J O U R N A L O F C H E M I S T R Y Materials Crystal structure of the mixed conductors phases, Li0.5–3xLa0.5+x+yTi1–3yM3yO3 (M=Mn, Cr) with x=0.133 and y=0.20 Manuel Morales,a Lourdes Mestres,a Maja Dlouha�,b Stanislav Vratislavb and Maria-Luisa Martý�nez-Sarrio�n*a aDept. Inorganic Chemistry, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain. *E-mail: mluisa@kripto.qui.ub.es bDept.of Solid State Engineering, Czech Technical University in Prague, Bøehova� 7, 115 19 Prague 1, Czech Republic Received 23rd June 1998, Accepted 14th September 1998 Perovskite-like solid solutions of general formula Li0.5–3xLa0.5+x+yTi1–3yM3yO3 (M=Mn, Cr) show three polymorphs; A, b and C. The crystal structure of the C polymorphs in manganese– and chromium–lanthanum systems, determined from powder neutron diVraction using Rietveld refinement, are of the ordered perovskite type.The structures of both phases are similar, containing a three dimensional framework of corner-sharing MO6 (M=Ti or Mn/Cr) octahedra in which the structures are partially collapsed as a result of a cooperative tilting and rotation of octahedra. Orthorhombic unit cell, M=Mn: a=5.5411(11) A° , b=7.8120(14) A° , c=5.4924(10) A° ; M=Cr: a=5.5014(9) A° , b=7.7735(15) A° , c=5.4729(9) A° ; space group Pnma (no. 62). Ionic conductivity takes place by a hopping mechanism between Li+-occupied and empty A-sites, while electronic conductivity is along octahedra. The family of Li+ ion-conductors with perovskite-like structure of general formula Li0.5–3xRE0.5+xTiO3 (RE=La, Pr, Nd, Sm) has been extensively studied because of their high Li-ion conductivity.The maximum bulk ionic conductivity is found in the lanthanum system with a value of 1.1×10-3 S cm-1 for x=0.07.1–4 A few years ago, phase diagrams of La, Pr and Nd systems were reported,5,6 which show two polymorphs in the lanthanum system, labelled A and b, and three polymorphs, A, b and C, in the praseodymium and neodymium systems.All of these have a perovskite-related structure. Polymorph A is a simple cubic perovskite, while b is a tetragonal perovskite with ao=bo#ac and co#2ac, and C is an orthorhombic distortion of A with ao=Ó2ac+d, bo#2ac and co=Ó2ac-d, where ac is the parameter of a cubic perovskite. Recently, the study of titanium substitution by manganese and chromium in lanthanum and praseodymium systems have led to compounds of general formula RE0.5+x+y- Li0.5–3xTi1–3yMn3yO3 (RE=La, Pr and M=Mn, Cr)7,8 with large regions of perovskite-like solid solutions.These compounds showed both electronic and Li-ion conductivity. The ionic conductivity was similar in manganese and chromium systems while the electronic conductivity was much higher in the manganese system.Polymorph C appeared for the first time in lanthanum systems when titanium was substituted by manganese or chromium. The aim of the present work was to obtain the crystal structure of this polymorph in lanthanum systems. Experimental Samples were prepared in 10 g quantities from La2O3 (99.9% Fluka), TiO2 (Aldrich 99+%), MnO2 (>99% Fluka) or Cr2O3 (>99% Fluka) and Li2CO3 (Aldrich >99%). La2O3 and TiO2 were dried overnight at 900 °C prior to weighing.These chemicals were weighed, mixed in an agate mortar with acetone, dried and heated at 650 °C for 2 h to drive oV CO2. Fig. 1 Phase diagrams along joins: a) La0.538Li0.25TiO3–LaMnO3 and b) La0.538Li0.25TiO3–LaCrO3. After grinding, samples were pressed into pellets and covered J.Mater. Chem., 1998, 8, 2691–2694 2691Table 1 Crystallographic data for La0.833Li0.10Ti0.40Mn0.60O3 a Atom Site x/a y/b z/c Uiso/10-2 A° 2 Occupancy La 4c 0.0067(12) 0.25 0.9968(14) 1.2(5) 0.832(5) Li 4c 0.388(4) 0.25 0.439(5) 2.1(6) 0.11(4) Ti 4b 0.5 0.0 0.0 1.1(5) 0.395(5) Mn 4b 0.5 0.0 0.0 1.1(5) 0.605(5) O(1) 4c 0.5156(9) 0.25 0.0134(8) — 1.00 O(2) 8d 0.2541(6) 0.0413(6) 0.7374(8) — 1.00 Anisotropic temperature factors/10-2 A° 2 Atom U11 U22 U33 U12 U13 U23 O(1) 9.8(5) 0.8(9) 3.2(8) 0.00 -5.1(9) 0.00 O(2) 0.6(7) 1.4(6) 3.6(8) -0.25(8) -0.24(7) -2.1(7) Bond lengths and angles La coordination M coordination La–O(1) 2.710(7) M–O(1) ×2 1.956(5) La–O(1) 2.604(7) M–O(2) ×2 1.945(4) La–O(2) ×2 2.542(7) M–O(2) ×2 1.987(4) La–O(2) ×2 2.913(8) La–O(2) ×2 2.616(8) O(1)–M–O(1) 180 La–Li 0.699(1) O(1)–M–O(2) ×2 80.1(8) Li coordination O(1)–M–O(2) ×2 99.9(8) Li–O(1) 2.442(8) O(1)–M–O(2) ×2 95.9(8) Li–O(1) 2.079(7) O(1)–M–O(2) ×2 84.1(7) Li–O(2) ×2 2.426(7) O(2)–M–O(2) ×2 180 O(2)–M–O(2) ×2 89.0(7) O(1)–Li–O(1) 114.1(8) O(2)–M–O(2) ×2 90.9(7) O(1)–Li–O(2) 67.2(8) O(1)–Li–O(2) 137.3(9) O(2)–Li–O(1) 84.4(9) aSpace group: Pnma (no. 62); ao=5.5411(11) A° , bo=7.8120(14) A° , co=5.4924(10) A° ; Rwp=8.13%, Rp=6.04%.Fig. 2 Observed, calculated (upper curve) and diVerence ( lower curve) Fig. 3 Polymorph C structure of La0.833Li0.10Ti0.40Mn0.60O3; neutron profiles for a) La0.833Li0.10Ti0.40Mn0.60O3 and b) octahedra, MO6 (M=Ti/Mn); black balls, La; and gray balls, Li. La0.833Li0.10Ti0.40Cr0.60O3. 2692 J. Mater. Chem., 1998, 8, 2691–2694Table 2 Crystallographic data for La0.833Li0.10Ti0.40Cr0.60O3 a Atom Site x/a y/b z/c Uiso/10-2A° 2 Occupancy La 4c 0.0067(16) 0.25 0.9910(12) 0.63(10) 0.830(5) Li 4c 0.355(8) 0.25 0.433(7) 1.8(8) 0.10(4) Ti 4b 0.5 0.0 0.0 0.88(17) 0.402(5) Cr 4b 0.5 0.0 0.0 0.88(17) 0.588(5) O(1) 4c 0.5043(8) 0.25 0.0124(8) — 1.00 O(2) 8d 0.2593(7) 0.0378(6) 0.7424(8) — 1.00 Anisotropic temperature factors/10-2 A° 2 Atom U11 U22 U33 U12 U13 U23 O(1) 8.9(9) 1.0(5) 3.2(7) 0.00 -1.2(8) 0.00 O(2) 1.2(7) 1.4(4) 2.3(6) -0.46(5) 0.72(7) -1.8(8) Bond lengths and angles La coordination M coordination La–O(1) 2.588(6) M–O(1) ×2 1.946(5) La–O(1) 2.815(6) M–O(2) ×2 1.942(4) La–O(2) ×2 2.555(6) M–O(2) ×2 1.984(4) La–O(2) ×2 2.883(8) La–O(2) ×2 2.625(8) O(1)–M–O(1) 180 La–Li 0.866(11) O(1)–M–O(2) ×2 81.2(9) Li coordination O(1)–M–O(2) ×2 98.8(9) Li–O(1) 2.447(7) O(1)–M–O(2) ×2 95.7(9) Li–O(1) 2.001(6) O(1)–M–O(2) ×2 84.3(8) Li–O(2) ×2 2.444(6) O(2)–M–O(2) ×2 180 O(2)–M–O(2) ×2 89.1(8) O(1)–Li–O(1) 120.5(8) O(2)–M–O(2) ×2 90.8(8) O(1)–Li–O(2) 70.8(8) O(1)–Li–O(2) 137.4(9) O(2)–Li–O(1) 84.9(9) aSpace group: Pnma (no. 62); ao=5.5014(9) A° , bo=7.7735(15) A° , co=5.4729(9) A° ; Rwp=7.32%, Rp=5.18%.with powder of the same composition to avoid loss of lithium Samples of composition La0.833Li0.10Ti0.40M0.60O3 (M=Cr or Mn) were synthesized and annealed at 1000 °C and were during thermal treatment. The pellets were fired at 1100 °C for 8 h giving green products which were reground, repelleted and studied by powder neutron diVraction, since they were clearly located in the polymorph C region for this temperature and fired at 1200 and 1250 °C for 12 h.Phase purity was checked by X-ray powder diVraction using composition. a Siemens D-500 diVractometer. Stoichiometries were obtained by ICP with a JOVIN IVON apparatus. Crystal structure Phase diagram studies vs. temperature were carried out for The structures were refined initially using the parameters of joins La0.583Li0.25TiO3–LaMO3 where M=Cr or Mn. Small the polymorph C in the system Pr0.5+xLi0.5–3xTiO310 with La pelleted samples were wrapped in platinum foil envelopes, and Li placed on the larger A sites of the perovskite structure placed in a furnace and annealed isothermally for 15 min in (ABO3) and Ti and Mn or Cr on the octahedral B sites.order to reach equilibrium.Finally they were droliquid Occupancies for La/Li and Ti/Mn or Cr were constrained at nitrogen to quench the phase. Samples for neutron diVraction the values 0.83/0.10 and 0.40/0.60 respectively, which were were pressed in several pellets, annealed at 1000 °C for 15 min obtained by ICP. In the first stage, lithium coordinates were and quenched in liquid nitrogen. Powder neutron diVraction fixed and lanthanum, titanium and manganese or chromium data were collected on the KSN-2 diVractometer located at coordinates and temperature factors were refined.Finally the LVR-15 research reactor near Prague. The crystal struclithium coordinates, isotropic thermal parameters and occu- tures were refined by the Rietveld method with the program pancies were refined.GSAS,9 using data collected at l=0.1362 nm, between 10 and Final refined atomic coordinates, temperature factors, bond 85° in 2h and taking into consideration the absorption correcangles and lengths for La0.833Li0.10Ti0.40Mn0.60O3 and tion for the natural mixture of the lithium isotopes. La0.833Li0.10Ti0.40Cr0.60O3 are given in Tables 1 and 2, respectively, with fitted neutron diVraction profiles for Results manganese– and chromium–lanthanum compounds [Fig. 2(a) and 2(b)].Phase diagrams The structure (Fig. 3) contains a three dimensional framework of corner-sharing MO6 (M=Ti or Mn/Cr) octahedra in Polymorphs A, b and C were identified along both joins [Fig. 1(a) and (b)]. Polymorphs A and C only form at high which the structure is partially collapsed in all three directions as a result of a cooperative tilting and rotating of octahedra. temperatures (T800 °C), although they can be preserved at room temperature by quenching.Polymorph C spreads over Although polymorphs C in the manganese– and chromium– lanthanum systems show similar features to those in praseo- a large region for y0.133 while polymorph b extends along the whole range of composition at low temperatures.dymium and neodymium systems, there are some diVerences. J. Mater. Chem., 1998, 8, 2691–2694 2693For instance, rare earth elements are placed close to the Acknowledgements theoretical A-site in all of these systems, however, Pr and This work was partially sponsored by financial support from Nd10,11 are clearly displaced from this site by 0.098 and CICYT MAT95–0218 and from 1997SGR 00265 and from 0.141 A° , respectively, which allows them to adopt a distorted GAE`R 202/97/K038.eight-coordination for RE–O, while La is closer to theoretical A-site with distances of 0.041 and 0.061 A° for manganese and chromium systems, giving a distorted 12-coordination with References La–O distances in the range 2.555–2.883 A° in the lanthanum– 1 M.Itoh, Y. Inaguma, W. Jung, L. Chen and T. Nakamura, Solid chromium system. This behaviour could be associated to rare State Ionics, 1994, 70/71, 203. earth size, since it is more diYcult for a large element to move 2 H. Kawai and J. Kuwano, J. Electrochem. Soc., 1994, 141, L78. oV the cell centre. 3 Y. Inaguma, L. Chen, M. Itoh and T.Nakamura, Solid State On the other hand, Li+ in manganese– and chromium– Ionics, 1994, 70/71, 196. lanthanum systems is clearly oV-centre with a displacement of 4 Y. Inaguma and M. Itoh, Solid State Ionics, 1996, 86–88, 257. 5 A. D. Robertson, S. Garcý�a Martý�n, A. Coats and A. R. West, 0.7053 and 0.8382 A° , respectively, cf. 0.5260 A° in the praseo- J. Mater. Chem., 1995, 5, 1405.dymium system. This displacement allows lithium to adopt a 6 M.Morales and A. R. West, Solid State Ionics, 1996, 84, 33. distorted tetrahedral coordination. 7 I.Moreno, M. Morales and M. L. Martý�nez Sarrio� n, J. Solid State MO6 (M=Ti or Cr/Mn) octahedra are more distorted than Chem., 1998, 140, in the press. in Pr and Nd systems with M–O distances between 1.945 and 8 M. Morales and M. L. Martý�nez Sarrio� n, J. Mater. Chem., 1998, 1.987 A° in the manganese system and 1.942–1.984 A° in the 8, 1583. 9 A. C. Larson, R. B. Von Dreele, GSAS Generalized Crystal chromium system. The distance M–O(1) is slightly larger in Structure Analysis System, Los Alamos National Laboratory, Los the manganese than in the chromium system probably due to Alamos, New Mexico, 1994. electronic eVects. 10 J. M. S. Skakle, G. C. Mather, M. Morales, R. I. Smith and A. R. These phases show both Li+-ion and electronic conductivity. West, J. Mater. Chem., 1995, 5, 1807. From the structure, it is presumed that ionic conductivity 11 R. I. Smith, J. M. S. Skakle, G. C. Mather, M. Morales and A. R. takes place by a hopping mechanism among Li+-occupied West, Mater. Sci. Forum, 1996, 228–231, 701. and empty A-sites, while electronic conductivity is along octahedra. Paper 8/04764B 2694 J. Mater. Chem., 1998, 8, 269