Crystal Structure and Madelung Potential in R_2-xCe_xCuO_4-δ (R=Pr, Nd, Sm, Eu and Gd) System

Abstract

Powder X-ray diffraction measurements were taken in the R2-x Ce x CuO4-δ (R=Pr, Nd, Sm, Eu and Gd) system. Rietveld refinement of these structures was performed assuming that the space group was 14/mmm. The lattice constant and rare earth metal location were determined to study the electrostatic effects of T'-phase materials. We found that the difference in the Madelung site potential, ΔV (V R-V Cu), as well as T c has a peak for the Cu-O bond length. These data suggest that the carrier concentration is not constant in T'-phase materials and that changes in the carrier concentration influence T c in these materials.

Predicted properties ofNd1.5Ce0.5Sr2Blackstead1, Dow2 1998Phys. Rev. B16120View full textAdd to dashboardCite

“…The widely accepted positive sign of the charge carriers in La 2Ϫ␤ Sr ␤ CuO 4 is to be contrasted with the situation in Nd21-4 and its homologues, where the sign of the carriers is controversial, 111 with a majority of authors following the original suggestion of Tokura et al 27 that electrons carry the charge in the Nd21-4 homologues. However, the Nd21-4 homologue fabricated with the most careful oxygen control is Pr 2Ϫz Ce z CuO 4 , 24 and the evidence is strong that this robust superconductor is p-type.…”

“…The cations Ba and Cu can assume, at most, the ͑integral͒ charge-states Ba ϩ2 and Cu ϩ2 . The rare-earths are R ϩ3 ͑Tables I and II [38][39][40] ͒, leaving a total of 13 electrons to charge the 7 oxygen anions: Based on ionization potential data 40 and Madelung potential calculations 27,41 such as those in Table I, the first of the cations to be further ionized would be Cu ϩ2 →Cu ϩ3 , but nuclear magnetic resonance ͑NMR͒ data show that there is no Cu ϩ3 in YBa 2 Cu 3 O 7 ͑and presumably there is none in Nd123-7 either͒, 42 and point-ion-model calculations show that none of the Cu sites has a Madelung potential as large as the Cu ϩ2 →Cu ϩ3 ionization potential. Indeed, the Madelung potential is too small in magnitude by at least ϳ10 V. ͑See Table I.͒ Therefore a maximum of 13 electrons can charge at most 6.5 oxygen anions to ϷϪ2͉e͉, leaving ͑in some sense͒ Ϸ0.5 O 0 or Ϸ1.0 O Ϫ1 in each unit cell: hypocharged oxygen, O ϪZ , with ZϽ2.…”

“…These materials offer an especially appropriate proving ground for theories which purport to offer explanations of high-temperature superconductivity, because they provide clear chemical trends in their crystal structures, because R222M -10 has been studied sufficiently little that many of its properties are unmeasured ͑providing an opportunity for the theorists to make genuine predictions͒, and because they behave very differently: ͑i͒ R123-7 is a Ϸ90 K superconductor for most rare-earth ions, 14 including RϭPr, [15][16][17][18][19][20][21][22] but with the notable ͑current͒ exceptions RϭCe and Cm being larger-radius magnetic ions that are likely to occupy Ba sites in significant quantities, where they would break Cooper pairs and destroy superconductivity if the primary supercurrent were in the charge-reservoir Cu-O chain layers 23 ; ͑ii͒ in contrast, R122M -8 is an insulator for the rare-earth ions studied to date, 3 ͑iii͒ R222M -10 is a superconductor at Ϸ28 K, for RϭNd, Sm, Eu, and Gd, but not for RϭPr, which is an insulator, 9 and ͑iv͒ R21-4 is typically a T c Ϸ21-24 K superconductor with Ce doping, for RϭPr, Nd, Sm, and Eu, with T c ranging from Ϸ9 K for RϭEu, to 24.3 K for RϭPr ͑carefully prepared͒, 24 to 32 K for RϭNd ͑pressure fabricated͒; 25 but R21-4 does not superconduct for RϭGd and trivalent rare-earth ions smaller than Gd ϩ3 . [25][26][27][28][29][30] There are indications 30 that Y 2Ϫz Ce z CuO 4 , if it could be fabricated, would not superconduct. Moreover, since growth under pressure causes Nd 2Ϫz Ce z CuO 4 to superconduct at 32 K, 25 we think that such pressure-growth might produce Eu 2Ϫz Ce z CuO 4 with a critical temperature T c higher than the current Ϸ9 K.…”

Structure Determination of the Layered Manganocuprates Ln3Ba2Mn2Cu2O12±y, Ln=Sm, Eu, by Powder Neutron Diffraction

“…The end result is the formation of disordered structures, like the impurity phase Sm 2 BaMnCuO 7 , where manganese and copper share a site. For smaller lanthanides, incorporation into the rocksalt layers presumably becomes unfavorable as is illustrated by the existence of the K 2 NiF 4 structure for Ln 2 CuO 4 only for Ln=La (16) and in distorted form; for Ln 2 CuO 4 , Ln=Nd-Gd the T 0 -type structure forms (17).…”

Hole superconductivity in Nd2−Ce CuO4

“…closed-shell Ce +4 ) [12]. Thus, recognizing that most authors simply assume that Ce is in the Ce +4 charge-state, we took a first step towards computing what the charge state of Ce should be [13][14][15]: by examining a calculation of the Madelung potential at a Ce site by Uzumaki et al [16], we concluded that isolated, substitutional Ce cannot be Ce +4 . In a point-ion model for Nd 2 CuO 4 , those authors computed the electrostatic potential at a site where Ce replaces Nd, and found a potential of magnitude 30 V, far too small to exceed the potential of 37 V required to ionize Ce to Ce +4 [17].…”