  We investigate the effects of carbon and boron doping on the thermal
expansion in the hexagonal (P63/mmc) intermetallic YbGaGe. X-ray powder
diffraction was used to measure the lattice constants on pure and doped (C or B
at nominal levels of 0.5 %) samples from T~10 K to T~300 K. Also measured were
resistivity, specific-heat, and magnetic susceptibility. While the pure YbGaGe
samples exhibit positive thermal volume expansion, (V300K-V10K)/V300K = 0.94%,
the volume expansion in the lightly C and B-doped samples, contract and tend
towards zero volume expansion. Such a strong response with such light doping
suggests that the underlying mechanism for the reported zero volume expansion
is substitutional disorder, and not the previously proposed valence
fluctuations.
