Precise electrical resistivity and ac calorimetry measurements under pressures have revealed (P, T) phase diagram of superconducting clathrate Ba24Ge100. Following conventional superconducting theory, the pressure dependence of the density of state at Fermi energy, N(E F), estimated by the electronic specific heat C/T has been discussed with the Tc(P). Up to P= 2.1 GPa, where Tc(P) starts to saturate, C/T increases with Tc in agreement with the theory. On further compression, C/T is strongly suppressed while Tc slightly decreases. Across Pc ∼ 2.9 GPa, where structural deforming transitions at Ts1(s2) are suppressed, pressure variation of C/T becomes smaller. In addition to the enhancement of the N(EF), other changes in crystal and/or electrical structure may play important roles to pressure dependence of Tc in Ba24Ge100.
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