Pressure induced electronic crossover

This fact implies that the bulk modulus of a-Ce is extremely large. Taking these facts into account, the Ce atom in the hpp of CeCug may be in the same electronic state as in a-Ce which is considered as the mixed-valence state material. Thus the large bulk modulus may be originated mainly from a valence change of Ce atom induced by pressure. This consideration is supported by the pressure-induced electronic crossover as was observed in the electrical resistance measurements (Oomi et al., 1993b). 

As in the case of the E —> A2 emission of Cr +, a weak dependence on Dq is expected. The spectroscopy of d ions is complicated by distortions from regular tetrahedral symmetry that influence electronic energies and alter the prediction of Eq. (7). In Mn + 28105, Shen et al. reported that distortion effects were unimportant above -100 kbar and that the E —> A2 luminescence energy was constant within experimental uncertainty in the absence of site distortion effects [158]. A similarly weak dependence of the E —> A2 luminescence energy was observed in Cr + Y3AI5O12 at high pressure after an electronic crossover of the T2 and E states was induced [157]. The weak shift observed for the E emission indicates only small changes in B with pressure in the d systems. 

In this section we review recent high pressure studies of luminescence lifetimes in transition metal and lanthanide systems. We focus on individual electronic transitions from isolated luminescent centers. Other phenomena, such as changes in energy transfer processes or electronic crossovers induced by pressure, can also influence emission lifetimes and will be discussed in later sections. 

The first pressure-induced excited state electronic crossover in a luminescent transition metal system was reported by Dolan et al. [175] in a study of Cr + K2NaGaFg. Cr + is an excellent candidate for observing an electronic crossover with pressure because, depending on its crystal field strength, it can exhibit either spin allowed T2 A2 emission (low field Cr ) or spin forbidden... 

In this section, as examples of pressure-induced crossover, we will show some data on electrical resistivity under pressure for three HF materials CeAla, CeAL, and CeInCu2. Brief discussions about these novel electronic states induced by high pressure will be introduced. 

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