Structural and Spectroscopic Studies of NaCuCr2(PO4)3: A Noncentrosymmetric Phosphate Belonging to the α-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 α-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 (CHARDI) 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+ (3d9) and Cr2+ (3d4) Jahn-Teller polaron effects as a key factor to the centro Imma to noncentrosymmetric Imm2 phase change.