Chandan Singh Verma

Synthesis and Structural Characterization of Honeycomb Lattice Li2MO3 (M = Ti, Ti0.5Ru0.5, Mn) Kundan Verma (15090) Project Instructor: Dr. Ravi Shankar Singh Abstract: Polycrystalline samples of honeycomb lattice Li2MO3 (M = Ti, Ti0.5Ru0.5, Mn) have been prepared by solid state reaction method. Crystal structures of all the three systems were found to be monoclinic with space group symmetry C2/c for Li2TiO3 and Li2Ti0.5Ru0.5O3 and C2/m for Li2MnO3 by rietveld refinement of the x ray diffraction patterns. Comparison of the x ray diffraction patterns of samples prepared in different conditions suggest that the cell parameters strongly depend on synthesis process suggesting different stoichiometry of the lithium content. Introduction: Systems with honeycomb lattice are often found to be fascinating due to their characteristic properties. In the recent years, correlated systems Li2MO3 with honeycomb lattice have attracted a lot of attention due to their potential applications as lithium ion batteries. Crystal structures of Li2MO3 are consist of pure Li layers and mixed metal honeycomb layers LiM2 separated by cubic closed packing oxygen layers. This is mixed metal layers which truly make these systems interesting, it consists honeycomb layers of edge sharing MO6 octahedra with LiO6 octahedron at centre of each hexagon of MO6 octahedra. It is the variation in position of the mixed metal layers with respect to the neighbouring mixed metal layers that allows for variation in the crystal structure. Two such well known structures are, the Li2MnO3 and the Li2SnO3 type structure. The LiMn2 mixed metal layers in Li2MnO3 stack along c direction after horizontal shifting along a-axis with C2/m symmetry while LiSn2 mixed metal layers stack along c direction after horizontal shifting along both a and b-axis with C2/c symmetry resulting in doubled c parameter in C2/c symmetry [1]. In this project, we discuss three systems with honeycomb lattice Li2TiO3 (LTO), Li2Ti0.5Ru0.5O3 (LTRO), Li2MnO3 (LMO). The honeycomb lattice LTO and LTRO were found to be monoclinic with space group C2/c while LMO was found to be monoclinic with space group C2/m. This different phase formation ascribed to different ionic radius of M4+ ions. System Li2TiO3 (Ti4+: d0) with no d electron is suppose to be band insulator but as we move towards the systems Li2MnO3 (Mn4+: d3), Li2Ti0.5Ru0.5O3 (Ti4+:d0, Ru4+: d4), we expect these systems to be metallic with partially filled d band but transport measurements exhibiting all of them to be insulating [1,2] which suggests that the electron-electron interaction plays an important role in deciding the ground state of these systems. Thus, to study the effect of change in ionic radii and to understand role of electron correlation in these systems, we plan to investigate their electronic structure by photoemission spectroscopy. With this understandings and motivations, in this report, we present the synthesis and structural characterization of these systems. Experimental Details: Polycrystalline sample of honeycomb lattice Li2TiO3, Li2Ti0.5Ru0.5O3, Li2MnO3 have been prepared by solid state reaction method using high purity ingredients Li2CO3 (99.999%), TiO2 (99.99%), MnO2 (99.99%) and RuO2 (99.9%). A 10 % excess of Li2CO3 was used to compensate for evaporation of Li during heat treatment. We prepared two sets of samples of LTO and LMO. Stochiometric amounts of ingredients were well mixed and grounded using mortar and pestle. The well-grounded mixtures were pressed into 15 mm-diameter pellets and calcinated at 600 oC for 12 hours. The mixtures were regrounded and repelletized and sintered at 1000 oC for 100 hours in case of preparation of set I and the samples are named as LTO-1000 and LMO-1000. In case of set II, the mixture after calcination at 600 oC, were first sintered at 1000 oC for 20 hours followed by regrounding and repelletization by adding 10 % excess of Li2CO3 and were treated at 800 oC for 12 hours. These samples were named as LTO-800 and LMO-800. Single set of LTRO was prepared as done for the set II and named as LTRO-800. The absence of any impurity peak was confirmed at each stage by powder x-ray diffraction (XRD) pattern collected at room temperature by PANalytical X'Pert Pro diffractometer equipped with Cu Kα radiation (λ= 1.5405Å). Rietveld refinement of x-ray diffraction patterns were performed using the HighScore Plus software. Synthesis conditions of different sets of different samples Samples Max temp of 1st heating step (°C) Max temp of 2nd heating step (°C) Set I Li2TiO-hrs) Li2MnO-hrs) Set II Li2TiO-hrs) 800(12hrs) Li2MnO-hrs) 800(12hrs) Li2Ti0.5Ru0.5O-hrs) 800(12hrs) Results and Discussions:jdjfkjdj Structural Characterization: Figure 1 shows the x-ray diffraction (XRD) patterns of the LTO-1000 and LTO-800 collected at room temperature. The observed XRD data shown by black circles, rietveld refinement is plotted by red solid lines and tick marks indicate the position of allowed Bragg reflections. A differenced curve is also shown at bottom in green lines. The bottom panel shows the diffraction pattern of stoichiometric LTO from ICSD database [3]. In case of LTO-1000, the peaks at 18.4o and 43.6o are not in appropriate ratio as seen in the bottom panel. Apparently, Li gets evaporated from the samples prepared at high temperatures and also if the heat treatment is done for longer time as reported earlier [4]. Samples prepared at 1000 oC for 20 hours also have the similar ratio of these diffraction peaks suggesting delithiation happening at higher temperature, thus we reground and repelletized the sample by adding 10% excess of Li2CO3 and sintered at 800 oC for 12 hours (LTO-800). Comparison of XRD patterns of LTO-800 suggests that the ratio of peaks appearing at 18.4 o and 43.6 o is close to diffraction pattern of stoichiometric LTO from ICSD database as shown in bottom panel as well as earlier reported data [3]. Rietveld refinement results reveal that LTO crystallizes in monoclinic structure with space group C2/c. Detailed structural parameters of LTO-1000 as well as LTO-800 are listed in Table 1. Table 1: Results of rietveld refinement of Li2TiO3 Li2TiO3 LTO-1000 LTO-800 a (Å- b (Å- c (Å- α (°) 90 90 β (°- γ (°) 90 90 Figure 2 shows the x-ray diffraction patterns of the LMO-1000 and LMO-800 collected at room temperature. We compare the diffraction patterns with diffraction pattern of stoichiometric LMO from ICSD database as shown in the bottom panel [5]. Similar to the case of LTO we find that the samples prepared at higher temperature tend to show higher intensity of the diffraction peak observed around 18.7o. The sample prepared at 800o C exhibits similar intensity ratio of peaks observed at 18.7 o and 44.8 o as shown in the bottom panel as well as in earlier reported data [5]. Rietveld refinement results reveal that LMO crystallizes in monoclinic structure with space group C2/m. Detailed structural parameters of LMO-1000 as well as LMO-800 are listed in Table 2. Table 2: Results of rietveld refinement of Li2MnO3 Table 3: Results of rietveld refinement of Li2Ti0.5Ru0.5O3 LTRO-800 LTO-800 a (Å- b (Å- c (Å- α (°) 90 90 β (°- γ (°) 90 90 Figure 3 shows the x-ray diffraction patterns of the LTRO-800 and LTO-800 collected at room temperature. It is worth noticing that peaks in LTRO shifts towards smaller 2θ in comparison to LTO, suggesting change in lattice parameters due to different ionic radii of Ru4+ (0.62 Å) compared to Ti4+ (0.605 Å). Rietveld refinement of x ray diffraction patterns reveals different changing tendencies of lattice parameters with substitution of Ru at Ti site in LTO. As seen in Table 3, the lattice parameter a and b decrease while parameter c increases. The increase in lattice parameter c leads to increase in interlayer distance. Rietveld refinement results reveal that LTRO crystallizes in monoclinic structure with space group C2/c. Detailed comparison of structural parameters of LTRO and LTO is listed in Table 3. Conclusions: Various 3d and 4d based honeycomb lattices systems have been prepared by solid state reaction route. Rietveld refinement of x ray diffraction patterns of suggests that the crystal structures of all three systems were found to be monoclinic with space group symmetry C2/c for LTO and LTRO and C2/m for LMO. Comparison of XRD data of two differently prepared sets of these samples suggests that the cell parameters and stoichiometry of lithium content in the sample strongly depends on synthesis process. Samples prepared at high temperatures exhibit high intensity of diffraction peak at around 18.4 o and 18.7 o for LTO and LMO respectively, suggesting delithiation in these systems. Thus to obtain the stoichiometric compounds the samples need to be prepared at relatively low temperatures. Future plan: We propose to perform the iodometric titration on differently prepared systems to confirm the stoichiometry and the role of delithiation. In order to understand the role of electron correlation and disorder in these 3d and 4d based honeycomb lattices, we propose to investigate the electronic structure of these systems using high resolution photoemission spectroscopy. References: 1. Akanksha Gupta, Sachin Pal, S. Uma, J. Phys. Chem. Solids, 134, 238-244 (2019). 2. Hu Zhao, Yanchao Shi, Li Xue, Yingzhi Cheng, Zhongbo Hu, Xiangfeng Liu, J. Energy Chem. 33, 9-16 (2019). 3. Kunimitsu Kataoka, Yasuhiko Takahashi, Norihito Kijima, Hideaki Nagai, Junji Akimoto, Yasushi Idemoto, Ken-ichi Ohshima, Mat. Res. Bull. 44, 168-172 (2009). 4. Marco-Polo Jimenez-Segura, Atsutoshi Ikeda, Simon A. J. Kimber, Carlotta Giacobbe, Shingo Yonezawa, and Yoshiteru Maeno, Phys. Rev. B 94,115163 (2016) 5. Chih-Jung Chen, Wei Kong Pang, Tatsuhiro Mori, Vanessa K. Peterson, Neeraj Sharma, Po-Han Lee, She-huang Wu, Chun-Chieh Wang, Yen-Fang Song, and Ru-Shi Liu, J. Am. Chem. Soc. 138,- (2016).