Powder neutron diffraction study of LiMnV04 ~~~~ ~ ~ M. Sato,' S. Kano,' S. Tamaki,b M. Misawa,c Y. Shirakawad and M. Ohashi" 'Department of Chemistry and Chemical Engineering, Faculty of Engineering, Niigata University, Niigata 950-21, Japan bDepartmentof Physics, Faculty of Science, Niigata University, Niigata 950-21, Japan 'Department of Chemistry, Faculty of Science, Niigata University, Niigata 950-21, Japan dDepartmentof Materials Science and Processing, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan "Institutefor Material Research, Tohoku University, Sendai 980-77, Japan Powder neutron diffraction data have been recorded for LiMnVO,. The crystal structure was determined by Rietveld analysis. The Rietveld refinement confirmed that the compound adopts an orthorhombic spinel-related structure.The framework structure of the compound is comprised of nearly regular, edge-sharing octahedra of MnO,. The lithium and vanadium atoms in LiMnVO, occupy, as partially mixed species, the two kinds of tetrahedral interstitial sites constructed by cubic close-packed oxygen atom arrays; this structural feature is quite similar to those of CrV0, and LiCuVO,. The magnetic susceptibility and the metal-oxygen bond distances estimated from the refined structure of LiMnVO, clearly indicate that the valence states for the transition-metal ions are divalent for manganese ions and pentavalent for vanadium ions. The lithium oxide spinels, Li,Mn,O, (O<x<2), have been widely investigated for cathode materials in rechargeable lith- ium batteries because of their high cell voltage, long shelf life and wide operating-temperature range.' In a spinel of general formula A[B,]X, with prototypic symmetry Fd?rn, the struc- ture is characterized by the [B2]X4 framework, of which the anion arrays are stacked with cubic close packing (ccp)., This framework, which provides a three-dimensional interstit-ial space for lithium ion transport, remains intact during the lithiation Vanadium oxides, e.g.V2OS7 and LiV308,* are also attractive cathode materials for lithium batteries because of the high valence state of the vanadium ions. Their crystal structures are not spinel-type, but rather a sort of bronze-type structure in which where a nearly two- dimensional framework responsible for lithium insertion is realized; although a spinel-type compound, LiV,O,, exists it shows a rather poor performance as a cathode material, i.e.low operating cell voltages of around 2.5 V us. Li+/Li., Therefore, if suitable syntheses can be determined, oxide sys- tems consisting of manganese and vanadium ions in a rigid framework may be interesting candidates for cathode materials in lithium batteries. While making a survey of the pseudo-binary systems of LiMn20, and LiV204, we reported previously a new com- pound formulated as LiMnVO, with a spinel-related ~tructure.~ The previous powder X-ray diffraction measurements on LiMnVO, have shown it to be orthorhombic and a spinel- related structure has been assumed.However, X-ray diffraction is very insensitive to lithium positions and the scattering is dominated by the manganese and vanadium. As a result, it is possible that not only uncertainty over the lithium positions but also relatively large distortions of VO, tetrahedra and MnO, octahedra could occur, which would still be accommo- dated by the X-ray diffraction. The sensitivity of neutrons to lithium and oxygen positions makes neutron scattering an ideal tool for the observation of such ambiguities. In this work, we report the results of a neutron diffraction study on LiMnVO,, and discuss its structural features in comparison with those of CrVO," and LiCuVO,," which possess spinel-related structures. Experimental LiMnVO, was prepared by a conventional solid-state reaction of reagent-grade Li,C03, Mn(CH3COO),.4H20 and V,O, powders. Stoichiometric amounts of the powders were ground thoroughly, pelletized under a pressure of 30 MPa, and fired in an alumina crucible at 500-750 "C for 24 h in air with several intermittent grindings.The phase purity of the product was confirmed by the powder X-ray diffraction method. The powder X-ray diffraction analysis for the LiMnVO, sample showed a trace amount of a cubic spinel phase remaining in the product, in addition to a new LiMnVO, compound. The composition analysis for the product was carried out by a flame spectroanalyser (Hitachi 180-50) for the lithium content and by a induced coupled plasma spectroanalyser (Seiko SPSl5OOV) for the other compo- nents, except for oxygen.Powder neutron diffraction patterns for the Rietveld analysis were recorded using the KPD diffractometer (Tl-3) installed at JRR-3M Guide Hall in the Japan Atomic Energy Researct Institute (JAERI). An incident neutron wavelength A= 1.767A was obtained from the (311) reflection of five germanium single crystals. The powder sample (of size 3 cm3) was enclosed in the cylindrical vanadium vessel and mounted on a double-axis diffractometer. The data were collected on thoroughly ground powders by a multi-counter-scanning mode in the 28 range 10-121.90" with a step width of 0.10" and a monitoring time of 260s at room temperature. The powder pattern obtained was analysed by the Rietveld method using the RIETAN94" profile refinement program.? The magnetic susceptibility was measured using the Faraday method using a home-made computerized magnetometer which was calibrated with a sample of high purity Mohr's salt [Fe( NH,)2(S04)2.6H,0].Magnetic susceptibility data were collected at an appropriate applied magnetic field over the temperature range 77-773 K. Results and Discussion Data analysis was carried out on the room-temperature data set. As previously reported, other than the weak impurity peaks observed by X-ray diffraction, no additional reflections were observed, indicating that the unit cell of LiMnVO, is an t Structural refinement data as output from the RIETAN94 program are available as supplementary data (SUP 57140) from the British Library.For details of the Supplementary Publications Scheme, see Information for Authors, Issue 1. J. Muter. Chem., 1996, 6(7), 1191-1194 1191 orthorhombic system with space group Cmcm aFd approximate cell parameters a =5 75, b =8 70 and c =6 35 A The impurity peaks observed at 28=20 70 and 42 00" were found to corre- spond to a cubic spinel phase Considering t$e cell constants for the spiFel compounds LiMn,O, (a=8 247 A)13 and L1V204 (a=8 241 A),', these peaks may be attributable to the (1 1 1) and (3 I 1) reflections, respectively, of the cubic spinel compounds, probably forming a solid solution such as L1(Mn,V)204The following scattering lengths were used l5 Li, -1 900, Mn, -3 730, V, -0 382, 0,5 803 fm An initial site assignment was referred to the results obtained by the previous X-ray diffraction study,g I e ,4c sites for Li, 4a sites for Mn, 4c sites for V, 8f site for 0(1) and 8d site for 0(2), respectively, in Cmcm space group Since a small amount of the impurity spinel phase remained in the sample, the recorded pattern was analysed while assuming a two-phase mixture In the two- phase refinement mode of RIETAN94, a matrix refinement of the LiMnVO, structure, with the variables of scale factor, lattice parameters, fractional coordinates and individual iso- tropic thermal parameters, was undertaken, while the refine- ment of the impurity spinel phase was made using the same variables as those for LiMnVO, except for an overall isotropic thermal parameter, where the site assignment was based on the cubic space group Fd%, 8a sites for Li, 16d sites for the mixed species of Mn and V with an equimolar ratio, and 32e sites for 0 The R factor of the weighted pattern fitting, R,,, was reasonably reduced to ca 660% However, the isotropic thermal parameter for tht lithium site was converged to a relatively large value, 2 4 A2, and that for th? vanadium site was converged to a large negative value, -2 8 A2 These results apparently indicate the possibility of partial disorder of lithium and vanadium in the two 4c sites Therefore, the following refinement was performed on this assumption, resulting in a improved convergence with R,, =5 74% and normal isotropic thermal parameters for the two 4c sites The lithium and vanadium atoms are distributed on the two 4c sites with mutual displacement ratios of 14% The proportion by mass of the impurity phase in LiMnVO, was determined on the basis of the scale factors finally obtained for LiMnVO, and the impurity From the analysis proposed by Hill and Howard,16 the proportion of the impurity phase was found to be 252 mass% At present, it is unknown whether a mixture of LiMn20, and LiV204 or a solid solution of Li(Mn,V),O, is the true composition of the impurity spinel phase, because of the very similar lattice parameters and of the fairly low concentration of the impurity in the product In Table 1 the crystallographic data together with the data collection conditions are given The positional parameters finally obtained are listed in Table 2 Table 3 shows the bond distances and angles for the compound Fig 1shows the results of the pattern fitting of LiMnVO,, where the reflection peaks corresponding to the impurity spinel phase in LiMnVO, are shown as the lower series of bars The crystal structure, determined using the crystallographic parameters refined for LiMnVO,, is illustrated in Fig 2 The structure is quite similar to that of CrV0410 which has the same space group and the same atomic locations except for lithium, and it also rather resembles that of LICUVO,,~~ these structures are also shown Although LiCuVO, crystallizes in an orthorhombic system with space group Imma, which is different from that of both LiMnV0, and CrVO,, these three materials have several common structural features In LiMnVO, and CrVO,, almost regular, edge-sharing M0, (M =Mn, Cr) octahedra run along the c axis There are two kinds of M-0 bond distances in the ,MO, octahedroq, i e one is the short M-O( 1) bond (2 144 A for Mn, 1?57 A for Cr) an! the other is the long M-0(2) bond (2 183 A for Mn, 2 039 A for Cr), the average lengths of the M-0 bonds are 1192 J Muter Chem , 1996, 6(7), 1191-1194 Table 1 Neutron data collection and analysis of LiMnVO, neutron data collection wavelength/,& 1767 28 rangeldegrees 10-121 90 step scan increment (28) 0 10 count time/s step 260 contnbuting reflections 124 total number of observations 1120 sample can diameter 10mm, length 40 mm, vanadium can model refinement spGce group Cmcm a14 5 7465( 2) blh 8 7057( 5) CIA 6 3425( 3) volumejA3 317 3 Z 4 calculated density/g cm 3 701 scattering lengthlfm Li -1 900, Mn -3 730 V -0 382, 0 5 803 program RIETAN94 function minimized M=C[w(Y,-ly],w=l/Y, no of parameters refined 36 background 8-term polynomial peak shape pseudo-Voigt function reliable factors' (%) R,, 5 74 RP 4 45 RE 4 21 R, 4 49' RF 3 08' 'Defined in ref 12 'Those for the impurity spinel phase are R,= 3 71% and RF, 3 06% The unit-cell parameter of the impunty phase is a=8224(2)A Table 2 Positional parameters for LiMnV04 atom site f xla Ylb z/c B/A2 Li(1) V(l) Mn V(2) Li(2) 0(1) 4c 4c 4a 4c 4c 8f 086(7) 014 10 086 014 10 00 00 00 00 00 00 -0336(3) -0 336 00 0361(8) 0 361 0245(1) 025 0 25 00 025 0 25 00334(9) 12(8)12 1 O(3) 0 3(2) 03 lO(2) O(2) 8g 10 0259(1) -00212(7) 025 0 8(2) 2 170 A for Mn and 2 012 A for Cr, which are almost equal to the distances estimated from the effective ionic radii17 of Mn2+ (VI fold), Cr3+ (VI fold) and 02-(I1fold) The oxygen atoms in LiMnVO, and CrV0, form cubic close packed (ccp) arrays running in the direction parallel to the (1 1 0) plane A pair of edge-sharing tetrahedra are formed in the interstitial space constructed by the ccp arrays of oxygen atoms In LiMnVO,, lithium and vanadium atoms are located in equivalent positions with a displacement ratio of 14% in these tetrahedra, while one of the corresponding two types of tetrahedra is missing in the case of CrVO, The CuO, octahedron in LiCuVO, is quite distorted toward a square-planar coordination owing to the Jahn-Teller Cu2+ ion In spite of such a distortion, the coor- dination environment around the vanadium atoms is very similar to those in LiMnVO, and CrV0, In addition, all the oxygen atoms in LiCuVO, also form ccp arrays However, the lithium atoms in LiCuVO, occupy octahedral sites but not tetrahedral ones, as in the case of LiMnVO, This is probably because the Jahn-Teller distortion results in more space being available to the octahedral site than in the case without the distortion of the CuO, octahedron In view of the cation distribution in interstitial sites constructed by ccp arrays of oxygen atoms, the following formulae can be suggested (A,),,, CBlO,,O, for LiMnVO,, Wtet CBlO,tO, for CrVO, and Table 3 Bond distances (in A)and angles (in degrees) for LiMnVO," distances M( l~---O(l~ 1.964( 14) x 2 M( 1)' -O(2) 2.124(27) x 2 Mn-O(1) 2.144(9) x 2 Mn-0(2) 2.183(5) x 4 M(2)-0(1)...1.704(46)x 2 M (2)- O(2!,, 1.722(45)x 2 O(1)-O( 1y 2.747( 11) 0(1)-0(2) 3.079( 10) 0(2)-0(2)' 3.192( 1) angles O(ly-M( l,)-O( 1); 132.94( 18) O(2)-M( 1)' YO(2)"" 81.3( 12) O( 1)" -M( 1)'-0(2) 107.8(5) O(1)-Mn-O( 1)' 180.0 O(1)-Mn-O(2) 90.7(2) O( 1)-Mn-O(2)' 89.2(2) O(2)- Mn- O(2)' 93.9( 3) O(2)- Mn- O(2)~' 86.0( 3) 0(2)- Mn -0(2)"' 180.0 O(1)-M(2)-0( l)k 107.4( 41) 0(2)g -M (2)-O(2)" 106.9(40) O(1)-M( 2)-O( 2)"' 110.6( 2) symmetry codes: (none)x,y,z; ' 1/2 +x,1/2 +y, -2; ii 1/2+x,1/2 -y, -z; iii 1/2-x,1/2,+y,z; iv x,y,1/2-z; x,-y,-z; vI 1/2+x,1/2-y,1/2+2; Vii 1-x,y,z; vlll -x,y,z; ix x,y,1/2 -z; -1/2+x,1/2 +y,z; xi -x, -y, -z.a M( 1) and M(2) represent mixed species for the Li-rich 4c site and the V-rich 4c site, respectively. -50001 I I a0 -c. I I I I I I I# I I I1 I I 111 I I 11111 I1111111111 1111111111111 1111 I1 11111 1111 111111 1111111 I 1 I1 1 1 1 1 Ill Ill I1 I1 I IIILIIII~II Fig. 1 Powder neutron diffraction pattern fitting for LiMnVO,. The calculated and observed intensities are shown as solid line and dots, respectively. The upper and lower bars are the diffraction positions for LiMnVO, and the impurity phase, respectively.The trace is a plot of the difference between calculated and observed intensities. Fig. 2 Structural models of (a)LiMnVO,, (b)CrVO, and (c) LiCuVO,. The positional parameters are taken from ref. 10 for CrVO, and from ref. 11 for LiCuVO,. In LiMnVO,, the large tetrahedra are those of the Li-rich 4c species [M( l)], the small tetrahedra are those of the V-rich 4c species [M(2)], and the octahedra are those of the Mn 4a species. In CrVO, and LiCuVO,, the tetrahedra and octahedra are those of V04 and MOs (M =Cr, Cu and Li), respectively. (A)tet[B2IoctO4for LiCuVO, with a normal spinel distribution, where A is a tetrahedral cation and B an octahedral cation. The similarities and differences between LiMnVO, and the spinel structure are as follows: (1) the oxygen atoms in LiMnVO, form ccp arrays like the spinel structure, but they are slightly distorted; (2) the manganese atoms reside in half of the B sites of the spinel structure but in an ordered arrangement; and (3) one half of the A sites which are occupied by lithium and vanadium correspond to the A sites of the spinel structure.Fig. 3 shows the environment around the vanadium atoms for three compounds. The Li-rich and V-rich tetrahedra in LiMnVO, are linked together by sharing two O(2) atoms. The Li-rich [denoted as M(l) in Fig. 33 tetrahedron is fairly distorted from an ideal tetrahedron, where the position of the central atom is shifted toward the opposite side of the V-rich [denoted as M(2) in Fig. 31 tetrahedron.Such distortion seems to be due to a coulombic repulsion between the two cations since the Li-rich tetrahedron has a less positive charge than the V-rich tetrahedron. LiMnV0, showed typical paramagnetic behaviour in the temperature range 77-773 K as shown in Fig. 4.The extrapo- lated Curie temperature is -84K, implying an antiferromag- netic interaction between magnetic ions. The effective mag- netic moment calculated from the x-' us. l/T plot is 5.62 pB. Three combinations of possible valence states of manga-nese and vanadium ions in LiMnVO, can be considered, i.e. Mn2+-V5+, Mn3+-V4+ and Mn4+-V3+. The theoretical effective magnetic moments, calculated by taking into account Fig. 3 Environment around lithium and vanadium atoms in (a) LiMnVO,, (b) CrV04 and (c) LiCuVO,.M(l) and M(2) in LiMnV04 are the Li-rich and V-rich atomic species, respectively. The atomic distances in CrVO, and LiCuVO, are taken from ref. 10 and ref. 11, respectively. (Symmetry code is given in Table 3.) 250--200 F5 150-0 I 200 400 600 800 TIK Fig. 4 Temperature dependence of the inverse molar magnetic suscepti- bility of LiMnVO, J. Mater. Chem., 1996, 6(7), 1191-1194 1193 Table 4 Bond-valence sums for Mn ions in LiMnVO, 1 atom J bond-valence parameter' RV bond-valence sum K Mn2+ O2 1790 2 15 Mn3+ O2 1760 198 Mn4+ O2 1753 1 94 'The values are taken from ref 18 the spins of d electrons only for each magnetic transition ion, are 5 91, 5 19, and 4 79 pB for Mn2+-V5+, Mn3+-V4+ a nd Mn4+-V3+ combinations, respectively The observed value is in good agreement with that for the Mn2+-V5+ combination The concept of bond valence provides another powerful method for the prediction of valences of metal ions in crystals According to Brown and Altermatt,18 the valence vZJof a bond between two atoms z andj can be defined so that the sum of all the valences from a given atom z with valence K, obeys K=c'CJ=cexpC(RtJ-dd,J)/blY J J where b is taken to be a universal constant equal to 0 37 A,dtJ is the observed distance and R,, the bond-valence parameter Table 4 shows the calculated bond-valence sums for Mn atoms in LiMnVO, All the bond-valence sums calculated are nearly equal to 2 no matter which bond-valence parameters are used These results are also consistent with the valence combination deduced from the magnetic susceptibility data The electrical conductivity measurement for LiMnVO, showed typical semi- conducting behaviour with r~ =1x S cm-'at 300 K From the preliminary measurement of thermoelectric power at room temperature, the main charge carriers were found to be holes These facts indicate that the unpaired electrons of Mn2+ ions in LiMnV0, are localized in 3d orbitals LiMnVO, does not show any lithium intercalation or deintercalation by chemical or electrochemical methods, although it contains manganese and vanadium components which are usually effective for lithium intercalation For the manganese component, this may be because the manganese ions in LiMnV0, are not in a high valence state Moreover, in spite of the vanadium ions possessing a high valence state, it is likely that vanadium ions in a rigd tetrahedral coordi- nation cannot take part in lithium intercalation In fact, the V5+ ions in LiCuVO, are not responsible for the chemical lithium intercalation, which is performed via the reduction of only Cu2+ ions l9 Conclusions The crystal structure of LiMnVO, was reexamined by Rietveld analysis on the basis of powder neutron diffraction data The results obtained are summarized as follows (1) The structure is closely analogous to those of CrVO, and LiCuVO, The atomic arrangement is related to that of a spinel structure The manganese atoms occupy the nearly regular octahedral sites which are constructed by cubic close-packed arrays of oxygen atoms, while the lithium and vanadium atoms are located in a pair of edge-sharing tetrahedral sites with a displacement ratio of 14% in these sites (2) From the magnetic susceptibility data and the valence-bond sum calculations, the valence states of the manganese and vanadium atoms are found to be divalent and pentavalent, respectively (3)LiMnVO, exhibits neither lithium intercalation nor demter- calation reactions, probably because it has divalent manganese atoms which are non-active for lithium intercalation, and ngid VO, tetrahedra in spite of the presence of high-valent vanadium atoms We are grateful to Dr Y Tsuchiya, Niigata University, for making available the magnetic susceptibility measurement facilities, to Mr K Uematsu, Niigata University, for his help with the chemical analysis of the samples, and also to Mr K Nemoto, IMR, Tohoku University, for his assistance with the neutron diffraction experiments References 1 I Faul and J Knight, Chem Znd, 1989,24,820 2 F S Galasso, in Structure and Properties of Inorganic Solids, Pergamon, Oxford, 1970, ch 8 3 W I F David, M M Thackeray, L A De Picciotto and J B Goodenough, J Solid State Chem ,1987,67,316 4 B Zachau-Chnstiansen, K West, T Jacobsen and S Atlung, Solid State Zonics, 1990,40/41, 580 5 T Ohzuku, J Kato, K Sawai and T Hirai, J Electrochem SOC, 1991,138,2556 D6 M M Thackeray, A De Kock, M H ROSSOUW, Liles, R Bittihn and D Hoge, J Electrochem Soc ,1992,139,363 7 C R Walk, in Lithium Batteries, ed J-P Gabano, Academic Press, London, 1983, p 265 8 S Panero, M Pasquali and G Pistoia, J Electrochem Soc , 1983, 130,1225 9 M Sat0 and S Kano, Chem Lett, 1994,427 10 M J Isasi, R Saez-Puche, M L Veiga, C Pic0 and A Jerez, Muter Res Bull, 1988,23, 595 11 R Kanno, Y Kawamoto, Y Takeda, M Hasegawa, 0 Yamamoto and N Kinomura, J Solid State Chem ,1992,96,397 12 F Izumi, in The Rietveld Method, ed R A Young, Oxford University Press, Oxford, 1993, ch 13 13 Natl Bur Stand (US) Monogr 25,1984,21,78 14 L A De Picciotto and M M Thackeray, Muter Res Bull, 1986, 21,583 15 International Tables for Crystallography, ed A J C Wilson, Kluwer Academic Publishers, Dordrecht, 1992, vol C, p 384 16 R J Hill and C J Howard, J Appl Crystallogr ,1987,20,467 17 R D Shannon, Acta Crystallogr Sect A, 1976,32,751 18 I D Brown and D Altermatt, Acta Crystallogr Sect B 1985, 41,244 19 R Kanno, K Kawamoto, Y Takeda, M Hasegawa and 0 Yamamoto, Solid State Ionics, 1990,40/41, 576 Paper 5/07904G, Received 4th December, 1995 1194 J Muter Chem, 1996, 6(7), 1191-1194