text: LAGP crystals belong to the space group D - Rc. The factor group analysis of NASICON-type materials with general formula MMPO (where M stands for a monovalent metal ion like Na, Li or K, and M represents a tetravalent cation such as Ti, Ge, Sn, Zr or Hf) is usually performed assuming the separation between internal vibrational modes (i.e. modes originating in PO units) and external modes (i.e. modes arising from the translations of the M and M cations, from PO translations, and from PO librations).
  Focusing on internal modes only, the factor group analysis for Rc space group identifies 14 Raman-active modes for the PO units: 6 of these modes correspond to stretching vibrations and 8 to bending vibrations.   
  On the contrary, the analysis of external modes leads to many available vibrations: since the number of irreducible representations within the rhombohedral Rc space group is restricted, interactions among different modes could be expected and a clear assignment or discrimination becomes unfeasible.