Structural Aspects of the Phase Separation in La 2 CuO 4.032
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STRUCTURAL ASPECTS OF THE PHASE SEPARATION IN La2CuO4.032 1 C. CHAILLOUTI, P. BORDET , J. CHENAVAS41 , S.W. CHEONG 3 , Z. FISK 2 , 4 1 3 M.MAREZIO , , B. MOROSIN AND J.E. SCHIRBER 1 Laboratoire de Cristallographie, CNRS, associ6 Atl'Universit6 J. Fourier, 166X, 38042 Grenoble cedex, France 2 Los Alamos National Laboratory, NM 87545, USA 3 AT&T Bell Laboratories, Murray Hill, NJ 07974, USA 4 Sandia National Laboratories, Albuquerque, NM 87185, USA
ABSTRACT The average structure of superconducting La 2 CuO4.032 has been determined by single crystal neutron diffraction data. The excess oxygen is located between two adjacent LaO layers. Its presence distorts the apical-oxygen sublattice in such a way that a short 0-0 bond is formed (1.64A). By scanning several hkl reflexions, we have confirmed that a phase separation occurs below room temperature. The peaks of the phases are in agreement with the unit cells proposed by Jorgensen et al. and Zolliker et al. However, Cmca seems to be the correct space group for both phases. La2 Cu04+a and the Ni counterpart are, thus, not isostructural. NEUTRON DIFFRACTION The structure of superconducting (Tc = 40K) La 2 CuO4.032 has been determined from neutron diffraction single crystal data taken at room temperature and 15K [1] (Figure 1 and Table I). It was found that the La/Cu ratio is exactly 2 and that an excess oxygen 04 is present at x=l/4, y=0.243(3), z=l/4 with an occupation factor p(04)= 0.016(2). Because of this insertion the site 03 at x=0.031(5), y=0.182(2), z=0.100(5) is occupied with an occupation factor p(O3)=0.02 4 (2) x 2. Since this position is just 0.75A from that of the apical 01, it can only be occupied when 01 is empty. We found indeed that p(Ol1)+p(O3)=l. We also found that p(O3) is about equal to 3 x p(O4). Consequently, it seems that each 04 is responsible for the
FIGURE 1 The structure of La2CuO4.032. Only half cell is shown. Mat. Res. Soc. Symp. Proc. Vol. 169. c1990 Materials Research Society
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TABLE I Crystallographic data at 15K for La2CuO4.032 obtained by refinement of the neutron data (X=0.484
cell parameters
A) with space
group Cmca
positions (Cmca) La: Oy z 02: 1/4 y 3/4 Cu:000 03:xyz O1:Oyz 0 4 : 1/4y 1/4
a = 5.350 (1) A b = 13.148 (6)oA c = 5.398 (3) A positional parameters y La 0.36111(4) zLa 0.0069(1) y 01 0.18307(8) z O1 - 0.0356(3) y 02 - 0.00697(7) x 03 0.030(5) y 03 0.182(2) z 03 0.100(5) y 04 0.243(4)
thermal parameters ULa 11 U La 22 "ULa 33 "ULa 23 "UCu 11 "UCu 22 "UCu 33 UO1 11 UO1 22 UOI 33 UO1 23 U02 11 U 02 22 U 02 33 U02 13 U 03 U 04
occupancy parameters nOl 0.943(5) n 02 0.993(4) n 03 0.024(2) n 04 0.016(2)
0.0055(7) 0.0030(2) 0.0022(6) 0.0002(2) 0.0042(9) 0.0050(3) 0.0001(8) 0.0077(6) 0.0040(3) 0.0123(5) 0.0014(4) 0.0055(7) 0.0094(3) 0.0023(6) 0.0007(2) 0.005 (fixed) 0.005 (fixed)
agreement factors Rw(F 2 ) 5.13% Ruw(F 2 ) 3.88% X2 4.20 Interatomic Cu-O1 Cu-02 Cu-03 Cu-04
Distances 2.415(1) 1.9022 2.46(2) 3.72(4)
x2 x4 x2 x4
La-O1
La-04 01-02 01-04
3.061(1) x2 3.087(1) x2 3.47(3) x2
02-02
02-03
2.352(1) 2.537(2)
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