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Low-temperature phase structure of the T∗-phase compound (La, Tb, Pb)2CuO4
Physica C 185-189 (1991) 541-542 North-Holland
LOW-TEMPERATURE PHASE STRUCTURE OF THE T*-PHASE COMPOUND (La, Tb,Pb)2CuO4
P.BordeP, R. Argoud l, C. ChaillouP, J. Chenavas I, S-W. Cheong2, Th. Fournierl,3, J.L. Hodeau !, M. Mareziol,2, J. Muller1 and A. Varela t. 1Laboratoire de Cristallographie CNRS, BP 166X, 38042 Grenoble Cedex France.2AT&T Bell Laboratories, Murray Hill, NJ07974 USA,;CRTBT, CNRS, BP 166X, 38042 Grenoble Cedex France. The low temperature structure of the T*-phase compound (La,Tb,Pb)2CuO4 was determined by single crystal x-ray diffraction at 50K. It is characterized by ordered canting of the CuO5 pyramids and corrugation of the CuO2 planes. The existence of two Cu sites with different valences in the CuO2 planes induces charge ordering which is detrimental for superconductivity. 1. INTRODUCTION The 2:1:4 cuprate superconductors are known to crystallize with three structures, T, T' and T*. The T*-phase structure 1 (P4/nmm, a--3.86,~,, c=12.5,~,) contains alternately one half unit cell of the T- and T'-structure.The compound ( N d , S r , C e ) 2 C u O 4 - y was reported to become superconducting by A k i m i t s u et al. 2. From neutron powder data, Izumi et al.3 refined the structure of a fully oxidized and superconducting sample, and of a d e o x i d i z e d , non superconducting one, w i t h o u t finding any a p p r e c i a b l e difference b e t w e e n the two refinements. Recently we reported the results of a single crystal x-ray diffraction study of the non superconducting T*-phase compound ( L a , T b , P b ) 2 C u O 4 4 We showed that this compound exhibits a superstructure at room temperature and determined the structure o f the high temperature phase, which is similar to that found by Izumi ,., °' a!. for ,.¢~a.~,~.,--,,-~" c',~v~CuOa_,,.., The room temperature unit cell was a=b=5.451.~ and c=12.426/k, with systematic extinctions for (hk0) reflections with h and k odd. This was interpreted as due to orthorhombic symmetry with space group Pmma or one of its subgroups, and twinning by the (110) plane. The width of the superstructure reflections indicated short-
range ordering. 2. EXPERIMENTAL The single crystal platelet used in the previous structure determination was mounted on a Phillips PW1100 diffractometer equipped with AgKtx radiation. The low temperatures were achieved by using a helium flow cryostat with magnetically coupled crystal holder 5. 7409 reflections up to 0 < 30 ° were collected with the o-scan mode (speed:0.02°/s, width:2 °, number of scans:_<4). Absorption correction was made by gaussian integration using the crystal shape. By comparing the intensities of reflections for which only one twin individual contributes, the twin ratio was found to be 50/50. Thus, the intensity of each (hkl) reflection can be described as Iobs(hkl) = 0.5.I(hkl)1 + 0.5.l(khl)2, where the subscripts refer to the two twin individuals. As a consequence, lobs(hkl) -~ Iobs(khl) and the intensities can be averaged in the 4/.m___m___.m_point group. _The refinement~ were carried out with orthorhombic space groups by describing each (hkl) reflection intensity as Icalc(hkl) = 0.5.lcalc(hkl)l + 0.5.lcalc(khl)2• Refinements carried out with the Pmma space group dead not yield enough intensity for the superstructure reflections. We then switched to the P21212 space group which allows all atoms
0921-4534/91/$03.50 © 1991 - ElsevierSciencePublishers B.V. All rights reserved.
542
P. Border et aL / T*-phase compound (La, Tb,Pb)zCuO4
to be placed in general positions. The results obtained indicated that all atoms obeyed the symmetry of the P2lma space group which was then used for the final calculations. In this case, there are two different positions for each of the La, Tb, Cu, O1 and 0 2 atoms. The isotropic thermal factors were constrained to be equal for the two positions. The La/Pb and Th/La occupancy ratios were also refined with the same type of constraint. The final agreement factors were R ( F 2 ) = 7 . 0 7 % , w R ( F 2 ) = 1 0 . 5 % and X2--1.73 for 669 observations and 34 variables. The results and interatomic distances are shown in Table I. 3. DISCUSSION The most important feature of the low temperature structure is the existence of an ordered canting of the CuO5 pyramids in the (110) direction of the perovskite cell. This is similar to the canting of octahedra in the orthorhombic phase of La2CuO4. The canting is accompanied by distortions of the pyramids and by the corrugation of the CuO2 plane. However, in the present case two inequivalent Cu sites are generated in the CuO2 plane. As shown in Table I, the corresponding Cu cations have different valences. This leads to charge ordering in the C u O 2 planes and could explain why this compound is not superconducting, although the average Cu valence is more than 2+. The existence of the superstructure is probably due to the difference in size between the two rare earth cations and to the mismatch at low temperature between the CuO2 planes and the rare earth oxyde planes. A similar superstructure was also observed by electron diffraction at 77K for a (LaI,Sm0.8,Sr0.2)CuO4 sample but the superstructure spots were very broad and diffuse, indicating very short range ordering due to a smaller deformation. It is known that for (La,R,Sr)2CuO4 compounds,
TABLE 1 Positions, thermal factors and interatomic distances (A,) for (La,Tb,Pb)CuO4, at 50K. X .2647(2) .7409(2) .2534(2)
Z U(,~ 2) .38475(9) .00516(8) La2 .38399(9) id. Tbl .09717(8) .00336(7) Tb2 .7438(2) 1/2 .09693(8) id. Cu I .2509(3) 0 . 7 5 6 0 ( 2 ) .0029(2) Cu2 .7480(3) 1/2 .7566(2) id. O 11 . 4 9 6 ( 4 ) .263(4) .7562(6) .005(1) O 12 .001 (4) .270(3) .7698(6) id. O21 .214(4) 0 .574(1) .011(1) 022 .794(3) 1/2 .571(1) id. 03 .016(3) .255(4) .0052(7) .005(1) The compositions of the La and Tb sites are : 0.913(4)l_a/0.087(4)Tb and 0.82(1)Tb/0.18(1)La. Atom
Lal
Lal O11 O12 O21 O21 O21
(x2) (x2) (xl) (xl) (xl) 0 2 2 (x2)
average
Tbl O11 O12 03 03
(x2) (x2) (x2) (x2)
average
C u l O11 (x2) O12 (x2) O21 (xl) average valence
Y 0 112 0
2.70(1) La2 O11 (x2) 2.58(2) 2.74(1) O12 (x2) 2.64(1) 2.35(2) O21 (x2) 2.784(3) 2.50(2) 022 (xl) 2.35(2) 3.05(2) 022 (xl) 2.51(2) 2.786(3) 022 (xl) 3.06(2) 2.706 average 2.659 2.71(2) Tb2 O l l (x2) 2.62(2) 2.59(2) O12 (x2) 2.46(2) 2.21(2) 03 ( x 2 ) 2.31(2) 2.37(2) 03 ( x 2 ) 2.22(2) 2.470 average 2.403 1.96(2) Cu2 O l i (x2) 1.89(2) 2.01(2) O12 (x2) 1.87(2) 2.28(2) 022 (xl) 2.31(2) 2.044 average 1.966 1.81 valence 2.47
average Cu valence : 2.14
superconductivity exists only for large R cations such as Sm, Eu and Gd. Our results seem to indicate that for these cations the superstructure is either very short range or does not exist at all. The charge ordering process described above would be irrelevant and superconductivity could be established. REFERENCES 1 E.Takayama-Muromachi et a l . , Jpn. J. Appl. Phys., 27 (1988) L2283. 2 J.Akimitsu et al.,Jpn. J. Appl. Phys., 27 (1988) L1859. 3 F.Izumi et al.,Physica C158 (1989) 440. 4 P.Bordet et al.,J. Less Comm. Met. 164-165 (1990) 792. 5 R.Argoud et al.,J. Appl. Cryst. 22 (1989) 584
LOW-TEMPERATURE PHASE STRUCTURE OF THE T*-PHASE COMPOUND (La, Tb,Pb)2CuO4
P.BordeP, R. Argoud l, C. ChaillouP, J. Chenavas I, S-W. Cheong2, Th. Fournierl,3, J.L. Hodeau !, M. Mareziol,2, J. Muller1 and A. Varela t. 1Laboratoire de Cristallographie CNRS, BP 166X, 38042 Grenoble Cedex France.2AT&T Bell Laboratories, Murray Hill, NJ07974 USA,;CRTBT, CNRS, BP 166X, 38042 Grenoble Cedex France. The low temperature structure of the T*-phase compound (La,Tb,Pb)2CuO4 was determined by single crystal x-ray diffraction at 50K. It is characterized by ordered canting of the CuO5 pyramids and corrugation of the CuO2 planes. The existence of two Cu sites with different valences in the CuO2 planes induces charge ordering which is detrimental for superconductivity. 1. INTRODUCTION The 2:1:4 cuprate superconductors are known to crystallize with three structures, T, T' and T*. The T*-phase structure 1 (P4/nmm, a--3.86,~,, c=12.5,~,) contains alternately one half unit cell of the T- and T'-structure.The compound ( N d , S r , C e ) 2 C u O 4 - y was reported to become superconducting by A k i m i t s u et al. 2. From neutron powder data, Izumi et al.3 refined the structure of a fully oxidized and superconducting sample, and of a d e o x i d i z e d , non superconducting one, w i t h o u t finding any a p p r e c i a b l e difference b e t w e e n the two refinements. Recently we reported the results of a single crystal x-ray diffraction study of the non superconducting T*-phase compound ( L a , T b , P b ) 2 C u O 4 4 We showed that this compound exhibits a superstructure at room temperature and determined the structure o f the high temperature phase, which is similar to that found by Izumi ,., °' a!. for ,.¢~a.~,~.,--,,-~" c',~v~CuOa_,,.., The room temperature unit cell was a=b=5.451.~ and c=12.426/k, with systematic extinctions for (hk0) reflections with h and k odd. This was interpreted as due to orthorhombic symmetry with space group Pmma or one of its subgroups, and twinning by the (110) plane. The width of the superstructure reflections indicated short-
range ordering. 2. EXPERIMENTAL The single crystal platelet used in the previous structure determination was mounted on a Phillips PW1100 diffractometer equipped with AgKtx radiation. The low temperatures were achieved by using a helium flow cryostat with magnetically coupled crystal holder 5. 7409 reflections up to 0 < 30 ° were collected with the o-scan mode (speed:0.02°/s, width:2 °, number of scans:_<4). Absorption correction was made by gaussian integration using the crystal shape. By comparing the intensities of reflections for which only one twin individual contributes, the twin ratio was found to be 50/50. Thus, the intensity of each (hkl) reflection can be described as Iobs(hkl) = 0.5.I(hkl)1 + 0.5.l(khl)2, where the subscripts refer to the two twin individuals. As a consequence, lobs(hkl) -~ Iobs(khl) and the intensities can be averaged in the 4/.m___m___.m_point group. _The refinement~ were carried out with orthorhombic space groups by describing each (hkl) reflection intensity as Icalc(hkl) = 0.5.lcalc(hkl)l + 0.5.lcalc(khl)2• Refinements carried out with the Pmma space group dead not yield enough intensity for the superstructure reflections. We then switched to the P21212 space group which allows all atoms
0921-4534/91/$03.50 © 1991 - ElsevierSciencePublishers B.V. All rights reserved.
542
P. Border et aL / T*-phase compound (La, Tb,Pb)zCuO4
to be placed in general positions. The results obtained indicated that all atoms obeyed the symmetry of the P2lma space group which was then used for the final calculations. In this case, there are two different positions for each of the La, Tb, Cu, O1 and 0 2 atoms. The isotropic thermal factors were constrained to be equal for the two positions. The La/Pb and Th/La occupancy ratios were also refined with the same type of constraint. The final agreement factors were R ( F 2 ) = 7 . 0 7 % , w R ( F 2 ) = 1 0 . 5 % and X2--1.73 for 669 observations and 34 variables. The results and interatomic distances are shown in Table I. 3. DISCUSSION The most important feature of the low temperature structure is the existence of an ordered canting of the CuO5 pyramids in the (110) direction of the perovskite cell. This is similar to the canting of octahedra in the orthorhombic phase of La2CuO4. The canting is accompanied by distortions of the pyramids and by the corrugation of the CuO2 plane. However, in the present case two inequivalent Cu sites are generated in the CuO2 plane. As shown in Table I, the corresponding Cu cations have different valences. This leads to charge ordering in the C u O 2 planes and could explain why this compound is not superconducting, although the average Cu valence is more than 2+. The existence of the superstructure is probably due to the difference in size between the two rare earth cations and to the mismatch at low temperature between the CuO2 planes and the rare earth oxyde planes. A similar superstructure was also observed by electron diffraction at 77K for a (LaI,Sm0.8,Sr0.2)CuO4 sample but the superstructure spots were very broad and diffuse, indicating very short range ordering due to a smaller deformation. It is known that for (La,R,Sr)2CuO4 compounds,
TABLE 1 Positions, thermal factors and interatomic distances (A,) for (La,Tb,Pb)CuO4, at 50K. X .2647(2) .7409(2) .2534(2)
Z U(,~ 2) .38475(9) .00516(8) La2 .38399(9) id. Tbl .09717(8) .00336(7) Tb2 .7438(2) 1/2 .09693(8) id. Cu I .2509(3) 0 . 7 5 6 0 ( 2 ) .0029(2) Cu2 .7480(3) 1/2 .7566(2) id. O 11 . 4 9 6 ( 4 ) .263(4) .7562(6) .005(1) O 12 .001 (4) .270(3) .7698(6) id. O21 .214(4) 0 .574(1) .011(1) 022 .794(3) 1/2 .571(1) id. 03 .016(3) .255(4) .0052(7) .005(1) The compositions of the La and Tb sites are : 0.913(4)l_a/0.087(4)Tb and 0.82(1)Tb/0.18(1)La. Atom
Lal
Lal O11 O12 O21 O21 O21
(x2) (x2) (xl) (xl) (xl) 0 2 2 (x2)
average
Tbl O11 O12 03 03
(x2) (x2) (x2) (x2)
average
C u l O11 (x2) O12 (x2) O21 (xl) average valence
Y 0 112 0
2.70(1) La2 O11 (x2) 2.58(2) 2.74(1) O12 (x2) 2.64(1) 2.35(2) O21 (x2) 2.784(3) 2.50(2) 022 (xl) 2.35(2) 3.05(2) 022 (xl) 2.51(2) 2.786(3) 022 (xl) 3.06(2) 2.706 average 2.659 2.71(2) Tb2 O l l (x2) 2.62(2) 2.59(2) O12 (x2) 2.46(2) 2.21(2) 03 ( x 2 ) 2.31(2) 2.37(2) 03 ( x 2 ) 2.22(2) 2.470 average 2.403 1.96(2) Cu2 O l i (x2) 1.89(2) 2.01(2) O12 (x2) 1.87(2) 2.28(2) 022 (xl) 2.31(2) 2.044 average 1.966 1.81 valence 2.47
average Cu valence : 2.14
superconductivity exists only for large R cations such as Sm, Eu and Gd. Our results seem to indicate that for these cations the superstructure is either very short range or does not exist at all. The charge ordering process described above would be irrelevant and superconductivity could be established. REFERENCES 1 E.Takayama-Muromachi et a l . , Jpn. J. Appl. Phys., 27 (1988) L2283. 2 J.Akimitsu et al.,Jpn. J. Appl. Phys., 27 (1988) L1859. 3 F.Izumi et al.,Physica C158 (1989) 440. 4 P.Bordet et al.,J. Less Comm. Met. 164-165 (1990) 792. 5 R.Argoud et al.,J. Appl. Cryst. 22 (1989) 584