Evolution of electronic structure with dimensionality in divalent nickelates K. Maiti, Priya Mahadevan,* D. D. Sarma † Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India Received 12 August 1998 We investigate the evolution of electronic structure with dimensionality d of Ni-O-Ni connectivity in divalent nickelates, NiO (3-d ), La 2 NiO 4 , Pr 2 NiO 4 (2-d ), Y 2 BaNiO 5 (1-d ) and Lu 2 BaNiO 5 (0-d ), by ana- lyzing the valence band and the Ni 2 p core-level photoemission spectra in conjunction with detailed many- body calculations including full multiplet interactions. Experimental results exhibit a reduction in the intensity of correlation-induced satellite features with decreasing dimensionality. The calculations based on the cluster model, but evaluating both Ni 3 d and O 2 p related photoemission processes on the same footing, provide a consistent description of both valence-band and core-level spectra in terms of various interaction strengths. While the correlation-induced satellite features in NiO is dominated by poorly screened d 8 states as described in the existing literature, we find that the satellite features in the nickelates with lower dimensional Ni-O-Ni connectivity are in fact dominated by the over-screened d 10 L 2 states. It is found that the changing electronic structure with the dimensionality is primarily driven by two factors: i a suppression of the nonlocal contri- bution to screening; and ii a systematic decrease of the charge-transfer energy  driven by changes in the Madelung potential. S0163-18299909619-8 I. INTRODUCTION Electronic structure of transition metal oxides has been an active field of research for several decades arising from many puzzling issues such as the nature and origin of the insulating state in partially filled 3 d systems, like NiO and the existence of metal-insulator transitions. 1 It is well known now that the electron-electron interaction effects play an im- portant role in determining the electronic properties in these systems. 2 The interest in this particular class of compounds has seen an explosive growth in recent times due to the dis- covery of several exotic properties exhibited by transition metal oxides, such as the colossal negative magneto- resistance, 3 and high-temperature superconductivity. 4 Such results have indicated a close interplay between the elec- tronic structure and the geometric structure in these com- pounds. For example, it is observed that while divalent three- dimensional copper oxide CuO is a wide band-gap insulator, essentially two-dimensional insulating La 2 CuO 4 becomes superconducting upon hole doping. 5 Here, the dimensionality refers to the connectivity between the transition metal TM sites for the transport of charge carriers governed primarily by transition metal-oxygen-transition metal hopping interac- tions. A decrease in dimensionality is expected to reduce the bandwidth by affecting the effective coordination number for such electron transfers. Hence, the effective electron correla- tion is expected to increase with decreasing dimensionality of the system. However, the band gap in several systems are found to be less sensitive to the change in dimensionality. For example, the band gaps in three-dimensional cuprate CuO, 6 two-dimensional La 2 CuO 4 , 7 and one-dimensional Sr 2 CuO 3 5 and Ca 2 CuO 3 8 are about 1.4, 1.8, 1.5, and 1.8 eV, respectively, indicating that the band gap does not exhibit any systematic trend with dimensionality of the electronic structure. This is somewhat unexpected since the Cu-O bond distances, controlling the hopping interaction parameters, are similar in all these three compounds and the bandwidth W to a large extent is expected to be controlled by the dimension- ality of the Cu-O-Cu network. This relative insensitivity of the band gaps between these compounds with different struc- tural motifs may be driven by a concomitant change in the charge-transfer energy  . 5,9,10 It has been suggested 5 that the change in  , which more than compensates the reduction in the near-neighbor coordination number in determining the bandgap, may be driven by changes in the Madelung poten- tial with dimensionality. Electronic structure of transition metal compounds in gen- eral and late transition metal oxides in particular have been usually described within a finite cluster 11–13 or an impurity model 14 containing a single transition metal site. In recent times, it has, however, been found to be necessary to go beyond the single transition metal-site model in order to pro- vide an accurate description of the electronic excitation spec- tra of three-dimensional Ni and Cu oxides, 15–19 suggesting the importance of impurity-impurity interactions in deter- mining the electronic structure. While it is straightforward to anticipate that such TM-TM interactions via the intervening oxygen sites would be less dominant with decreasing dimen- sionality of the TM-O-TM network, there has been no inves- tigation of the systematic changes in the electronic structure with dimensionality, so far. In order to obtain a more de- tailed understanding of the role of dimensionality in deter- mining the electronic structure, we investigate a series of divalent nickelates with different dimensionalities of the Ni- O-Ni connectivity. The systems studied here are three- dimensional NiO, two-dimensional La 2 NiO 4 and Pr 2 NiO 4 , one-dimensional Y 2 BaNiO 5 , and zero-dimensional Lu 2 BaNiO 5 . We provide a brief overview of the structure and physical properties of these systems in Sec. II. Section III and Sec. IV describe in detail the experimental and theo- retical methodologies, respectively, used for the present study. The results and discussions are presented in Sec. V and conclusions in Sec. VI. PHYSICAL REVIEW B 15 MAY 1999-I VOLUME 59, NUMBER 19 PRB 59 0163-1829/99/5919/1245714/$15.00 12 457 ©1999 The American Physical Society