  We report a new Au-Ga-Ce 1/1 approximant crystal (AC) which possesses a
significantly wide single-phase region of 53 - 70 at% Au and 13.6 - 15.1 at%
Ce. Single crystal X-ray structural analyses reveal the existence of two types
of structural degrees of freedom, i.e., the Au/Ga mixing sites and the
fractional Ce occupancy site: the former enables a large variation in the
electron concentration and the latter allows a variation in the occupancy of a
magnetic impurity atom at the center of the Tsai-type cluster. Following these
findings, the influences of two types of structural modifications on the
magnetism are thoroughly investigated by means of magnetic susceptibility and
specific heat measurements on the Au-Ga-Ce 1/1 AC. The spin-glass (SG) state is
found to be the ground state over the entire single-phase region, showing a
robust nature of the SG state against both structural modifications. In
addition, a gigantic specific heat (C/T) is commonly observed at low
temperatures for all the compositions, which is consistently explained as a
consequence of the spin-freezing phenomenon, not of a heavy Fermion behavior as
reported elsewhere. Moreover, the origin of the SG state in the 1/1 Au-Ga-Ce AC
is attributed to the existence of non-magnetic atom disorder in the Au/Ga
mixing sites. Furthermore, a Kondo behavior is observed in the electrical
resistivity at low temperatures, which is enhanced by increasing the Ce
concentration, verifying that a Ce atom introduced at the cluster center
behaves as a Kondo impurity for the first time.
