:Philips Research RfP~rts

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1 VOL: 24 No. 1 FEBRUARY 1969 :Philips Research RfPrts EDITED BY THE RESEARCH LABORATORY OF N.V. PHILlPS' GLOEILAMPENFABRIEKEN, EINDHOVEN, NETHERLANDS R 683 Philips Res. Repts 24, 1-14, 1969 SEMICONDUCTING PROPERTIES OF LANTHANUM-COBALT OXI0E by G. H. JONKER *) Abstract The semiconducting properties of p- and /I-doped LaCo03 samples have been reinvestigated. For the analysis of the measurements use is made of curves representing the Seebeck coefficient as a function of the resistivity of these samples. The temperature coefficient of the conductivity is explained as being due to the variation of both the number and the mobility of the charge carriers. The activation energy of the mobility is probably caused by "spin-state trapping". 1.: Introduction The compound lanthanum-cobalt oxide LaCo03 has been the subject of many crystallographic 1.2), magnetic 3-8) and semiconductor 3.5.9) investigations. At room temperature it has a rhombohedrally distorted perovskite structure (space group R3c) with the following lattice parameters: a = A, Cl( = 90 41', V = A3, with 8 molecules La.co03 per unit cell. At 937 C a first-order transition to a pseudocubic structure occurs 2). The ground state of Co3+ is the diamagnetic low-spin state. The energy of this state differs very little from that of the paramagnetic high-spin state 8) so that at room temperature a mixture of these two states is present. It has been found that mixed crystals of LaCo0 3 with SrCo0 3 show a weak ferromagnetism 3). At room temperature pure LaCo0 3 has a resistivity of about 3 Q cm, which points to a low value of the forbidden energy gap Eg. The possibility of investigating the normal semiconducting properties of LaCo0 3 is severely restricted by the fact that above room temperature an anomalously fast increase of the conductivity occurs 5). This has been explained as being due to the influence of the high concentrations of charge carriers formed at this temperature on the value of Eg. This value goes rapidly to zero, leading to metallic or pseudo metallic conduction near 1000 "K, At low temperature another limit caused by impurity conduction is to be expected. From conductivity and thermoelectric measurements on Th (n)- and "') Now at Technical University Twente, Enschede, The Netherlands.,.

2 2 G. H. JONKER Sr (p)-doped LaCo03 it has been -concluded that at room temperature Eg is 0 35 ev and that the ratio between the hole and electron mobilities #+/IL is 10 to 15 9 ). These conclusions were based on the assumptions that at room temperature (a) the donor and acceptor centres were completely ionized, (b) the conduction was of the small-polaron type, so that the densities of states in the two bands could be taken equal to the atomic density and (c) the transport coefficients could be neglected. The presence of small polarons was clearly affirmed by optical investigations 10). Since new experiments have shown that there is no hopping of charge carriers in simple oxides like NiO, CoO and Fe203 and that a better description of the semiconducting properties can be given using a band picture with high densities of states 11) it seemed to us of interest to reconsider the properties of LaCo03. In this paper we report on the semiconducting properties of a large series of LaCo0 3 samples doped with foreign ions which replace either the La or the Co ion. Further it was of interest to know whether the electrical properties are influenced by the rhombohedral deformation of the crystal structure. Therefore, we also prepared a number of samples from mixed crystals of LaCo03 and YCo03 having an orthorhombic distortion ofthe lattice 8). For the combined analysis ofresistivity and Seebeckeffect measurements a new method is used which is especially applicable in the case of semiconductors with a smaii forbidden energy gap 12). 2. Experimental part 2.1. Preparation Lanthanum-cobalt oxide was prepared using normal ceramic techniques consisting of milling weighed amounts of La203 and CoC0 3, prefiring the mixture at 900 C for 15 hours, milling the product and firing isostatically pressed rods for 10 to 20 hours in air or in oxygen at temperatures varying from 1200 to 1400 C. The samples were cooled either slowly in the furnace or quenched by shifting them rapidly to the cold end of the furnace. The La203 was 99 9 % quality, the main impurities being other rare-earth elements; CoC0 3 of 99 8 % purity was used, the main impurities being 0 1 % Ni and 0'1 % Na. After it was found that these impurities formed acceptor centres we used especially purified,coc03 *) containing less than 10 ppm Na and Ni. Solid solutions of (1 - x) LaCo03 - x YCo03 (x = 0-0'10) were prepared. by the same method. X-ray analysis showed that samples with x < 0 06 were rhombohedral and with x > 0 06 orthorhombic in structure. The composition La. 9YO'l Co03 was chosen for further experiments. It is known that small amounts of Sr and Th substituted for La act as P: *) Thanks are due to Dr W. Kwestroo of this Iaboratory, who purified the CoC0 3

3 SEMICONDUCTING "' PROPERTIES OF LANTHANUM-COBALT OXIDE 3 and n-type dopes, respectively 9). In addition we investigated a number of small ions (Mg, Ti, Mn, Ni, Cu, Zn, Zr, Ce) as substitutes for Co. For most of these dopes series of samples with varying amounts were prepared by substituting the appropriate oxides or carbonates in the raw material. The densities of the samples varied between 95 and 97 % of the theoretical value of Chemical analysis As the ratio between the metal oxides was fixed by the weights of the oxides or carbonates in the initial mixtures, only the oxygen content of the samples needed examination by chemical analysis. The standard method of wet chemical analysis described in ref. 13 was applied. This method determines the amount of "chemically active oxygen", which is to in pure LaCoÛ3 As the normal -: accuracy of this method of ± 0 5 % is relative to to, the accuracy with respect to the total amount of oxygen ions in the compound is ± 0 1 %. This seems to be a good result but it corresponds at the same time to an inaccuracy.of ± 1% in the mean valency of Co, which is much too high for semiconductor investigations. Fortunately, doped samples have a natural tendency to be constant in composition so that most of the care of preparation was coneentrated on the undoped or slightly doped samples. Inhomogeneous deviations in oxygen content appear more clearly in the semiconductor properties than in chemical analysis Physical measurements Both for the resistivity and the thermoelectric measurements rods of 15 to 25 mm length were used, the thickness being adjusted in dependence of the resistance value. For resistivity measurements at lower temperatures four indiummercury electrodes were applied. These were replaced by silver electrodes for high temperatures. For the thermoelectric measurements use was again made of indium-mercury electrodes, care being taken to obtain a complete liquidlayer contact between the end faces of the samples and the silver or copper blocks ofthe apparatus. For thermoelectric measurements near room temperature one metal block was kept at a temperature of about 14 C by fast-running tap water, the other at 34 5 C by the vapour of boiling diethylether. The lowtemperature measurements were performed in an apparatus described in ref. 11. In fig. 1 the room-temperature resistivities of a few series of LaCo03 doped with different types of foreign ions are shown as a function of the dope coneentration. Roughly, for each type of dope a linear relation between resistivity and concentration is present. For Mn and Ti deviations occur both at low and at high concentrations. In table I the dope elements are divided into two groups, one leading to x-type, the other to p-typé conduction, respectively. For a number of extrinsic p- and n-type doped LaCo03 samples the resis-

4 4 G.H.JONKER 8. Cl.. 'mlr-.0 '_ -+ :;:;. I. m t-----"--j-----7''r----rf- - Dope concentrapon Fig. 1. Resistivity of LaCo03 samples at room temperature as a function of the dope concentration; 000 Sr;... Ti; x x x Mn. TABLE I type n p site La I Co I La Co Th Mn Ti Sn Zr Ce Sr Mg Ni Cu Zn tivity (! and the Seebeck coefficient a have been measured as a function of temperature. The results are shown in figs 2 and 3. In these plots the ordinates for log (! and a have been chosen in such a way that (kie) In 10 = 198 {LV;oK for a corresponds 'to one decade in (!.. Further, the measurements of a and (! at fixed' temperatures for a.iarge series of samples of LaCo03 and of Lao.gYo'lCo03 are presented in combined a-log (! plots in figs 4, 5 and 6.: Both a and (! depend on the actual chargecarrier concentrations, so that in these combined plots irregularities that are mainly due to slight chemical variations in the samples disappear. These

5 SEMICONDUCTING PROPERTIES OF LANTHANUM-COBALT OXIDE 'i Ë 105 s- o -2: )' S.!::S._ Cl.... c:.!!!,. :;:; la'... lij. 0 11) lij ct: 1.Q lij 10 3,,- // I' 1 / 10 2 I I / 10' ---' I, lj -'C lj lij 1 1Or O o la IT Fig. 2. Resistivity e and Seebeck coefficient IX as a function of the reciprocal temperature for LaCo0 3 samples doped with 0, 0 2 (curve b), 0 4 and 1'0% Sr (curve a: 0'2%, after ref. 9). chemical variations include errors, impurities and also deviations from the ideal oxygen content, but not inhomogeneities which remain as a disturbing factor. Indeed, most of the points form together smooth symmetrical curves that can be,considered as characteristics of the compounds at 298 and 200 "K, respectively.

6 6 G.H.JONKER J 'loog!t Fig. 3. Resistivity and Seebeck coefficient as a function of the reciprocal temperature for LaCo03 samples doped with 0'2, 0'5 and 1'0% Mn. 3. Analysis 3.1. The infiuence of foreign ions It is not difficult to understand that foreign ions having fixed valencies like Sr2+ and Th 4 +, substituted for La3+ are compensated byco+" or holes and by Co2+ or electrons respectively. The same holds for Mg2+ and Zr4+ ions, substituted for Co3+. The other dopes used, belonging to the first series of transition elements, need a more careful inspection. It is a peculiar property that elements with a lower atomic number act as donors (Ti, Mn) and those with a higher atomic number as acceptors (Ni, Cu, Zn). This is just the reverse ofwhat is known of common broad-band semiconductors in which the chemical bonding is preponderantly covalent. For compounds with a more ionic character, obviously a different mechanism determines the character of the dopes. Here the ionization potentials of the ions are of more importance than the atomic numbers The temperature dependence of «and e.. Ithas been mentioned in the introduction that the temperature range available for the analysis of the normal conduction properties is rather small. From the

7 SEMICONDUCfING PROPERTIES-OF LANTHANUM-COBALT OXIDE l::l... c:.!!! 600 QJ -g 4-00 c'l o VB+ / / - Á i\ /' / \./.. 0 7'... C /1 <, /11 ;!" +_... \ '" 1 B:S::::_ r-, x... ẋ" 7 9- '1, ",E la 2 10-' 100 la' -----<_ Resistivity P(.11cm) Fig. 4. IX -log (! plot for LaCo0 3 at 298 "K; 000 undoped samples, H after ref. 5; i: intrinsic value; p-type:., Sr n-type: x x x Ti - M.g Mn III NI measurements it can be concluded that room temperature is near the upper limit and that a temperature of about 170 C may be considered as the lower limit. In this range the slope of the IX vs lit curves of doped samples is due to variation of the concentration of one type of charge carriers only, according to IX = (kie) {In (Nln) + A}. Here N represents the density of states in the conduction band, and A the transport coefficient. On the other hand, the slope of the log e vs 1IT curves is due to the variation of the number and of the mobility of the charge carriers, according to e- 1 = ne ft. In the case of NiO of CoO it has been found that the two curves are parallel. This points to the absence of an activation energy of the mobility. In our case we see that nonparallel curves of IX and log e occur both for p- and for n-type conduction (figs 2 and 3). From the difference in slope of the IX and the log e curves a small activation energy q; = 0 05 ev is found for the p-type mobility leaving 0, = 0 07 ev for the acceptor distance EA' Such low values are un-

8 8 G. H. JONKER o !7 _,/' 0 /- -\ t> 0 / NV A C V"", -: -. V r-, I x / J - 1'", E, la la Resistivity p(.[1cm) Fig. 5. IX-log (! plot for Lao'9 YO.IC003 at 298 ok; 000 undoped samples; p-type:... Sr; /I-type: x x x Mn. certain of course, as also impurity conduction would lead o a deviation in slope 11). The presence of impurity conduction is obvious below 150 "K in the Cl: - T curves of fig. 2. Further both the curves of Cl: and of log e point to a beginning of saturation of the charge-carrier concentration near room temperature. As the anomalous intrinsic conduction starts at room temperature as well, the real value of the saturated extrinsic conductivity cannot- be determined. We may assume, however, that the value of the conductivity at room temperature corresponds quite well to the value of the saturated conductivity. This leads to a value of the hole mobility f-t+ of 0 3 cm2fv s. The n-type samples show amore pronounced difference in slope between the Cl: and the log e curves. Here we calculate activation energies q: for the electron mobility f-t- varying between 0'12 and 0 13 ev and very small dissociation energies varying between 0 for Mn-doped and 0,02-0,05 ev for Ti-doped samples, respectively. As saturated conduction isreally present in these samples at room temperature, the electron mobility f-t- can be calculated again assuming complete equality between donor and electron concentration. This gives a value 0,,-

9 SEMICONDUCTING PROPERTIES OF LANTHANUM-COBALT OXIDE 9 o "'-n // V i\.// \ / / I -: \ 1/ \ /-,B;/ A' C' / k' -, // -. <, VB+/B'+ B-"'-.., -,<, B'- <, "+<,, '" / x <, To x I + / / ' ID' Resistivity p(l1cm) Fig. 6. IX - log e plot for LaCo0 3 at 200 "K (the curve of 298 "K is given for comparison); i: intrinsicvalue; 000 undoped samples, H after ref. 5; p-type:... Sr; 1I-type: x x x Ti, Mn. of 0 03 cm 2 fy S for IL, in agreement with ref. 9. The combination leads to a value of b = fh+ffh- = 10. As this value too' is inaccurate we shall consider in the following sections besides this value also a higher value, e.g. ') = Analysis of the 0: - log ç plots In ref. 11 it has been shown that a semiconductor can be characterized at a certain temperature by a pear-shaped 0: vs log e curve, representing samples with p-type, n-type and mixed conduction. The three 0: - log e plots presented in figs 4, 5 and 6 indeed show for the greater part the expected symmetric shape. This was not so in the beginning of our experiments, when we found deviations near the extreme values of 0:,' especially near the positive maximum, where too low values of 0: were measured. The origin of this effect was probably a slight surface oxidation during cooling of the samples in the furnace. This causes parallel conduction in a thin layer, which.affects both the apparent 0: and e. The «-Iog e combination of such a layer can be represented on the 0: - log e curve by a point that is shifted towards the p side with respect to the point representing the bulk properties of the sample. In parts of the 0: - log e curve that are straight, these incorrect samples stiii give points lying nearly on the

10 10. G. H. JQNKER ideal curve. This holds for the extrinsic regions and fortunately also for the region around the maximum of log ç. The 'deviations occur in parts of high curvature, near the extreme values of IX. Samples of stoichiometrie composition show the highest sensitivity to oxidation or reduction so that for LaCo03 the deviations are maximal near the positive maximum of IX. Further it is clear that at lower temperatures stronger deviations occur and therefore the analysis of the low-temperature curve is less certain. Another uncertainty about the correctness of the analysis is caused by im- purity conduction. The presence of this effect is obvious below 150 "K (fig. 2) but a rest may be present at 200 "K. This effect is stronger in doped.samples, increasing with the dope concentration. Therefore, especially the extrinsic n part of the curve of 200 "K is uncertain. This effect too has only little influence on the properties of samples with maximum resistivity. Accepting now the IX - log {}curves of figs 4, 5 and 6, the analysis goes along lines reported previously 12). The IX -log {}curves are described by k ( {}2 )1/2 k [{}max { ( {}2 )1/2}] k IX = ± -E In - 1 ± ln(bc), e {}max e {} {}max e with 1 (}max = HVb + Vb) {}I, #+ N+ e A + b=- and c=, #_ N_ e A - where Eg is the forbidden energy gap, A+ and A_ are the transport coefficients and N+ and N_ are the densities of states in the p- and n-type conduction bands. The value of E can be derived in different ways, for instance from the extreme values of IX according to k (IX+ + IX_)cxtreme = - [E - 1-In {2(E - I))]. e For LaCo03 at room temperature (25 C) (fig. 4) the value of E is 15,4, corresponding to Eg + (A+ + A_) k T = ev. The complete curve, calculated with E = 15 4 fits very well to the experimental points, especially in the region of lower dope concentrations. This curve gives (}mnx = 9 0.Q cm.. For LaO.9YO'lC003 at 25 C(fig. 5) E = 14 3 corresponding to Eg + (A+ + + A_) k T = ev. Here too {}mnx = 9 0.Q cm. This shows that there is no fundamental difference between the rhombohèdral and the orthorhombic compound. Therefore the analysis has been continued for pure LaCo03 only.

11 TABLE II Results derived directly from ct - log e plots of LaCo I I Tl = 298 ok T I 2 = K I E = Eglk T + A+ + A_ a = Egolk T + A+ + A_ Ego = 0 32 ev -PIk Eg + (A+ + A_) k T b or 0'39 ev Ego + (A+ + A_ - Plk)kT 0'37 ev A+ + A_ - PIk = 2'7 c f3max 9 0 ncm 1' ncm Ego + q; + q_ = 0 50 ev Ego = 0 32 ev (point A) d (N+ e A + e Jl.+) ncm 3' ncm I q; = 0 05 ev I (point B+) I (Ne e A - ejl._)-l (point B_) I 5' ncm I 6' ncm I q: = 0 13 ev I f. I asymmetry of IX 165 (J.VrK 230 (J.VrK 1 I -I be = N+ e A + Jl q : -q+ = 0 08 ev g N_ ea- p-: I (/.I!:ti s:: I ig I I:::

12 12 G.H.JONKER Fór this?ase a - log e plots at two temperatures are available. In table 11 the data that can be directly calculated, using the methods previously described 12), are given in columns 3 and 4. Further, column 5 gives data derived by combining the data obtained at 200 and 28? "K and column 6 data derived from column 5. For these calculations the following considerations have been used:. ".. (1) Generally a forbidden energy gap is weakly temperature-dependent according to Eg = Ego _:_f3 T.. " (2) In the product N /.l = N /-to e- q / k T the temperature variation of N /.lo has been neglected with respect to the exponential function. This is of course only permissible for q» k T. (3) For the temperature dependence of N e A be = + 1""+ N_ e A - /-tit is assumed that c is constant. The activation energy of b then is equal to the difference of q; and q_. In table III further results are shown using a value 'of 0 03 cm 2 fv s for /-t'- and b= 10 and 16, respectively, and further a few rasonable values for the sum of the transport constants. TABLE III Further results calculated for LaCo assumptions at 298 C quantity T= 298 K T= K a b = 10 2 em.x 5 2 ncm 250 ncm b = 16 et = Vb + lïvb 4 25 ncm 180 ncm f.t- = 0 03 cm 2 jv s 1 b 3' cmb= 10 N 3 t = b = 16 et e (p,+ + f.t-) 2' cm- 3 f.t- = 0'03 cm 2 jv s N_ e A - 4' cm- 3 c b= 10 N+ e A + 1' cm- 3 b= 16 N+ e A ,1 cm-3 ct A+ + A_ - {Jjk=2'7 A+ +A_ =4 =5 = ev 0 30 ev Eg 0 26 ev 0 28 ev 0 23 ev 0 26 ev, In fig. 4 the points. representing,. n-type samples'doped with high amounts of, -, '.-. Mn or Ti (> 3 %) show a strong deviation from the calculated curve. The resistivity passes through a minimum at a dope concentration of 8 %. This effect has been explained in ref. 8 where the complete solid-solution system of

13 SEMICONDUCTING PROPERTIES OF LANTHANUM-COBALT OXIDE 13 Laéo03 with LaMn03 has been described. At a ratio of 1 : 1 an ordered compound La2Co2+Mn4+06 is formed. With Ti a comparable compound La2Co2+Ti4+06 is formed. Both these compounds have a high resistivity, pointing to a much larger forbidden energy gap than has been found' for LaCo03. Both these compounds are p';type semiconductors if a small excess Co is present (Co3+). This shows that somewhere between this composition and pure LaCo03 the resistivity must show a minimum and that a change in.sign of oe has to occur. One would expect a quasi-metallic behaviour at this \ transition point where the Fermi level passes through the n-conduction level. The fact that this does not occur 'may be due to the formation of clusters of Co2+Mn4+ or C02+Ti4+, with short-range order. 4. Discussion The present results confirm in general the data given by Gerthsen and Härdtl 9). Only quantitative differences occur as a result of an improved method of analysis. Both in the rhombohedral and in the orthorhombic crystal structure LaCo03' proves to be a semiconductor with a narrow forbidden energy gap Eg of about 0 30 ev, separating two transport bands with high densities of states. The mobilities of the charge carriers are low, as is generally found in this type of semiconductors: at room temperature t-t+ 0 3 cm 2 JV s, and t-t cm2jv s. For t-t+ a weak thermal activation energy of 0 05 ev has been found. This value is rather uncertain as some influence of impurity conduction cannot be excluded with certainty. The conduction electrons, however, show clearly a high activation energy q.: of 0 13 ev. This property, which distinguishes LaCo03 from other transition-metal oxides, needs. further examination. In our opinion this property is connected to the variation in spin state of the different valency states of Co. This idea has been applied earlier in the discussion of the dipole centres in CoO doped with Li 11). In pure LaCo03 the trivalent Co ions have the diamagnetic low-spin state as a ground state. The energy difference between the ground state and the paramagnetic highspin state is only 0 01 ev 8). Thus, at higher temperatures mixtures of the two states occur which are estimated to contain at200 ok at 298 ok 25 % Co3+ h.s., 32 % Co3+ h.s. In oxides Co2+ ions are only known in the high-spin state; C04+ ions are less well known, but are most probably in the low-spin state. The exchange of electrons between two neighbour ions goes easily if the exchange occurs between equal types of energy levels (te or t 20 ) and if the new states of the ions after the exchange are equilibrium state. This condition isautomatically present for two neighbour-ions having the same spin state, but not if one of the ions is in the high-spin. state and the other in the low-spin state. Then at first sight,

14 14. G. H. JONKER one would expect an easy exchange o'f holes between C04+ l.s. and Co3+ l.s. ions and also that of electrons between C0 2 + h.s. and Co3+ h.s. ions, respectively. However; as has been discussed by Goodenough 4) and Blasse 7), such neighbour pairs do not occur, as in the perovskite structure the crystal-field splitting of the d levels at a certain site is very sensitive to the occupation of the neighbour sites. In a configuration Co4+-O-Co3+ the polarization of the oxygen ion increases the crystal field at the Co4+ site and decreases that of the Co3+ site. Therefore the Co4+ l.s. state is stabilized, but the 6 surrounding Co3+ ions obtain the high-spin state. This means at the same time a localization.of the hole. This "spin-state localization" has to be added to the normal polarization localization considered in Holstein's small-polaron theory. The opposite effect occurs at the site of a Co2+ ion where the high-spin state is stabilized and at the same time the low-spin states of the 6 surrounding Co3+ ions. The extra electrons of C0 2 + h.s. and the holes of Co4+ l.s. ions are situated on f 2g levels. The f 2y wave functions are extended in directions pointing to the next-nearest Co3+ neighbours. The majority of these Co3+ ions are in the low-spin state and this may be the origin 'of the rather high activation energy of 1"- and a rather low one of /1-+, respectively. The "spin-state localization" will have the same sensitivity to lattice vibrations as the dielectric-polarization trapping normally considered in smallpolaron theories. Therefore, it may be that the sum of these two effects is strong enough to cause a thermal activation energy in the electron and hole mobilities of LaCo0 3, which effect has not been found in the d.c. properties of other transition-metal compounds (compare De Wit 14)). Eindhoven, June 1968 REFERENCES 1) H. L. Yakel, Acta cryst. 8, 394, ) P. M. Raccah and J. B. Goodenough, Phys, Rev. 155,932, ) G. H. Jonker and J. H. van Santen, Physica 19, 120, ) J. B. Goodenough, J. Phys. Chem. Solids 6, 287, ) R. R. Heikes, R. C. Miller and R. Mázelsky, Physics 30, 1600, ) C. S. Naiman, R. Gilmore and B. D. BartoIo, J. appl. Phys. 36, 1074, )'G. Blasse, J. appl. Phys. 36, 879, ) G. H. Jonker, J. appl, Phys. 37, 1424, ) P. Gerthsen and K. H. Härdtl, Z. Naturf. 17a, 514, ) H. G. Reik, E. Kauer and P. Gerthsen, Phys. Letters 8, 29, H. G. Reik and R. Milhlstroh, Solid State Comm. 5, 105, R. Müh lst roh and H. R. Reik, Phys. Rev. 162, 703, ) A. J. Bosman and C. Crevecoeur, Phys. Rev. 144, 763, ) G. H. Jonker, Philips Res. Repts 23, 131, ) G. W. van. Oosterhout and J. Visser, Anal. chim. Acta 33,330, ) H. J. de Wit, Philips Res. Repts 23, 449, 1968.