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The Nd2O3-SrO-CuO system: compounds and phase relations
Journal of Alloys and Compounds, 205 (1994) 101-106 JALCOM 956
101
The Nd203-SrO-CuO system: compounds and phase relations X i a o l o n g C h e n a'b, J i n g k u i L i a n g a'c, C o n g W a n g a, G u a n g h u i R a o a, X i a n r a n X i n g d, Zhihong Song ~ and Zhiyu Qiao ° "Institute of Physics, Chinese Academy of Sciences, 100080 Beijing (China) bCCASTA (World Laboratory), PO Box, 8730, 100080 Beijing (China) Clnternational Center for Materials Physics, Academia Sinica, 110015 Shengyang (China) dUniversity of Science and Technology Beijing, 100083 Beijing (China)
(Received May 25, 1993; in final form August 30, 1993)
Abstract
The subsolidus phase relations of Nd203-SrO---CuO ternary system sintered in air at 950-1000 °C have been investigated by X-ray powder diffraction. The system can be divided into 13 three-phase regions and five twophase regions. In this system there exist at least four compounds and their solid solutions: Nd2SrCu206, Ndl+xSr2_xCu206 (0~
1. Introduction
Up to now, a series of RaO3-BaO-CuO ternary systems have been investigated to clarify the phase relations and to search for new superconductors. These ternary systems include Y203-BaO-CuO [1-3], La203-BaO-CuO [4, 5], Nd203-BaO-CuO [6], Gd203BaO-CuO [7], Ho203-BaO-CuO [8, 9], DyzO3-BaOCuO [8], YbzO3-BaO-CuO [10], Pr6Oal-BaO-CuO [11] and Sm203-BaO-CuO [12]. The common superconducting phase in these systems is RBa2Cu307_n except for PrBa2Cu307_6, which has not been observed to have superconductivity even down to 4.2 K. Another common phase R2CuO 4 exists in some R~O3-BaO-CuO systems (with large enough ionic radii of R 3+, otherwise the R2Cu205 phase forms [13]). Though compounds R2CuO4 are not themselves superconducting, they can be made superconducting by doping with oxygen or alkaline earths and Ce, etc., depending on their crystal structure, namely T, T* and T' phases. Hall measurements showed that the electrical carriers are holes in the T and T* phases, and are electrons in the T'phase. This difference has been attributed to the presence of apical oxygen atoms near the CuO2 layers, forming CuO6-0cathedra and CuO5 square pyramids in the T and T* phases, and only forming square planar CuO4 arrangements in the absence of these. The squareplanar CuO4 seems indispensable in electronic superconductors. Quite recently, Cava et al. [14] reported that superconductivity with To=60 K was observed in (La, 0925-8388/94/$07.00 © 1994 Elsevier Sequoia. All rights reserved SSDI 0925-8388(93)00956-Y
Sr)2CaCu20 6. Examination of its crystal structure revealed that it is related to La2CuO4 by intercalating a Ca-CuO2 layer between the upper and lower halves of CuO5 square pyramids. A general chemical formula La2Ca,_lCu,,O4+2,, may be adopted to designate the series of compounds with n = 1 (L%CuO4) and n = 2 (La2CaCu206). This is a typical case with high Tc superconductors. Similar situations have already been observed in T1- and Bi-based superconductors. Now the question arises of whether or not a series of Nd2CuO4-base d compounds NdEM, _ lCu, 0 4 + 2."t exist. If so, is the Cu-O arrangement square planar? Can these compounds be made into electron-type superconductors by doping with appropriate elements like Ce? With these questions in mind, we first tried to synthesize Nd2MCu206 and chose alkaline earths as M since alkaline earths are known to be good host elements in some layered superconductors. Ba was not considered as a candidate because RBa2Cu3Oy forms easily in the R203-BaO-CuO system as we mentioned above. Preliminary experiments showed that it is difficult to synthesize RzCafu20 6 (R-=Nd, Sm, Gd) under atmospheric pressure, but NdzSrCu206 is formed easily. This compound is known to possess double CuO sheets with space group I 4 / m m m . However, this compound is a semiconductor. It has not been made a superconductor even by doping. In addition, several compounds such as NdSrCuO3.5 [15], Nda.8Srl.2Cu206_ ~ [15], Nd6Sr3Cu60,7 [16], Nd2SrCuzOs.76 [17] and Nd1.4Srl.6Cu2Os.79 [18] ternary
102
X. Chen et al. / Nd203-SrO-CuO system: compounds and phase relations
SrO
systems have been reported in the N d 2 O a - S r O - C u O system. To clarify the phase relations and search for new possible superconductors, we have systematically investigated the subsolidus relation of the N d z O 3 - S r O - C u O system and the crystal structure of the compounds.
uO~
2. Experimental details
A series of N d 2 0 3 - - S r O - C u O samples of different composition were prepared by solid-state reaction of an appropriate mixture of high purity of Nd203, SrO and CuO. The powders were mixed, ground and fired at 950 °C for 12 h in air. Several additional firings for 24 h intervals with intermediate grindings were performed before the samples were pressed into pellets of diameter 10--12 mm and thickness 2-4 mm. Then the pellets were sintered at 1000 °C for 3 days. The above process was repeated for some samples until the volumes of component phases were unchanged. Phase identifications were partially performed on a Guinier-de Wolff monochromatic focusing transmission camera and partially on a Rigaku automatic diffractometer using CuKa radiation. High purity Si was added to the samples as an internal standard to correct the 20 position of diffraction peaks in case precision measurements should be required. The lattice parameters were then calculated by using a standard least-square reduction method. The electrical resistivity and a.c. magnetic susceptibility were measured on rectangular bars cut from sintered pellets employing the standard d.c. four-probe technique with In contacts attached to electric leads. Data was collected from 300 K down to 4.2 K.
3. Results
3.1. Binary oxides For the binary N d 2 0 3 - S r O system, powder X-ray diffraction indicated that the samples consist of Nd203, minor SrO and some minor unknown phase after sintering. Since the diffraction lines are very weak and few, we cannot index the pattern of the unknown phases. The instability of SrO in air may be responsible for this result. Tresvyatskii et al. [19] reported that there exist at least two stable compounds, SrO. 2Nd203 and 2SrO-Nd203, in the system. We have, however, not yet verified the existence of these compounds in the present investigation. The three-phase regions I, XII and XIII in Fig. 1 are only tentative representations of the subsolidus phase relations in the corresponding compositional region; further study is still needed to clarify the ambiguity in compatibility I, XII and XIII.
NdO,s
Nd~CuO~
CuO
Fig. 1. The subsolidus phase relation for the NdOLs--SrO--CuO system sintered at 950-1000 °C in air; compatibitities I, XII and XIII are tentative and divided by broken lines. The tie lines for the two-phase fields are indicated schematically: A -- Nd2SrCu2Or, B -- Ndl+xSr2_xCu2Oy (0
In the binary system Nd203--CuO, only one compound, Nd2CuO4, is identified. It crystallizes in an tetragonal unit cell, I4/mrnm, with square planar Cu-O arrangements, that is, the so-called T' phase. Its cell parameters are a = 3.938, c = 12.1465/~. Nd2CuO4 melts congruently at about 1200 °C [20], but others [21] argued that it melts incongruently since it is needed to add appropriate flux to grow the single crystal of Nd2CuO4. In the binary system SrO-CuO, the following compounds have been reported: SrzCuO3, SrCuO2, SrCul.6sO2.65+,s, Sr3CusO8 and Sr14Cu24041. The compound Sr2CuO3 belongs to an orthorhombic lattice, space group Immm, with lattice parameters a = 12.68-12.71 A, b=3.91-3.913 /~ and c=3.48--3.50 /~ [22, 23]. These results are in good agreement with the present study. Sr2CuO3 has an orthorhombic distorted KzNiF4 structure. Each Cu is coordinated to four O, forming distorted square planar Cu-O arrangements along its long axis a. From the structural point of view, it can be thought of as forming from the removal of a pair of oxygen atoms on the square plane from the CuO6 octahedra in L a 2 C u O 4. Isostructures were found in Ca2CuO3 [23, 24] and Ba2CuO3+~ [3]. The compound SrCuO2 crystallizes in an orthorhombic unit cell, space group Cmcm, with a =3.562 /k, b=16.32 /~ and c=3.918 ~, which are consistent with the results by other works [23, 25]. The structure consists of square plane CuOz. The CuO2 squares are connected in isolated infinite double zig-zag ribbons [24, 25]. Under high pressure and high temperature,
103
X. Chen et al. / Nd203-SrO-CuO system: compounds and phase relations
SrCuO2 exhibits a new structure, an infinite sequence of CuO2 planes separated by planes of SrO [26], i.e. the "infinite layer" compound. The structure is still stable at ambient pressure upon synthesis under high pressure. In the present investigation, we did not synthesize compound SrCu202, which Teske and Mtiller-Buschbaum [27] reported to belong to a tetragonal lattice, ace group I41/amd, with lattice parameters a =5.48 and c=9.82 ~. Each Cu +1 ion in the unit cell has only two 0 2 - neighbors forming an infinite zig-zag chain. It was made by firing SrO and CuO at 700-800 °C in air. Our results showed that this compound cannot be synthesized in air since Cu ÷1 is stable in a reducing gas atmosphere. De Leeuw et al. [28] reached the same conclusion. The remaining compounds in the system are Sr3CusOs, SKCUl.6502.65+6 and SElnCU24041. Sr3CusO8 was indexed as a face-centered lattice, space group F m m m , with lattice constants a = 13.402 ~, b = 11.470 ~ and c = 3.939 ,~ [23]. SrCu1.6502.65+,~ was indexed as a=13.416 ~, b = 11.478 ~ and c = 3.949 ~, most of its indices obey h + k = 2n, k + 1= 2n, h + 1= 2n, but some weak reflections like (510), (520) and (141) etc., violate these reflection conditions. Therefore, SrCua.6502.65 + 8 do not belong to a face centered lattice [28]. Based on diffraction data by a single crystal, McCarron et al. [29] characterized the crystal structure of 8r14Cu24041 as an orthorhombic unit cell, space group Pcc2, with lattice parameters a = 11.469/~, b = 13.368 ~ and c = 27.501/~. The authors argued that the structure consists of two unique subcells, one with Sr-(Cu203 sheets)-Sr layers in an orthorhombic cell with dimensions of a=11.459, b=13.368 and c = 3.931 ~ and a second structure with layers of CuO2 chains in a cell possessing identical a and b values but with a different c = 2.749 /~. Since these two subcells are nearly commensurate at 7*c(sheet)= 27.372 ~ and 10*c(chain)=27.534 ~, the chemical formula is designated by Sr14Cu24041. The atomic ratios of Cu and Sr are 1.67, 1.65 and 1.71 for Sr3CusOs, 8rCUl.6502.65+,5 and Sra4Cu24041 respectively. Considering the composition and their unit cell dimensions, we think these three compounds should belong to one phase. Our powder diffraction data are nearly identical with that by De Leeuw et al. [27]. Some reflections violating the face-centered symmetry really exist. Therefore, the compound should possess a space group with lower symmetry than Fmrnm. We adopt Sr3CusO 8 as the chemical formula for the time being. The real space group is needed to redetermine this. Like Ca [27] and Y [28], Nd can partially substitute for Sr in Sr3CusOs. Figure 2 shows the variations of lattice constants vs. the Nd content for a solid solution Sr3 -x NdxCusO8. The lattice parameters a and b decrease with increasing Nd, while c is nearly unchanged and
615 "' o<
600
585 570 ~
I
°
0
O0
0 O
13.5 000•0 13.0 12.5
~
12.0 11.5
vv~v
~c
V
v
V
v
[]
[]
11.0 4.5
4.0
[]
3.5
D D D D D i
0.0
0.5
[] i
1.0
I
1.5
2.0
X Fig. 2. The variations of lattice constants a, b, c and unit cell
volume
V vs. x
for Sr3_.Nd.CusOs.
a decreases in a more rapid manner than b. The solid solution limit is about x = 1.5 as indicated by a leveling off of lattice constants a and b, and the appearance of impurity phases. In view of the difference in ionic radii between Sr and Nd, it is expected that the unit cell volume will decrease with increasing Nd content. As the structure is layered, substitution would directly affect the layer spacings, stacking along the a axis, while having a smaller effect on the b--c plane parameters which are determined, for the most part, by the copper--oxygen connectivity. Since the ionic radius of Nd 3+ is closer to Sr 2+ than that of y3+, Sr3_~Nd, fusO8 will have a larger solution limit than Sr3_,Y~CusOs, in which case, the solid-solution limit is about 1.0 [28].
3.2. Ternary compounds Two compounds Nd2SrCu206 and Ndl +xSrz_,CuzOy always coexist in the preparation of Ndz_,Srl+xCu2Oy samples. Their lattice parameters are very close, but they do not belong to a solid solution because the diffraction intensities of these two compounds change in an opposite manner with increasing x. For a small x, NdzCuO4, NdzSrCu206 and a small amount of N d l +xSr2 --xCuzOy often coexist. Isolation of Nd2SrCu206 or Ndl+xSr2_,Cu2Oy requires a long annealing time. From the diffraction data (see Table 1), we can index Nd2SrCu206 as a tetragonal body-centered pattern with a = b = 3 . 8 3 /k and c=19.71 /~. Compared with the parameters of Nd2CuO4, a and b shrink, while c increases a lot. Since the layered stacking of cations in a unit cell for NdzCuO4 is C u - N d - N d - C u - N d - N d - C u , the
X. Chen et al. / Nd203--SrO--CuO system: compounds and phase relations
104
T A B L E I. List of d spacings, diffraction intensity and hid for Nd2SrCu206, a = b = 3 . 8 3 /~,, c = 19.71 /~, space group 14/mmm, Z=2 No. hkl
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
002 004 101 103 006 105 110 112 008 114 107 116 0010 200 109 202 118 204 211 213 206 0012 1011
d¢,~¢
dobs
(A)
(A)
9.8550 4.9275 3.7597 3.3088 3.2850 2.747 2.7082 2.6114 2.4637 2.3734 2.2686 2.0896 1.9710 1.915 1.9011 1.8798 1.8224 1.7849 1.7065 1.6575 1.6544 1.6425 1.6230
9.81 4 24 4.930 1 25 3.764 16 26 3.312 3 27 3.886 5 28 2.748 100 29 2.713 60 30 _a -a 31 2.464 3 32 2.371 2 33 2.270 18 34 2.091 25 35 1.971 10 36 1.916 2 37 1.897 3 38 1.886 4 39 1.822 1 40 1.788 1 41 1.710 5 42 1.655 6 43 -" -~ 44 1.638 10 45 1.622 8
Io~
No. hkl
1110 215 208 217 1013 0014 1112 2010 220 219 222 224 301 303 226 1114 2012 1015 2111 0016 305 310
dc~lc
dob~
(A)
(A)
1.5936 1.594 1.5709 1.573 1.5120 1.513 1.4633 1.465 1.4097] 1.4079 I 1.406 1.4044 1.3735 1.372 1.3541 1.3350 1.3492 _a 1.3415 -~ 1.3057 1.304 1.2740 -" 1.2532~ 1.2519 .I 1.252 1.2492 -~ 1.2467 -~ 1.2429 1.243 1.2381 1.235 1.2319 1.232 1.2146 ]~ 1.212 1.2127 J
lobs 13 30 2 8 10 8 10 _~ -~ 3 -" 2 -~ -~ 3 7 5 6
aDiffraction line is not observed T A B L E 2. The structural parameters for Nd2SrCu206 Atom
Position
X
Y
Z
N
Sr Nd Cu O1 02 03
2b 4e 4e 8g 4e 2a
0 0 0 0 0 0
0 0 0 0.5 0 0
0.5 0.31 0.090 0.09 0.19 0.1
1.0 1.0 1.0 1.0 1.0 0.1
8(g) and 4(e), and the occupancies were set as 1.0. We found that there should be partial oxygen atoms occupied (2a) positions though the occupancy is as low as 0.1, otherwise the calculated intensities of the diffraction line disagree much with the observed ones no matter how the atomic positions of other atoms are adjusted. This suggests that not all the Cu ions are coordinated by five oxygen atoms to form CuO5 square pyramids. Two Cu ions are coordinated by six oxygen atoms and form two CuO6 octahedra in every ten unit cells. According to Labbe's study [15], there should exist a small solid solution Nd2_xSr1+xCu206_~. In the present investigation, however, we cannot conclude the existence of this solid solution due to the impurity of the samples. Another compound, or rather, a solid solution which often coexists with Nd2SrCu206 is Ndl+xSr2_xCu206 (0~
14/mram, Z = 2
average spacing between the layers is equal to c/6 ~- 12/ 6 = 2 /~. Based on the approximate average layered spacing, we might estimate that the number of layers in the unit cell for Nd2SrCu206 is 19.71/2= 10. The possible arrangement of cations along the z axis is Sr-Cu-Nd-Nd-Cu-Sr--Cu-Nd-Nd-Cu-Sr. Hence, Nd2SrCu206 is related to Nd2CuO4 by inserting a layer of Sr-O between the upper and lower halves of the CuO4 squares. The number of chemical formula in the unit cell is z = 2 for Nd2SrCu206.The remaining problem is how the Cu is coordinated by O and whether they form CuO4 square planes, CuO5 square pyramids or CuO6 octahedra. Referring to thecrystal structure of Nd~.sSr~.2Cu206_ [15], we have roughly determined the atomic parameters for Nd2SrCu206 by using the LAZY program; these are listed in Table 2. The oxygen atoms were set to occupy
NO. hkl
doe
dobs
(A)
(.~)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
002 004 101 006 103 105 130 132 008 134 107 136 0010 109 200 202 138 204
10.06 5.03 3.7425 3.3533 3.296 2.759 2.679 2.589 2.515 2.3652 2.2902 2.0935 2.012 1.9255 1.895 1.8622 1.8339 1.7733
9.91 4.99 3.722 3.356 3.286 2.756 2.678 _b 2.509 2.371 2.290 2.093 2.015 1.921 1.983 1.861 __b __b
19 20 21 22
231 0012 206 1011
1.6890 1.6767 1.6498 1.6473
1.683 1.672 1.649
lobs No. hkl
4 1 2 4 3 100 50 __b 3 2 15 22 11 2 2 3 _b b
23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
6 41 11 42 13 43
bDiffraction line is not observed.
d~lc
dobs
(A)
(A)
233 1,643 1310 1,6090 235 1.562 208 1.5135 237 1,4600 0014 1.4371"[ 1013 1,4328 J 1312 1.4214 2010 1.3795 236 1.3506 260 1.3400 262 1.3282 224 1.2948 1614 1.263 ] 1015 1.263 I 301 1.263 0016 1.257 ] 2012 1.2557 i 266 2611 303
lobs
1.638 1.610 12 1.561 27 1.513 2 1.465 8 1.433 9 1.420 5 1.372 7 1.350 4 b _b _b _b _b _b 1.263
4
1.254
3
1.2443 ] 1.2432 I 1.243 1.2415
5
X. Chen et al. / Nd203-SrO-CuO system: compounds and phase relations
Structural study of La2_xSLCuO4_y by Goodenough [31] and Nguyen et al. [32] suggests that La2_xSrxCuO4_y can be synthesized in the T-type structure (with orthorhombic distortion or superstructure) in awide composition 0 ~ x < 1.34. In comparison, Hwang et al. [33] indicate that a structural evolution T' ~ T* -0 T ~ Sr2CuO3 phase exists with increasing Sr substitution in Pr2_xSrCuO4_r Takahashi et al. [30] reported a structural evolution T'-~T~Sr2CO3, not forming a T* phase. The T phase reported by them
105
T A B L E 5. Structural parameters for Nd0.aSrl.zCuO4_y
et al.
is ( N d l _ x S r x ) 2 C u O a _ y
(0.55 ~
parameters
a = b =3.03-3.71 ]k and c = 12.82-12.76/~, but they did not give enough diffraction data. We reexamined the structural evolution of the Nd2CuO4_y-Sr2CuO 3 system and obtained similar results. The T phase, i.e. the solid solution Nd2_xSLCuOa_y exists in the compositional region 1.2~
T A B L E 4. List of d spacings, diffraction intensity and hkl for NdosSri2CuO4_y a = b = 3 . 7 2 9 /[, c=12.81 A?, space group 14/ mmm, Z = 2 No
hkl
d (cal)
d (obs)
1 No. hM (obs)
1 2 3 4 5 6 7 8 9 10 11
002 101 004 103 130 132 006 105 134 200 202
6.405 3.582 3.203 2.809 2.637 2.438 2.135 2.112 2.036 1.865 1.7902
6.396 4 3.589 10 3.208 10 2.813 100 2.640 60 _c c 2.136 13 2.113 11 2.037 19 1.865 21 __c c
12 13 14 15 16 17 18 19 20 21 22
CDiffraction line is not observed.
136 231 107 204 008 233 206 235 138 109 260
d (cal)
d (obs)
1 (obs)
1.659 ~ 1.66 12 1.654 J 1.641 1.64 7 1.611 1.611 6 1.599 1.595 4 1.554 1.554 19 1.404 1.404 8 1.898 __c __c 1.367 1.368 4 1.328 __c __c 1.319 1.318 4
Atom
Position
X
Y
Z
N
Nd Sr Cu O1 02
4e 4e 2a 4c 4e
0 0 0 0 0
0 0 0 0.5 0
0.357 0.357 0 0 0.174
0.4 0.6 1.0 0.8 1.0
of O l to be 0.8 (6=0.4). A fairly good agreement was obtained between the measured and calculated X-ray powder diffraction intensity. We note, however, that the exact positions of the occupancy of oxygen remain uncertain. Measurements of the electrical resistivity as a function of temperature for all ternary compounds and solid solutions were performed by using a standard DC fourprobe technique. All samples exhibit an insulating or semiconducting-type behavior; no evidence for superconductivity above 4.2 K was detected, irrespective of heat treatment and gas atmospheres.
Acknowledgment This work was supported by the National Center for Research and Development of Superconductivity.
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X. Chen et al. / Nd203-SrO-CuO system: compounds and phase relations
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101
The Nd203-SrO-CuO system: compounds and phase relations X i a o l o n g C h e n a'b, J i n g k u i L i a n g a'c, C o n g W a n g a, G u a n g h u i R a o a, X i a n r a n X i n g d, Zhihong Song ~ and Zhiyu Qiao ° "Institute of Physics, Chinese Academy of Sciences, 100080 Beijing (China) bCCASTA (World Laboratory), PO Box, 8730, 100080 Beijing (China) Clnternational Center for Materials Physics, Academia Sinica, 110015 Shengyang (China) dUniversity of Science and Technology Beijing, 100083 Beijing (China)
(Received May 25, 1993; in final form August 30, 1993)
Abstract
The subsolidus phase relations of Nd203-SrO---CuO ternary system sintered in air at 950-1000 °C have been investigated by X-ray powder diffraction. The system can be divided into 13 three-phase regions and five twophase regions. In this system there exist at least four compounds and their solid solutions: Nd2SrCu206, Ndl+xSr2_xCu206 (0~
1. Introduction
Up to now, a series of RaO3-BaO-CuO ternary systems have been investigated to clarify the phase relations and to search for new superconductors. These ternary systems include Y203-BaO-CuO [1-3], La203-BaO-CuO [4, 5], Nd203-BaO-CuO [6], Gd203BaO-CuO [7], Ho203-BaO-CuO [8, 9], DyzO3-BaOCuO [8], YbzO3-BaO-CuO [10], Pr6Oal-BaO-CuO [11] and Sm203-BaO-CuO [12]. The common superconducting phase in these systems is RBa2Cu307_n except for PrBa2Cu307_6, which has not been observed to have superconductivity even down to 4.2 K. Another common phase R2CuO 4 exists in some R~O3-BaO-CuO systems (with large enough ionic radii of R 3+, otherwise the R2Cu205 phase forms [13]). Though compounds R2CuO4 are not themselves superconducting, they can be made superconducting by doping with oxygen or alkaline earths and Ce, etc., depending on their crystal structure, namely T, T* and T' phases. Hall measurements showed that the electrical carriers are holes in the T and T* phases, and are electrons in the T'phase. This difference has been attributed to the presence of apical oxygen atoms near the CuO2 layers, forming CuO6-0cathedra and CuO5 square pyramids in the T and T* phases, and only forming square planar CuO4 arrangements in the absence of these. The squareplanar CuO4 seems indispensable in electronic superconductors. Quite recently, Cava et al. [14] reported that superconductivity with To=60 K was observed in (La, 0925-8388/94/$07.00 © 1994 Elsevier Sequoia. All rights reserved SSDI 0925-8388(93)00956-Y
Sr)2CaCu20 6. Examination of its crystal structure revealed that it is related to La2CuO4 by intercalating a Ca-CuO2 layer between the upper and lower halves of CuO5 square pyramids. A general chemical formula La2Ca,_lCu,,O4+2,, may be adopted to designate the series of compounds with n = 1 (L%CuO4) and n = 2 (La2CaCu206). This is a typical case with high Tc superconductors. Similar situations have already been observed in T1- and Bi-based superconductors. Now the question arises of whether or not a series of Nd2CuO4-base d compounds NdEM, _ lCu, 0 4 + 2."t exist. If so, is the Cu-O arrangement square planar? Can these compounds be made into electron-type superconductors by doping with appropriate elements like Ce? With these questions in mind, we first tried to synthesize Nd2MCu206 and chose alkaline earths as M since alkaline earths are known to be good host elements in some layered superconductors. Ba was not considered as a candidate because RBa2Cu3Oy forms easily in the R203-BaO-CuO system as we mentioned above. Preliminary experiments showed that it is difficult to synthesize RzCafu20 6 (R-=Nd, Sm, Gd) under atmospheric pressure, but NdzSrCu206 is formed easily. This compound is known to possess double CuO sheets with space group I 4 / m m m . However, this compound is a semiconductor. It has not been made a superconductor even by doping. In addition, several compounds such as NdSrCuO3.5 [15], Nda.8Srl.2Cu206_ ~ [15], Nd6Sr3Cu60,7 [16], Nd2SrCuzOs.76 [17] and Nd1.4Srl.6Cu2Os.79 [18] ternary
102
X. Chen et al. / Nd203-SrO-CuO system: compounds and phase relations
SrO
systems have been reported in the N d 2 O a - S r O - C u O system. To clarify the phase relations and search for new possible superconductors, we have systematically investigated the subsolidus relation of the N d z O 3 - S r O - C u O system and the crystal structure of the compounds.
uO~
2. Experimental details
A series of N d 2 0 3 - - S r O - C u O samples of different composition were prepared by solid-state reaction of an appropriate mixture of high purity of Nd203, SrO and CuO. The powders were mixed, ground and fired at 950 °C for 12 h in air. Several additional firings for 24 h intervals with intermediate grindings were performed before the samples were pressed into pellets of diameter 10--12 mm and thickness 2-4 mm. Then the pellets were sintered at 1000 °C for 3 days. The above process was repeated for some samples until the volumes of component phases were unchanged. Phase identifications were partially performed on a Guinier-de Wolff monochromatic focusing transmission camera and partially on a Rigaku automatic diffractometer using CuKa radiation. High purity Si was added to the samples as an internal standard to correct the 20 position of diffraction peaks in case precision measurements should be required. The lattice parameters were then calculated by using a standard least-square reduction method. The electrical resistivity and a.c. magnetic susceptibility were measured on rectangular bars cut from sintered pellets employing the standard d.c. four-probe technique with In contacts attached to electric leads. Data was collected from 300 K down to 4.2 K.
3. Results
3.1. Binary oxides For the binary N d 2 0 3 - S r O system, powder X-ray diffraction indicated that the samples consist of Nd203, minor SrO and some minor unknown phase after sintering. Since the diffraction lines are very weak and few, we cannot index the pattern of the unknown phases. The instability of SrO in air may be responsible for this result. Tresvyatskii et al. [19] reported that there exist at least two stable compounds, SrO. 2Nd203 and 2SrO-Nd203, in the system. We have, however, not yet verified the existence of these compounds in the present investigation. The three-phase regions I, XII and XIII in Fig. 1 are only tentative representations of the subsolidus phase relations in the corresponding compositional region; further study is still needed to clarify the ambiguity in compatibility I, XII and XIII.
NdO,s
Nd~CuO~
CuO
Fig. 1. The subsolidus phase relation for the NdOLs--SrO--CuO system sintered at 950-1000 °C in air; compatibitities I, XII and XIII are tentative and divided by broken lines. The tie lines for the two-phase fields are indicated schematically: A -- Nd2SrCu2Or, B -- Ndl+xSr2_xCu2Oy (0
In the binary system Nd203--CuO, only one compound, Nd2CuO4, is identified. It crystallizes in an tetragonal unit cell, I4/mrnm, with square planar Cu-O arrangements, that is, the so-called T' phase. Its cell parameters are a = 3.938, c = 12.1465/~. Nd2CuO4 melts congruently at about 1200 °C [20], but others [21] argued that it melts incongruently since it is needed to add appropriate flux to grow the single crystal of Nd2CuO4. In the binary system SrO-CuO, the following compounds have been reported: SrzCuO3, SrCuO2, SrCul.6sO2.65+,s, Sr3CusO8 and Sr14Cu24041. The compound Sr2CuO3 belongs to an orthorhombic lattice, space group Immm, with lattice parameters a = 12.68-12.71 A, b=3.91-3.913 /~ and c=3.48--3.50 /~ [22, 23]. These results are in good agreement with the present study. Sr2CuO3 has an orthorhombic distorted KzNiF4 structure. Each Cu is coordinated to four O, forming distorted square planar Cu-O arrangements along its long axis a. From the structural point of view, it can be thought of as forming from the removal of a pair of oxygen atoms on the square plane from the CuO6 octahedra in L a 2 C u O 4. Isostructures were found in Ca2CuO3 [23, 24] and Ba2CuO3+~ [3]. The compound SrCuO2 crystallizes in an orthorhombic unit cell, space group Cmcm, with a =3.562 /k, b=16.32 /~ and c=3.918 ~, which are consistent with the results by other works [23, 25]. The structure consists of square plane CuOz. The CuO2 squares are connected in isolated infinite double zig-zag ribbons [24, 25]. Under high pressure and high temperature,
103
X. Chen et al. / Nd203-SrO-CuO system: compounds and phase relations
SrCuO2 exhibits a new structure, an infinite sequence of CuO2 planes separated by planes of SrO [26], i.e. the "infinite layer" compound. The structure is still stable at ambient pressure upon synthesis under high pressure. In the present investigation, we did not synthesize compound SrCu202, which Teske and Mtiller-Buschbaum [27] reported to belong to a tetragonal lattice, ace group I41/amd, with lattice parameters a =5.48 and c=9.82 ~. Each Cu +1 ion in the unit cell has only two 0 2 - neighbors forming an infinite zig-zag chain. It was made by firing SrO and CuO at 700-800 °C in air. Our results showed that this compound cannot be synthesized in air since Cu ÷1 is stable in a reducing gas atmosphere. De Leeuw et al. [28] reached the same conclusion. The remaining compounds in the system are Sr3CusOs, SKCUl.6502.65+6 and SElnCU24041. Sr3CusO8 was indexed as a face-centered lattice, space group F m m m , with lattice constants a = 13.402 ~, b = 11.470 ~ and c = 3.939 ,~ [23]. SrCu1.6502.65+,~ was indexed as a=13.416 ~, b = 11.478 ~ and c = 3.949 ~, most of its indices obey h + k = 2n, k + 1= 2n, h + 1= 2n, but some weak reflections like (510), (520) and (141) etc., violate these reflection conditions. Therefore, SrCua.6502.65 + 8 do not belong to a face centered lattice [28]. Based on diffraction data by a single crystal, McCarron et al. [29] characterized the crystal structure of 8r14Cu24041 as an orthorhombic unit cell, space group Pcc2, with lattice parameters a = 11.469/~, b = 13.368 ~ and c = 27.501/~. The authors argued that the structure consists of two unique subcells, one with Sr-(Cu203 sheets)-Sr layers in an orthorhombic cell with dimensions of a=11.459, b=13.368 and c = 3.931 ~ and a second structure with layers of CuO2 chains in a cell possessing identical a and b values but with a different c = 2.749 /~. Since these two subcells are nearly commensurate at 7*c(sheet)= 27.372 ~ and 10*c(chain)=27.534 ~, the chemical formula is designated by Sr14Cu24041. The atomic ratios of Cu and Sr are 1.67, 1.65 and 1.71 for Sr3CusOs, 8rCUl.6502.65+,5 and Sra4Cu24041 respectively. Considering the composition and their unit cell dimensions, we think these three compounds should belong to one phase. Our powder diffraction data are nearly identical with that by De Leeuw et al. [27]. Some reflections violating the face-centered symmetry really exist. Therefore, the compound should possess a space group with lower symmetry than Fmrnm. We adopt Sr3CusO 8 as the chemical formula for the time being. The real space group is needed to redetermine this. Like Ca [27] and Y [28], Nd can partially substitute for Sr in Sr3CusOs. Figure 2 shows the variations of lattice constants vs. the Nd content for a solid solution Sr3 -x NdxCusO8. The lattice parameters a and b decrease with increasing Nd, while c is nearly unchanged and
615 "' o<
600
585 570 ~
I
°
0
O0
0 O
13.5 000•0 13.0 12.5
~
12.0 11.5
vv~v
~c
V
v
V
v
[]
[]
11.0 4.5
4.0
[]
3.5
D D D D D i
0.0
0.5
[] i
1.0
I
1.5
2.0
X Fig. 2. The variations of lattice constants a, b, c and unit cell
volume
V vs. x
for Sr3_.Nd.CusOs.
a decreases in a more rapid manner than b. The solid solution limit is about x = 1.5 as indicated by a leveling off of lattice constants a and b, and the appearance of impurity phases. In view of the difference in ionic radii between Sr and Nd, it is expected that the unit cell volume will decrease with increasing Nd content. As the structure is layered, substitution would directly affect the layer spacings, stacking along the a axis, while having a smaller effect on the b--c plane parameters which are determined, for the most part, by the copper--oxygen connectivity. Since the ionic radius of Nd 3+ is closer to Sr 2+ than that of y3+, Sr3_~Nd, fusO8 will have a larger solution limit than Sr3_,Y~CusOs, in which case, the solid-solution limit is about 1.0 [28].
3.2. Ternary compounds Two compounds Nd2SrCu206 and Ndl +xSrz_,CuzOy always coexist in the preparation of Ndz_,Srl+xCu2Oy samples. Their lattice parameters are very close, but they do not belong to a solid solution because the diffraction intensities of these two compounds change in an opposite manner with increasing x. For a small x, NdzCuO4, NdzSrCu206 and a small amount of N d l +xSr2 --xCuzOy often coexist. Isolation of Nd2SrCu206 or Ndl+xSr2_,Cu2Oy requires a long annealing time. From the diffraction data (see Table 1), we can index Nd2SrCu206 as a tetragonal body-centered pattern with a = b = 3 . 8 3 /k and c=19.71 /~. Compared with the parameters of Nd2CuO4, a and b shrink, while c increases a lot. Since the layered stacking of cations in a unit cell for NdzCuO4 is C u - N d - N d - C u - N d - N d - C u , the
X. Chen et al. / Nd203--SrO--CuO system: compounds and phase relations
104
T A B L E I. List of d spacings, diffraction intensity and hid for Nd2SrCu206, a = b = 3 . 8 3 /~,, c = 19.71 /~, space group 14/mmm, Z=2 No. hkl
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
002 004 101 103 006 105 110 112 008 114 107 116 0010 200 109 202 118 204 211 213 206 0012 1011
d¢,~¢
dobs
(A)
(A)
9.8550 4.9275 3.7597 3.3088 3.2850 2.747 2.7082 2.6114 2.4637 2.3734 2.2686 2.0896 1.9710 1.915 1.9011 1.8798 1.8224 1.7849 1.7065 1.6575 1.6544 1.6425 1.6230
9.81 4 24 4.930 1 25 3.764 16 26 3.312 3 27 3.886 5 28 2.748 100 29 2.713 60 30 _a -a 31 2.464 3 32 2.371 2 33 2.270 18 34 2.091 25 35 1.971 10 36 1.916 2 37 1.897 3 38 1.886 4 39 1.822 1 40 1.788 1 41 1.710 5 42 1.655 6 43 -" -~ 44 1.638 10 45 1.622 8
Io~
No. hkl
1110 215 208 217 1013 0014 1112 2010 220 219 222 224 301 303 226 1114 2012 1015 2111 0016 305 310
dc~lc
dob~
(A)
(A)
1.5936 1.594 1.5709 1.573 1.5120 1.513 1.4633 1.465 1.4097] 1.4079 I 1.406 1.4044 1.3735 1.372 1.3541 1.3350 1.3492 _a 1.3415 -~ 1.3057 1.304 1.2740 -" 1.2532~ 1.2519 .I 1.252 1.2492 -~ 1.2467 -~ 1.2429 1.243 1.2381 1.235 1.2319 1.232 1.2146 ]~ 1.212 1.2127 J
lobs 13 30 2 8 10 8 10 _~ -~ 3 -" 2 -~ -~ 3 7 5 6
aDiffraction line is not observed T A B L E 2. The structural parameters for Nd2SrCu206 Atom
Position
X
Y
Z
N
Sr Nd Cu O1 02 03
2b 4e 4e 8g 4e 2a
0 0 0 0 0 0
0 0 0 0.5 0 0
0.5 0.31 0.090 0.09 0.19 0.1
1.0 1.0 1.0 1.0 1.0 0.1
8(g) and 4(e), and the occupancies were set as 1.0. We found that there should be partial oxygen atoms occupied (2a) positions though the occupancy is as low as 0.1, otherwise the calculated intensities of the diffraction line disagree much with the observed ones no matter how the atomic positions of other atoms are adjusted. This suggests that not all the Cu ions are coordinated by five oxygen atoms to form CuO5 square pyramids. Two Cu ions are coordinated by six oxygen atoms and form two CuO6 octahedra in every ten unit cells. According to Labbe's study [15], there should exist a small solid solution Nd2_xSr1+xCu206_~. In the present investigation, however, we cannot conclude the existence of this solid solution due to the impurity of the samples. Another compound, or rather, a solid solution which often coexists with Nd2SrCu206 is Ndl+xSr2_xCu206 (0~
14/mram, Z = 2
average spacing between the layers is equal to c/6 ~- 12/ 6 = 2 /~. Based on the approximate average layered spacing, we might estimate that the number of layers in the unit cell for Nd2SrCu206 is 19.71/2= 10. The possible arrangement of cations along the z axis is Sr-Cu-Nd-Nd-Cu-Sr--Cu-Nd-Nd-Cu-Sr. Hence, Nd2SrCu206 is related to Nd2CuO4 by inserting a layer of Sr-O between the upper and lower halves of the CuO4 squares. The number of chemical formula in the unit cell is z = 2 for Nd2SrCu206.The remaining problem is how the Cu is coordinated by O and whether they form CuO4 square planes, CuO5 square pyramids or CuO6 octahedra. Referring to thecrystal structure of Nd~.sSr~.2Cu206_ [15], we have roughly determined the atomic parameters for Nd2SrCu206 by using the LAZY program; these are listed in Table 2. The oxygen atoms were set to occupy
NO. hkl
doe
dobs
(A)
(.~)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
002 004 101 006 103 105 130 132 008 134 107 136 0010 109 200 202 138 204
10.06 5.03 3.7425 3.3533 3.296 2.759 2.679 2.589 2.515 2.3652 2.2902 2.0935 2.012 1.9255 1.895 1.8622 1.8339 1.7733
9.91 4.99 3.722 3.356 3.286 2.756 2.678 _b 2.509 2.371 2.290 2.093 2.015 1.921 1.983 1.861 __b __b
19 20 21 22
231 0012 206 1011
1.6890 1.6767 1.6498 1.6473
1.683 1.672 1.649
lobs No. hkl
4 1 2 4 3 100 50 __b 3 2 15 22 11 2 2 3 _b b
23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
6 41 11 42 13 43
bDiffraction line is not observed.
d~lc
dobs
(A)
(A)
233 1,643 1310 1,6090 235 1.562 208 1.5135 237 1,4600 0014 1.4371"[ 1013 1,4328 J 1312 1.4214 2010 1.3795 236 1.3506 260 1.3400 262 1.3282 224 1.2948 1614 1.263 ] 1015 1.263 I 301 1.263 0016 1.257 ] 2012 1.2557 i 266 2611 303
lobs
1.638 1.610 12 1.561 27 1.513 2 1.465 8 1.433 9 1.420 5 1.372 7 1.350 4 b _b _b _b _b _b 1.263
4
1.254
3
1.2443 ] 1.2432 I 1.243 1.2415
5
X. Chen et al. / Nd203-SrO-CuO system: compounds and phase relations
Structural study of La2_xSLCuO4_y by Goodenough [31] and Nguyen et al. [32] suggests that La2_xSrxCuO4_y can be synthesized in the T-type structure (with orthorhombic distortion or superstructure) in awide composition 0 ~ x < 1.34. In comparison, Hwang et al. [33] indicate that a structural evolution T' ~ T* -0 T ~ Sr2CuO3 phase exists with increasing Sr substitution in Pr2_xSrCuO4_r Takahashi et al. [30] reported a structural evolution T'-~T~Sr2CO3, not forming a T* phase. The T phase reported by them
105
T A B L E 5. Structural parameters for Nd0.aSrl.zCuO4_y
et al.
is ( N d l _ x S r x ) 2 C u O a _ y
(0.55 ~
parameters
a = b =3.03-3.71 ]k and c = 12.82-12.76/~, but they did not give enough diffraction data. We reexamined the structural evolution of the Nd2CuO4_y-Sr2CuO 3 system and obtained similar results. The T phase, i.e. the solid solution Nd2_xSLCuOa_y exists in the compositional region 1.2~
T A B L E 4. List of d spacings, diffraction intensity and hkl for NdosSri2CuO4_y a = b = 3 . 7 2 9 /[, c=12.81 A?, space group 14/ mmm, Z = 2 No
hkl
d (cal)
d (obs)
1 No. hM (obs)
1 2 3 4 5 6 7 8 9 10 11
002 101 004 103 130 132 006 105 134 200 202
6.405 3.582 3.203 2.809 2.637 2.438 2.135 2.112 2.036 1.865 1.7902
6.396 4 3.589 10 3.208 10 2.813 100 2.640 60 _c c 2.136 13 2.113 11 2.037 19 1.865 21 __c c
12 13 14 15 16 17 18 19 20 21 22
CDiffraction line is not observed.
136 231 107 204 008 233 206 235 138 109 260
d (cal)
d (obs)
1 (obs)
1.659 ~ 1.66 12 1.654 J 1.641 1.64 7 1.611 1.611 6 1.599 1.595 4 1.554 1.554 19 1.404 1.404 8 1.898 __c __c 1.367 1.368 4 1.328 __c __c 1.319 1.318 4
Atom
Position
X
Y
Z
N
Nd Sr Cu O1 02
4e 4e 2a 4c 4e
0 0 0 0 0
0 0 0 0.5 0
0.357 0.357 0 0 0.174
0.4 0.6 1.0 0.8 1.0
of O l to be 0.8 (6=0.4). A fairly good agreement was obtained between the measured and calculated X-ray powder diffraction intensity. We note, however, that the exact positions of the occupancy of oxygen remain uncertain. Measurements of the electrical resistivity as a function of temperature for all ternary compounds and solid solutions were performed by using a standard DC fourprobe technique. All samples exhibit an insulating or semiconducting-type behavior; no evidence for superconductivity above 4.2 K was detected, irrespective of heat treatment and gas atmospheres.
Acknowledgment This work was supported by the National Center for Research and Development of Superconductivity.
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X. Chen et al. / Nd203-SrO-CuO system: compounds and phase relations
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