Svein Stølen

Standards, calibration, and guidelines in microcalorimetry. Part 2. Calibration standards for differential scanning calorimetry* (IUPAC Technical Report)

Differential scanning calorimeters (DSCs) are widely used for temperature, heat capacity, and enthalpy measurements in the range from subambient to high temperatures. The present recommendations describe procedures and reference materials (RMs) for the calibration of DSCs. The recommendations focus on the calibration of the response of the instrument and on the estimation of the measurement uncertainty. The procedures for temperature, enthalpy, and heat-flow rate calibration are given in detail. Calibration on cooling has also been considered. Recommended RMs are listed, and the relevant properties of these materials are discussed.

Equation of state of magnetite and its high-pressure modification: Thermodynamics of the Fe-O system at high pressure

Abstract Fe3O4 has been studied by high-pressure diffraction to 43 GPa. No major changes in the spinel-type structure of magnetite is observed below 21.8 GPa. At higher pressure a sluggish transition to a highpressure modification, h-Fe3O4, is observed. The X-ray diffraction pattern of the high-pressure modification is consistent with the orthorhombic unit cell (CaMn2O4-type structure, space group Pbcm) recently proposed for h-Fe3O4 by Fei et al. (1999), however, it is also consistent with a more symmetric CaTi2O4- type structure (space group Bbmm). Bulk modulus values for magnetite, KT0 = 217 (2) GPa, and h-Fe3O4, KT0 = 202 (7) GPa, are calculated from the pressure-volume data using a third-order Birch-Murnaghan equation of state. A thermodynamic analysis of the Fe-O system at high pressure is presented. The proposed equation of state of h-Fe3O4 gives an increased stability of wüstite relatively to a two-phase mixture of iron and h-Fe3O4 compared to earlier equations of state and removes an inconsistency in the thermodynamic description of the Fe-O system at high pressure.

Oxygen-deficient perovskites: linking structure, energetics and ion transport

The present review focuses on links between structure, energetics and ion transport in oxygen-deficient perovskite oxides, ABO(3-delta). The perfect long-range order, convenient for interpretations of the structure and properties of ordered materials, is evidently not present in disordered materials and highly defective perovskite oxides are spatially inhomogeneous on an intermediate length scale. Although this makes a fundamental description of these and other disordered materials very difficult, it is becoming increasingly clear that this complexity is often essential for the functional properties. In the present review we advocate a potential energy barrier description of the disordered state in which the possible local (or inherent) structures are seen to correspond to separate local minima on the potential energy surface. We interpret the average structure observed experimentally at any temperature as a time and spatial average of the different local structures which are energetically accessible. The local structure is largely affected by preferences for certain polyhedron coordinations and the oxidation state stability of the transition metals, and the strong long-range electrostatic interactions present in non-stoichiometric oxides imply that only a small fraction of the local energy minima on the potential energy surface are accessible at most temperatures. We will show that models neglecting the spatial inhomogeneity and thus the local structure serve as useful empirical tools for particular purposes, e.g. for understanding the main features of the complex redox properties that are so crucial for many applications of these oxides. The short-range order is on the other hand central for understanding ionic transport. Oxide ion transport involves the transformation of one energetically accessible local structure into another. Thus, strongly correlated transport mechanisms are expected; in addition to the movement of the oxygen ions giving rise to the transport, other ions are involved and even the A and B atoms move appreciably in a cooperative fashion along the transition path. Such strongly correlated or collective ionic migration mechanisms should be considered for fast oxide ion conductors in general and in particular for systems forming superstructures at low temperatures. Structural criteria for fast ion conduction are discussed. A high density of low-lying local energy minima is certainly a prerequisite and for perovskite-related A(2)B(2)O(5) oxides, those containing B atoms that have energetic preference for tetrahedral coordination geometry are especially promising.

Neutron total scattering study of the delta and beta phases of Bi2O3

The highly disordered structure of the delta phase of Bi2O3, which possesses the highest known oxide-ion conductivity, has been studied using neutron powder diffraction. A detailed analysis of data collected at 1033(3) K using Rietveld refinement indicates that the time-averaged structure of delta-Bi2O3 can be described using the accepted model of a disordered, anion-deficient fluorite structure in space group Fm3m. However, reverse Monte Carlo modelling of the total (Bragg plus diffuse) scattering demonstrates that the local anion environment around the Bi3+ resembles the distorted square pyramidal arrangement found within the stable alpha and metastable beta phases at ambient temperature, which is characteristic of the cations 6s2 lone-pair configuration. Similarities between the structures of the highly disordered delta phase and the ambient temperature metastable beta phase are used to support this assignment and assess the validity of previous structural models based on short-range ordering of vacancies within the cubic lattice of delta-Bi2O3.

Critical assessment of the enthalpy of fusion of metals used as enthalpy standards at moderate to high temperatures

Enthalpy of fusion values for metals used as enthalpy standards are critically assessed. Recent developments of high temperature DSCs imply that potential standards for higher temperature use must also be considered. The fusion values for Ga, In, Sn, Cd, Bi, Pb, Zn, Sb, Al, Ag, Au, Cu, Ni and Co are treated here. We are hence covering materials for use from ambient temperature and up to 1768 K. A detailed review of the enthalpy of fusion determinations reported for each individual metal is presented in terms of sample quality and the methodology used. The accompanying evaluation leads to recommended values for the enthalpy of fusion of the metals and estimates of the uncertainty of the mean. The statistical method used is discussed in some detail. In reaching the recommended values for the enthalpy of fusion of the metals, the uncertainty of each individual determination has been estimated. Arguments regarding the effects of sample quality, intrinsic defects, and thermal treatment to aid in the assignment of uncertainties as well as a short review of the main calorimetric techniques used are presented.

Heat capacity and thermodynamic properties of nearly stoichiometric wustite from 13 to 450 K

Abstract The heat capacity of a three-phase sample of Fe0.990±0.005O, Fe3O4, and Fe (mole fractions 0.915, 0.078, and 0.007, respectively) has been measured by adiabatic shield calorimetry at temperatures from 13 to 450 K. Fe0.99O and magnetite are formed as a metastable intermediate on heating of a quenched nonstoichiometric wüstite with composition Fe0.9374O. The small amount of Fe present stems from the second disproportionation reaction, in which the stable two-phase mixture of Fe and magnetite is formed from Fe0.99O. The value of ΔSm (Fe0.99O, 298.15 K), 60.45 J/(K·mol), is derived from the entropy of the three- phase sample and recommended standard entropies of Fe and magnetite. The character of the magnetic order-disorder transition changes with composition and is strongly cooperative in Fe0.99O, with Tn ≈ 191 K. A minor, seemingly higher order transition is observed at ~ 124 K. It is caused by the Verwey transition in the magnetite. This magnetite, formed in metastable equilibrium with Fe0.99O, is presumably more Fe-rich than magnetite in stable equilibrium with Fe.

Nearly stoichiometric iron monoxide formed as a metastable intermediate in a two-stage disproportionation of quenched wüstite. Thermodynamic and kinetic aspects☆

Quenched metastable wustites are shown to undergo a two-stage disproportionation reaction on heating. A mixture of nearly stoichiometric iron monoxide and magnetite is formed during the first stage, which takes place at ≈ 470 K. The resulting nearly stoichiometric Fe1−yO remains metastable up to ≈ 530 K. Above this temperature the stable two-phase mixture of iron and magnetite is slowly formed. The thermodynamic and kinetic aspects of this two-stage disproportionation reaction have been studied in a step-wise heated adiabatic calorimeter. The decomposition behaviour is rationalized in a Gibbs energy of formation representation of stable and metastable phases in the iron-oxygen system. The antiferromagnetic to paramagnetic order-disorder transition which takes place in Fe1−yO at ≈ 196 K is found to be greatly influenced by the oxygen content; it becomes much more cooperative as the exact 1:1 stoichiometry is approached.

Monoclinic nearly stoichiometric wüstite at low temperatures

Abstract The crystallographic and magnetic structures of Fe0.99O at 10 K have been determined by highresolution neutron powder diffraction. Fe0.99O is found to be monoclinic, space group C2/m, with unit-cell dimensions a = 5.2615(1), b = 3.0334(1), c = 3.0602 (1) Å, and β = 124.649(2)°. The Fe-O distances in the distorted FeO6 octahedron are 2.154 Å × 4 and 2.165 Å × 2. The magnetic unit cell is obtained by doubling one of the crystallographic axes, a (magn) = am, b (magn) = bm, and c (magn) = 2cm. The refined magnetic components at 8 K are Mx = 2.7(1) μB, My = -0.9(2) μB, and Mz = 4.77(2) μB, with resultant M = 4.03(2) μB.

Heat capacity, thermodynamic properties, and transitions of silver iodide

The heat capacity of silver iodide was measured using adiabatic calorimetric cryostats over the ranges 7 to 350 K and from 2 to 75 K. The (0 to a)- and (g/y to a)-transitions and the heat capacities of single crystals of p-, of finely divided p/y-, and of the a-phases were measured in adiabatic calorimetric cryostats and thermostats from 70 to 700 K and from 310 to 523 K. The values of C,,, Sm. and CPA at 298.15 K are 6.707R. 33.764R, and 8.511R for the g-phase and since the molar volumes of p- and y-phases are identical and the structures differ only in their stacking sequence, their heat capacities are essentially identical. Moreover, the molar transition-enthalpy increments to the a-phase of the two samples are also equal within the accuracy of the present measurements in spite of the structural differences occasioned by the presence of some y-AgI in one of them (below the transition temperature). The (partial) enthalpy of the (P/y to a)-phase transition was found to be (758.7k0.8)R.K and that of the (g to a)-phase transition was found to be (758.0+0.3)R’ K over the range 405 to 425 K (mean: (758.4f 0.4)R. K}. The (total) enthalpy of transition is about I1 50R. K. An abnormal trend in the heat capacity in the low-temperature region was noted. The heat-capacity values are compared with those of prior overlapping measurements by Nernst and Schwers, by Pitzer, and by Madison et al. for the P/y-phase, and in and beyond the transition region with those of numerous prior investigators.

Fragility transition in GeSe2–Se liquids

The heat capacity and entropy of liquids in the GeSe2–Se system are reported. The thermodynamics of the system is evaluated and discussed in light of earlier reported viscosities and recent structural studies of liquid GeSe2. The thermodynamics of liquid GeSe2 suggest that extensive structural disordering takes place in the liquid on heating and points to a fragile melt at high temperatures whereas the thermodynamic characteristics of the system close to the glass transition temperature suggests a much less fragile character. A consistent picture is evident from earlier viscosity data; the viscosity of GeSe2–Se glasses just above Tg supports a fragility minimum near x = 0.225, (x is x in GexSe1−x) while a monotonically increasing fragility is evident from the viscosity in the stable liquid and non-viscous region. We propose that the above discrepancy between the behaviour of the melts close to Tg and at high temperatures can be explained by a fragile-to-strong transition in supercooled GeSe2–Se. The transition is related to the onset of chemical disorder and the density minimum of GeSe2. The proposed fragile-to-strong transition of other tetrahedral liquids has also been linked to a first order liquid–liquid transition, which recently also have been discussed for GeSe2.