Crystal chemistry of the Pmnb polymorph of Li2MnSiO4

The crystal structure of the Pmnb polymorph of Li2MnSiO4 (prepared by solid-state synthesis in argon at 900 °C) is characterized by Rietveld refinement of structural models using high resolution synchrotron X-ray and neutron powder diffraction data. The crystal structure is confirmed to be isostructural with Li2CdSiO4 with lattice parameters a=6.30694(3), b=10.75355(4), and c=5.00863(2) Å, which are in good agreement with previously published data. No evidence was found for mixed lithium/manganese sites. Testing of the material as a cathode in a lithium cell shows that 1.3 lithium ions per formula unit can be extracted on the first charge cycle but very little lithium can be re-inserted. These results are compared with those of other phase-pure Li2MnSiO4 polymorphs.