Phase Transitions and Structural Peculiarities of Divalent Nitrates

The paper is devoted to the problem of choose of the space group of symmetry for the high temperature phase of divalent nitrates. The photos of reverse lattices of Barium, Strontium and Lead nitrates at room temperature were obtained, as well as at high temperatures, which correlate with the temperatures of realization of high temperature phase in divalent nitrates. The analysis of reflexes indicates, that phase Pa3 realizes at room temperature; at the same time phases Pm3 or Pm3m are to be realized as the high temperature phase. The symmetry considerations, that are described in the paper, testify to Pm3 space group for high temperature phase in divalent nitrates. This phase can be considered as maternity phase for Pa3 and P213 phases, that realizes in divalent nitrates, as it contains all symmetry elements for these phases. So, the chain of phase transitions Pm3 ↔ Pa3 ↔ P213 can be described by the set of symmetry elements of Pm3 phase. These investigations confirm our supposition, that this chain of phase transitions realizes due to especial mobility of nitrate groups in crystal lattice. It was shown, that high temperature Pm3 → Pa3 phase transition can be realized because of “freezing” of free move of NO3-groups around Nitrogen atoms, which has spherical symmetry and turning it into their oscillations around Nitrogen atom in one plane.