Spectroscopic investigations on SrAl₂O₄ polymorphs

Nominally undoped and Eu³⁺ or Eu³⁺–Dy³⁺ doped SrAl₂O₄ powders are prepared using source chemicals of standard grade (99–99.99%) by solid state and sol–gel procedures, in order to vary the proportions of monoclinic (P2₁) and hexagonal (P6₃ and P6₃22) polymorphs in the compounds. Electron spin resonance reveals systematic contamination of the compounds by transition metal ions. The emission properties of the materials are investigated at 77 K upon UV and blue excitations. The optical features are ascribed, for the essential, to Mn⁴⁺ ions. The spectroscopy of this ion in conjunction with X-ray diffraction data and theoretical considerations is used to individuate P6₃22, P6₃ and P2₁ phases in the doped and undoped SrAl₂O₄ powders. - Highlights: • Different polymorphs of SrAl₂O₄, undoped or doped with Eu³⁺, were synthesized. • Contamination by transition metals is proved by ESR and photoluminescence. • The Mn⁴⁺ spectroscopy allows the identification of the SrAl₂O₄ polymorph