摘要:

Precipitation of finely divided A1,0, powders by a molten salt method Yuansheng Du and Douglas Inman Department of Materials, Imperial College, London, UK SW7 2BP Finely divided A120, powders have been prepared by reactions of Al,( SO,), or AlC1, with molten nitrates, nitrites or nitrates containing Lux-Flood bases. The reactions in different melts were compared and the possible stoichiometries of the reactions have been proposed. The powders precipitated were characterised by XRD, AAS and TEM. A120, as an important ceramic material has attracted a great deal of research interest. The traditional routes for the prep- aration of advanced A120, powders normally employ aqueous solutions such as in precipitation and sol-gel methods. As has been shown by a number of researchers,lP7 alternative low- temperature methods using molten salts have led to ultrafine ceramic powders.These researchers have reported that single- component powders, including Zr021-, and MgO,' and mul- tiple-component powders, including Y20,-Zr026 and MgO- ZI-O~,~can be precipitated from molten nitrites and nitrates below 600°C. However, there has been no report on the preparation of A120, powders from molten salts. Therefore it is necessary and interesting to carry out further research into the production of A120, powders by the molten salt method. The starting materials employed in this study were anhy- drous Al2(SO,), (BDH, LR) and AlCl, (BDH, LR). Other chemicals such as molten nitrites, nitrates and Lux-Flood bases, the experimental procedures, and the extraction and characterisation methods of the powders produced were the same as those described previ0usly.~3~ In normal furnace runs, anhydrous A12( was observed to react with the NaN0,-KNO, eutectic at temperatures above 100"C with the evolution of brown NO2.After melting of the eutectic (220 "C), the reaction became more violent with further evolution of gas and the formation of a white powder. Above about 400"C, hardly any gas formation or further precipitation of powders was observed. The TG graph [Fig. l(a)] shows three stages of mass loss: the first from ca. 100 to 210"C, before the melting of the NaN02-KN02 eutectic; the second with a very sharp peak from ca. 220 to 250 "C; and the third, which overlapped with the second peak, from ca.260 to 420°C. The overall mass loss of Al,(SO,), added (64.8 +0.3%) was close to that predicted on the basis 140~ 50 100 150 200 250 300 350 400 450 500 550 600 TI'C of the following reaction (66.6%): A1,(SO4),+6NO,-+A1203+3N02+3NO+3S042-(1) In a similar way to the case of nitrites, it was observed that anhydrous A12(S04)3 began to react with NaN03-KN03 at about 220"C, around the melting point (220°C) of the NaN03-KN03 eutectic, with the evolution of brown NO2 and the formation of a white powder. Above 400"C, the reaction became very violent and bubbled nearly to the top of the test tube. The reaction continued to temperatures above 500 "C. The TG diagram [Fig. l(b)] indicates that the reaction took place in two stages with the first at ca.22O-32O0C, and the second at 320-520°C. The overall mass loss was ca. 91.4 +0.4% of A12(SO,), added, which is close to that predicted on the basis of the following reaction (94.7%): A12(S04),+ 6N0,-+A1203 + 3s042-+ 6N02+502 (2) Similar reaction phenomena were observed for the reaction of Al,(SO,), in single-component NaNO, to those in NaN03-KN03. The reaction started at ca. 300 "C and was complete at ca. 530"C, higher than the starting (ca. 220°C) and completion (ca. 520 "C) temperatures in NaN0,-KN03. From these results, it can be seen that the reaction tempera- tures are lower in nitrites than in nitrates, and in the different nitrate systems they increase with increasing melting point.The differences of acidity of these melts are probably the reason for this., When using AlCl, as the starting material, the reaction was observed to occur at lower temperatures than when employing A12(S04), in a similar melt. For example, AlCl, began to react with NaN0,-KNO, at about 160 "C, almost 50 "C lower than when using A12(S04),, also with the evolution of brown NO, and the formation of a white powder. The TG graph of the reactions of A12(SO,), in the NaN03-KN03 system containing different Lux-Flood bases (Na20z and Na2C0,) is shown in Fig. 2. It can be seen that reaction temperatures were reduced by adding bases. This is because the Lux-Flood bases can give rise to oxide ions in the melt and thus can accelerate the reaction between A13+ and 02-to form A1203.The contributions of Na202 and Na2C03 to the reactions have been explained previ~usly.~ The powders precipitated by the reactions of A12(S04), and AlCl, with different melts under various conditions were examined by XRD. The results (Fig. 3) indicate that the ) 50 100 150 200 250 300 350 400 450 500 550 600 TI'C Fig. 1 TG diagram (differential mass loss vs. temperature) for the Fig. 2 TG diagram (differential mass loss vs. temperature) of A12(S04)3 reaction of A12(S04)3 (7.5 mol%) in the NaN02-KN02 (a) and (7.5 mol%) in NaN03-KN03 containing bases: (a)no base; (b)Na20, NaN0,-KNO, (b) systems (heating rate 5 "C min-') (22.5 mol%); (c) Na,CO, (45.0 mol%) (heating rate 5 "C min-') J. Muter. Chem., 1996, 6(7), 1239-1240 1239 4 0-(a1 0 040 ' 20 ' 40 I 60 I 80 2Bldegrees Fig.3 XRD patterns of powders precipitated by the reactions of Al,(SO,), with NaNO,-KNO, (a)and LiN0,-KN03 (b) Table 1 Crystallite sizes of Al,03 powders precipitated from different nitrate melts (450 "C; 90 min) crystallite size/nm LiN03-KN03 132 3.3 2.0 NaN03-KN03 220 3.8 2.3 NaN0, 307 4.7 2.8 "Starting matenal.precipitates were very fine or poorly crystalline A120, powders, in accord with JCPDS card 37-1462 for Al,03. From the line broadening of the XRD patterns, the crystallite sizes of the powders were estimated. The results are listed in Table 1. It is seen that the crystallite sizes of all the powders were <10 nm and the crystallite sizes produced from various nitrate melts under the same conditions (450 "C; 90 min) increased with the melting points of the melts.The XRD results also revealed that the structure of the product was the same but the crystallite sizes were smaller when employing AlCl, in place of Al,( SO4), under the same reaction conditions. The crystallite sizes of powders precipitated by the reaction of A12(S04)3 in NaN0,-KNO, (450 "C; 90 min), estimated to be ca. 1-2 nm, were even smaller than those in the nitrate melts. This is in agreement with the results discussed for the precipi- tation of ZrO,, and Mg0' powders. The XRD patterns of the powders produced by adding bases were different from those produced from the pure NaNO,-KNO, melt. There were changes in peak positions and the peaks became broader, corresponding to structural changes and smaller crystallite sizes. The very broad and overlapping XRD peaks also indicate that the Al,O, powders produced were very fine or poorly crystalline.Fig. 4 TEM image of Al,O, prepared from Al,(SO,), with NaN0,-KNO, (45 "C; 90 min) The total impurity levels of Na and K elements (measured by AAS) were all <0.5 mass% when no Na,O, was added to the melts. The addition of Na,02 to the NaNO,-KNO, melt as a Lux-Flood base increased the impurity levels of Na and K to 0.53 and 0.64 mass%, respectively. This might be because Na,O produced by the decomposition of Na202 in the melt could combine directly with Al,03. The TEM image obtained after using ultrasonic agitation and isopropyl alcohol in the final wash of the powders precipitated is shown in Fig.4, It indicates that the powders were well dispersed and that individual powders were uniform and spherical with soft agglomeration. The size of each individ- ual powder particle was as small as a few nanometers, which was close to the crystallite size measured by XRD. This seems to show that the elementary grain was nanocrystalline. References 1 D. H. Kerridge and J. Cancela Rey, J. Inorg. Nucl. Chem., 1977, 39,405. 2 H. Al-Raihani, B. Durand, F. Chassagneux, D. H. Kerridge and D. Inman, J. Muter. Chem., 1994,4, 1331. 3 Y. Du and D. Inman, J. Muter. Chem., 1995,5,1927. 4 Y. Du, P. Rogers and D. Inman, J. Muter. Sci., 1996, in press. 5 Y.Du and D. Inman, J. Muter. Sci., 1996, submitted. 6 B. Durand and M. Boubin, Muter. Sci. Forum, 1991,73-75,663. 7 Y.Du and D. Inman, Br. Ceram. Trans., 1996, in press. Communication 6/01 804A; Received 14th March, 1996 1240 J. Muter. Chem., 1996, 6(7), 1239-1240

ISSN:0959-9428    

DOI:10.1039/JM9960601239

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Modified carbothermal reduction for the synthesis of ultrafine particle tungsten compounds dispersed in a microporous carbon matrix Yusaku Sakata," Akinori Muto, Md. Azhar Uddin and Kazumasa Harino Department of Applied Chemistry, Faculty of Engineering, Okayama University, Tsushima-naka, 3-1-1, Okayama 700, Japan A selective, low-temperature carbothermal reduction (CTR) process with metal ion-exchanged resin has been proposed. The tungsten compounds prepared by this method consisted of ultrafine particles of around 10-20 nm in diameter which were dispersed highly in the porous carbon matrix. The new CTR process is applicable for any other metal elements, depending on the combination of resin type, metal ionic species and heat treatment conditions. Tungsten compounds, such as tungsten carbide, are effective catalysts for the hydrogenation of alkenic hydrocarbons and the dehydrogenation of paraffinic hydrocarbons.' Conven-tionally, tungsten carbide is prepared from a mixture of tungsten oxide and carbon powder by a carbothermal reduction (CTR) process with treatment at high temperatures between 1400 and 1600°C.Recently, the CVD method has been applied to the synthesis of tungsten ~arbide~.~and nit~-ide.~*~However, the CVD method requires not only sophis- ticated equipment and a highly reduced pressure system but also costly halogenotungstens as the tungsten source. Dal .~et ~1reported a low-temperature process for the preparation of mixed niobium and tungsten carbides. They used composites of polyacrylonitrile and tungsten oxide as the metal source.Since the metallic ions adsorbed in an organic ion exchanger are highly dispersed, we propose, in this communication, a simple and low-temperature CTR process for the preparation of microporous carbon in which various kinds of tungsten compounds are dispersed uniformly, from an ion-exchanger containing W042- as a precursor. The composite produced by this method contained ultrafine particles of tungsten com- pounds dispersed in the microporous carbon matrix. The tungsten compounds in the carbon matrix, the oxide (W18049), metal (W), carbide (a-W2C, WC) and nitride (P-WN), are all selectively produced from the same material, i.e. the W04,- type ion-exchanged resin.The experimental procedure is as follows. A commercial strong anion-exchange resin (DIAION PA3 18, spherical bead, average diameter cu. 0.4 mm; Mitsubishi Chemical Co.) was first converted to the Cl-type resin by conventional condition- ing in the glass column (i.d. 42 mm, length 500 mm), and was fully changed to the W042- type resin by feeding an aqueous Na2W04 solution (1.0 mol dm-3). After filtration, the resin was washed with deionized water, and dried at room tempera- ture for 1 day, and then at 110°C for 12 h in an electronic drying box. After cooling in a nitrogen stream, the sample was stored in a sealed bottle. At this stage, the tungsten content of the dry sample resin was 0.21 [g(W) (g dry resin)-']. The W042- type resin was separated into three batches and heat- treated in a programmable electric tube furnace (i.d. 40mm, length 300 mm) under gas streams of N2 (loo%), a mixture of H2 (50%)-N2 (50%) and a mixture of NH, (15%)-H2 (42%)-N2 (43%).The furnace was first held at 110°C to dry the sample resins, and then raised to a pre-determined maxi- mum temperature (TR) at a constant rate of 5 "C min-', and finally held at TR (400-900°C) for 3 h. During this treatment, the organic matrix of the W042- type resin (styrene-di- vinylbenzene copolymer) was thermally decomposed and car- bonized to microporous carbon below 400°C. The tungsten compounds in the carbon matrix may react with carbon and the gaseous component at higher temperatures. The tungsten compounds in the carbon spherical beads were identified by powder X-ray diffraction (XRD) analysis, the results of which are summarized in Table 1.The reported yields are the mass ratios of carbonized product to dry W042- type anion- exchange resin. The specific surface areas shown were measured by the N2 BET method. All of these values include both the carbon matrix and the metallic compounds surfaces. Tungsten carbides (a-W2C, WC) and metallic tungsten can be prepared in the carbon matrix by heat treatment at 900°C under N2 (100%) gas. This temperature is much lower than that used in the conventional method. When a mixture of H, (50%)-N2 (50%) was used, the resulting compound contained only tungsten carbides in the carbon matrix. The XRD pattern of the sample is shown in Fig. l(u).The particle diameters of a-W2C and WC, calculated using Scherrer's equation (assumed constant K= 1.0) are 11.7 and 12.1 nm, respectively. Both particles of tungsten carbides were found to be very fine. The scanning electron micrographs (SEM) of the sample are shown in Fig. 2. The sample maintained its spherical shape with many fine cracks. The distribution of W atoms by electron dispersive 20 30 40 50 60 70 80 2Bldegrees (Cu-Ka) Fig. 1 X-Ray diffraction patterns of the products of carbothermal reduction. (a) H, (50%)-N2 (50%), 900°C; (b) N, (loo%), 700°C; (c) H2 (5O%)-Nz (5O%), 700°C; (d)NH, (15%)-H2 (42%)-N, (43%), 700°C. 0,W18049; 0,W; A,WC; A, u-WZC; 0,P-WN. J. Muter. Chem., 1996, 6(7), 1241-1243 1241 Table 1 Preparation of tungsten compounds by carbothermal reduction W04' type anion-exchange resin T,/"C control' Nz (100%) Nz (100%) Hz (50%0)-N2(50%) NH, (15%) Hz (42%) N, (43%) ~~~ 400 (O 18) Lo l1 w18°49 (045) ~321 w18°49 (048) ~41 w18°49 (049) [24i w18°49 (046) c41 P-WN, w18°49 (045) c31 700 (013 cl51 w18°49 (045) [14] W (042) c701 P-WN (042) PO1 800 w18°49, (043) [04] w.-W,C W (041) [I2101 E-WZC, WC (040) [175] 900 (0 13) [2] E-W~C, WC (034) [1801 E-W~C,WC (041) [1991WC, W (039) [208] N-W~C, C1-type anion-exchange resin Figures in parentheses are yields in g [g(dry resin)] I, figures in square brackets are specific surface areas in m2 g analysis of X-rays (EDAX) analysis is displayed in Fig 3 The tungsten compound is highly dispersed in the carbon matrix The tungsten compound resulting from treatment at 700 "C was dependent on the gas atmosphere When N, gas was applied, the produced tungsten compound was only W18049 dispersed in the carbon matrix When a mixture of H, (50%)-N, (50%) was used, only metallic tungsten was obtained The XRD patterns of these products are shown in Fig l(b) and (c), respectively Fig 4 shows a transmission electron micrograph (TEM) of the former sample Layered fine straight lines suggest that tungsten oxide prepared in the carbon matrix has a high degree of crystallinity Furthermore, tungsten nitride (P-WN) was prepared in the carbon matrix by adding NH3 gas into the H,-N, mixture The XRD pattern of P-WN is shown in Fig l(d) Surprisingly, tungsten carbides (a-W,C and WC) were prepared at lower temperatures than those needed in the conventional CTR method In all cases, Fig.3 EDAX image of W atoms for the same sample as Fig 2(c) Fig. 2 SEM images of the sample prepared with a mixture of H, Fig. 4 TEM image of the sample prepared with N, at 700 "C (x 2 (50%)-Nz (50%)at 900 "C (a), x 100, (b), x 1000, (c) x 5000 100 OOO) 1242 J Muter Chem, 1996, 6(7), 1241-1243 peaks due to carbon were not observed in the XRD patterns. The carbon matrix was found to be amorphous. Table 1 shows that we can control easily the production of many kinds of tungsten compounds by altering the heating conditions (temperature and gas atmosphere). This synthesis method, i.e.carbothermal reduction using an ion-exchange resin, is very useful for preparing ultrafine particles of tungsten compounds in the carbon matrix. We have described the carbonization reaction of W042- by carbothermal reduction in Table 1. The reaction course depends on the gas mixture used in the thermal process: in N2 (100%) and the mixture of H2(%%)-N2 (50%)~WO,2- +WI8O49 +W+a-W,C, WC; in the mixture of NH3 (15%)-H2 (42%)-N2 (43%),wo42--'w18049 +P-WN+a-W,C, wc. The specific surface area is very small when w18049 is prepared. When the heating temperature is higher, and w18049 is reduced to W, P-WN and tungsten carbides, the specific surface area becomes larger, because the oxygen atoms of w&49 activate the carbon matrix. This phenomenon is very favourable for the synthesis of the tungsten carbide catalyst. We confirmed that the carbon beads containing highly dis- persed tungsten carbides prepared for this method, was cata- lytically active for the hydrogenation of ethene.' In conclusion, we present a simple synthesis procedure for various tungsten compounds at low temperatures.The advan- tages of this method are: (1) the temperature for the synthe- sis of tungsten carbide and nitride is low (700-900 "C); (2) sel- ective production of the tungsten compounds is possible by choice of both reaction temperature and gas atmosphere; (3) sophisticated equipment and a reduced pressure system are not required for this method; (4) the Na2W04 used as the tungsten source is cheaper and easier to handle than other tungsten compounds.The proposed modified carbothermal reduction method can be applied for not only tungsten as reported in this communi- cation, but for other metals such as Mo, Cu, Ni, Pt, etc., depending on the type of ion-exchange resin, temperature and gaseous atmosphere applied for carbonization/heat-treatment of the resin. Finally, we wish to thank Professor Jun Takada at Okayama University and Dr. Yoshiro Kusano at Kurashiki University of Science and the Arts for helpful discussions. References R. B. Levy and M. Boudart, Science, 1973,181,547. W. Chang-whan, C. Revankar and H. Y. Sohn, J. Mater. Res., 1993, 8,2702. G. Y. Zhao, V. V. S. Revankar and V. Hlavacek, J. Less-Common Met., 1990,163,269. G. W. Elger, D. E. Traut and G. J. Slavens, Metallurgical Trans. B, 1989, 20, 493. M. Nagai and K. Kishida, Appl. Surf. Sci., 1993,70,759. B. F. Dal, S. G. Hardin and D. G. Hay, J.Mater. Sci., 1993,28,6657. Y. Sakata, A. Muto, Md. A. Uddin and K. Harino, Reprint of the 61st Annual Meeting of the Society of Chemical Engineers, Nagoya, Japan, 1996, L-104, p. 199. Communication 6/02304E; Received 2nd April, 1996 J. Muter. Chern., 1996,6(7), 1241-1243 1243

ISSN:0959-9428    

DOI:10.1039/JM9960601241

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Effect of aluminium for manganese substitution upon the GMR properties of the praseodymium manganites Christine Martin, Antoine Maignan and Bernard Raveau Laboratoire CRISMAT, URA CNRS 1318 associde au CNRS ISMRA, Universitk de Caen, 6 Bd du Mardchal Juin, 14050 Caen Cedex, France The substitution of A1 for Mn in the GMR perovskites Pr0.7Ca0.2sr0.1MnO3 pro. 7CaO. 1sr O.ZMnO3 and Pro~,Sro~,Mn03has been studied. For the first two series of compounds that belong to the type I GMR manganites [ferromagnetic (FM)-paramagnetic (PM) transition] T, decreases significantly as the aluminium content increases, by about 12 K per percent of A1 atoms introduced into the Mn sites, whereas the maximum magnetoresistance ratio Ro/HHis not modified dramatically, reaching lo3at 71 K for 6% A1 per Mn in a magnetic field of 7 T.This increase of Ro/RHis, in fact, correlated to the decrease of T', i.e. to the increase of the semiconductive character of the samples. For the series Pro.,Sro~,Mnl -,Al,O,, which belongs to type I1 [antiferromagnetic (AFM)-FM-PM transitions], an increase of TNfrom the AFM to the FM state is observed by A1 doping: TNincreases from 136 K for x =0 to 170 K for x =0.06. In contrast, T, decreases as x increases. The manganese perovskites have been studied extensively since the discovery of giant magnetoresistance (GMR) properties in these compounds more than seven years ago.' Among these compounds, the praseodymium phases Pr, -,A,MnO, (A= Ca, Sr, Ba) are of great interest owing to their exceptional Two kinds of effects can be distinguished in these materials, according to the x value.The first (type I) effect appears generally for 0.2 <x <0.5 and corresponds to the transition from a ferromagnetic (FM) metallic state to a paramagnetic (PM) semiconducting state as T increases.' The second effect (type 11) was observed for a particular x value (x=0.5),* and is explained in terms of charge ordering. This type I1 effect corresponds to a transition from an antiferromag- netic (AFM) semiconducting state to a ferromagnetic (FM) metallic state as Tincreases. The study of the crystal chemistry of these phases has shown that two factors are of paramount importance for their GMR properties: the mean size of the interpolated cation and the hole carrier density determined by the Mn"'/Mn'' rati0.~-'9~ It was indeed shown that the transition temperature, T,, of the FM-PM transition increases as the mean size of the interpolated cation increases, whereas the transition tempera- ture TNof the AFM-FM transition increases as the mean size of the interpolated cation decreases.Such effects, which corre- spond to the modification of the overlap of the d orbitals of manganese and oxygen p orbitals, play a key role in the control of the transition temperatures of the phases and to optimize their GMR properties. In contrast, very little is known about the influence of the presence of foreign elements in the Mn sites upon the GMR properties of these compounds.For this reason, the study of the substitution of aluminium for manganese in the phase Pr0.7( Ca,Sr),,,Mn03 and Pro.5Sro.5Mn03 was undertaken. Besides the regular decrease of Tc by aluminium doping, the most important feature that is shown herein for the first time deals with the substantial increase of the transition temperature TN in the type I1 manganites Pro.5Sro.,Mn03 by aluminium doping. The samples were prepared by solid-state reaction, by mixing Pr6011, SrC03, CaO, MnO, and A1203 in stoichiometric proportions. The powders were first heated for 12 h at 9OO0C, and after regrinding they were pressed into bars (2x 2 x 10 mm3) and sintered for 12 h at 1200°C and 1500°C successively. All the compounds were tested for purity by X- ray diffraction (XRD) and electron diffraction (ED).The resistances were measured by the four-probe method on bars in earth magnetic field and in a field of 7 T; the samples were first zero-field cooled and then the magnetic field was applied. The measurements were recorded by increasing the tempera- ture from 5 to 300 K. The magnetization curves were recorded with a vibrating-sample magnetometer, after zero-field cooling down to 5 K, a magnetic field was applied (1.45 T and 100 G for the Pro.7Cao.3 -,Sr,,Mnl -,A1,03 and Pro.,SrO.,Mnl -xAl,03 series, respectively) and the temperature was increased to 280 K. The substitution of aluminium for manganese in these praseodymium perovskites influences dramatically the GMR properties, so that only substitutions at very low level, i.e.<16% of aluminium, were studied. Under these conditions, the structure of the substituted perovskite is not significantly different from the pristine one. The powder XRD patterns of all the samples evidence a single phase with the perovskite structure. The systematic ED investigation performed on more than 100 microcrystals confirms the high purity of the different samples. The two series of oxides Pro~7(Ca,Sr)o~3Mnl -,Al,03 and Pro~,Sro.,Mnl -,Al,03 exhibit different behaviour and for this reason will be examined separately. The resistance us. temperatures curves in zero magnetic field for the type I GMR perovskites Pro~7Cao~2Sro~lMnl -,Al,03 [Fig. 1(a)] and Pro~7Cao~lSro~2Mnl -xAl,03 [Fig.1 (b)] show that the introduction of small amounts of aluminium does not modify the shape of the curves, which are all characterized by a maximum at T= T,, for x<O.16. Thus the transition from a metallic to a semiconducting state, as T increases, still exists in the presence of aluminium. Nonetheless, the resistance is increased significantly by aluminium substitution, so that for the higher x values, the samples do not exhibit metallic behaviour at low temperature. However, the transition tem- perature T,,, decreases rapidly as the aluminium content increases, by about 12 K per percent of A1 atoms introduced onto the manganese sites. Indeed for both series Pr0.7Ca0.2Sr0.1Mn1 -xA1x03 and Pr0.7Ca0.1Sr0.2Mn1 -xAlxO3, TmaXdecreases linearly us.x (Table 1). The magnetization us. temperature curves in a magnetic field of 1.4T for the series Pro.,Cao~2Sro,lMnl-xAl,03 (Fig. 2) confirm that this evolution of the resistance corresponds to a FM-PM transition as T increases: the Curie temperature, T,, coincides with T,,,. The magnetic moment per Mn atom, although it decreases slightly, remains rather close to the theoretical value of 3.70~~ (Table 1). J. Muter. Chern., 1996,6(7), 1245-1248 1245 =o 12 1 X=O 08 x=O 04 1 o2 10' -.--..--+.-1 oo -. lo-' 1 0 50 100 150 200 250 300 TIK Fig. 1 Temperature dependence of R for different x values for the Pro 7Cao ,Sr0 lMn, -xAl,03 samples (a) and for the Pro 7Cao lSro ,Mn, -,AlXO3 samples (b) Table 1 Magnetic moment per Mn atom and T,,, observed in Pro $ao ,Sr0 lMn,-,Al,O, and in Pro 7Cao lSro ,Mn, -,Al,03 Pro 7Cao ,Sr0 ,Mn, -,AlXo3 0 3.75 146 0.01 3.68 134 0.02 3.58 116 0.04 3.46 91 0.06 3.21 58 Pro 7Cao ,Sr0 ,Mn, -,Al,03 0 3.57 217 0.04 3.55 171 0.08 3.10 115 0.12 2.95 72 J 0 50 100 150 200 250 300 TIK Fig. 2 Temperature dependence of magnetization for different x values (labelled on the graph) for the Pro ,Ca, ,Sr0 ,Mn, -,Al,03 series in a magnetic field of 1.45 T The resistance us.temperature curves in a magnetic field of 7 T [Fig. 3(a)] show that the aluminium substitution, if it decreased T,, does not diminish or enhance dramatically the Ro/RH ratio [Fig. 3(b)]. In fact, a complex behaviour is 1246 J.Muter. Chem., 1996, 6(7), 1245-1248 lo'.-oo 0 50 100 150 200 250 1000 L , 'A 1 , , ,I I I I I I I I I i, x=o.o1 200 0 50 100 150 200 TIK Fig. 3 Temperature dependence of (a) R at H =O and 7 T for the Pro 7Cao ,Sr0 ,Mn0 94A10 0603sample, and (b)the ratio Ro/R, for the Pro 7Cao $r0 ,Mn, -,AlXO3 series observed. For the series Pro &ao 2Sro ,Mn, -xAlx03, the maxi- mum Ro/RH ratio first decreases from 275 at 151 K for x=O to 76 for x=O.Ol at 141 K; it is then increased to 330 at 97 K for x=O.O4 and finally to lo3at 68 K for x=O.O6. In a similar manner, in the series Pro &ao ,Sr0 ,Mn, -,AlXO3, the maxi- mum Ro/RH ratio remains approximately constant, and close to 6-8, from x=O at 217 K to x=0.04 at 167 K; it increases then up to 100 at 72 K for x =0.12.Clearly, in each series, the fact that Ro/RH can be increased by one order of magnitude as x increases, is correlated to the decrease of T,,,, i.e. to the increase of the resistance of the material in a zero magnetic field. The evolution of the resistivity of the type I1 GMR perov- skites Pro 5Sro ,Mnl -xAlx03 us. temperature in a zero magnetic field (Fig. 4) shows that, up to x =0.08 they exhibit a behaviour similar to that observed for Pro $1-0 5Mn03,8 characterized by a transition from a semiconducting to a metallic state as T increases. The corresponding magnetization us. temperature curves (Fig. 5) confirm that for x <0.08 this transition coincides with an AFM-FM transition as T increases and evidences an FM-PM transition at higher temperature.Thus, the transition temperature, TN,from the AFM semiconducting state to the 1 o2 10' 1 oo G U lo-' 1o-2 1o-~ 0 50 100 150 200 250 300 TIK Fig. 4 Temperature dependence of R for different x values (labelled on the graph) for the Pro ,Mn, -,AI,O3 series 0 30 0.25 0.20 2m 0 15 z 0 10 0.05 0.00 0 50 100 150 200 250 300 TIK Fig. 5 Temperature dependence of magnetization for different x values (labelled on the graph) for the Pr,,,Sr,,,Mn, -xAl,O, series in a magnetic field of 100 G FM metallic state, can be defined either by the inflexion on the R(T) curves (Fig. 4) or by the left branch of the M(T) curve (Fig. 5). In the same way, the transition temperature, Tc (or T,,,) can be obtained either from the right branch of the M(T) curve (Fig.5) or from the maximum of the R(T) curve (Fig. 4). The first interesting point concerns the increase of the resistance as x increases, so that for x=O.lO, it has become two orders of magnitude larger than for the pristine phase Pr,,,Sr,,,MnO, at 100 K. For xb0.10 the behaviour of the phase is that of a classical semiconductor, the resistivity increasing dramatically as shown for the x =0.12 phase (Fig. 4), whose resistivity is three orders of magnitude larger than that of the pristine phase at 100 K. The most important feature deals with the significant increases of TN,as the A1 content increases, from 136 K for x =0 to 170 K for x =0.06. The Curie temperature Tc (or T,,,) undergoes a corresponding decrease as x increases, similarly to the decrease of T, observed for the type I compositions ranging from 260 K for x=O to 190 K for x=O.O6.In other words, the substitution of aluminium for manganese favours the expansion of the AFM and PM semiconducting states at the expense of the FM metallic state which vanishes around x =0.08 (Fig. 5). A similar behaviour has been observed previously for the 0~mangani tes pr0.5 -~ ~ ~ ~ ~ (ref. 9) . 5 and Pr,,,Sr0,,-,Ca,Mn0, (ref. 10). It was explained as the conse- quence of size effect: the decrease of the mean size of the interpolated cation (Pr, Y, Ca) hinders the overlapping of the Mn-0-Mn orbitals and weakens significantly the hole delocal- ization and consequently benefits to the semiconducting AFM and PM phases. In the case of the aluminium substitution, the smaller size of aluminium compared to manganese may not be the prominent factor that governs this property, but rather its electronic configuration.Owing to the absence of d electrons on the aluminium sites, the substitution of Al"' for Mn"' tends to break the hole propagation in the manganese-oxygen lattice. Nevertheless it is quite remarkable that for an occupancy factor of 6% of the Mn sites by aluminium the metallic conductivity still exists. The application of a magnetic field of 7T confirms the negative magnetoresistance properties of these compounds [Fig. 6(u)]. One only observes a small decrease of the R,/R, ratio as aluminium is substituted for manganese [Fig.6(b)]. Moreover, this decrease of R,/R, is regular and correlated to the increase of TN.In a zero-field cooled sample, one observes maximum R,/R, ratios of 18 at 61 K, 11 at 105 K, 7 at 144 K and 3 at 173 K for x=O, 0.01, 0.04 and 0.08, respectively. 10' a: loo lo-' 1o-2 50 100 150 200 250 300--0 \ i c I-% 10cr" 5 0 50 100 150 200 250 300 T/K Fig. 6 Temperature dependence of (a) R at H=O and 7 T for the Pro,5Sro~5Mno,96Al,,0403sample, and (b) the ratio R,/R, for the Pr,,,Sr,,,Mn, -xAl,03 series In contrast to the evolution of TN,the evolution of R,/RH is very different from that observed for the manganites Pro., ~,Y,Sr,~,MnO, (ref. 9) and Pro.5Sro.5 -,Ca,Mn03'o.One indeed observes that the introduction of aluminium does not modify or decrease only slightly the R,/R, ratio as x increases in the solid solution Pr,.SSro.,Mn, -,AlXO3, whereas the R,/RH ratio can be increased by two orders of magnitude by yttrium or calcium doping of Pr,~,SrO,,MnO3. Such a different behav- iour can be explained by the fact that the introduction of aluminium onto the Mn sites tends to decouple the Mn-0-Mn magnetic interactions owing to the absence of d electrons for this element. This study has shown that the substitution of aluminium for manganese in the praseodymium manganites does not deteriorate their GMR properties, although it decreases T,. One indeed observes a significant increase of the R,/R, ratio for the type I Pr,~,(Sr,Ca),~,Mn, -,Al,O, phases, whereas the ratio is only slightly decreased in the case of the type I1 GMR phases PT,.~ST,.~M~~ -,Al,03.But the most remarkable feature ~3~which is shown for the first time deals with the fact that the transition temperature TNfrom the AFM semiconducting to the FM metallic state can be increased by 40 K by doping the Mn sites with aluminium. Such results suggest that substi- tutions on the manganese by foreign cations should play a key role for the optimisation of the GMR properties of the manganites, depending upon their electronic configuration. A systematic study of the doping of the manganese sites by other cations, transition and post-transition elements, is in progress in order to understand their role in the GMR properties of these materials.References 1 R. M. Kusters, J. Singleton, D. A. Keen, R. M. Greevy and W. Hayes, Physica B, 1989,155,362. 2 B. Raveau, A. Maignan and V. Caignaert, J. Solid State Chem., 1995,117,424. 3 H. Y. Hwang, S. W. Cheong, P. G. Radaelli, M. Marezio and B. Battlogg, Phys. Rev. Lett., 1995,75,914. 4 A. Maignan, Ch. Simon, V. Caignaert and B. Raveau, Solid State Commun., 1995,96,623. 5 A. Maignan, V. Caignaert, Ch. Simon, M. Hervieu and B. Raveau, J.Muter. Chem., 1995,5, 1089. J. Muter. Chem., 1996,6(7), 1245-1248 1247 6 7 8 V Caignaert, E Suard, A Maignan, Ch Simon and B Raveau, J Magn Magn Muter, 1996,152, L5 A Maignan, Ch Simon, V Caignaert and B Raveau, Z Phys B, 1996,99,305 Y Tomioka, A Asamitsu, Y Montomo, H Kowahara and 9 10 J Wolfman, A Maignan, Ch Simon and B Raveau, J Magn Magn Muter, submitted J Wolfman, Ch Simon, M Hervieu, A Maignan and B Raveau, J Solid State Chem ,1996, in press Y Tokura, Phys Rev Lett, 1995,74,5108 Communication 6/02498J, Received 10th Aprzl, 1996 1248 J Muter Chem , 1996,6(7), 1245-1248

ISSN:0959-9428    

DOI:10.1039/JM9960601245

出版商:RSC    

年代:1996

数据来源: RSC