Characterisation and properties of the non-stoichiometric perovskite, Ca,Fe, -,Nb,O, (0.45< x < 0.65) Jose A. Chavez,"+ Terence C. Gibbband Anthony R. West" "University of Aberdeen, Department of Chemistry, Meston Walk, Aberdeen, UK AB24 3UE bUniversity of Leeds, Department of Chemistry, Leeds, UK LS2 9JT The cubic perovskite phase Ca2Fe2-,Nb,0, (0.45< x < 0.65) has been prepared by solid-state reaction at 1400 "C followed by rapid cooling to room temperature. On slower cooling, or heating below 1360 "C, it decomposes to a mixture of Ca2Fe205 and Ca2FeNb06, but the quenched single-phase material is kinetically stable up to at least 950 "C. The oxygen content of composition x =0.60, determined by thermogravimetry, can be varied over the range 5.55 <y < 5.89 by heat treatments under a range of conditions, including H2-Ar at 700 "C to obtain low y, and high-pressure oxygen, 420 bar at 950 "C, for high y.The main oxidation state of Fe, as determined by Mossbauer spectroscopy was Fe3+, but about 13% Fe4+ was present in the as-prepared sample, increasing to 42% under high-pressure 02.For most y values, the crystal structure was that of a cubic perovskite with Fe/Nb disorder on the B sites and some oxygen vacancies; at high y contents, an orthorhombic distortion gave a structure resembling that of Ca2FeNb06. Electrical properties range from semiconducting at high y consistent with Fe3+ /Fe4+ exchange, to insulating at low y; although there was no direct evidence of oxide ion conduction over the range 25-300 "C, the rapid adjustment of oxygen content y on changing the atmosphere at 700-900 "C indicated significant mobility of oxide ions.The phase is therefore likely to be a mixed oxide ion-electron conductor. The perovskite structure shows great diversity in its stoichi- ometry with a wide range of substitutions possible on the A and B cation sites, together with the possibility of both cation and anion vacancies. The brownmillerite structure of Ca2Fe205 can be regarded as derived from perovskite by ordering of oxide ion vacancies, resulting in a tetrahedral coordination for some Fe atoms. The possibility of high levels of oxide ion conduction in certain anion-deficient perovskites remains an intriguing possibility, especially for applications as fuel cell cathodes and catalysts where mixed oxide ion-electron conduc- tion may be desirable.Following the report of an extensive range of cubic, perovskite-like solid solutions in Nb-doped Ca2Fe205,' we have investigated their stoichiometry, structure and electrical properties. Our results are considerably different to those reported in ref. 1; two solid solution phases form, one based on orthorhombic Ca2FeNb06 and the other on a cubic perovskite structure stable over a more limited composition range than given in ref. 1. These latter results are reported here. Experimental Samples were prepared by solid-state reaction of CaCO,, Fe203 and Nb205. Reagents were dried, weighed out in the required proportions, mixed with an agate mortar and pestle and fired in Pt boats at 90O-135O0C, with a final firing at 1400°C for 20 h of the reground samples.Products were characterised by X-ray powder diffraction, Stoe STADI P diffractometer, Cu-Ka, radiation. Oxygen contents and vari- ations in oxygen content were determined by thermogravimetry (TG) in various atmospheres. Samples were also heated under O2 at pressures up to 420 bar and temperatures up to 950°C in a Morris HPS-5015E7 high-pressure furnace in order to attempt a modification of their oxygen content. The oxidation states of the Fe in some samples were estimated by Mossbauer spectroscopy. Data were collected at room temperature using a 57Co/Rh source matrix and isomer shifts were calibrated relative to metallic Fe.Electrical conductivity was determined t Permanent address: National University of Mexico, UNAM, IIM, Apdo Postal 70-360 Mexico, D.F. by ac impedance measurements on sintered pellets with elec- trodes fabricated from either Au or In/Ga in a two-terminal arrangement. The frequency range 30 mHz-10 MHz was scanned using a combination of Solartron 1250/1286 and Hewlett-Packard 4192 instrumentation; data were processed using in-house software. Results and Discussion Synthesis of Ca,Fe, -,Nb,O, Attempts to investigate the possible doping of Ca2Fe,05 with Nb involved a preliminary investigation of the relevant region of the CaO-Fe,O,-Nb,O, phase diagram and then focused on a study of the system Ca2Fe205-Ca2Nb207. A detailed phase-diagram study2 was made which indicated the occur- rence, stoichiometry range and thermodynamic stability of a cubic, perovskite-like phase.Its formula may be represented as Ca2Fe2-,Nb,0,. It forms, and is stable, only above ca. 1360"C, although it is readily preserved to room temperature by removing from the furnace at e.g. 1400°C and cooling in air over a period of a few minutes. This quenched, single-phase material is kinetically stable up to at least 950°C. At higher temperatures, e.g. 1200-13OO0C, or on slow cooling from ca. 14OO0C, Ca2Fe2-,Nb,0, decomposes to give a mixture of Ca,Fe205 and Ca2FeNb06 .2 Ca2Fe2- ,Nb,O, is a solid-solu- tion phase with composition 0.45 <x < 0.65; it appears not to have an ideal composition within this range.A variety of characterisation studies on composition x =0.60 were made and are reported here. Determination of oxygen content, y At the outset, the oxygen content of Ca2Fe2-,Nb,0, was not known; if the Fe were present exclusively as Fe3+, then the oxygen content would be y = 5 + x. If, however, some Fe2+ or Fe4+ formed during synthesis, then the oxygen content would differ. Two methods were used to assess oxygen content and the oxidation state of Fe: TG and Mossbauer spectroscopy. Agreement was generally excellent; unless otherwise noted, the quoted y values are based on TG results. J. Mater. Chem., 1996, 6(12), 1957-1961 1957 Miissbauer spectroscopy. Mossbauer spectra are shown in Fig 1 for (a)the related compound Ca,FeNbO,, which can be used as an Fe3+ standard and (b)as-prepared Ca,Fe, 4Nbo ,OY (z e x=O 6) Also shown, (c), is a spectrum of x=O 6 after treatment in high-pressure O2 (discussed later) Fitting param- eters for the Mossbauer spectra are listed in Table 1 The presence of a disordered mixture of cations in a perovsk- ite can have a major effect on the observed 57Fe Mossbauer spectrum3 This factor is more important than any small deviation from cubic symmetry in the (averaged) unit cell Thus, in SrLaFeSnO, where all the iron is present as Fe3+ (an S-state ion with no intrinsic quadrupole effect), the disordered Sr,La and Fe,Sn cation charges generate a substantial electric- field gradient at the Fe which is manifested as an apparent (if somewhat broadened) quadrupole doublet Similar behaviour has also been found in the series A2FeMO6 (A=Ca, Sr, Ba, M =Nb, Ta) where only the M,Fe cations are di~ordered,~ in the equivalent M =Sb compounds where partial Fe,Sb order is found, there is clear evidence for multiple Fe sites The larger nominal charge difference for Fe3+ /Nb5+ results in much larger apparent splittings in A2FeMO6 than for Fe3+ /Sn4+ in Sr LaFeSnO, The spectrum of Ca2FeNb0, [Fig l(a)] is similar to that of CazFe, 4Nbo 60y[Fig 1(b)],but the spectrum of the latter has an asymmetry at low velocity which is clear evidence that some of the iron is in a higher oxidation state than 3+ The curve-fit shown in Fig l(b) is not perfect as the component lines are clearly compound because of the disorder and deviate from the normal Lorentwan profile which has been assumed throughout, however, the use of a more complex treatment is not warranted in the absence of an appropriate model Thus, IIIIILI I -2 0 2 velocity/mm s-l Fig.1 Mossbauer spectra of (a) Ca2FeNb06, (b) Ca2Fel 4Nbo 60568 and (c) Ca2Fe1 4Nb0 6O5 89 Table 1 Mossbauer fittmg parameters" compound d/mm s-' d/mm s-' r/mm s-l % area Ca2FeNb06 +0385(1) 0606(2) 0451(3) 100 Ca2Fe, 4Nb0 605 68 +O 386(2) 0 647(3) 0 543(5) 87 5 -0342(5) 0216(8) 0300(15) 125 Ca2Fe, 4Nbo 605 89 +o 365(5) 0 283(5) 0 384( 1) 55 3 -0 007(4) 0 207(5) 0 327(1) 44 7 "y values calculated from TG data 1958 J Muter Chem, 1996, 6(12), 1957-1961 the standard deviations quoted in Table 1 for the isomer shift, 6, quadrupole splitting, A, and linewidth, I-', ignore a potentially greater systematic error The relative area of the Fe3+ component is, however, well defined by the asymmetry and is comparatively insensitive to the model adopted The similarity of the parameters for the Fe3+ component in Ca,FeNbO, and Ca,Fe, ,Nbo ,OY suggest that the Fe and Nb cations are similarly disordered in the two structures If the high oxidation state Fe component corre- sponds to Fe4+ (rather than Fe5+ which is also known in perovskites) then the peak area ratio associated with Fe3+ and the additional peak in Fig l(b) is 87 13 This then leads to a calculated oxygen content at x =0 6 of y =5 691 Thermogravimetry. TG studies of the same as-prepared material were carned out in an atmosphere of flowing Ar, the sample was heated to 900 "C, held at that temperature for 2 h and cooled, these conditions were chosen since studies5 on other Fe-containing materials had shown reduction of Fe4+ to occur under similar conditions The results (Fig 2) showed a significant mass loss at 400-600 "C, with a constant mass thereafter and no significant change, apart from a slight baseline drrft, on cooling Assuming that all the Fe4+ had been reduced to Fe3+ during TG, the oxygen content of the original sample was calculated to be y= 5 684, with an Fe3+ /Fe4+ ratio of 88 12, in good agreement with the Mossbauer value This provides confirmation that the second oxidation state is indeed Fe4+ Variation of oxygen content These TG results indicated that the oxygen content of the perovskite phase is capable of significant variation Two sets of expenments were therefore carned out to try and extend further the range of oxygen contents For both, the same batch of starting matenal was used as that for which the oxygen content had been determined, above The TG-determined value of the oxygen content, y= 5 684, was taken as the starting oxygen content In one set of expenments, samples were heated under high- pressure oxygen, the design and mode of operation of the Morns furnace was such that the maximum pressure was attained at the highest temperature in a particular heat-cool cycle, on subsequent cooling, at 1-2 "C min-', the pressure inside the sealed vessel gradually reduced unless the vessel was deliberately opened to the atmosphere (not in these experi- ments) The oxygen content of the products was determined by TG in Ar to 900°C as above and, for one sample, by Mossbauer spectroscopy Results are summarised in Table 2 for two heat treatments At the highest pressure used, 420 bar, the value obtained for the oxygen content was similar for the two techniques The Mossbauer spectrum [Fig l(c)] is clearly rather different to that of the starting material [Fig l(b)] Again, the model used for fitting may not be an entirely ., . . . , . . . , . . . , . . . 5-70 I -5.68 -0.1 : 0 -5.66 h. 0 E-0.2: 0 Y E: .5.64 0 -0.3 0 -0 5.62 Fig.2 Mass as a function of temperature for Ca2Fe, 4Nbo 60yin flowing Ar 0,heating, A,cooling Table 2 Data for Ca2Fe,,4Nbo,60, treated in high-pressure 0, annealingconditions a/A y from TG Fe3+/Fe4' 950°C, 2 h, 3.8452 5.891 58/42 P,,, z 420 bar 5.915" 600°C, 2 h, 3.8476 5.768 76/24 P,,, z 45 bar "Calculated from Mossbauer spectroscopy. accurate description of the phase overall, but the asymmetry is well defined. As well as a substantial increase in oxidation of Fe to the +4 oxidation state, there is a noticeable change in the isomer shift and quadrupole splitting parameters. Thus, the isomer shift of the Fe4+ component is much more positive whereas that of Fe3 has decreased. One possible explanation + is that an electron-hopping process is taking place on a timescale slightly longer than the Mossbauer timescale of lo-' s so that each Fe environment is partially averaged.The decrease in the average quadrupole splitting could then be due merely to the reduction in charge discrepancy between the B-site cations. Conductivity data that may support this notion are discussed later. The isomer shift for the component at lower velocity confirms the existence of a higher oxidation state. Although the associated quadrupole splitting is in prin- ciple anomalously small for both the high-spin and low-spin states of Fe4+, both of which might be predicted by analogy with high-spin Fe2 to show a large valence-electron contri- + bution, it should be remarked that such an effect has not been observed in any of the perovskite oxides studied to date.The reason for this remains unknown. In the second set of experiments, TG studies were carried out in a range of atmospheres and the change in mass or oxygen content monitored directly. In each case, samples were heated at 10°C min-' to 700-9OO0C, held isothermally for 2 h, then cooled. The starting material was again taken to have the composition x=O.6, y=5.684. On heating in air [Fig. 3(a)] a significant mass loss occurred over the tempera- ture range 400-600 "C, which was partially recovered on cooling, to give a final oxygen content at room temperature of y= 5.667, corresponding to a residual Fe4+ content of 9.6%. On comparing Fig. 3(a) with Fig.2, a similar pattern of behaviour is seen on heating but in Ar (Fig. 2) the mass loss occurs rather more rapidly and levels off, at a lower tempera- ture, at 7 =5.60; in air [Fig. 3(a)] the oxygen content at 900 "C appears to be slightly greater than y=5.60, indicating a residual amount (1-2%) of Fe4+. Under flowing oxygen [Fig. 3(b)] the sample gained mass over the range 400-500°C and then lost most of this mass at higher temperature. The latter loss was recovered on cooling, to give a final oxygen content at room temperature of 5.808, corresponding to 30.0% Fe4+. Under strongly reducing conditions [5%H2-95%N2, Fig. 3(c)] a continuous mass loss was observed over the range 25-65OoC, with most of the mass loss occurring between 400 and 600°C.No further mass change occurred on cooling, giving a final oxygen content of 5.549. To account for this, the Fe is assumed to be present as an Fe3+/Fe2+ mixture, in the ratio 93 :7. The above results demonstrate a significant variation in oxygen content, depending on heat treatment, associated with a variation in oxidation state of Fe. They also demonstrate that this variation is largely confined to low temperatures. At the temperature of synthesis, 1400 "C, extrapolation of the TG results indicates that Fe4+ is most unlikely to be present, even in an atmosphere of pure 02.On cooling in air, rapid uptake of oxygen, with oxidation of some Fe3+ to Fe4+, occurs below ca. 800°C and the final oxygen content will depend on the cooling rate and the atmosphere.0.1 : (a) 5.70 000 0 0 0 0 0 oo~o~ i 0 7 5.68 AAA. Ao-0.1 ; A0 15.66 A0 -0.2 A0 15.64A0 A0 -0.3 A0 PO 5.62......15.60 -0.4 0 200 400 600 800 lo00 5.84 A A A A A A A 0.6 1 0 0 5.80 Obo 0 0.4 A0 5.76 Y A0d 0 0 00.2 so 15.72 0 4 O' 15.68 L..... .......................... I 0 100 200 300 400 500 600 700 800 ................................ 5.70 0 (') 15.66 0 0 1 0 15.62-0.4 I Fig.3 Mass as a function of temperature for Ca2Fel,4Nbo.60, in flowing atmospheres of (a) air, (b) oxygen and (c) H2-N2 (5:95) mixture: 0,heating: A, cooling Crystallographic data X-Ray powder diffraction data were indexed on a simple, primitive cubic unit cell for most oxygen contents, although at high y, 35.8, a distortion to orthorhombic symmetry occurs.Values for the cubic cell parameter us. composition are shown in Fig. 4; an approximately linear decrease in a with increasing oxygen content occurs, consistent with a gradual increase in bond strength and lattice energy on progressive oxidation of Fe. Clearly, there is interstitial space available for the extra oxygen and the unit cell contracts when oxygen enters these sites due to the increase in Fe-0 bond strength and shortening 3.880 3.875 3.870 H-NZO Ar(8995)3.865 E air23.860 1 3.855 4 3.850 0 HOP* 3.845 0 3.8401, I.............................,I 5.4 5.5 5.6 5.7 5.8 5.9 6.0 Y Fig. 4 Cubic cell parameter a us.oxygen content y for Ca,Fe, 4Nb0.60y.*,pseudo-cubic parameter from averaged orthorhombic parameters. J. Muter. Chem., 1996, 6(12), 1957-1961 1959 0: of the Fe-0 bonds associated with the increased oxidation state of Fe Rietveld refinement was performed on XRD data for one composition, x =0 6, y =5 68, using the basic perovskite struc- ture as the starting model Fe and Nb were placed on the B sites with overall full occupancy Oxygen occupancies and oxygen temperature factors could not be refined independently, the oxygen occupancy was therefore fixed at the value given by the formula and the temperature factor refined A satisfac-tory refinement was obtained [Table 3, Fig 51, confirming the essential correctness of the cubic perovskite structure model Electrical properties The electncal properties of Ca2Fe, 4Nbo 60rwere evaluated using ac impedance measurements A typical impedance data set at 200 "C is shown in Fig 6 for y =5 549 The impedance complex plane plot [Fig 6(a)] shows a broad arc which extrapolates to, or close to, the ongin The low-frequency intercept on the 2' axis gives the total resistance of the sample On replotting the same data in the 2" and M" spectroscopic plot format [Fig 6 (b)],' it is clear that the M" spectrum is a double peak, the low-frequency component occurs at about the same frequency as the 2" peak whereas much of the higher frequency peak is beyond the available frequency range The data were fitted to the circuit shown in [Fig 6(c)], with values for the component parameters also listed, the quality of fit is shown by the solid curves in Fig 6(a), (b) The low-frequency peak in the M" spectrum, and the main peak in the 2" spectrum, dominates the total resistance of the sample but its capacitance is somewhat larger than that of the less resistive, high-frequency M" peak (since M" peak heights are inversely proportional to capacitance') Since both capacitance values are small, a few pF, we can conclude that the sample is electrically heterogeneous with two components, 1 and 2, in the approximate proportions, given by the ratio of their reciprocal capacitances, of 1 143 Since the resistances of the two regions are in the ratio 137 1, their resistivities are in the approximate ratio 20 1 We do not know the origin of this heterogeneity but it may be associated with a variation in oxygen content through the sample, especially as this particular sample prepared by H2-reduction was not subjected to pro- longed low-temperature annealing to ensure homogeneity Table 3 Atomic parameters for Ca,Fe, 4Nbo 605691 (e s d s in parentheses) atom occupancy x/a y/b zfc UV Ca 10 05 05 05 0046(2) Fe 07 0 0 0 0 0151(8) Nb 03 0 0 0 0 0151(8) 0 0 9467 0 0 05 0088(6) 80! -20t 1 10 20 30 40 50 60 70 80 90 100 110 120 2Bldeglees Fig.5 (a) X-Ray diffraction pattern for Ca,Fe, ,NbO 60y,synthesised in air at 1400°C for 20 h, (b) difference between observed and calculated profiles after Rietveld refinement 1960 J Mater Chem, 1996,6(12), 1957-1961 0 04 2 00 05 10 15 20 0 10-2 1oo 102 lo4 lo6 lo* frequency/Hz R1 R2-F7+T c1 Cl R1= 1 504 x106 R R2 = 1 096 xlOs R C1= 8 138 xlO-I2 F C2 = 5 701 xlO-"F A =4 415 x lo4 S A1 = 2 515 x10-" s B1= 2 948 x10" S Bl = 3 630 x10-" s nl = 0 3748 n2 = 0 6142 Fig.6 (a) Impedance complex plane plot and (b)M ,Z spectroscopic plot for the H,-reduced sample at 200 "C, (c) equivalent clrcuit used to fit data and parameters extracted for this data set Plots such as Fig 6 are typical of all samples studied here Resistances values were extracted from the low-frequency intercepts of the complex plane plots, on the 2' axis, and thus represent the total resistances of the samples These data are shown in Arrhenius conductivity format in Fig 7 and Table 4 A large variation in conductivities, depending on oxygen content, is seen The data fall approximately into two groups In the more oxidised samples, the Arrhenius plots are almost -1 0 15 20 25 30 35 1000 KIT Fig.7 Arrhenius plots for CazFe, ,Nbo 60y,the same sample was used throughout, with annealing treatments in the sequence (1)-(4) (1) as-prepared sample, 1400°C in air, (2) 420 bar 02,95OoC, (3) H2-N2 (5 95), 700°C, (4) air, 700°C 0/@,heating, A/V,cooling Table 4 Conductivity Arrhenius parameters activation energ y/eV annealing conditions heating cooling as-synthesised, air, 1400 "C, 20 h oxygen pressure, 950 "C, 2 h, P,,, HZ-NZ, 700 "C, 2 h z420 bar 0.26 0.28 1.37 0.27 0.29 0.38 air, 700 "C, 2 h 0.36 0.38 parallel, with activation energies in the range 0.26-0.38 eV; differences in conductivities are therefore attributed largely to differences in mobile carrier concentration, which is highest in the samples treated in high-pressure oxygen.For the reduced sample, the conductivity was many of orders of magnitude lower; in fact, it was too low to be measured (< S cm-l) below ca. 140"C, and had a much higher acti- vation energy. After heating in air to ca. 350°C during the impedance measurements, the data were not reproduced on subsequent cooling but came instead into the category of behaviour of the more oxidised samples; it appears likely that partial reoxidation of the reduced sample had occurred during the impedance measurements, consistent with the TG results [Fig.3(b)] which indicate that oxidation can occur above ca. 340 "C at a heating rate of 10 "C min-l. The impedance complex plane plots [Fig. 6(u)] showed no evidence of low-frequency polarisation effects, such as an 'electrode spike' which could have been attributed to double- layer effects associated with ionic transport. This, coupled with the observation that the activation energies for conduction of the more oxidised samples are quite low, indicates that the principal current carrier is electronic rather than ionic. The plots also showed no evidence for grain-boundary impedances, which would have had significantly higher capacitance values than those observed, in the pF range.The impedance data do, therefore, correspond to the bulk of the sample. The conductivity data (Fig. 7) may be interpreted as follows. The material is an electronic semiconductor. Its conductivity depends very much on the presence of Fe4+ ions which gives rise to a mixed 3 +/4 + valence state and allows the hopping of electrons (or holes) between adjacent Fe atoms in the perovskite structure. With increasing oxygen content, the Fe4+ concentration rises, as does the level of conductivity, which is therefore p-type. The conductivity appears to approach its limiting value in the sample prepared in high-pressure oxygen. Thus, the conductivities from data sets (1) and (2) differ by a factor of only about 3, as do the Fe4+ contents, and in the high-pressure-treated sample, nearly half the Fe ions are tetra- valent.The conductivity may well start to decrease if a further increase in Fe4+ content could be induced, since there would be less opportunity for Fe4+ /Fe3+ exchange to occur. In the H,-treated sample, the conductivity behaviour is totally different; the level of conductivity is very low and the activation energy is high. No positive holes (Fe4+ ions) are present; instead, the residual conductivity may be n-type owing to the small amount of Fe2+ present. In spite of the fact that the samples appear to equilibrate with the atmosphere and modify their oxygen contents rapidly, especially at ca. 400-600°C, there was no evidence for signifi- cant levels of oxide ion conduction in the temperature range 25 to ca. 300 "C.This is probably because the level of electronic conduction, to lo-, S cm-I at 25 "C, is several orders of magnitude higher than that which could reasonably be expected for oxide ion conduction. This situation may change at high temperatures (2400 "C) however, since the activation energy for oxide ion conduction is likely to be in the range 0.8-1.5 eV, several times larger than that for the electronic conduction measured here and hence, with increasing tempera- ture, the transport number of oxide ions should increase. J. A. C. thanks UNAM, Mexico for a scholarship. The high- pressure furnace was provided by EPSRC. We thank A. Herod for assistance with the Mossbauer measurements. References 1 I. J. Moraes, M. C. Terrile, 0. R. Nascimento, M. S. Li, R. H. P. Francisco and J. R. Lechat, Muter. Res. Bull, 1992,27, 523. 2 J. A. Chavez, PhD Thesis, Aberdeen University, 1995. 3 T. C. Gibb, J. Muter. Chem., 1992,2,415. 4 P. D. Battle, T. C. Gibb, A. J. Herod, S-H. Kim and P. H. Munns, J. Mater. Chem., 1995,5, 865. 5 M. A. Alario-Franco, J. M. Gonzalez-Calbet and M. Vallet-Reglo, J. Solid State Chem., 1983,49,219. 6 S. J. La Placa, J. F. Bringley, B. A. Scott and D. E. Cox, Actu Crystallogr., Sect. C, 1993,49, 1415. 7 P. D. Battle, T. C. Gibb and S. Nixon, J. Solid State Chem., 1989, 79, 75. 8 J. T. S. Irvine, D. C. Sinclair and A. R. West, Adu. Muter., 1990, 2, 132. Paper 6/04653C;Received 3rd July, 1996 J. Muter. Chern., 1996,6( 12), 1957-1961 1961