Structural And Spectroscopic Studies Of Nacucr2(Po4)(3): A Noncentrosymmetric Phosphate Belonging To The Alpha-Crpo4-Type Compounds

An electron and joint neutron and X-ray diffraction study of the synthetic copper/chromium phosphate NaCuCr2(PO4)(3) (NaCuP) is reported. A noncentrosymmetric Imm2 space group belonging to the well-known alpha-CrPO4 type is observed contrary to what is reported in NaMCr2(PO4)(3) (M = Co and Ni) phosphates. The structural model is validated by bond valence sum analysis and charge-distribution (CHARD degrees calculations and supported by complementary infrared and Raman spectroscopy investigations. Both Raman spectroscopy and theoretical study by deformation density approach further suggest the presence of Cu2+ (3d(9)) and Cr2+ (3d(4)) Jahn-Teller polaron effects as a key factor to the centro Imma to noncentrosymmetric Imm2 phase change.