Manganese Arsenides

The Effect of Temperature on (A) Specific Heat and (B) Magnetic Induction of Trimanganese Di-arsenide, and (C) the Rate of Change of the Magnetic Induction with Temperature.

Three of these have been described: Mn₂As, Mn₃As₃ and MnAs. The hemi-arsenide, Mn₂As, is prepared, according to Arrivaut, by heating a mixture of arsenic and manganese at 500° C.; combination occurs with incandescence. It is also formed when the monarsenide is heated in the absence of air. Hilpert and Dieckmann could not obtain the hemi-arsenide, but Arrivaut, in an investigation of the E.M.F. diagram of Mn-As alloys containing 6 to 53 per cent. As, showed the presence of the two arsenides Mn₂As and Mn₃As₂. The former is grey and non-magnetic and is stable at high temperatures. The latter, trimanganese di-arsenide, Mn₃As₂, is obtained by heating the mixture of elements in a current of hydrogen at 700° to 800° C. When freshly prepared, this arsenide is ferromagnetic, but on heating to about 45° C. it becomes paramagnetic; on cooling it returns to the ferromagnetic state. Each subsequent transition from the ferromagnetic to the paramagnetic state increases the intrinsic magnetisation, and the thermal properties are also affected. Thus the specific heat of a specimen of the arsenide was observed to change as shown in fig.; the value increased slowly from 0.122 at 28° C. to 0.14 at 36° C., then rapidly rose to a maximum of about 0.8 at 42.2° C., falling steeply to 0.13 at 45° C. (the critical temperature) and reaching a minimum at about 46.5° C. Thus heat is rapidly absorbed as the substance passes from the ferromagnetic to the paramagnetic state. In the figure, the curve B shows, in arbitrary units, the effect of temperature on the magnetic induction, and the curve C the rate of change of magnetic induction with temperature, also in arbitrary units. The latter shows a maximum at 42.2° C. and corresponds closely to the specific heat curve. Bates suggests that the changes are due to interaction between the spin moment of one atom and the orbital moment of another. The density also changes from 6.26 to 6.20, and the course of the volume- temperature curve between 15° and 50° C. resembles those of the intensity of magnetisation-temperature and the specific heat-temperature curves. The change is gradual and there is a temperature hysteresis. The transition from the paramagnetic to the ferromagnetic form appears to involve a series of irreversible metastable conditions. No volume change results from the application of a magnetic field.

Manganese Monarsenide, MnAs, was described by Wedekind as a black crystalline powder formed by heating together manganese and arsenic, the product being freed from excess of the former by treatment with dilute hydrochloric acid, and of the latter by heating in chlorine. Its density was 5.5 and it was stable only below 400° to 450° C., further heating causing loss of arsenic. A similar product may be obtained by the action of arsenic trichloride on manganese. Hilpert and Dieckmann heated pure manganese in arsenic vapour under pressure, but Bates, using the same method, found that the product contained nearly equal parts of manganese and arsenic and therefore corresponded with Mn₃As₂. Arrivaut was unable to prepare the monarsenide, as the mixture (containing 57 per cent. As) decomposed on fusion. There is consequently some confusion in the literature. Wedekind's product was non-magnetic. The crystal structure of the so-called monarsenide has been investigated.