CRYSTALLIZATION OF LEAD AZIDE

S. M. Ryabykh, Yu. A. Zakharov, A. P. Lysykh.

(Presented by the scientific seminar of the Department of Radiation Chemistry)

Among works on the thermal, photo-, and radiation decomposition of heavy-metal azides, only a very limited number of studies have been carried out using large single crystals. At the same time it is obvious that only studies on perfect crystals make it possible, with a controlled surface area, to obtain correct results on the kinetics and geometry of decomposition reactions in solid crystalline compounds. In the overall body of work with solid solutions in heavy-metal azide systems, we encountered the task of obtaining large and defect-free single crystals, perfect and sufficiently large for studying electrophysical properties and the concentrations of photo- and radiolytic products in them by the methods described in [1, 2].

Unfortunately, there is no detailed description in the literature of methods for preparing PbN₆ crystals. Only in works [2, 3] are there the most general indications concerning the preparation of PbN₆ crystals.

The present work describes a method for preparing PbN₆ crystals from solution in a saturated solution of ammonium acetate by the method of slow cooling; crystals were grown both of pure PbN₆ and with homophase additions of Ag⁺ and Cu⁺⁺.

Materials and apparatus

As the starting substance, polycrystalline PbN₆ was used, prepared by a precipitation reaction when mixing 0.2 N solutions of technical twice-recrystallized NaN₃ and Pb(NO₃)₂ of chemically pure grade. To introduce homophase impurities Ag⁺ and Cu⁺², the corresponding amount of AgNO₃ or Cu(NO₃)₂ was added to the initial Pb(NO₃)₂ solution. To dissolve PbN₆, chemically pure ammonium acetate was used.

In growing the crystals, a U-10 thermostat was used; the required rate of cooling of the solutions was provided automatically by means of a clock mechanism connected through gears to the magnetic head of a contact thermometer.

Single crystals were grown in a chemical beaker of 100 ml capacity and in U-shaped tubes.

Experiment

A 17% solution of ammonium acetate in water was used as the solvent. This solution was saturated with $\mathrm{PbN_6}$ at a temperature of 70–90°C for 2–3 hours. The excess $\mathrm{PbN_6}$ was then filtered off in a hot-filtration funnel, and the solution was poured into vessels in which crystallization was carried out.

In growing single crystals of $\mathrm{PbN_6}$ we encountered the difficulty of preventing the spontaneous explosions described in [3, 4]. It was established that the probability of a spontaneous explosion, as well as the temperature at which it occurs, depends on the amount of solution in the crystallizer and on the cooling rate. At the same cooling rate of 2 deg/hour, spontaneous explosions in 100 ml of $\mathrm{PbN_6}$ solution occur in the temperature interval 35–45°C, while in 25 ml of the same solution they occur at 15–20°C.

With increasing cooling rate, the probability of a spontaneous explosion when passing through the critical temperature increases.

Crystallization of $\mathrm{PbN_6}$ usually occurs in the temperature interval 20–25°C. Addition of $\mathrm{Ag^+}$ raises this temperature to 40–42°C, while addition of $\mathrm{Cu^{+2}}$ lowers the crystallization temperature to 17–19°C.

Taking all the above into account, the following method for growing $\mathrm{PbN_6}$ crystals was chosen, making it possible to minimize the probability of spontaneous explosions.

From the temperature at which the solution was saturated with $\mathrm{PbN_6}$ to a temperature 1–2° above the crystallization temperature, the solution is cooled at a rate of 2 deg/hour. The cooling rate is then reduced to 1° over 12–14 hours. It is preferable to work with an amount of solution not exceeding 25 ml. In U-shaped tubes, crystals of larger size are grown. When crystallization was carried out under the conditions described, explosions could be completely eliminated.

The $\mathrm{PbN_6}$ crystals obtained were white and opaque. On prolonged storage in the dark they acquire a pale-pink tint. Addition of $\mathrm{Ag^+}$ does not affect the color of the crystals, whereas when an addition of $\mathrm{Cu^{+2}}$ is introduced the crystals acquire a reddish-brown color.

The $\mathrm{PbN_6}$ crystals usually had the form of an irregular elongated hexagonal prism, reaching dimensions of $6 \times 1 \times 1$ mm, or, more rarely, a parallelepiped with maximum dimensions of $5 \times 3 \times 1$ mm. The weight of the $\mathrm{PbN_6}$ crystals reached 4–5 mg.

Addition of $\mathrm{Ag^+}$ increases the size of the crystals: crystals with dimensions of $11 \times 2 \times 2$ mm and a weight of 7–8 mg were obtained. Crystals with an addition of Cu did not exceed dimensions of $3 \times 2 \times 0.5$ mm.

REFERENCES

1. H. G. Neal, Nature, 172, 30 (1953).
2. F. P. Bowden, K. Sing, Proc. Roy. Soc., A227, 22 (1954).
3. B. L. Evans, A. D. Joffé, P. Gray, Chem. Rev. 59, 515 (1959).
4. F. P. Bowden, A. D. Joffe. Fast Reactions in Solids. Foreign Literature Publishing House, (1960).

ERRATA NOTED