Trivalent holmium

Johnson et al. (55) have reported a phonon-assisted energy exchange from trivalent erbium to trivalent thulium or to trivalent holmium. In this case, these authors were able to rule out resonance exchange completely Of some importance is that these systems are useful for laser oscillators, and the energy exchange results in a substantial decrease in threshold. 

The term rare earth elements is sometimes applied to the elements La-Lu plus yttrium. The convenience of including La, which, strictly speaking, is not a lanthanide, is obvious. The reason for including Y is that Y has radii (atomic, metallic, ionic) that fall close to those of erbium and holmium and all of its chemistry is in the trivalent state. Hence it resembles the later lanthanides very closely in its chemistry and occurs with them in Nature. 

Also as a result of the lanthanide contraction, yttrium has an ionic radius comparable to that of the heavier REE species in the holmium-erbium region. If the effective ionic radius (Shannon 1976) of is plotted (0.90 A)., it plots in between element 67 (Ho) and 68 (Er). Scandium (effective ionic radius is 0.745 A), plots outside of the Lanthanide series. As also the outermost electronic arrangement of yttrium is similar to the heavy rare earths, the element behaves chemically like the heavy rare earths. It concentrates during (geo)chemical processes with the heavier REEs, and is difhcult to separate from the heavy REEs. Scandium, on the other hand, has a much smaller atomic radius, and the trivalent ionic size is much smaller than that of the heavy rare earths. Therefore, scandium does not occur in rare earth minerals, and in general has a chemical behavior that is significantiy different from the other rare earth elements (Gupta and Krishnamurthy 2005). 

The high-temperature polymorphic form for most of the rare earth metals just before melting is the bcc structure. Four of the trivalent lanthanides (holmium, erbium, thulium and lutetium) are monomorphic and do not form a bcc structure before melting at atmospheric pressure (see fig. 4). However, the bcc phase can be formed in holmium and erbium by the application of pressure (< 1 GPa), see section 3.7.1. The existence of the bcc phase in the lanthanides has been correlated with the d occupation number, which decreases along the lanthanide series, but increases... 

Recently the luminescence properties of Pr ", Nd, Tm and Yb " ions in fluorite have been obtained by steady-state measurements. In addition, the luminescence spectra of Ce ", Sm ", Sm ", Dy ", Er and Yb were measured. It was pointed out that Xex = 415 nm is most suitable for measuring the Ho " emission beside the Er ". The emission of trivalent holmium and erbium ions was measured independently using time-resolved measurements and tentative assignment of luminescence lines to 3 and C4V symmetry sites was proposed. Besides for natural fluorite crystal, the transition between Stark energy levels of lanthanide ions were presented (Czaja et al. 2012). 

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