Effect of pressure on the size of energy gap in semiconducting mixed-valent rare-earth compounds

We have synthesized CeRhBi and CeRhAs and found that these compounds crystallize in the epsilon-TiNiSi-type structure. It is found that CeRhBi is a heavy-fermion compound, whereas CeRhAs is a mixed-valent semiconductor with an energy gap of 144 K, about 10 times larger than that of CeRhSb. Various experimental results throughout the CeRhX series also vary in the sequence CeRh (Bi-Sb-As). However, the gap observed in CeRhAs is suppressed by external-pressure, suggesting that the chemical and the external pressure seem to cause opposite effects on the size of energy gap in these compounds. These results are compared with those for mixed-valent semiconductors Ce(3)Pt(3)Bi(4) and Ce(3)Pt(3)Sb(4).