Structural and electronic effects of Sr substitution for Ba inY(Ba1−xSrx

Abstract

To unveil the structural mechanisms associated with T c variations induced by either mechanical or chemical pressure, samples of the Y(Ba 1Ϫx Sr x ) 2 Cu 3 O w system with xр0,0. 02,0.1,0.25,0.35,0.5,0.625,0.75,1, as well as those with xϭ0.5 and wϭ6.685,6.80,6.96,6.98, have been prepared and characterized. Characterization includes crystal structural refinements based on powder neutron diffraction data taken at room temperature, electron microscopy for detecting a possible Ba/Sr ordering, and resistive and magnetic measurements of the superconducting transition. The effects of Sr substitution on the structural parameters are equivalent to those of a pressure of approximately 10 GPa/x. The main difference is the thickness of the superconducting block CuO 2 -Y-CuO 2 , which increases with increasing x and decreases with increasing pressure. As a consequence of the displacement of O4 from the ͑0, 1 2 ,0͒ to the (x, 1 2 ,0) position, the Ba/Sr-O4 distance decreases with increasing Sr content. At constant w, T c decreases at the rate of 20 K/x. For xϾ0, the maximum of T c occurs at a value of w higher than for xϭ0. For xϭ0.5 the thickness of the superconducting block increases with increasing w and consequently with increasing T c . This indicates that, at least in this system, the thickness of the superconducting block is not the key parameter controlling T c . The stress of the Ba/Sr and the Y sites, estimated from the bond valence sums ͑BVSs͒, decreases with increasing x. For xϭ0.5, the BVS values are almost equal to the formal values of 2ϩ and 3ϩ, respectively. The average BVS of Cu1 and Cu2 increases with increasing x. However, this does not correspond to a real increase because the total charge of the Cu cations, as determined by iodometric titration or neutron diffraction, remains constant with x. Our analysis of the structural data suggests that the relaxation of the Ba/Sr layer hinders the charge transfer from Cu1 to Cu2, which accounts for the decrease of T c with increasing x. ͓S0163-1829͑98͒05645-8͔

Strain effects on point defects and chain-oxygen order-disorder transition in 123 cuprate compounds

“…Secondly, we focus on the effects of strain on Schottky defects and related phenomena for a series of homovalent substitutions at Y and Ba sites of YBa 2 Cu 3 O 7−δ . Although the 123 structure is not stable in the Ca-and Sr-analogs under ambient pressure [23,24,25], partial substitution of Sr on the Ba site has been reported [4,23,26,27,28]. Finally, strain effects on chain-oxygen order-disorder transition of REBa 2 Cu 3 O 7−δ will be investigated.…”

“…For example, in many bulk forms and multilayer applications, Y is replaced by Nd, Sm, or other rare-earth elements (RE). Doping YBa 2 Cu 3 O 7−δ with Ca, Sr, or other alkaline earth elements (AE) has also been shown to improve bulk and grain boundary transport and other properties [3,4,5].…”

Effect of Sr-for-Ba isovalent substitution on the local structure, hole distribution and magnetic irreversibility of Cu(Ba,Sr)2YbCu2O6.95(2)

“…This is illustrated in Fig. 7 system, the very first NPD study using a sample with y = 0.5 (z not given) concluded full ordering of the oxygen vacancies [33], but later Licci et al [24] revealed partial shift of the charge-reservoir oxygen from the b-axis site to the a-axis site due to Sr substitution. Thermoelectric power measurement has proven to be a powerful tool to reveal the metallicity of (fully-oxygenated) CuO chains in Cu-1212 and related copper oxides [36 -38].…”

“…The H irr value was determined by the procedure introduced elsewhere [23]. In brief, the magnetic field (H) In a previous study on Cu(Ba,Sr) 2 YCu 2 O 6+z samples Licci et al [24] used (x, 1/2, 0) rather than the conventional position of (0, 1/2, 0) for the O (1) site. This was also tested in the present work, but no improvements were seen in the reliability factors, R p , R wp and χ 2 .…”

Effect of Sr-for-Ba isovalent substitution on the local structure, hole distribution and magnetic irreversibility of Cu(Ba,Sr)2YbCu2O6.95(2)

“…The H irr value was determined by the procedure introduced elsewhere [23]. In brief, the magnetic field (H) In a previous study on Cu(Ba,Sr) 2 YCu 2 O 6+z samples Licci et al [24] used (x, 1/2, 0) rather than the conventional position of (0, 1/2, 0) for the O (1) site. This was also tested in the present work, but no improvements were seen in the reliability factors, R p , R wp and χ …”