h-1). The close agreement of both data sets proves that slow relaxational processes (e.g. the rearrangement of dense domain patterns) do not influence ΔCp(T) significantly in SrTiO3. Measurements at low temperatures do not reveal any further anomaly. The freezing temperature reported around 30 K due to quantum saturation of the order parameter does not produce any anomalous effect in the heat capacity.

Phase transitions in perovskites near the tricritical point: an experimental study of KMn1−xCaxF3 and SrTiO3

Abstract We present experimental data for the Pm3m-I4/mcm phase transitions in the perovskite crystals KMn1-xCaxF3and SrTiO3. Comparison of calorimetric data (latent heat and specific heat) with order parameter data (measured with X-ray rocking methods) indicates that these transitions follow meanfield behaviour, and may be described using Landau potentials where the free energy expansion includes terms up to Q6. This potential is characteristic of transitions close to the tricritical point. Comparison of the behaviour of SrTiO3 and KMnF3indicates that KMnF3 is closer to the tricritical point; a small amount of substitution of Ca for Mn causes the transition to cross the tricritical point from first order to second order behaviour.

First order transitions by conduction calorimetry: Application to deuterated potassium dihydrogen phosphate ferroelastic crystal under uniaxial pressure

The specific heat c and the heat power W exchanged by a Deuterated Potassium Dihydrogen Phosphate ferroelectric-ferroelastic crystal have been measured simultaneously for both decreasing and increasing temperature at a low constant rate (0.06 K/h) between 175 and 240 K. The measurements were carried out under controlled uniaxial stresses of 0.3 and 4.5±0.1 bar applied to face (110). At Tt=207.9 K, a first order transition is produced with anomalous specific heat behavior in the interval where the transition heat appears. This anomalous behavior is explained in terms of the temperature variation of the heat power during the transition. During cooling, the transition occurs with coexistence of phases, while during heating it seems that metastable states are reached. Excluding data affected by the transition heat, the specific heat behavior agrees with the predictions of a 2-4-6 Landau potential in the range of 4–15 K below Tt while logarithmic behavior is obtained in the range from Tt to 1 K below Tt. Data ...

Cubic–tetragonal phase transition in Ca0.04Sr0.96TiO3: a combined specific heat and neutron diffraction study

The specific heat corresponding to the tetragonal-to-cubic transition in Ca0.04Sr0.96TiO3 perovskite has been measured by conduction calorimetry. The order parameter of the transition has been obtained by means of neutron diffraction at low temperatures. Comparison of calorimetric data with the evolution of the order parameter indicates that this transition seems to follow a mean field Landau potential as in SrTiO3. The linear behaviour of the excess of entropy versus temperature suggests that a 2–4 Landau potential is sufficient to describe the transition.