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High-pressure synthesis and thermal decomposition of LaCuO3
MATERIALS SCIEYCE & EMGIREERIIIG ELSEVIER
B
Abstract
The high-pressure synthesis of the rhombohedral LaCuO, perovskite has been studied and optimized in terms of the applied pressure and temperature. the synthesis time and the amount of the oxidizer. Essentially single phase samples were obtained at temperatures abct\e I4fWC under an oxygen pressure of 5 GPa. The high oxygen pressures were generated using excess amounts of KCIO, as an external oxidizing agent. Upon heating under ambient pressure the stoichiometric high-pressure fc>rrn of LaCuO, , loses oxygrn yielding various oxygen-deficient phases, and finally decomposes into La,CuO, and CuO or Cu,O at approximately 800°C.
1. Introduction
The oxygen-defect LaCuO, ~, perovskite resembles cuprate superconductors both chemically and structurally. and has therefore been considered as a potential candidate for exhibiting superconductivity when optimally doped. In the LaCuO, , system four distinct structures exist depending on the synthesis pressureand the oxygen content of the product. Formation of the highly oxidized. stoichiometric LaCuO, is promoted by very high oxygen pressures [I], and was for the first time prepared by Demazeau et al. [2] at 6.5 GPa. Since that LaCuO, samples have been prepared by the highpressure synthesis technique starting from the binary oxides [3-61. from the prereacted La,CuO, and CuO powders [2.7,8] or from citrate gel precursors [9]. The very high oxygen pressures required have been generated in-situ by the thermal decomposition of either KCIO, [4-61, KOz [3] or CrO, [7]. Single crystal X-ray data of Weigl and Range [3] confirm rhombohedral symmetry for LaCuO,.
The tetragonal, low-pressure form of LaCuO, ,. is stabilized for 0
2. Experimental
The high-pressure syntheses of the LaCuO, samples wcrc carried out in a cubic-anvil-type apparatus using stoichiomctric amounts of LiilOl and CuO i1s starting materials and an csccss (% 10076) of KCLO, as an oxygen generator. The La,O? powder \\as fired in air at 1000°C prior to use in order to remove absorbed moisture and CO= Lanth;1num and copper oxides were first mixed thoroughly, since after the addition of the oxidizer only vcrg gentle mixing could be applied due to the easy decomposition of KCIO, (T,,,, of approximately 400°C [IZ]). The resulting mixtures of the I‘~M materials were tightly packed into gold capsules. Pressure was applied through pyrophyllitc containers with an internal graphite tube heater. The gold capsule. placed inside the heater, 1~21s clcctrically isolated from the graphite wall be means of II BN sIce\;e. The high pressure(4 5 GPa) ~~1s first slowly (in 30 min) applied. and then temperature \vas increased to the desired value ( 1000 1600°C) in 10 min. The sampleswere treated in the high-pressure and high-temperature conditions foi IO-60 min. and afterwards quenched to room temperature in IO s. Finally, the pressure\vas decreasedback to ambient value in 30 min. The oxygen evolution characteristics and the thermal decomposition of the high-pressure LaCuO, phase under oxygen and argon atmospheres bverc studied up to 950°C using ~1MAC Science TG DTA 2000s apparatus equipped with an infrared furnace. The sample size was approximately 20 mg and the heating rate was 2’C
‘_ The phase contents of the high-pressure products and the TG rcsiducs wcrc checked by 21 MAC Science M 1XXHF X-ray diffractometer. min
3. Results and discussion
Essentially single phase LaCuO, material with the rhombohedral structure could be obtained under oxygcn pressure of 5 GPa and at temperatures above 1400°C using KCIO, as an oxygen generator. Completion of the synthesis requires that the stoichiometric amount of oxidizer is used twice. No indication of the very stable La(OH), compound, often found as an impurity phase [3,h]. could be seenin the X-ray diffraction (XRD) patterns of the high-pressure synthesis products. On the other hand, small unidentified impurity peaks at 20 = 30.5 and 34.9” (Cu Kx radiation was used) as well as the peaks due to KCI, formed as 21 by-product due to the decomposition of KCIO, (Eq. (1)). wcrc always present, as indicated in Fig. 1. Since the oxidation power of KCIO, is larger than that of the most widely applied KCIO,, the amount of oxidizer needed and thus the amount of KCI being produced could be. however. kept at quite a low level. 4La,O, + 8CuO + KCIO, 3 XLaCuO, + KCI
(1)
The formation of the LaCuO, phase starts already at IO()O”C, but the reaction does not proceed completely below 1400°C. The higher limit for the applied temperature in the experimental set-up used is 1500°C. re-
stricted by the melting of the Au capsules. At 5 GPa the capsules were found to melt totally by 16OO”C, but even at 1500°C some reaction between the reactants and gold was observed. most probably promoted by partial melting of gold. The extent of this harmful reaction increased with increasing heat-treatment time (lo-60 min). The synthesis of LaCuO, is very fast at high pressures and high temperatures. At 5 GPa and 1400°C the desired reaction was essentially completed in IO min. On the other hand. when the synthesis time was lengthened to 60 min some additional unidentified diffraction peaks appeared. The oxygen gas left over after the complete oxidation of Cu(I1) to Cu(III) remains at least for the most part inside the Au capsules. which as a consequence were found to be concave in shape. Most likely the capsules are, however, not completely tight but allow slow diffusion of the excess oxygen out of the synthesis environment. This supposition is supported by the fact that after the long heat-treatments the capsules were not no longer concave but Aat. The maximum pressure achieved by the cubic-anviltype apparatus used \vas 5 GPa. As described above, it is high enough for the successful synthesis of LaCuO,. The reaction was no longer completed at 4 GPa and 1400°C. but small amounts of the reactants were left in the product. Since the melting point of Au decreases with decreasing pressure. it was not possible to study whether the deterioration in the reaction conversion due to lower synthesis pressure could be counteracted by using higher temperatures. The oxygen evolution characteristics and thermal decomposition of the rhombohedral LaCuO, were studied in oxygen and argon atmospheres by thermogravimetric measurements (Fig. 3). Upon heating the oxygen evolution starts at approximately 200°C. In both atmospheres the slope of the TG curve for the weight loss
L
.aCu03+0.25
20
Fig. tion
KU
200
LOO TEMPERATURE
7. TG curve5 For the oxygen of the high-pressure synthesized
evolution LKuO,
600 l”CJ
800
950
and thermal decomposimaterial in 0: and Ar.
changes several times with temperature. These changes, seen as turning points in the corresponding DTG curves, are attributed to the structural phase transitions corresponding to the tetragonal. monoclinic and orthorhombic phases of LaCuO, I which were prepared for the first time by Bringley et al. [IO.1 l] by the “lower-pressure post-annealing” route discussed in the Section 1. In the present study the following transition temperatures were observed in an argon atmosphere: rhombohedral-tetragonal ca. 2OO”C, tetragonnl monoclinic ca. 300°C. monoclinic-orthorhombic ca. 350°C. Consistently, the XRD patterns obtained for the samples annealed in argon at 280, 400-440 and 500~~640°C could be readily indexed according to these tetragonul. monoclinic and orthorhombic structures, respecti\,ely. A more detailed study on the stability limits of the different oxygen-deficient LaCuO, , phases and the corresponding oxygen content and copper valence WIues is in progress [13]. Above 700°C the final decomposition of the LaCuO, ( framework and the evaporation of KCI. present in the high-pressure product used as a starting material in TG experiments. overlap. Judging from the magnitude of the total weight loss, KCI is totally removed during the TG runs up to 950°C. although its boiling point is abo\,e 1500°C (m.p. 770°C [13]). This conclusion is consistent with the XRD data sho\ving no KC1 peaks in the TG residues. The decomposition product in oxygen consists of the La,CuO, and CuO phases. while in argon the Cu(I1) oxide is further reduced to Cu,O at approximately 800°C. 4. Conclusions The high oxygen pressures needed to stabilize the highly oxidized LaCuO, perovskite cm be generated in a cubic-anvil-type high-pressure apparatus using excess amounts of KClO, as an external oxidizing apcnt. Essentially single phase samples lvere obtained at 5 GPa and 1400°C. Upon heating under ambient pressure the stoichiometric high-pressure form of LaCuO, loses oxygen yielding various oxygen-deficient phases ad intermediates. and finally decomposes to La$ZuO, and CuO or CuZO at approximately 800°C. The tetragonnl, monoclinic and orthorhombic forms of osygcn-deficient LaCuO, ,. can be isolated as pure phases by annealing in argon at temperatures between ca. 200 and 650°C. So far, most of the attempts to “superconductorize” LaCuO, ) have concerned only the fully oxidized, rhombohedral form. However, the oxygen-deficient tetragonal, monoclinic or orthorhombic lattices might provide more suitable frameworks for hosting superconducting holes or electrons. By a combination of aliocalent substitutions and oxygen content adjustment it should be possible to tune the Cu valence value in a wide range in any of the LaCuO, , phases.
Acknowledgements
[61 c4.W
Webb.
.SO//d Sru/c
This work was partially supported gaku Shinkokai Foundation.
by the Kato Ka-
171 K. RlT.
PI
E.F.
Skclton.
C%CVll..
I/C
S.B. Qxiri
Allan. A C’anyion. H. JI ( I YYO) I 1572.
J.tf.
c‘hoy.
Rw.
R. 50 ( I YY4)
I1.K
Kim.
and
E.R.
Cqwnter.
Jr., /.
( I YY?) 5 I Y. J. %hou S.II.
and
~lwane
'1
J.B. and
Goodenough. G.
/‘/!J,\.
Delll;lrwLl.
P/r,,,.
I 663 I
PI References [I]
O.M.
Sreedharan.
I’ (‘I J.F. C.
Mallika
Sr7.. 2.1 (IYXX) 2735. [2] G. Demazeau, C. Parent. .Cfur~~. Rrs. Bull.. 7 (1972) [3] C. Wcigl and K.J. Range. [A] .A.W. Webb. E.F. Skclton. Osofskq. R.J. Soulen and 8’)‘). [5]
A.W. Webb. E.F. Skelton, Owfskq. R.J. Soulcn and (I’)%~) 205.
and
K.
S\\ominatban.
J. .\/trtw.
M. Poucbard and P. Hag!cnmuller. Y 13. J. .4/I. (;vrrp.. 200 (lYY3) Ll. S.B. Qadrl. E.R. C‘arpentcr. Jr.. M.S. V. LeTourncau. P/I.I,.c C‘. lh2 (lY8Y) S.B. Qadr~. E.R. V. LeTourneau.
Shaw.
Carpenter. Jr.. M.S. P/I>,.\. /.cs//. A. l.??
Bringlq. h1.M’.
( I YYO) 263.
II II
B.A. McElt’re\h.
Scott.
S.J. S.S.
LcPlxa.
TI-ail
and
R.F. D.E.
Bocbme. (‘OS.
.\‘u/u~.
T.M. .iJ7
B
Abstract
The high-pressure synthesis of the rhombohedral LaCuO, perovskite has been studied and optimized in terms of the applied pressure and temperature. the synthesis time and the amount of the oxidizer. Essentially single phase samples were obtained at temperatures abct\e I4fWC under an oxygen pressure of 5 GPa. The high oxygen pressures were generated using excess amounts of KCIO, as an external oxidizing agent. Upon heating under ambient pressure the stoichiometric high-pressure fc>rrn of LaCuO, , loses oxygrn yielding various oxygen-deficient phases, and finally decomposes into La,CuO, and CuO or Cu,O at approximately 800°C.
1. Introduction
The oxygen-defect LaCuO, ~, perovskite resembles cuprate superconductors both chemically and structurally. and has therefore been considered as a potential candidate for exhibiting superconductivity when optimally doped. In the LaCuO, , system four distinct structures exist depending on the synthesis pressureand the oxygen content of the product. Formation of the highly oxidized. stoichiometric LaCuO, is promoted by very high oxygen pressures [I], and was for the first time prepared by Demazeau et al. [2] at 6.5 GPa. Since that LaCuO, samples have been prepared by the highpressure synthesis technique starting from the binary oxides [3-61. from the prereacted La,CuO, and CuO powders [2.7,8] or from citrate gel precursors [9]. The very high oxygen pressures required have been generated in-situ by the thermal decomposition of either KCIO, [4-61, KOz [3] or CrO, [7]. Single crystal X-ray data of Weigl and Range [3] confirm rhombohedral symmetry for LaCuO,.
The tetragonal, low-pressure form of LaCuO, ,. is stabilized for 0
2. Experimental
The high-pressure syntheses of the LaCuO, samples wcrc carried out in a cubic-anvil-type apparatus using stoichiomctric amounts of LiilOl and CuO i1s starting materials and an csccss (% 10076) of KCLO, as an oxygen generator. The La,O? powder \\as fired in air at 1000°C prior to use in order to remove absorbed moisture and CO= Lanth;1num and copper oxides were first mixed thoroughly, since after the addition of the oxidizer only vcrg gentle mixing could be applied due to the easy decomposition of KCIO, (T,,,, of approximately 400°C [IZ]). The resulting mixtures of the I‘~M materials were tightly packed into gold capsules. Pressure was applied through pyrophyllitc containers with an internal graphite tube heater. The gold capsule. placed inside the heater, 1~21s clcctrically isolated from the graphite wall be means of II BN sIce\;e. The high pressure(4 5 GPa) ~~1s first slowly (in 30 min) applied. and then temperature \vas increased to the desired value ( 1000 1600°C) in 10 min. The sampleswere treated in the high-pressure and high-temperature conditions foi IO-60 min. and afterwards quenched to room temperature in IO s. Finally, the pressure\vas decreasedback to ambient value in 30 min. The oxygen evolution characteristics and the thermal decomposition of the high-pressure LaCuO, phase under oxygen and argon atmospheres bverc studied up to 950°C using ~1MAC Science TG DTA 2000s apparatus equipped with an infrared furnace. The sample size was approximately 20 mg and the heating rate was 2’C
‘_ The phase contents of the high-pressure products and the TG rcsiducs wcrc checked by 21 MAC Science M 1XXHF X-ray diffractometer. min
3. Results and discussion
Essentially single phase LaCuO, material with the rhombohedral structure could be obtained under oxygcn pressure of 5 GPa and at temperatures above 1400°C using KCIO, as an oxygen generator. Completion of the synthesis requires that the stoichiometric amount of oxidizer is used twice. No indication of the very stable La(OH), compound, often found as an impurity phase [3,h]. could be seenin the X-ray diffraction (XRD) patterns of the high-pressure synthesis products. On the other hand, small unidentified impurity peaks at 20 = 30.5 and 34.9” (Cu Kx radiation was used) as well as the peaks due to KCI, formed as 21 by-product due to the decomposition of KCIO, (Eq. (1)). wcrc always present, as indicated in Fig. 1. Since the oxidation power of KCIO, is larger than that of the most widely applied KCIO,, the amount of oxidizer needed and thus the amount of KCI being produced could be. however. kept at quite a low level. 4La,O, + 8CuO + KCIO, 3 XLaCuO, + KCI
(1)
The formation of the LaCuO, phase starts already at IO()O”C, but the reaction does not proceed completely below 1400°C. The higher limit for the applied temperature in the experimental set-up used is 1500°C. re-
stricted by the melting of the Au capsules. At 5 GPa the capsules were found to melt totally by 16OO”C, but even at 1500°C some reaction between the reactants and gold was observed. most probably promoted by partial melting of gold. The extent of this harmful reaction increased with increasing heat-treatment time (lo-60 min). The synthesis of LaCuO, is very fast at high pressures and high temperatures. At 5 GPa and 1400°C the desired reaction was essentially completed in IO min. On the other hand. when the synthesis time was lengthened to 60 min some additional unidentified diffraction peaks appeared. The oxygen gas left over after the complete oxidation of Cu(I1) to Cu(III) remains at least for the most part inside the Au capsules. which as a consequence were found to be concave in shape. Most likely the capsules are, however, not completely tight but allow slow diffusion of the excess oxygen out of the synthesis environment. This supposition is supported by the fact that after the long heat-treatments the capsules were not no longer concave but Aat. The maximum pressure achieved by the cubic-anviltype apparatus used \vas 5 GPa. As described above, it is high enough for the successful synthesis of LaCuO,. The reaction was no longer completed at 4 GPa and 1400°C. but small amounts of the reactants were left in the product. Since the melting point of Au decreases with decreasing pressure. it was not possible to study whether the deterioration in the reaction conversion due to lower synthesis pressure could be counteracted by using higher temperatures. The oxygen evolution characteristics and thermal decomposition of the rhombohedral LaCuO, were studied in oxygen and argon atmospheres by thermogravimetric measurements (Fig. 3). Upon heating the oxygen evolution starts at approximately 200°C. In both atmospheres the slope of the TG curve for the weight loss
L
.aCu03+0.25
20
Fig. tion
KU
200
LOO TEMPERATURE
7. TG curve5 For the oxygen of the high-pressure synthesized
evolution LKuO,
600 l”CJ
800
950
and thermal decomposimaterial in 0: and Ar.
changes several times with temperature. These changes, seen as turning points in the corresponding DTG curves, are attributed to the structural phase transitions corresponding to the tetragonal. monoclinic and orthorhombic phases of LaCuO, I which were prepared for the first time by Bringley et al. [IO.1 l] by the “lower-pressure post-annealing” route discussed in the Section 1. In the present study the following transition temperatures were observed in an argon atmosphere: rhombohedral-tetragonal ca. 2OO”C, tetragonnl monoclinic ca. 300°C. monoclinic-orthorhombic ca. 350°C. Consistently, the XRD patterns obtained for the samples annealed in argon at 280, 400-440 and 500~~640°C could be readily indexed according to these tetragonul. monoclinic and orthorhombic structures, respecti\,ely. A more detailed study on the stability limits of the different oxygen-deficient LaCuO, , phases and the corresponding oxygen content and copper valence WIues is in progress [13]. Above 700°C the final decomposition of the LaCuO, ( framework and the evaporation of KCI. present in the high-pressure product used as a starting material in TG experiments. overlap. Judging from the magnitude of the total weight loss, KCI is totally removed during the TG runs up to 950°C. although its boiling point is abo\,e 1500°C (m.p. 770°C [13]). This conclusion is consistent with the XRD data sho\ving no KC1 peaks in the TG residues. The decomposition product in oxygen consists of the La,CuO, and CuO phases. while in argon the Cu(I1) oxide is further reduced to Cu,O at approximately 800°C. 4. Conclusions The high oxygen pressures needed to stabilize the highly oxidized LaCuO, perovskite cm be generated in a cubic-anvil-type high-pressure apparatus using excess amounts of KClO, as an external oxidizing apcnt. Essentially single phase samples lvere obtained at 5 GPa and 1400°C. Upon heating under ambient pressure the stoichiometric high-pressure form of LaCuO, loses oxygen yielding various oxygen-deficient phases ad intermediates. and finally decomposes to La$ZuO, and CuO or CuZO at approximately 800°C. The tetragonnl, monoclinic and orthorhombic forms of osygcn-deficient LaCuO, ,. can be isolated as pure phases by annealing in argon at temperatures between ca. 200 and 650°C. So far, most of the attempts to “superconductorize” LaCuO, ) have concerned only the fully oxidized, rhombohedral form. However, the oxygen-deficient tetragonal, monoclinic or orthorhombic lattices might provide more suitable frameworks for hosting superconducting holes or electrons. By a combination of aliocalent substitutions and oxygen content adjustment it should be possible to tune the Cu valence value in a wide range in any of the LaCuO, , phases.
Acknowledgements
[61 c4.W
Webb.
.SO//d Sru/c
This work was partially supported gaku Shinkokai Foundation.
by the Kato Ka-
171 K. RlT.
PI
E.F.
Skclton.
C%CVll..
I/C
S.B. Qxiri
Allan. A C’anyion. H. JI ( I YYO) I 1572.
J.tf.
c‘hoy.
Rw.
R. 50 ( I YY4)
I1.K
Kim.
and
E.R.
Cqwnter.
Jr., /.
( I YY?) 5 I Y. J. %hou S.II.
and
~lwane
'1
J.B. and
Goodenough. G.
/‘/!J,\.
Delll;lrwLl.
P/r,,,.
I 663 I
PI References [I]
O.M.
Sreedharan.
I’ (‘I J.F. C.
Mallika
Sr7.. 2.1 (IYXX) 2735. [2] G. Demazeau, C. Parent. .Cfur~~. Rrs. Bull.. 7 (1972) [3] C. Wcigl and K.J. Range. [A] .A.W. Webb. E.F. Skclton. Osofskq. R.J. Soulen and 8’)‘). [5]
A.W. Webb. E.F. Skelton, Owfskq. R.J. Soulcn and (I’)%~) 205.
and
K.
S\\ominatban.
J. .\/trtw.
M. Poucbard and P. Hag!cnmuller. Y 13. J. .4/I. (;vrrp.. 200 (lYY3) Ll. S.B. Qadrl. E.R. C‘arpentcr. Jr.. M.S. V. LeTourncau. P/I.I,.c C‘. lh2 (lY8Y) S.B. Qadr~. E.R. V. LeTourneau.
Shaw.
Carpenter. Jr.. M.S. P/I>,.\. /.cs//. A. l.??
Bringlq. h1.M’.
( I YYO) 263.
II II
B.A. McElt’re\h.
Scott.
S.J. S.S.
LcPlxa.
TI-ail
and
R.F. D.E.
Bocbme. (‘OS.
.\‘u/u~.
T.M. .iJ7