Isostructural volume collapses

Isostructural volume collapses observed under pressure in lanthanide pnictides and chalcogenides LnX. For each compound the table lists, from top to bottom the pressure (range) of collapse (GPa), the volume decrease on collapse (%), and references. (x means compound unknown, - studied under pressure, but no isostructural transition observed, and -I- compound exists, but no HPXRD study known.)... 

Isostructural volume collapses under pressure were observed for monochalco-genides of Ce, Sm, Eu, Tm, and Yb, and for CeP and SmBi. The known electronic transitions of this type are tabulated in table 4. 

In the case of AmN (Dabos-Seignon et al. 1993) there are Indications of a 2-3% isostructural volume collapse around 34GPa. Whether an 1% volume decrease... 

When the itinerant state is formed, a volume collapse AV/V is always encountered, as predicted by the theory of the preceding sections. In one of the lanthanides, cerium, this volume collapse is particularly accentuated for its isostructural transition from the y to the a form, possibly associated with a change in metallic valence from three to four (both oxidation numbers are stable in cerium chemistry) (see Fig. 1 of Chap. A),... 

In CeS, the first transition occnrs to the B1 phase with delocalized f-electrons (Svane et al., 1999), that is, the theory predicts an isostructural phase transition in CeS similar to CeP. The calculated transition pressure is 10.1 GPa with a volume collapse of 6%. These findings are in excellent agreement with the experiment of Croft and Jayaraman (1980), but at variance with the results of Vedel et al. (1986), who observe a soft anomaly in the pV-curve but no discontinuity. These results may indicate the proximity of a critical point. At higher pressures, CeS transforms into the B2 phase. According to the present calculations, this occurs in two steps. First, at a pressure of 24.3 GPa, CeS goes into the bivalent B2 phase with a 4.6% volume change. In the second step, at a pressure of 29.5 GPa, the tetravalent B2 phase is reached with a 3.6% volume collapse (Svane et al., 1999). Unfortunately, no experiments have been performed beyond 25 GPa (Leger, 1993). 

There is an interesting difference for one metal of the lanthanide series with respect to the heavy actinide metals. While high density ( volume collapse ) and low structural symmetry simultaneously appear at the same pressure in Cm, Bk, and Cf (Benedict 1987), and only low structural symmetry, with no appreciable volume decrease, is observed in Am, the lanthanide metal Ce is the only one where volume collapse (by 16%, in an isostructural phase transition) and formation of a low-symmetry structure occur successively at two different pressures (Staun Olsen et al. 1985a). 

In CeAl2, Croft and Jayaraman (1979) and Bartholin etal. (1980) observed an isostructural transition around 7 GPa, accompanied by a volume collapse of about 4%. This observation was interpreted, like in the case of the lanthanide chalcogenides, as a valence change from - - 3 to -I- 4 of the Ce ion. This interpretation was supported by the fact that the corresponding compound of the non-4f metal La, LaAl2, did not show any phase transition up to 3 GPa, the highest pressure reached for both... 

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