  We have measured the specific heat of the S = 1/2 alternating Heisenberg
antiferromagnetic chain compound pentafluorophenyl nitronyl nitroxide in
magnetic fields using a single crystal and powder. A sharp peak due to
field-induced magnetic ordering (FIMO) is observed in both samples. The H-T
phase boundary of the FIMO of the single crystal is symmetric with respect to
the central field of the gapless field region HC1 < H < HC2, whereas it is
distorted for the powder whose ordering temperatures are lower. An analysis
employing calculations based on the finite temperature density matrix
renormalization group indicates the possibility of novel incommensurate
ordering due to frustration in the powder around the central field.
