  Pb2V3O9 was found to have antiferromagnetic alternating chains of S=1/2 and
show the antiferromagnetic long range ordered (AFLRO) state under the magnetic
field between 4 and 38 T. We succeeded in growing single crystals of Pb2V3O9 by
floating zone method and determined the asymmetric H-T phase diagram of Pb2V3O9
from measurements of the magnetization and the heat capacity. Previous reports
focused on the AFLRO state in the low magnetic field region and that in high
magnetic field region was not discussed so much. In this paper, we examined the
AFLRO state in terms of the mean field theory and the picture of Bose-Einstein
condensation of magnons to discuss the whole phase diagram. Consequently, the
asymmetry of the phase boundary in Pb2V3O9 is explained by the mixing
contribution of excited states and by interchain interactions.
