Measurement of $P$, $V$, $T$, and sound velocity across the melting curve of $n$-${\mathrm{H}}_2$ and $n$-${\mathrm{D}}_2$ to 19 kbar

We used a piston-cylinder apparatus for simultaneous measurement of $P$, $V$, $T$, and longitudinal sound velocity $v_l$ across the melting line of $n$-${\mathrm{H}}_2$ and $n$-${\mathrm{D}}_2$ at pressures from about 4 to 19 kbar. An equation of the modified Simon type was found to fit the melting-line data satisfactorily. We measured the volume and sound-velocity changes at melting and used these data to infer the Debye temperature, transverse sound velocity, Poisson ratio, compressibility, and entropy in the solid phases. Comparisons with recent theoretical calculations and low-pressure measurements show generally good agreement.