Arsenic;magnesium

DescriptionMagnesium arsenide has the molecular formula of Mg3As2 and the molecular weight of 223.6592 g/mol. It may be prepared by direct reaction between the elements. Since As sublimes at 614°C and Mg melts at 650°C, the latter is melted at about 675°C in an inert atmosphere like argon (not N2 which would form nitrides) and arsenic vapors are carried into the reaction zone by a stream of Ar gas: 3Mg + 2As + heat Mg3As2 Mg3As2 is obtained by this method as a deep-brown, X-ray amorphous powder. Magnesium arsenide has the CAS number of 12044-49-4. This compound decomposes in contact with water: Mg3As2 + 3H2O 2AsH3 + 3Mg(OH)2 Thus, it must be stored under dry conditions. Its density is 3.15 g/cm3. This compound is dimorphic with the low temperature form, β-Mg3As2 being cubic (α = 12.33) and the high-temperature form, α-Mg3As2 being hexagonal. It crystallizes in the anti-La2O3 crystal structure, with unit-cell parameters of: a = 4.264A ° , c = 6.738, c/a = 1.580. Both of these compounds are semiconductors. The latter has a band gap, Eg, of 2.2eV (564 nm).

UsesThe structure of magnesium arsenide is not ionic but covalent. Magnesium arsenide was used as a rodenticide and insecticide. Its toxicity to humans has led to its prohibition in the US and Europe.