Preparation and electrical properties of KCa, -xLaxNb3010 Daisuke Harnada," Masahiko Machida," Yoshiyuki Sugahara"" and Kazuyuki Kuroda*"*b "Departmentof Applied Chemistry, School of Science and Engineering, Waseda University, Ohkubo-3, Shinjuku-ku, Tokyo 169, Japan bKagami Memorial Laboratory for Materials Science and Technology, Waseda University, Nishi-waseda, Shinjuku-ku, Tokyo 169, Japan Electron-doped layered perovskite KCa, -,La,Nb3Ol0 was prepared from KNbO,, Nb20s, Nb, La203 and KCa2Nb3010 at 1200"C for 10 h under Ar. Essentially single-phase KCa, -xLa,Nb,Olo with plate-like morphology was obtained with 0 <x d 0.3, and lattice parameters increased with increasing x. All the La-doped products showed semiconducting behaviour. Linear (log p)-T relationships were clearly observed, but the resistivity behaviour deviates from the linear relationship at lower temperatures.A possible conduction mechanism is discussed. Recently, a series of compounds M[An-1Bn03n+l] (M is the interlayer cation, A is the cation in the layer structures, B is Nb or Nb/Ti, and n describes the thickness of the perovskite slab), whose structures are closely related to the Ruddlesden- Popper phase, have been reported.' They consist of perovskite- derived slab layers and interlayer cations, hence they can be considered as two-dimensional (2D) layered perovskites. The n value ranges from 2 to 7,lp3 and MA,Nb3Ol0 (n=3) and MANb207(n=2) are typical niobates in the series. Electrical properties of conductive 2D oxides have attracted increasing attention since the discovery of high-T, super- conducting cuprates possessing layered structure~.~ Since the structures of such superconducting cuprates are related to the perovskite struct~re,~the electrical properties of layered perovskite-related oxides with various B-site ions are of inter- est.The electrical properties of various Ruddlesden-Popper phases have been investigated extensively (B-site: Ti,6 Fe,7 V,8 Ni,9 Ru," Ir," RhI2). Furthermore, variations in electrical properties and electron-doping by induced oxygen-deficiencies have been reported.13 Niobium-containing oxides can be metallic,14 semiconduct- ing" or even superconducting,16-18 when the average valence of niobium is lower than +5.Among various structures, an electro-conductive Nb-based perovskite-related phase AXNbO3 is known (A =Sr,19-21 Ca,,, Ba,23v24 Eu ,25-31 S~-EU,,~Sr-Ca34). Thus, the electrical properties of electron- doped layered perovskite niobates are of interest. So far, d-electrons have been doped into layered perovskite niobates by intercalation of excess amounts of M ions (M =Li,35 H,36 Rb37). However, none of these reports have described the electrical properties of the d-electron-doped compounds. Furthermore, as far as we know, no studies have been reported on electron-doping by solid-solution formation. We report here the preparation of the d-electron-doped layered perovskite KCa, -xLa,Nb3010 and its electrical proper- ties in the temperature range 4-280 K.Trivalent lanthanum is selected as a foreign ion because Goparakrishnan and co- workers reported the successful substitution of La3 + for Ca2+ in K, -,Ca2 -xLa,Nb30,038 and KCa,-,La,Nb, -xTix01039 (note that the valence of niobium in these two compounds was +5 and no d-electrons were doped). The structures and electrical properties of the products are discussed. Experiment a1 Polycrystalline KCa, -xLaxNb3010 were prepared from KCa2Nb3010, KNb03, Nb205, Nb and La203 by solid-state reactions according to eqn. (1): x/2 KNb03+2/5x Nb205+ 1/5x Nb +x/2 La203+ (1-x/2) KCa2Nb3010 -+KCa,-,La,Nb,O,, (1) KCa2Nb3010 was synthesized by the solid-state reaction of stoichiometric amounts of K,C03, CaCO, and Nb205 at 1100"C for 20 h in air.KNb0, was obtained by the solid- state reaction of stoichiometric amounts of K2C03 and Nb205 at 1000°C for 1h in air. La203 was dehydrated by heating at 21000"C for more than 1 h before use. KCa,Nb,O,,, KNbO,, Nb205, Nb and La203 were thoroughly mixed and pressed into a button pellet at 59 MPa. The pellet in an alumina boat was placed in an alumina tube with Ti powder (to remove oxygen). After evacuation (cu. 8.5 x lo-, Pa), the alumina tube was filled with Ar. Then the pellet was heated at 1200°C for 10 h under an Ar atmosphere at a heating and cooling rate of 5 "C min-'. Crystalline phases were identified by a Mac Science MXP3 diffractometer (monochromated Cu-Ka radiation). Lattice par- ameters were refined by the non-linear least-squares method from the positions of 21 peaks that were reasonably indexed.The amounts of potassium, calcium, lanthanum and niobium were determined by inductively-coupled plasma emission spec- troscopy (ICP; Nippon Jarrell Ash ICAP-575 11). The mor- phology of the products was studied with a scanning electron microscope (SEM) equipped with an electron probe micro- analyser (JEOL, JXA-8600). The resistivity measurements were performed using the standard four-probe method in the range 4-280 K. Results and Discussion Table 1 summarizes the compositional analysis results. The compositions of the products are essentially consistent with the corresponding nominal ones. The loss of potassium is possible, and therefore an excess amount of potassium was used for the preparation of the potassium-containing layered perov~kites.l*~~*~'In the present study, however, no obvious potassium loss was observed.We used an excess amount (10%) of potassium, but no effect on the phase purity was observed. X-Ray diffraction (XRD) patterns of the products are shown in Fig. 1. When Odxd0.3, the products are essentially single phase, but very trace amounts of LaNbO, and/or an unknown phase are also detected. If 0.4<x, LaNbO, is obviously detected, and its amount increases with increasing x. The structure of KCa,Nb301, is reported to be pseudotetragonal J. Muter. Chern., 1996, 6(l), 69-72 Table 1 Compositions of the products determined by ICP sample, x= composition"Yb 0 0.1 0.2 0.3 0.4 0.5 0.6 a Compositions were normalized by setting the amount of niobium at 3.Amount of oxygen was set at 10. I I x=o.a x =0.7 X =0.6 x =0.5 X =0.4 X =0.3 x =0.2 x =0.1 KCazNIJS010 10 20 30 40 50 60 28/degrees Fig. 1 Powder XRD patterns of KCaz-,La,Nb3Ol0. a, LaNbO,; V,unknown phase. orthorhombic,' and XRD patterns reveal that the symmetry is maintained in all the products. Uma and Goparakrishnan prepared the solid-solution K, -$a2 -xLa,Nb3010 and reported the appearance of new peaks and marked changes of peak intensity ratios.38 The almost complete retention of the XRD profile in the present system may be ascribed to the presence of a constant amount of potassium ions in the interlayer space.The peaks ascribed to the KCa,Nb3010 structure shift to lower angles as x increases. The variation in lattice parameters with x is demonstrated in Fig. 2. All the three lattice parameters increase with increasing x,and seem to level off with larger x. The substitution of La3+ (ionic radius 0.136nm) for Ca2+ (ionic radius 0.134 nm) should be accompanied by the simul- taneous reduction of the same amount of niobium [NbSf (ionic radius 0.064 nm)+Nb4+ (ionic radius 0.068 nrn)];,, hence, both substitution and reduction should cause the increase in the lattice parameters. Since no obvious composi- tional changes occur during the preparation, the observed 0.392 2.98 2.97 0.39 increase in the lattice parameters indicates solid-solution for- mation.The behaviour of the LaNbO, peaks and that of the lattice parameters allows us to assume that the x limit for solid-solution formation is between 0.3 and 0.4 under the present experimental conditions. In the K, -xCa,-,La,Nb3010 system, a similar lattice expan- sion was reported with increasing x, and the phenomenon was ascribed to the replacement of Ca2+ with La3+.38 The increments of the lattice parameters from x=O to x=OS in the two systems are as follows: for this study, Aa =2.5( 1)pm, Ab=2.4(9) pm, Ac= 13.(3) pm; for the K,-,Ca2-xLa,Nb30,0 system, Aa =3.5( 2) pm, Ab =4.6(2) pm, Ac =19.(0) pm. Thus, although the effect of the reduction of niobium (Nb5+ +Nb4+) was absent in the K, -,Ca, -xLa,Nb30,0 system, the increments of the lattice parameters are larger in the K, -xCa2-xLa,Nb,0,0 system.The morphology of the products is shown in Fig. 3. For x= 0.1, the products consist of plate-like particles (ca.2 pm x 2 pm) only [Fig. 3(a)], and electron probe microanalysis (EPMA) indicated a homogeneous distribution of elements (K, Ca, La, Nb); these observations are consistent with the formation of essentially single-phase products with x <0.3. In contrast, if x =0.8, granular particles [Fig. 3(b), indicated by arrow] are observed besides the plate-like products. EPMA showed that the granular products contain mainly La and Nb. Taking the XRD results into account, the granular products appear to correspond to LaNbO,, the evident impurity at x 20.4. KCa2Nb3OIo was a white insulator, and La doping resulted in blue products.The blue colour darkened with increasing x. The resistivity at room temperature decreased drastically as x increased from ca. 105-106 (x=O.l) to ca. 103-104 (x=0.4), and further decreased to ca. lo2 mi2 m (x=0.6). The observed values are 10-1000 times larger than the reported values of Nal-,Sr,Nb03,33 if the same amount of Nb4+ is present in the KCa, -xLa,Nb,O,o and Na, -,Sr,NbO, units. 0*3ss2m94 0'384 0 0.1 0.2 0.3 X 0.4 0.5 0.6 2.93 -10pm Fig. 2 Variation in lattice parameters with x. 0,a; 0,b; 4, c. Fig. 3 SE micrographs of the products with (a) x =0.1 and (b) x =0.8 70 J. Mater. Chem., 1996, 6(l), 69-72 100 t 1 80 120 160 200 240 280 TIK Fig. 4 Temperature dependence of normalized resistivity of the samples (0.1 <x <0.6) in the higher temperature region.x =0.1 (O),0.2 (0), 0.3 (A),0.4 (A),0.5 (O), 0.6 (0). All the La-containing products exhibited a semiconducting temperature-dependence of their electrical properties (Fig. 4). Instead of the Arrhenius-type relationship for a thermally activated hopping conduction mechanism, however, a linear relationship is observed between the logarithm of the resistivity, log p, and the temperature, T,over a large temperature range. The slope in Fig. 4 decreases with increasing x, but becomes constant at 0.4<x, which is in agreement with the range of solid-solution formation estimated from the XRD results. Other layered transition metal oxides such as A,Nb, + 3,@3, +3m,43 Ruddlesden-Popper phases6-13 and phos- phate tungsten bron~es~~,~~ are metallic or semiconducting, but none of them shows a linear (log p)-T relationship.On the other hand, similar resistivity behaviour was reported for some mixed-valence oxides such as Fe30446 and Na, -,Sr,Nb03,33 and a possible mechanism for such behav- iour has been proposed based on the incoherent tunnelling of electrons between the nearest-neighbouring sites with an oscil- lating barrier (small polaron model with a vibrating barrier).47 If a simple harmonic oscillator model (with frequency o)is applied, the slope in Fig. 4 corresponds to 2a2kB/rno2, where the parameter a is related to the expanse of the wavefunction [kB, Boltzmann's constant; rn, mass of involved ions (niobium) (= 1.5 x kg)].From Fig. 4, w/a values can be calculated, and by assuming a constant frequency o for all the products (o=lo1, s-'), l/avalues (so-called site localization parameters) can be estimated (Table 2). The decrease in slope in Fig. 4 thus corresponds to the increase in l/.. The o/a values are lower than those calculated similarly for the Na, -,Sr,NbO, system [263 (x=0.2)-514 (x=0.6)],,, but are consistent with the reported values for various oxides and chalcogenide~.~~ The resistivities of the products with relatively higher con- ductivities (0.3 <x) are measured down to 4 K (Fig. 5). The resistivity shows typical semiconducting behaviour at lower temperature, and linear relationship is not observed below cu.20 (x=0.6)-80 K (x=0.3). H~rd~~ proposed that the transfer rate R (which dominates the conductivity) in the vibrating Table 2 Parameters related to the temperature dependence of resis-tivity in the high-temperature region 0.1 91 0.091 0.2 105 0.105 0.3 123 0.123 0.4 169 0.169 0.5 174 0.174 0.6 174 0.174 w= 1x 10l2 s-l throughout. Fig. 5 Temperature dependence of resistivity of the samples (0.3 G x G0.6) in the lower temperature region. x =0.3 (0),0.4 (O), 0.5 (0), 0.6 (A). barrier model is expressed as: R zC x exp (-2aSo) exp (-U/k,T) exp (2a2kBT/rno2) where So is the equilibrium width of the barrier, U is the activation energy for the displacement of sites and C is a constant.The term exp(-2aSo) (the overlap term) is nearly independent of the temperature, thus the other two exponential terms dominate the temperature dependence. The coincidence term, exp (-U/kBT), reflects the thermally activated prob- ability for coincidence and the tunnelling term, exp(2a2kBT/rno2), is due to the oscillation of the barrier (as discussed above). At high temperatures where U<<kBT,the transfer rate is subjected to the tunnelling term to give a linear (log p)-T relationship. At lower temperatures, the coincidence term also contributes to the transfer rate, and the (log p)-T relationship deviates from linearity. Similar behaviour was reported for other mixed-valence oxides of Ti,O,, -149 and vno,, -50 1* Conclusions We have demonstrated the preparation, structure and electrical properties of KCa, -xLa,Nb,Olo.With 0 <x <0.3, essentially single-phase KCa, ~xLa,Nb,Olo is prepared from KNbO,, Nb205, Nb, La203 and KCa2Nb3OI0 by solid-state reactions at 1200°C for 10 h under Ar. No obvious loss of elements is observed. Lattice parameters increase with x,indicating solid- solution formation. All the La-doped products show semicond- ucting behaviour. Linear (log p)-T relationships are observed in a relatively high temperature range, but log p did not follow a linear relationship at lower temperatures. A possible expla- nation for the resistivity behaviour is proposed based on the vibrating barrier model which gives the linear (log p)-T relationship by the incoherent tunnelling of electrons between the nearest-neighbouring sites.These results indicates that d-electrons can be doped in the layered perovskite structures by solid-solution formation, and the electrical properties reflect the mixed valency of niobium in KCa, -xLa,Nb30,0. References 1 M. Dion, M. Ganne and M. Tournoux, Rev. Chim. Miner., 1986, 23, 61. 2 A. J. Jacobson, J. W. Johnson and J. T. Lewandowski, Inorg. Chem., 1985, 24, 3727; A. J. Jacobson, J. T. Lewandowski and J. W. Johnson, J. Less Common Met., 1986,116, 137. 3 R. A. Mohan Ram and A. Clearfield, J. Solid State Chem., 1991, 94,45. 4 J. G. Bednorz and K. A. Muller, Z. Phys. B, 1986,64,189. 5 C. N. R. Rao and B. Raveau, Acc. Chem. Res., 1989,22,106.J. Muter. Chern., 1996,6( l), 69-72 6 7 8 S. Hayami, H. Yamamoto, Y. Sugahara and K. Kuroda, in preparation. Y. Takeda, K. Imayoshi, N. Imanishi, 0. Yamamoto and M. Takano, J. Mater. Chem., 1994,4, 19. F. Deslandes, A. I. Nazzal and J. B. Torrance, Physica C, 1991, 28 29 30 K. Ishikawa, G. Adachi, M. Tanida and J. Shiokawa, Bull. Chem. SOC. Jpn., 1981,54, 159. K. Ishikawa, G. Adachi, M. Hasegawa, K. Sato and J. Shiokawa, J. Electrochem. SOC., 1981,128, 1374. K. Ishikawa, G. Adachi and J. Shiokawa, Bull. Chem. SOC.Jpn., 179, 85. 1982,553317. 9 Y. Takeda, R. Kanno, M. Sakano, 0. Yamamoto, M. Takano, 31 K. Ishikawa, G. Adachi and J. Shiokawa, Mater. Res. Bull., 1983, Y. Bando, H. Akinaga, K. Takita and J. B. Goodenough, Mater. 18,653. 10 Res.Bull., 1990,25,293. Y. Maeno, H. Hashimoto, K. Yoshida, S. Nishizaki, T. Fujita, 32 B. Ellis, J. P. Doumerc, M. Pouchard and P. Hagenmuller, Mater. Res. Bull., 1984,19, 1237. 11 J. G. Bednorz and F. Lichtenberg, Nature, 1994,372,532. R. J. Cava, B. Batlogg, K. Kiyono, H. Takagi, J. J. Krajewski, 33 B. Ellis, J. P. Doumerc, M. Pouchard and P. Hagenmuller, Solid State Commun., 1984,51,913. W. F. Peck, Jr., L. W. Rupp, Jr. and C. H. Chen, Phys. Rev. B, 34 K. Isawa, R. Itti, J. Sugiyama, N. Koshizuka and H. Yamauchi, 12 1994,49,11890. T. Shimura, M. Itoh, Y. Inaguma and T. Nakamura, Phys. Rev. B, 35 Phys. Rev. B, 1993,48,7618. R. Jones and W. R. McKinnon, Solid State ionics, 1991,45, 173. 1994,49,5591. 36 P. Gomez-Romero, M. R. Palakin, N.Casaii and A. Fuertes, Solid 13 I. S. Kim, M. Itoh and T. Nakamura, J. Solid State Chem., 1992, State Ionics, 1993,63,424. 14 15 101, 77. R. J. Cava, B. Batlogg, J. J. Krajewski, P. Gammel, H. F. Poulsen, W. F. Peck, Jr., and L. W. Rupp, Jr., Nature, 1991,350,598. R. J. Cava, B. Batlogg, J. J. Krajewski, H. F. Poulsen, P. Gammel, W. F. Peck, Jr., and L. W. Rupp, Jr., Phys. Rev. B, 1991,44,6973. 37 38 39 A. R. Armstrong and P. A. Anderson, Inorg. Chem., 1994,33,4366. S. Uma and J. Goparakrishnan, J. Solid State Chem., 1993, 102, 332. J. Goparakrishnan, S. Uma and V. Bhat, Chem. Mater., 1993, 5, 132. 16 17 18 19 20 21 22 23 24 25 26 M. J. Geselbracht, T. J. Richardson and A. M. Stacy, Nature, 1990, 345, 324. M. A. Rzeznik, M. J. Geselbracht, M.S. Thompson and A. M. Stacy, Angew. Chem., int. Ed. Engl., 1993,32,254. J. Akimitsu, J. Amano, H. Sawa, 0. Nagase, K. Gyoda and M. Kogai, Jpn. J. Appl. Phys. Part 2, 1991,30, L1155. D. Ridgley and R. Ward, J. Am. Chem. SOC., 1955,77,6132. B. Hessen, S. A. Sunshine, T. Siegrist and R. Jimenez, Mater. Res. Bull., 1991,26, 85. K. Isawa, J. Sugiyama, K. Matsuura, A. Nozaki and H. Yamauchi, Phys. Rev. B, 1993,47,2849. M. Hervieu, F. Studer and B. Raveau, J. Solid State Chem., 1977, 22,273. R. R. Kreiser and R. Ward, J. Solid State Chem., 1970,1, 368. M. T. Casais, J. A. Alonso, I. Rasines and M. A. Hidalgo, Mater. Res. Bull., 1995,30,201. G. J. McCarthy and J. E. Greedan, Inorg. Chem., 1975,14,772. J. P. Fayolle, F. Studer, G. Desgardin and B. Raveau, J. Solid State Chem., 1975,13,57. 40 41 42 43 44 45 46 47 48 49 50 J. Goparakrishnan, V. Bhat and B. Raveau, Mater. Res. Bull., 1987, 22,413. J. Goparakrishnan and V. Bhat, Inorg. Chem., 1987,26,4299. R. D. Shannon, Acta Crystallogr., Sect. A, 1976,32,751. J. Kohler, G. Svensson and A. Simon, Angew. Chem., Int. Ed. Engl., 1992,31,1437. J. P. Giroult, M. Goreaud, Ph. Labbe, J. Provost and B. Raveau, Mater. Res. Bull., 1981,16,811. E. Canadell, I. E-I. Rachidi, E. Wang, M. Greenblatt and M-H. Whangbo, Inorg. Chem., 1989,28,2455. A. J. M. Kuipers and V. A. M. Brabers, Phys. Rev. B, 1979,20,594. W. R. McKinnon, C. M. Hurd and I. Shiozaki, J. Phys. C, 1981,14, L877; I. Shiozaki, C. M. Hurd, S. P. McAlister, W. R. McKinnon and P. Strobel, J. Phys. C, 1981,14,4641. C. M. Hurd, J. Phys. C, 1985,18,6487. A. D. Inglis, Y. Le Page, P. Strobel and C. M. Hurd, J. Phys. C, 1983,16, 317. A. D. Inglis, C. M. Hurd and P. Strobel, J. Phys. C, 1984,17,6801. 27 F. Studer, G. Allais and B. Raveau, J. Phys. Chem. Solids, 1980, 41, 1187. Paper 5105248C; Received 7th August, 1995 72 J. Mater. Chem., 1996, 6(l),69-72