Influence of oxygen defects on the physical properties ofLa2CuO4−y

Abstract

We present the results of magnetic-susceptibility, heat-capacity, electrical-resistivity, and thermoelectric-power measurements on La2Cu04 -. The crystallographic, magnetic, and electronic transport properties are found to be extremely sensitive to the oxygen-defect concentration y. Coupled with x-ray and neutron-difI'raction data presented in a companion paper, we find that long-range antiferromagnetic order develops with increasing y out of a nonmagnetic ground state at y=0 into a state with a maximum Neel temperature of 290 K at y=0.03.La2Cu04y is the parent compound of the doped high-temperature (T, (40 K) superconductors La2 (Sr, Ba)"Cu04~with x = 0.2. ' lt has a roomtemperature structure which is an orthorhombic distortion of the tetragonal K2NiF4 structure.The latter structure is regained upon heating above a transition temperature Tp of 506-536 K; the variation in Tp is thought to arise from differences in y between different samples, but a quantitative correlation has not before been established. The low-temperature structure and the temperature dependence of the orthorhombic distortion parameter have recently been studied using neutron diffraction. The electrical resistivity p and thermoelectric power S of sintered samples are nearly independent of temperature from 100-1100 K, ' but the carriers appear to localize at lower temperature T. ' ' A large scatter of unknown origin is present in the reported room-temperature S values (100-500 pV/K). Measurements of the magnetic susceptibility Z(T) have revealed peaks between 220 and 280 K in some samples of La2Cu04 -[Refs. 7(a), 10, 13-16] but not in others, ' which is another discrepancy of unknown origin. The X peak has variously been deduced to arise from a smooth vanishing of localized magnetic moments with decreasing T (Ref. 10) or from the occurrence of an antiferromagnetic (AF) or spin-densitywave transition; ' ' the T at which the anomaly occurs depends on the details of sample preparation and handling.We have carried out a series of experiments to clarify the physical properties of pure La2Cu04y as a starting point to understand and differentiate between various microscopic mechanisms for the occurrence of high-T superconductivity in the Srand Ba-doped compounds. Herein, we report the results of Z(T), heat-capacity, p(T), and S(T) measurements which establish that the crystallographic, magnetic, and electronic transport properties are extremely sensitive to the concentration of oxygen defects y in La2Cu04 -.' This sensitivity provides one explanation for the discrepancies noted above. Perhaps our most interesting results are that, in conjunction with companion x-ray and neutron-diffraction results, the X peak above is indeed due to long-range AF order, and that this order develops with increasing y out of a nonmagnetic ground state for y =0 into a state with a maximum Neel tempera-

Properties of La2CuO4 and related compounds

“…In the case of metallic, superconducting LaL85Sro ~5CuO4 to which a band model appears applicable, we can estimate the importance of electron-electron correlations by comparing the observed Pauli susceptibility (Xeaul~) with the carrier density inferred from a single-band-model interpretation of the Hall coefficient. The estimated XPauli ( ~ 1 × 10 -4 emu/mole Cu, after core diamagnetism correction and subtraction of a Cu2÷-Curie-Weiss contribution [4]) leads to an enhancement factor r/~ 5 due to electron-electron correlations [ 35 ]. Optical reflectivity measurements [36] on La-Sr-Cu-O powder also draw a similar conclusion.…”

“…10, the presence of a sharp peak [ 4,45 ] in the susceptibility indicates that La2CuO4 undergoes antiferromagnetic ordering at TN~257 K, which has been confirmed by neutron-scattering experiments [46 ] that find three-di- mensional Bragg peaks with unit-cell doubling. As noted earlier, TN is very sensitive to the oxygen content and the highest TN ~ 328 K is achieved by vacuum annealing.…”

“…It appears that as 6~0 in La2CuO4+6, TN approaches its maximum value and as soon as 6 becomes negative, the sample starts to dissociate. Susceptibility studies on La2CuO4 powders as a function of oxygen content show that TN vanishes at 6~ 0.03 [4], which is a hole-doping level comparable to that required to suppress antiferromagnetism by divalent-cation substitution for La. (The correct definition of TN is not simple [45,47 ]; however, for simplicity we defined TN as the temperature where Z reaches its maximum.)…”

“…It is also known that the oxygen stoichiometry corresponds to La2CuO4 within ~ 1%. Extensive transport [ 19 ] and magnetic measurements [4 ] show, however, that the physical properties are extremely sensitive to << 1% changes in the oxygen content. Important clues to understanding the exact oxygen content are the following [4,19].…”

“…The structure is one of a series of possible structures of Ruddlesdon-Popper type with formula AOn (ABO3): an interleaving of perovskite and rocksalt layers-n of the BO2 layers per formula unit. The tetragonal-to-orthorhombic transition [3,4 ] for La2CuO4 occurs at To= 530 K and involves a tilting of the elongated CuO6-octahedra. (This distortion is sometimes ascribed to a Jahn-Teller distortion of Cudg; however, the presence of a similar distortion in La~NiO4 [5], La2CoO4 [6], and YCaCrO4 [7] would argue against it being the only cause of the distortion.)…”