Defects in High T c Cuprate Superconductors

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y. The différence among thèse phases is the number of C a - O planes between C u - 0 2 planes. Because of the existence of thèse similar phases with layered crystal structures, one of the most commonly observed defects in thèse cuprates is the faults in stacking séquences.3-6 Thèse faults (single stacking faults or intergrowth of the différent phases) are created during processing and/or as a resuit of reaction with an atmosphère. Other frequently occurring defects are twin boundaries. The most studied and discussed are those formed due to structural phase transformations in 1:2:3, and this will be discussed in détail later. The 2:1:4 phase also exhibits complicated phase transformations as the température is lowered to

I TETRAGONAL

ORTHORHOMBIC

T(K)

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TWIN BOUNDARY AT CHAIN OXYGEN

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• =2Cu o » chain 0 • o2Ba.Y

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OXYGEN CONCENTRATION (YBa 2 Cu 3 O x ) TWIN BOUNDARY AT CATIONS

Figure 2. Calculated phase diagram for YBaiCu307-s. The dots are expérimental data"

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Figure 4. Idealized drawing of the twin boundary région consistent with the observations of (a) YBa2Cu307: twin boundary is centered on the (110) plane through the O atoms, and (b) YBazCuiOu: twin boundary is centered through the cation atoms. Most of the oxygen atoms, except for chain-oxygen atoms, are omiltcd.

(1)

where = 2(b - a)/(a + b) is the orthorhombicity and g and M are the grain size and the shear modulus, respectively. C is a constant and ~ 1 . The validity of this relationship was shown for bulk, and also for thin films, but for thin films the critical length is the thickness of the film rather than the grain size.30 Using M = 2 X 1012 dynes/cm2 as reported31,32 and measuring the twin boundary spacing, we find y « 80 ergs/cm2 for pure 1:2:3. When the Cu in the C u - O chain is substituted by Fe or Al, or the oxygen content is reduced, the spacing between the boundaries d has drastically decreased and y was found to decrease by an order of magnitude. 27 Interestingly, thèse values of y are in the range of the twin boundary énergies which are found in metals and alloys. In order to understand the reduced twin boundary énergies by alloying and oxygen réduction as well as the rôle the boundaries in determining some of superconducting properties in the 1:2:3 System, the boundary structures were examined using électron diffraction and high resolution électron microscopy (HREM). From thèse studies, in a stoichiometric 1:2:3, two effects are very striking: (1) sharp streaks of diffuse scattering streaks along

MRS BULLETIN/NOVEMBER1991

1000

Figure 3. Twin boundary structure observed along the [0011 axis for YBazCu307- (a) A selected area diffraction paltern. Note that ail the diffraction spots show sharp streaks along the [110] direction, perpendicular to the twin boundary. An enlarged Bragg spot is shown in the inset. (b) A two-beam image, (c) A high-resolution image. The vertical distorted région in the middle is a twin boundary. The square

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