Thorium forms two hydrides: ThH2, density 9.50 g/cm3; and ТІцН^, density 8.28 g/cm3. The limited information on temperature-composition relations for condensed phases in the system thorium-hydrogen is shown in Fig. 6.3. Approximate values for the equilibrium pressure of hydrogen in the system aTh-ThH2_x obtained from measurements of Mallett and Campbell [Ml] made with impure metal are given in Table 6.9.

Like other metal hydrides, thorium hydride is pyrophoric and must be handled with care.

1.8 Thorium Halides

The tetrahalides are the thorium halides of greatest practical importance. The tetrafluoride ThF4 is the preferred starting material for large-scale production of thorium metal (Sec. 10.4). ThF4 has been proposed as fertile material in the fuel mixture of the molten-salt reactor. The tetraiodide has been used as feed material in the iodide process for making very pure thorium metal (Sec. 10.4).

The more important properties of the tetrahalides, from reference [II], are listed in Table 6.10. Many of these properties, especially for ThCl4, ThBr4, and Thl4, are known only semiquantitatively.

Anhydrous ThF4 is made by passing an excess of HF vapor over Th02 or ThOFj at temperatures between 550 and 600°C. The anhydrous double fluoride KThF5 is precipitated from aqueous solutions of thorium nitrate by addition of an excess of KF. It has been used for electrolytic production of thorium metal.

ThCl4 can be made by reacting Th02 with chlorine mixed with CC14 or COCl2.

All tetrahalides react with water to form oxyhalides:

ThJCt + H20 ->• ThOX2 + 2HX

For this reason, ThF4, precipitated from aqueous solution, cannot be dried without contamina­tion by oxygen. When ThCl4 is dissolved in water, soluble ThOCl2 is formed and crystallizes out on evaporation. The oxyhalides are stable against disproportionation into oxide and tetrahalide at pressures near atmospheric and temperatures under 2000 K, as may be seen from the positive free-energy change AGdjsp in the reaction

2ThOX2 ->• Th02 + ThX4

The free-energy change ДС^р may be calculated from the enthalpy change A#disp and entropy change ASdisp f°r the disproportionation reaction given in Table 6.11 by Eq. (6.6):

AGdisp = Д^/disp T ASdisp (6.6)

Thorium di — and triiodides have been prepared by Scaife and Wylie [SI] and are of some practical significance in the iodide process for making thorium metal (Sec. 10.4). Other lower halides have only limited stability and are not well known.