Atomic charges in RBa₂Cu₃O₇ superconductor lattices. Electronic structure of the copper atoms

Abstract

The effective atomic charges for all sites of the RBa2Cu 0 7 lattices have been found using our Mossbauer data with the G7C~~("Zn) ,"':j Ba('"'CCs), 'n EufS5Gd) isotopes, our calculations of the lattice electric field gradient (EFG) at the lattice sitcs, as wcll as the I7O NMR data available in literature. The charges show holes placed in the chain oxygen sublattice mainly. The CuZf Sternheimer factor has been estimated and the Cu'+ valence EFG in YBa2Cib07 has been found. The zaxes of the total, lattice, and valence EFG tensors for Cii(2) coincide with the crystal axis c. For Cu(1) they coincide with the axes b, a, and a, respectively. a p H n b I aTOMOB BO BCeX Y n a X PeLUeTOK RBaZCu307 H?&eHbI C llOMOqbH) HaLUIIXMeCC-6ay3pOBCKkiX AaHHbIX H a C L I (~~Z~) ,133 BEL('~~CS), EU('"%~), HaIIIIIX BhIYliCJIeHHII~ peIIIe-Toworo r p a n H e H T a s n e~~p u s e c~o r o nonrr (r'3l-I) B ymax, u n w~e p a~y p~b i x HaHHbix no 5IKP H a 1 7 0 . ~T E 3aprrnb1 c o o T s e T m y m T AbIpKe, H a x o n a q e i r c r r npemyqecmemro B n o n p e m e T K e qenoseworo K n c n o p o n a . A a H a oqema + a K T o p a Uhepmaihepa nna Cu2+. H a W e H B a n e m -l3l-I cosnaxawr c K p u c T a n n o r p a @ , l i q e c K o i i ocblo c, a ~n n Cu(1) c o 4 a~u b, a, H a, c o o T B e T c T -Hbl# r3l-I HJIR CU2+ B YBazCu307. mn cU(2) 2-OCU IIOnHOrO, P e l I l e T O s H O r O II BaJIeHTHOrO BeHHO.

Atomic charges in RBa2Cu4O8lattices, determined by comparison of experimental and calculated nuclear quadrupole interaction parameters

“…The experimental spectra were resolved into two triplets identified with the 67 Zn 2+ centres at the Cu(1) and Cu(2) sites on the basis of the similarity of the Cu(2) local environments in RBa 2 Cu 4 O 8 and RBa 2 Cu 3 O 7 compound groups. For this reason, we suggested that the asymmetry parameter of the EFG tensor for the Cu(2) sites in RBa 2 Cu 4 O 8 is zero, as it was found to be for RBa 2 Cu 3 O 7 [1]. This requirement was found to be satisfied only for the (2-4-5) triplet ascribed to 67 Zn 2+ centres at Cu(2) sites (figure 1(a)).…”

“…The valence contribution to the total EFG tensor can be neglected for the 67 Zn 2+ probe [1], and then equation (1) transforms into C(Zn) ≈ eQ(1 − γ )V zz . The lattice EFG tensor was found using the point charge model.…”

“…The 67 Cu and 67 Ga isotopes were introduced into the samples by diffusion in an oxygen atmosphere at 700 • C for two hours. A similar procedure has been shown to introduce 67 Ga atoms into the REM sites and 67 Cu atoms into the Cu(1) and Cu(2) sites of YBa 2 Cu 3 O 7 in accordance with their natural populations [1,2]. A test annealing of an YBa 2 Cu 4 O 8 sample under the above conditions did not change its T c -value of ∼80 K. 67 Cu( 67 Zn) and 67 Ga( 67 Zn) Mössbauer spectra were recorded at 4.2 K with the 67 ZnS absorber having a surface density of 1000 mg cm −2 .…”

“…The EFG at the nuclei of such probes is created predominantly by lattice ions. For this reason, we have proposed and developed 67 Cu( 67 Zn) emission Mössbauer spectroscopy (EMS) as a method for experimental determination of nuclear quadrupole interaction parameters at copper sites of high-temperature superconductor lattices [1]. In this method, the 67 Zn 2+ lattice probe (spins of the ground and excited states: 5/2 and 1/2; quadrupole moment of the ground state: Q = 0.15b) occupies copper sites.…”

“…In particular, such a situation arises in the case of RBa 4 Cu 4 O 8 high-temperature superconductors (R is a rare-earth metal (REM) or yttrium). Their lattices have two copper positions Cu (1) and Cu (2). Another feature of these objects is that the nuclear quadrupole interaction parameters of the same 67 Zn 2+ probe can be determined at the REM sites using 67 Ga( 67 Zn) EMS [2].…”