The orthorhombic Pbn2 1 phase of Li 2 CoSiO 4 has been prepared by solid state reaction of Li 2 CO 3 , CoC 2 O 4 ⋅2H 2 O, and highly dispersed SiO 2 at 900 °C and the subsequent slow cooling in Ar gas stream. The DTA curve shows peaks at around 540 °C indicating a reversible structural phase transition. XRD pattern of the sample quenched from 650 °C can be indexed as a Pmn2 1 phase. However, electron diffraction patterns show complex diffuse scatterings. From the in-situ XRD patterns at high temperature, a reversible second order-type phase transition from Pbn2 1 to Pmn2 1 phase was confirmed and it is interpreted as an order-disorder transition of Co and Li atoms. At high temperature, Pmn2 1 phase transforms to the monoclinic γ-phase with large thermal hysteresis; DTA curve shows peaks at 950°C on heating and at 780°C on cooling.keywords: order-disorder, wurzite-type structure, DTA, high temperature powder XRD, TEM

The orthorhombic Pmnb phase of Li2MnSiO4 has been prepared by solid state reaction of Li2CO3, Mn(CH3COO)2⋅4H2O, and highly dispersed SiO2 at 800 °C and the subsequent slow cooling in Ar gas stream. The crystal structure was refined by the Rietveld method based on the model structure without disorder in the site occupancies, although some anomaly was observed for Mn sites. The DTA curve shows peaks at around 680 and 770 °C on heating and at around 510 °C on cooling indicating a reversible structural phase transition. From the in-situ XRD patterns at high temperature, the first order phase transition from Pmnb to a distorted orthorhombic wurzite-type structure with the random arrangement of Li, Si, and Mn.keywords: order-disorder, wurzite-type structure, DTA, high temperature powder XRD INTRODUCTIONLithium transition metal silicates, Li2MSiO4 (M: Mn, Fe, Co), have been investigated extensively in this decade as a new class of positive electrode materials for lithium ion batteries. 1-4) In these silicates, oxide ions form a pseudo-hexagonal closed packed (hcp) structure. The cations occupy a half of its tetrahedral sites and show several kinds of ordered arrangements depending on the transition metals species and the heat treatments during the sample preparation. 5,6) The homogeneity of the ordered structure may have significant effect on the electrode properties. Several investigations on the details of polymorphism have been reported [7][8][9][10][11][12][13] , mainly for the hydro-thermally prepared and heat-treated samples. However, the phase relations among polymorphs and the microstructures are still unclear. The mutual phase relationships above polymorphs are of interests to obtain the single-phase materials and to clarify the electrochemical properties. In this paper, the structural phase transition of Li2MnSiO4 was investigated using TG-DTA and powder XRD at high temperature.