Effect of Pressure on the Spectra of Five Nickel Complexes

Abstract

The effect of pressure to 150 kbars has been measured on the absorption spectra of five nickel complexes. The results are discussed in terms of the change of 10 Dq and of the Racah parameter B, with pressure. In general 10 Dq, the crystal field strength, increased with pressure, but the increase noted was not the same when calculated for the different peaks of a given complex. This discrepancy may be described in terms of changes of covalency as measured by changes of the Racah parameter B. In general B decreased with increasing pressure showing increased covalency at high pressure. There were some discrepancies in the value of B obtained from different transitions. These may be the result of experimental error, but may also be associated with distortions of the field, or with weaknesses in the assumption of point charges or dipoles.

Ab initioabsorption spectrum of NiO combining molecular dynamics with the embedded cluster approach in a discrete reaction field

“…2 In both phases, NiO behaves as an electric insulator, the conductivity of which increases as temperature rises 3 and remains insulating up to pressures on the order of 1 Mbar. [4][5][6] The clear nonconductivity observed in NiO is in contradiction to what can be expected from simple arguments regarding the partially filled 3d band. Various spectroscopical techniques have been employed since the 1950s to study the electronic structure of NiO and understand the nature of its nonconductivity.…”

“…This DOS is qualitatively in good agreement with the generally accepted behavior of NiO, with a conductivity gap defined by the charge-transfer process and a large mixing between the metal-hole and ligand-hole levels. Moreover, the calculation of NiO had to be done with data from only 24 out of the 32 conformations, because only those 24 structures have a 4 A n−1 electronic ground state of 3d 8 L character, with the hole localized in an O atom. The other 8 conformations have the hole mainly localized in the Ni atom independently of the active space used, being more similar to a 3d 7 level.…”

Investigations of the Pressure Dependence of Optical Spectra for NiO Crystal

“…The B value and Dq % 890 cm À1 are consistent with B % 665 cm À1 and Dq % 890 cm À1 in Ref. [2] by using the conventional theory, suggesting that the conventional theory is indeed a good approximation in analyzing the optical spectra of NiO. where b is the linear compressibility.…”

“…From the Racah parameters A 0 % 9442 cm À1 and B 0 % 1056 cm À1 of free Ni 2 ion [10], and the three observed transitions of NiO [2], we obtain N t % 0X8921, N e % 0X8923 and Dq % 890 cm À1 for NiO by using the above energy matrices. It can be seen that the difference between N t and N e is very small.…”

“…For example, for NiO crystal, although Stephens and Drickamer [2] explained reasonably the three observed transitions 3 T 2 , 3 T 1a and 3 T 1b from the ground level 3 A 2 by use of the conventional theory (in which B % 665 cm À1 and Dq % 890 cm À1 ), they found the discrepancy in the change in B under pressure for the 3 T 1a and 3 T 1b transitions (the latter is almost half of the former). The great disagreement cannot be attributed to the experimental errors [2] and until now, no satisfactory explanations have been made for it. In order to overcome this difficulty, in this paper, we present a modified crystal-field theory based on a cluster approach.…”