  We have investigated the magnetic phase diagram of a fully oxygenated
detwinned YBa2Cu3O7 single crystal by means of magneto-caloric and
magnetization measurements, and found thermodynamic evidence for a temperature
dependent first-order phase-transition line deep within the vortex-solid
region. The associated discontinuities in the entropy are apparently
proportional to the magnetic flux density, which may hint at a structural
transition of the vortex lattice.
