Phase Relations in the Th-N-H System

The partial 673-K isotherm based upon published results of investigations is illustrated in Fig. 14 [1]. A ternary compound that is assumed to be described by the compositions ThN(4/3 x/3)Hx, where x ranges from 1 to about 2 or more, is shown, but deficiencies in the H content appear to occur at temperatures above ca. 673 K [2 to 4]. 

The value of x at each temperature up to 600°C is unique as has been demonstrated by Peterson, Nelson [4] and there is one degree of freedom, viz. x(T) and for a specified temperature in the 3-phase region x is fixed. The composition at 400°C is estimated from the published tensimetric data as ThNo.692H 1.924 W- The corresponding reaction is 

The free energy change at 673 K from this equation is AG(673 K) = 5.19 kcal/mol H2. The heat capacity and entropy of ThH2 are estimated from that of CeH2, Ce [5], and Th. With these and other calorimetric data of the elements [6] and the Th compounds, the enthalpy change of reaction (12) is calculated as AH298 = 25.8 kcal/mol H2 [4]. 

The fee eompound of the approximate eomposition ThNH with a = 560 pm as first reported by Juza, Gerke [1] was obtained in an approximately equimolar mixture with the amorphous eompound Th2N2(NH), see p. 60, when the metal was reaeted with 4900 atm NH3 at 320°C for 90 h in a steel vessel impervious to H2. It is only a minor eomponent in produets of reaetion at 500 to 550°C. 

A phase evidently with the same strueture and having the lattiee parameter a = 516.4 pm and chemieal composition ThN 2 was later identified in mixtures with Th(NH)2 formed by reaction of Th metal powder in the presence of a small amount of NaNH2 (presumably as catalyst) with 6000 atm NH3 at 325°C for 13 d [2]. It was also identified in a mixture with ThH2 formed by reaction with 5000 atm NH3 at 200°C for 7 d. The compound was obtained pure with the composition ThN 23H by reaction of Th powder in the presence of NH4I with 3500 atm NH3 for 10 d at 300°C [2]. 

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