Synthesis, structural and electrical properties of two congruent isotypic diphosphates: Li2Na2P2O7 and Li3NaP2O7

The mixed alkali-diphosphate materials are among the widely used commercial materials due to their outstanding physical properties. The Li 2 Na 2 P 2 O 7 (LNOP1) and Li 3 NaP 2 O 7 (LNOP2) compounds have been synthetized by the conventional solid-state method. The structural and morphological analysis, optical properties, electrical and dielectric measurements were taken on the obtained compounds. Their powder X-ray diffraction analysis indicates that they crystallize in the monoclinic system with non-centrosymmetry P12/m1 and P12 1 /m1 space group, respectively. The morphological images show that the two compounds are consisting of irregular block-shaped aggregates. The UV–Vis absorbance spectra show that the cutoff edges for compounds (LNOP1) and (LNOP2) are below 400 nm. The optical gaps of the both compounds are found indirect type with a value of 3.39 eV and 3.42 eV, respectively. The electrical data are investigated using the complex impedance spectroscopy technique in the range of frequency (0.1 Hz–1 MHz) and through large temperature scale (373–673 K). The AC conductivity evolution increases rapidly with appearance of a dispersive behavior, which obeys to Jonscher’s law. Furthermore, the complex impedance analysis shows the contribution of only the grains in the conduction mechanism. The complex modulus diagrams confirmed the existence of only the contribution of the grain effect. Finally, the dielectric behavior of both compounds shows a non-Debye relaxation type.