Аннотация:High-resolution heat-capacity data have been determined for a single-phase sample of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.9}$ over the temperature range 77--150 K. Two anomalies were found: a \ensuremath{\lambda}-type transition occurring at 86.65 K and a discontinuity accompanied by a first-order transition occurring at 90.30 K. The discontinuity has a value of \ensuremath{\Delta}${C}_{p}$/T=62 mJ (mol f.u.${)}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}2}$, where f.u. denotes formula unit. An estimation of the latent heat and entropy associated with the first-order transition is given. The anomalies in the heat capacity are related to the changes in the resistance and Meissner effect and a mixed superconducting-normal state is proposed.
