Evidence for the existence of absorbed NO in double-layered cuprate

Abstract

The NO intercalation properties of layered cuprate La2yxBaxSrCu2O6ya reported by Machida et al. [1] suggest that the desorption process of NO contrasts with conventional NO absorption±desorption systems in the following ways: (a) NO absorption is not associated with the formation of nitrates or nitrites of barium; (b) the desorption temperature of N2 is much higher than that of O2. Also, the chemical states of NO over LaBaSrCu2O6ya were investigated using X-ray photoelectron spectroscopy (XPS) [2]. The obtained results suggested the surface adsorption of NO2 group corresponding to NO trapped by oxygen vacancies [2]. However, no prior measurement on the chemical states of absorbed NO in double-layered cuprates has been reported. Here, we report the chemical states of the intercalated NO in LaBaSrCu2O6ya investigated by the electron-energyloss spectroscopy (EELS). Powdered LaBaSrCu2O6ya was prepared by coprecipitation of the citrates of the elements. After calcination at 1073 K for 5 h in air, the powder was ground and then heated at 1273 K for 10 h in air. The crystal phase of the powder was determined by powder X-ray diffraction (XRD), and the singlephase XRD pro®les have been given in [1]. The NOintercalated samples were prepared by annealing at 627 K for 2 h upon introduction of 0.5 vol % NO± 99.5 vol % N2 gas. The NO-intercalated samples showed a similar XRD pro®le to LaBaSrCu2O6ya and there were no other additional refelctions. These powders were pressed into pellets and analysed by EELS and powder XRD. The electron-energy-loss spectra were acquired using the Gatan model 666 parallel-detection energy-loss spectrometer mounted on a Hitachi HF-2000 transmission electron microscope. The transmission electron microscopy specimens, i.e., ceramic akes, were prepared by crushing the sample pellets. The absorption edges of N 1s and O 1s levels were measured by high-energy EELS in transmission through samples about 200 AE thick. The EELS data from cuprates were recorded as two separate spectral regions. The N 1s absorption edges of NOintercalated LaBaSrCu2O6ya sample is shown in Fig. 1. Electron incidences are parallel to the [0 0 1] directions where diffraction spots are indexed in terms of the tetragonal double-layered cuprate, the so-called ` 326'' type. The features around 401 eV in the NO-intercalated LaBaSrCu2O6ya sample are attributed to N 1s bands similar to the results of BN and air in [3]. Therefore, the N 1s absorption edges indicates the existence of N elements in LaBaSrCu2O6ya. Also, the N 1s absorption edges are not observed for the sample in vacuum for 3 h. Therefore, it is considered that the NO-intercalated sample in vacuum releases N2 gas from the oxide. The O 1s absorption edges of the as-prepared and NO-intercalated LaBaSrCu2O6ya samples are shown in Fig. 2. The O 1s absorption edges of the NOintercalated LaBaSrCu2O6ya samples show a rise in intensity at 528±532 eV with NO intercalation as shown in Fig. 2. Nucker et al. [4, 5] reported that the rise in intensity of the O 1s absorption edges was realized in the low-energy part (E , 531 eV) which was related to holes on O sites. Therefore, the rise in intensity of the O 1s absorption edges below 532 eV as shown in Fig. 2 suggests that holes enter the O 2p states with the NO intercalation. Our previous report showed that the binding energies of Cu 2p3=2 XPS core-level spectra of the NO-intercalated LaBaSrCu2O6ya sample (933.1 eV) are away from that of as-prepared sample (934.0 eV) [2, 6]. As the formation of nitrates or nitrites are not observed in the NO-intercalated sample, the chemical shift of Cu 2p3=2 XPS data in our previous [2, 6] report re ects the intercalation effects of NO molecules into the layer structure. It is known that the core-level binding energy of Cu 2p is lower in an