The blue allotropic form of Co2+:Na2CoP2O7: optical and magnetic properties, correlation with crystallographic data

Abstract Na 2 CoP 2 O 7 is found in two allotropic forms. The first one is rose and belongs to the triclinic crystallographic system. The second one is blue and described in the orthorhombic system, space group Pna 2 1 , No. 33, with cell parameters a =15.378(6) A, b =10.271(4) A, c =7.713(2) A and Z =8. A recent work showed that the orthorhombic phase could be considered as tetragonal: space group P 4 2 / mnm , No. 136 with a =7.706(1) A and b =10.301(2) A and Z =4. The optical and magnetic properties, particularly the EPR spectrum, of the divalent cobalt ion and their phenomenological simulation were of considerable help to complement the study and allowed us to specify the real structure.