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Structural anomalies and non-linearity of Cu-O chains in YBa2Cu3O7 by high resolution neutron powder diffraction
PhysicaC153-155 (1988) 962 963 North-Holland, Amsterdam
STRUCTURAL ANOMALIES AND NON-LINEARITY OF CU-O CHAINS IN NEUTRON POWDER DIFFRACTION
M. FranGois*, A. Junod #, K. Yvon*, and A.W. Hewat &
YBa2Cu307
P. Fischer £, J.J. Capponi$,
BY HIGH RESOLUTION
P. Strobel $, M. Marezio$,
*Laboratoire de Cristallographie aux Rayons X, and # D4partement de Physique de la Mati~re Condens~e, Universit4 de Gen~ve, CH 1211 Gen~ve, Switzerland £Labor f~r Neutronenstreuung, ETH, CH 5303 WOrenlingen, Switzerland $Laboratoire de Cristallographie, CNRS, 166X, 38042 Grenoble, France, and &Institut Laue-Langevin, 156X, 38042 Grenoble, France
The structure of orthorhombic YBa2Cu30 x was redetermined on two very pure samples with xi~6.86 and x2~6.91, in the temperature interval 5-320K using high resolution neutron diffraction. No major structural change is observed at the onset of superconductivity but the lattice dimensions and some of the structural parameters show small anomalies at or near 90K and 240K. All vibrational amplitudes are normal except for those of the 04 atoms across the Cu-O chain, which are large. Good agreement with the data is obtained by assuming that the 04 atoms occupy potential minima at about 0.1A away from either side of the chain axis.
i. INTRODUCTION In orthorhombic oxygen deficient YBa2Cu307. 6 some oxygen atoms (04) on the so-called "linear Cu-O chains" are missing, and their displacement amplitudes (in particular those perpendicular to the chain axis) are anomalously large. Above room temperature these displacements become even larger and are apparently the precursor of a high temperature phase transition to an oxygen conducting phase, with finally complete loss of the 04 chain atoms from the structure near 1000K to leave semi-conducting YBa2Cu306. Below room temperature the displacement amplitudes parallel to the Cu-O chain axis decrease but those perpendicular to the chain axis remain large even at 5K. 2. EXPERIMENTAL Measurements were made on the new highresolution powder diffractometer D2B (ILL) {(sin(8)/~=0.91~} in the temperature range 5 to 320K on two samples. YBa2Cu306 91 (open squares a=3.8177(1)A, b~3.8836(1)%, "c=Ii.6827(3)A at 320K) was prepared by citrate pyrolysis and
0921-4534/88/$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
YBa2Cu306.8~ (filled squares, a=3.8180(1)A, b=3.8835(1)A, c=ii.6726(3)A) by a ceramic method. For structure refinement two models were tested (space group Pmmm). One (unsplitatom model) assumed the 04 site to be located on the Cu-O chain axis, i.e. at the one-fold position le (0, 1/2, 0; point symmetry mmm), with nearly full occupancy and anisotropic temperature factor. The other (split-atom model) assumed this site to be displaced along [i00] perpendicular to the Cu-O chain axis, i.e at the two-fold position 2k (±x, 1/2, O; x=O.020; point symmetry 2mm), with nearly half occupancy and isotropic temperature factor. 3. RESULTS AND DISCUSSION The refinements with unsplit and split 04 atoms converged at about the same agreement indices (Rf=3%). The orthorhombic lattice splitting parameter (b-a)/(b+a) shown in Fig i increases increases with decreasing temperature and shows a cusp around 90K, in agreement with previous X-ray measurements [i]. Below 90K its rate of increase is much smaller. There also
M. Frangois et al. / Structural anomalies and non-linearity of Cu-O chains in Yt~a2Cu307
appears to be an anomaly at 240K. The (a-b)/(b+a) values of the more reduced sample are systematically smaller,
963
These results suggests that the Cu-O chains are not linear. A structural drawing representing possible zig-zag patterns for these chains is shown Sn Fig 3.
0,0091 0,0090
Q m [ ] ,/.~
0,0089
[]
(b-a)/(b+a)
0,0088
[] Q
0,0087
El
0,0086
0,0085
O
0,00~
.
0
I
I
I
|
100 200 Temperature (K)
I
I
I
300
• Cul
o 04
~. v a c o n c y
FIGURE 3 FIGURE I As can be seen in Fig 2 the displacement parameter x(O4) of the split atom position 04 assumes non-zero values at all temperatures. It decreases as the temperature is decreased and remains approximatly constant below 90K. There also appears to be an anomaly at 240K.
0,040 0,035
x(O4)
0,030 0,025 0,020
........
100 200 Temperature (K) FIGURE 2
300
Whether or not the O atoms are ordered on the off-center positions cannot be decided from these quasi-elastic data. In case of disorder the structure is likely to become ordered at low temperature, which would perhaps explain the anomalies near 240K and 9OK. REFERENCES (I) P.M. Horn, D.T. Keane, G.A. Held, J.L. Jordan-Sweet, D.L. Kaiser, F. Holtzberg and T.M. Rice, Phys. Rev. Left. 59, 2772-2775 (1987)
STRUCTURAL ANOMALIES AND NON-LINEARITY OF CU-O CHAINS IN NEUTRON POWDER DIFFRACTION
M. FranGois*, A. Junod #, K. Yvon*, and A.W. Hewat &
YBa2Cu307
P. Fischer £, J.J. Capponi$,
BY HIGH RESOLUTION
P. Strobel $, M. Marezio$,
*Laboratoire de Cristallographie aux Rayons X, and # D4partement de Physique de la Mati~re Condens~e, Universit4 de Gen~ve, CH 1211 Gen~ve, Switzerland £Labor f~r Neutronenstreuung, ETH, CH 5303 WOrenlingen, Switzerland $Laboratoire de Cristallographie, CNRS, 166X, 38042 Grenoble, France, and &Institut Laue-Langevin, 156X, 38042 Grenoble, France
The structure of orthorhombic YBa2Cu30 x was redetermined on two very pure samples with xi~6.86 and x2~6.91, in the temperature interval 5-320K using high resolution neutron diffraction. No major structural change is observed at the onset of superconductivity but the lattice dimensions and some of the structural parameters show small anomalies at or near 90K and 240K. All vibrational amplitudes are normal except for those of the 04 atoms across the Cu-O chain, which are large. Good agreement with the data is obtained by assuming that the 04 atoms occupy potential minima at about 0.1A away from either side of the chain axis.
i. INTRODUCTION In orthorhombic oxygen deficient YBa2Cu307. 6 some oxygen atoms (04) on the so-called "linear Cu-O chains" are missing, and their displacement amplitudes (in particular those perpendicular to the chain axis) are anomalously large. Above room temperature these displacements become even larger and are apparently the precursor of a high temperature phase transition to an oxygen conducting phase, with finally complete loss of the 04 chain atoms from the structure near 1000K to leave semi-conducting YBa2Cu306. Below room temperature the displacement amplitudes parallel to the Cu-O chain axis decrease but those perpendicular to the chain axis remain large even at 5K. 2. EXPERIMENTAL Measurements were made on the new highresolution powder diffractometer D2B (ILL) {(sin(8)/~=0.91~} in the temperature range 5 to 320K on two samples. YBa2Cu306 91 (open squares a=3.8177(1)A, b~3.8836(1)%, "c=Ii.6827(3)A at 320K) was prepared by citrate pyrolysis and
0921-4534/88/$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
YBa2Cu306.8~ (filled squares, a=3.8180(1)A, b=3.8835(1)A, c=ii.6726(3)A) by a ceramic method. For structure refinement two models were tested (space group Pmmm). One (unsplitatom model) assumed the 04 site to be located on the Cu-O chain axis, i.e. at the one-fold position le (0, 1/2, 0; point symmetry mmm), with nearly full occupancy and anisotropic temperature factor. The other (split-atom model) assumed this site to be displaced along [i00] perpendicular to the Cu-O chain axis, i.e at the two-fold position 2k (±x, 1/2, O; x=O.020; point symmetry 2mm), with nearly half occupancy and isotropic temperature factor. 3. RESULTS AND DISCUSSION The refinements with unsplit and split 04 atoms converged at about the same agreement indices (Rf=3%). The orthorhombic lattice splitting parameter (b-a)/(b+a) shown in Fig i increases increases with decreasing temperature and shows a cusp around 90K, in agreement with previous X-ray measurements [i]. Below 90K its rate of increase is much smaller. There also
M. Frangois et al. / Structural anomalies and non-linearity of Cu-O chains in Yt~a2Cu307
appears to be an anomaly at 240K. The (a-b)/(b+a) values of the more reduced sample are systematically smaller,
963
These results suggests that the Cu-O chains are not linear. A structural drawing representing possible zig-zag patterns for these chains is shown Sn Fig 3.
0,0091 0,0090
Q m [ ] ,/.~
0,0089
[]
(b-a)/(b+a)
0,0088
[] Q
0,0087
El
0,0086
0,0085
O
0,00~
.
0
I
I
I
|
100 200 Temperature (K)
I
I
I
300
• Cul
o 04
~. v a c o n c y
FIGURE 3 FIGURE I As can be seen in Fig 2 the displacement parameter x(O4) of the split atom position 04 assumes non-zero values at all temperatures. It decreases as the temperature is decreased and remains approximatly constant below 90K. There also appears to be an anomaly at 240K.
0,040 0,035
x(O4)
0,030 0,025 0,020
........
100 200 Temperature (K) FIGURE 2
300
Whether or not the O atoms are ordered on the off-center positions cannot be decided from these quasi-elastic data. In case of disorder the structure is likely to become ordered at low temperature, which would perhaps explain the anomalies near 240K and 9OK. REFERENCES (I) P.M. Horn, D.T. Keane, G.A. Held, J.L. Jordan-Sweet, D.L. Kaiser, F. Holtzberg and T.M. Rice, Phys. Rev. Left. 59, 2772-2775 (1987)