The modelling of the behaviour of hazardous materials under environmental conditions is among the most important applications of natural and technical sciences for the protection of the environment. In order to assess, for example, the safety of a waste deposit, it is essential to be able to predict the eventual dispersion of its hazardous components in the environment (geosphere, biosphere). For hazardous materials stored in the ground or in geological formations, the most probable transport medium is the aqueous phase. An important requirement for predicting the pathways and rates of aqueous transport of potential contaminants is therefore the quantitative prediction of the reactions that are likely to occur between hazardous waste dissolved or suspended in ground water, and the surrounding rock material, in order to estimate the quantities of waste that can be transported in the aqueous phase. It is thus essential to know the relative stabilities of the compounds and complexes that may form under the relevant conditions. This information is often provided by speciation calculations using chemical thermodynamic data. The local conditions, such as ground water and rock composition or temperature, may not be constant along the migration paths of hazardous materials, and fundamental thermodynamic data are the indispensable basis for dynamic modelling of the chemical behaviour of hazardous waste components. 

This chapter outlines and lists the symbols, terminology and nomenclature, the units and conversion factors, the order of formulae, the standard conditions, and the fundamental physical constants used in this volume. They are derived from international standards and have been specially adjusted for the TDB publications.

This chapter presents the chemical thermodynamic data set for thorium species that has been selected in this review. Table III-1 contains the recommended thermodynamic data of the thorium compounds and species, Table III-2 the recommended thermodynamic data of chemical equilibrium reactions by which the thorium compounds and complexes are formed, and Table II-3 the temperature coefficients of the heat capacity data of Table III-1 where available.

This chapter presents the chemical thermodynamic data for auxiliary compounds and complexes which are used within the NEA-TDB Project. Most of these auxiliary species are used in the evaluation of the recommended thorium data in Tables III-1, III-2 and III-3. It is therefore essential to always use these auxiliary data in conjunction with the selected data for thorium. The use of other auxiliary data can lead to inconsistencies and erroneous results.

Thorium metal has a face centred cubic structure, (space group Fm3m , Cu type) at 298.15 K and undergoes a structural change to a bcc-phase (space group Im3m , W??type) at ca. 1630 K. The lattice parameters are 5.0842 Å at 298.15 K [1956JAM/STR] and 4.11 Å at 1723 K [1954CHI]. Rand [1975RAN] critically examined the data of 15 studies of this transformation with the reported transition temperatures ranging from 1605 to 1698 K, with most values lying between 1623 and 1653 K. The probable reason for this seemingly wide range of values is the effect of impurities in the thorium used. It is clear that small amounts of carbon, nitrogen and oxygen, which are difficult to remove from thorium, raise the transformation temperature, whereas additions of niobium, tantalum and zirconium tend to lower the temperature of the phase change. We have therefore given considerable weight to the work of Chiotti and Dooley [1967CHI/DOO] ((1623??????10) K) and Takeuchi et al. [1966TAK/HON] ((1633??????10) K) who both extrapolated the transition temperature to zero carbon content and select for the transformation temperature

There is no firm evidence for the stability of any valency state other than Th(IV) in aqueous media. Recently, Klapköte and Schulz [1997KLA/SCH] claimed to have identified Th3+ in the reaction of Th4+ with HN3 in slightly acidic solutions (pH = ca. 3.5), by absorption and ESR spectra. However, as noted in Appendix A, Ionova et al. [1998ION/MAD] have seriously questioned both the value of E?? (Th3+/Th4+) chosen by [1997KLA/SCH] in their thermodynamic analysis and the validity of their conclusions, pointing out that the absorption bands observed by these authors could equally well be due to azide complexes of Th(IV), with or without Cl– ligands. Moreover, the stability of HN3 in dilute aqueous solutions suggests that the proposed reduction of Th4+ is unlikely to be thermodynamically controlled as suggested in [1997KLA/SCH]. Thus there seems no incontrovertible evidence for the existence of any stable ions in aqueous solutions other than Th(IV) and its derivatives.

The only stable solid oxide of thorium is ThO2, although the metastable monoxide, probably containing impurities, can be obtained from the black residue formed when thorium metal is dissolved in aqueous HCl. Ackermann and Rauh [1973ACK/RAU3] found that when this material, which has the approximate composition ThO(Cl,OH)H, Katzin [1958KAT], is heated, a phase with a face centred cubic cell with a = (5.302 ?? 0.003) Å is first formed. On further heating, this disproportionates to, essentially, Th and ThO2, see Appendix A. 

The thermal functions of ThF(g), ThF2(g) and ThF3(g) were calculated assuming the molecular parameters shown in Appendix E, Table E-1. These are based on the parameters selected by [1977WAG/SCH], in turn derived principally from the estimates by [1973KRA/MOR]. No electronic contributions have been included, leading to considerable uncertainties in the thermal functions, especially for the monofluoride. The selected values, with uncertainties estimated by this review

Zachariasen [1949ZAC], [1949ZAC3] unambiguously identified four phases in the range S/Th = 0.8??????2.0 and elucidated their structures. From powder data, ThS(cr) is reported as cubic (NaCl type, Fm3m ), with a = (5.682??????0.002) Å and isomorphous with CeS and US, while ThS2(cr) is orthorhombic (PbCl2 type, Pnmb– 16 2h D ), with a??=??(4.267??????0.002), b = (7.264??????0.003), and c = (8.617??????0.003) Å. Using single crystal data, Zachariasen also reported Th2S3(cr) as orthorhombic (Sb2S3 type, Pbnm) with a??=??(10.99??????0.05), b = (10.85??????0.05), and c = (3.96??????0.03) Å, isomorphous with U2S3, and Th7S12(cr) hexagonal (P63/m), with a = (11.063??????0.001) and c = (3.991??????0.001) Å. It was also noted, at the time, that the homogeneity range for that phase extends from S/Th??= 1.71 to 1.76. 

There are two solid nitrides, the semimetal compound ThN(cr), and the Th(IV) compound Th3N4(cr), both of which have a small range of homogeneity at high temperatures. Benz and Zachariasen [1966BEN/ZAC] showed that the phase previously thought to be Th2N3(cr) is in fact the oxynitride Th2N2O(cr) (see Section X.1.1.4).

There have been two extensive reviews of the thermodynamics of the Th-C system by [1975RAN], [1984HOL/RAN], since which there has been little work on these compounds. Our treatment therefore lends fairly heavily on the latter review, with a few updates and corrections as appropriate.

The early identification of SrThO3(cr) by Naray-Szabo [1947NAR] has been questioned (see discussion below), but Purohit et al. [2000PUR/TYA] have suggested that it is formed by combustion of a sol-gel containing Th(NO3)4, Sr(NO3) and citric acid. They indicate that its structure is a monoclinic distortion of the ideal perovskite lattice, with a??= (6.319??????0.004) Å, b = (3.240??????0.001) Å and c = (4.928??????0.003) Å, ?? ??= (117.38??????0.05)°. The only thermodynamic data for SrThO3(cr) are measurements by Ali?? (Basu) et al. [2001ALI/MIS] of the pressure of Sr(g) obtained by heating SrThO3(cr) in a tungsten cell from 1677 to 2419 K by weight-loss Knudsen effusion. X-ray examination of the residue after partial decomposition showed that the condensed phases were different in different temperature ranges. As well as SrThO3(cr), W(cr) and ThO2(cr), the tungstate Sr2WO5(cr) was present from 1677 to 2047 K, but SrWO4(cr) was found in experiments from 2138 to 2419 K. Since SrWO4 would be molten at these higher temperatures, Energy Dispersive X-ray analysis was carried out to determine whether there was any solubility of thorium species in this melt. Less than 10% ThO2 was detected, but it was not clear whether this was due to dissolution of e.g. of SrThO3 or from interference from neighbouring ThO2 particles. Thus SrWO4(l) was treated as a pure phase in the subsequent analysis, as was ThO2(cr), although the solubility of SrO in ThO2(cr) at 2273 K is reported to be as high as 13 mol%. The error from these assumptions was estimated by the authors to be 2.6 kJ·mol–1. The authors discuss and dismiss the possible loss of oxygen to form phases such SrWO3(cr) containing lowervalent tungsten 

This appendix comprises discussions relating to a number of key publications, which contain experimental information cited in this review. These discussions are fundamental in explaining the accuracy of the data concerned and the interpretation of the experiments, but they are too lengthy or are related to too many different sections to be included in the main text. The notation used in this appendix is consistent with that used in the present book, and not necessarily consistent with that used in the publication under discussion. 

Thermodynamic data always refer to a selected standard state. The definition given by IUPAC [1982LAF] is adopted in this review as outlined in Section II.3.1. According to this definition, the standard state for a solute B in a solution is a hypothetical solution, at the standard state pressure, in which 1 mB = m?? = 1 mol??kg?? , and in which the activity coefficient ??B is unity. However, for many reactions, measurements cannot be made accurately (or at all) in dilute solutions from which the necessary extrapolation to the standard state would be simple. This is invariably the case for reactions involving ions of high charge. Precise thermodynamic information for these systems can only be obtained in the presence of an inert electrolyte of sufficiently high concentration that ensures activity factors are reasonably constant throughout the measurements. This appendix describes and illustrates the method used in this review for the extrapolation of experimental equilibrium data to zero ionic strength. 

This Appendix describes the origin of the uncertainty estimates that are given in the TDB tables of selected data. The original text in [1992GRE/FUG] has been retained in [1995SIL/BID], [1999RAR/RAN] and [2001LEM/FUG], except for some minor changes. Because of the importance of the uncertainty estimates, the present review offers a more comprehensive description of the procedures used. 

A SIT fitting code, NONLINT-SIT (Personal communication, September 2004. A. R. Felmy, Pacific Northwest National Laboratory, Richland, WA, USA), was used in some systems to optimise ??fGm?? / RT values of different solid and aqueous species and SIT ion-interaction parameters, using solubility, ion-exchange, and solvent extraction data. From these, the optimised values of the different quantities can be compared with the other reported values. The program NONLINT-SIT is an extended version of the parameter optimisation programs (NONLIN and NONLINT) developed by A. R. Felmy using the MINPACK nonlinear least-squares programs, in conjunction with a Gibbs energy minimisation program (GMIN, [1990FEL] and [1995FEL]). The mathematical development of the latter is based on the formulations described in [1981HAR], [1987HAR/GRE] and [1985GRE/WEA]. GMIN and NONLIN have also been the bases for the development of the related parameter optimisation code INSIGHT, [1997STE/FEL], which uses the same algorithms as NONLINT, but deals with fewer types of experimental data.

In most cases, the thermal functions for gaseous species have been calculated by wellknown statistical-mechanical relations (see for example Chapter 27 of [1961LEW/RAN]). 

There may be some problems with the data for ZrO(g) and ZrO2(g) selected in the NEA-TDB review of zirconium by Brown et al. [2005BRO/CUR]. The values given by Brown et al. are based on the assessment in the third edition of the Janaf tables [1985CHA/DAV], modified for the slightly different values of ?? f m ?? H (ZrO2, cr, 298.15 K) used in the two reviews. However, the assessments in [1985CHA/DAV] for these species were made in December 1965 and thus predate a number of detailed experimental studies made in 1970s. Indeed, their values of the Gibbs energies of formation of both ZrO(g) and ZrO2(g) are based solely on an early mass-spectrometric study by Chupka et al. [1957CHU/BER].

This chapter contains an alphabetical list of the authors of the references cited in this book. The reference codes given with each name corresponds to the publications of which the person is the author or a co-author. Note that inconsistencies may occur due to variations in spelling between different publications. No attempt has been made to correct for such inconsistencies in this volume.