Synthesis, crystal structure and ionic conductivity of MgAl2X8 (X = Cl, Br)

MgAl2X8 (X = Cl, Br) was synthesized from MgX2 and AlX3, and the physical properties were evaluated by XRD, AC conductivity, and NMR. MgAl2Br8 has the same crystal structure as MgAl2Cl8 and belongs to the space group C2/c. MgAl2Cl8-yBry was also synthesized in the same manner as MgAl2Br8. MgAl2Cl8-yBry was found to have the same crystal structure as MgAl2Cl8 and forms a solid solution in all the synthesized compositions. From the Rietveld analysis of the XRD pattern, it was found that the arrangement of Cl and Br in the crystal had no order and occupied the halogen sites almost randomly. The AC conductivity of the obtained compound was lowest for MgAl2Cl8 and highest for MgAl2Cl2Br6. The conductivity of MgAl2Cl2Br6 at 400 K is 1.3 × 10−6 S/cm, and the Nyquist plot suggests that MgAl2Cl8-yBry is a magnesium ionic conductor. In the temperature range from room temperature to 412 K, there was no significant change in the 27Al NMR spectra of a MgAl2Br8 single crystal, and the effects of Mg2+ ionic conduction and the reorientation motion of AlX4ˉ anion were not observed. MgAl2Cl2Br6 shows the highest Mg ion conductivity among MgAl2Cl8-yBry(y = 0–8), and its value is 1.3 × 10−6 S/cm at 400 K. [Display omitted] •MgAl2Cl8-yBry was also synthesized at the ratio y = 0–8.•MgAl2Cl8-yBry had the same crystal structure as MgAl2Cl8 and the arrangement of Cl and Br in the crystal had no order.•MgAl2Cl8-yBry are the Mg2+ ionic conductors and MgAl2Cl2Br6 shows the highest conductivity of 1.3 × 10−6 S/cm at 400 K.•Temperature dependence of 27Al NMR spectra are obtained and the fast ionic motion was not detected.