Pressure-induced valence and structure change in some anti-Th3P4 structure rare earth compounds

The anti-Th3P4 structure compounds Yb4Bi3 and Yb4Sb3 have been investigated to 350 kbar by high pressure X-ray diffraction, using the diamond anvil cell. From the P-V data it is found that Yb4Bi3 and Yb4Sb3 are much more compressible, compared to Sm4Bi3 before the valence transition. This suggests that a continuous change in the valence state of Yb takes place with pressure in the two compounds and that they may be in the mixed valent state already at ambient pressure. The ''collapsed'' anti-Th3P4 structure becomes unstable in Yb4Bi3 and Yb4Sb3 and new lines appear at high pressure, that fit the NaCl structure. The latter structure change seems to occur also in the electronically collapsed Sm4Bi3. The results are presented and discussed. (Auth.)