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Section II: Phase Diagram Evaluations

N-Rb (Nitrogen-Rubidium) James Sangster

Equilibrium Diagram The solubility of N in Rb was reviewed by [2000Bor] and [2001Bor]. The only report relevant to solubility is the analysis of Rb, under 1 atm of N2, by a modified Kjehldahl method [1963Tep, 1964Tep]. The lower limit of detection was 2 mass ppm and the sensitivity was ±2 ppm. The result of the analysis was 4.6 mass ppm of N, but since this was a measurement of impurity and not solubility, this value is suggestive only. There are two compounds: Rb3N (rubidium nitride) and RbN3 (rubidium azide). Rb3N is formed by direct reaction of the elements, but only if N2 is subject to silent electrical discharge at low pressure [1954Eph]. It is also formed in the decomposition of RbN3 [1930Clu] or of RbNH2 [1954Eph] or by the reaction of RbH with N2 gas at elevated temperatures [1903Moi, 1910Fis]. Neither its melting point nor crystal structure has been reported. [1990Sha] determined, from ab initio calculations, that metastable Rb3N is about 92% ionic. RbN3 is most commonly prepared by the neutralization of HN3 by RbOH [1898Cur] or Rb2CO3 [1986Bla] or by the precipitation reaction Rb2SO4 + Ba(N3)2 in aqueous solution. It is also formed by the reaction of RbNH2 with N2O [1954Eph]. Melting and transition temperatures of RbN3 were reviewed by [1993Bel]. It melts without decomposition; the melting point has been variously reported to be 300 °C [1916Tie], 317 °C [1965Mul], 321 °C [1926Suh], 330 °C [1898Cur], or 340 °C [1934Pas]. The true melting point is probably near 320 °C. The room temperature form, ␣RbN3,

Table 1

transforms to the high-temperature form ␤RbN3 at 315 °C [1965Mul]. Both forms have been characterized by x-ray [1929Gun, 1930Pau, 1957Eva, 1965Mul, 1972Mul] or neutron [1976Cho] diffraction. RbN3 decomposes at about 395 °C [1993Bel].

Crystal Structures and Lattice Parameters Crystal structures and lattice parameters are summarized in Tables 1 and 2. There is no information on the crystal structure of Rb3N. Crystal structures and lattice parameters of RbN 3 were reviewed by [1957Eva], [1959Wad], [1963Gra], and [1993Bel]. ␣RbN3 has a layered structure, with alternating planes of N3 and Rb+ ions [1972Mul]. The N3 group is linear and symmetrical [1936Fre], the N-N distance being 0.115 ± 0.002 nm [1963Gra]. ␣RbN3 is body-centered tetragonal and is isostructural with KN3; each N3 group is equidistant from eight K atoms and vice versa [72Mul]. In the earliest x-ray work on ␣RbN3 [1929Gun, 1930Gun], the reflections were indexed on the assumption of 4/mmm symmetry, rather than 4/mcm, as now preferred (Table 1). The lattice parameters, as originally reported by [1929Gun] were a ⳱ 0.4506 nm, c ⳱ 0.3714 nm. The correct interpretation, pointed out by [1930Pau], involves the change D14h to D18 4h (the value of c is doubled and the elementary cell is rotated 45° around the c axis.). ␤RbN3 is also a layered structure (like ␣RbN3) but has the cubic CsCl structure [1965Mul].

N-Rb crystal structure data

Phase

Composition, at.% Rb

Pears