Valence transitions lattice parameters

Substitution of the cation in the solid solution system Smi R Se by, e.g., Y or La reduces the lattice parameter and the 4f-5d excitation gap, without yielding the transition into the metallic intermediate-valence phase (Gronau 1979). In fig. 3 we show the polarized Raman spectra of Smj jjY Se at 80K for x = 0, 0.25, 0.50, 0.75 and 1.0, obtained by Giintherodt et al. (1981a). For the sake of completeness, Sm, jLag jSe has also been included. For the latter sample one observes below 200 cm first-order defect-induced Raman scattering from acoustic and optical phonons which is absent in pure SmSe. The / = 1 peak of Smo gjLap osSe has drastically broadened compared to that of pure SmSe at... 

Alloying of YbCotln in the series Ybi-jcR InCat (R = Y, La, Ce, Lu) has been studied for various x values (Mushnikov et al., 2002). Correlating with the size of the rare earth elements, the lattice parameters increase with x for R = Y, La, and Ce, while they decrease for lutetium. The influence of alloying on the valence transition temperature is discussed. Zhang et al. (2002) investigated the ytterbium substitution by yttrium, lutetium, and zirconiunL... 

The temperature variation of the lattice parameters, the magnetic susceptibility and the Eu Mossbauer measurements indicate the occurrence of a mixed valency in EuPd2Si2 with a continuous valence transition near 140 K. From these... 

The lattice parameter is a direct indicator of what is happening at the microscopic level in these systems. It reflects the change in the size of the Sm ion, which in turn is directly related to its valence state. The lattice parameter that characterizes the divalent SmS (Sm S) and fully trivalent SmS (Sm S) are respectively 5.97 A and 5.62 A. Any lattice parameter intermediate in value suggests an intermediate or fractional valence state for the Sm ion. The lattice parameter data especially as a function of temperature has been very revealing. The abrupt jumps are due to a first-order valence transition. 

Fig. 20.9. Lattice parameter change with temperature (upper figure) and composition (lower figure) in the Sm., Yi compounds. The abrupt changes are due to first order isostructural phase transition. In the bottom of lower figure the valence calculated from the lattice parameter is shown (from Tao and Holzberg, 1975).

The valence transition of Sm in the Snii Sr B5 hexaboride solution (Pm3m) was investigated by Tarascon et al. (1980). For sample preparation and methods of experimental investigation, see Smi Y Bj. Sr " substitution of Sm was found to increase the average Sm valence towards Sm +. Lattice parameters are shown in fig. 34b. 

Tarascon et al. (1980) investigated the valence transition of Sm in the hexaboride solid solutions Sm, M B (M = Yb, Sr +, La ", Y, Th +). Samples were prepared by borothermal reduction of the mixed oxides under vacuum and high temperatures. The exact values of x have been determined by x-ray fluorescence analysis and checked by density measurements. Density measurements. X-ray and chemical analysis of SmB indicate an atomic ratio B/Sm x 6. From X-ray absorption measurements at the L, edge at 300 K the Sm + Sm + atomic ratio was obtained as a function of x. (The L,n absorption spectrum of Eu + in EuB was used as reference.) Y + substitution of Sm decreases the average Sm valence towards Sm in accordance with estimations of the average Sm valence in the hexaborides (Sm, Sm +), M B from lattice parameter measurements, fig. 34c. Lattice parameters of Sm +B a = 4.186, and Sm + Bg a = 4.115, were derived from interpolations of neighboring divalent and trivalent rare earth hexaborides. 

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