BeP2: A tetrahedral structure of type order-disorder which obeys a coordination rule for short-range order

Single-crystal studies on BeP₂ indicate that this compound possesses an OD structure. The substructure has a tetragonal unit cell with: a = 3.546 Å, c = 15.01 Å, Z = 4, space group: $\text{I4}{}_1\text{amd}$. The final R factor has a value of 0.033. The atom sites in this substructure correspond to the sites of diamond if the latter is described with a tetragonal cell, where $\text{a = (}\text{2}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{2}\text{)a}{}_{\mathrm{<mtext>diamond</mtext>}}$ and c = 3a_diamond. A short-range order governs the occupation of these sites with Be and P atoms. Each Be has four tetrahedral P neighbors and every P has two Be and two P neighbors. Consideration of the maxima on the diffuse streaks between the sharp reflections of the substructure leads to an intermediate unit cell with a = 7.09 Å and c = 30.02 Å. Coordination considerations allow a structure proposal to be formulated for this intermediate structure which is triclinic but pseudotetragonal. The true unit cell is also pseudotetragonal with a = 7.09 Å and c = N · 15.01 Å, where N is a large integer.