An electron microscopic study of the polymorphic transformation in Ag₂Se (I)

The mechanism of the polymorphic transformation in silver selenide has been studied by electron microscopy and electron diffraction. The crystallographic relationship between the lattices of the low‐ and high‐temperature phases has been established and the results are compared to the large volume of inconsistent information available from the literature. It is shown that two different types of the low‐temperature phase, i.e. the semiconductor phase, exist and that at least one of these phases may undergo a second‐order transition. One of the modifications has a triclinic — in fact pseudo‐orthorhombic — and the other a monoclinic structure. All data from the literature refer to orthorhombic or tetragonal structures. The transformation relations between the lattices of the semiconductor phase and the b.c.c. high‐temperature phase are found to be not as simple as proposed by Boettcher and Junod. The polymorphic transition itself is a first‐order transition as can be concluded from the discontinuous change in specific volume at the transformation point.