Single crystal electron paramagnetic resonance study of Y 2 BaCuO 5 , a common impurity in the high temperature supercon
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Electron paramagnetic resonance (EPR) studies of pure Y 2 BaCu0 5 in powder and single crystal forms and of YBa 2 Cu 3 07_ 5 in powder and single crystal forms provide further evidence that it is Y 2 BaCu0 5 that is the common green impurity found in many preparations of YBa2Cu307_,5 as a powder or in pellet forms. Y 2 BaCu0 5 tends to be excluded in the growth of YBa2Cu307_« single crystals. A method is presented for the growth of Y 2 BaCu0 5 crystals from a flux. An apparent discrepancy between the observed single crystal EPR anisotropy and the reported crystal structure is resolved in three independent ways from the Y 2 BaCu0 5 powder and single crystal EPR data. These results show that the EPR spectrum is a superposition of the spectra of the two differently oriented Cu sites in the unit cell and is not a spectral average of them. The temperature independence of the EPR spectrum between 150 K and 300 K is also consistent with there being no temperature dependent exchange averaging of the EPR spectra of the two sites in this range. The orientations of the Cu crystal field axes, as indicated by the g axes, are in agreement with the crystal structure. Crystal field splittings of the Cu d-orbitals are estimated from the measured g values and indicate an appreciable covalency in the C u - 0 bonds. The linewidth and its anisotropy indicate a minor degree of exchange narrowing.
I. INTRODUCTION Several reports of the EPR spectrum of the high temperature superconductor YBa2Cu307_,5 were made during early investigations of this promising material.1"9 Many of the g values reported appeared to be characteristic of spins localized on Cu ions. Oxygen content in the nonstoichiometric YBa2Cu307_^ was emerging as an important determinant of their conductivity.10'11 The present work evolved from an unsuccessful attempt to study the dependence of Cu coordination and spin delocalization on oxygen content by means of the Cu EPR in single crystals of the YBa2Cu307_($. Although the reported room temperature EPR spectrum was found with various intensities in different powder samples, we were unable to find a strong localized Cu EPR spectrum in YBa2Cu307_,j single crystals above Tc. Other single crystal studies have also failed to find an EPR spectrum under these conditions.2"5 Simultaneously, indications began to appear in the literature12'15 21 that the EPR spectrum was from the green Y 2 BaCu0 5 phase frequently observed in small amounts in YBa2Cu307_,5 preparations, although it has been argued that a localized Cu2+ EPR spectrum with g values similar to the ones we observe may be associated with the (super)conductivity of YBa 2 Cu 3 0 7 _^. 22 The green material was occasionally found segregated from the melt and physical separation of the material from the melt showed it to have the EPR spectrum that had been observed in J. Mater. Res., Vol. 7, No. 3, Mar 1992
the high temperature superconductor preparations. The identity of the material as YBa 2 Cu 3 0 5 was confirmed by independent synthesis. As a result of these findings, we t
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