Why graphit to diamon is endothermic



I need some help to found the mistake in my reasoning :

I understand that the reaction carbon to diamon is endothermic. But, if i look the phase diagram of crabon the slope of the equilibrium curve for this transformation is positive. So dP/dT > 0. Clapeyron :


Moreover Vdiamon < Vgraphite so  to have a positive slope we need have Delta H transition negative, this is caracteristic of exothermic reaction, no ?

Where is the mistake on my reasoning ?

Thank you.

While the graphite -> diamond transition is slightly endothermic at 1 atm, I believe that it should be exothermic at high pressure.  Remember that enthalpy can depend on temperature and pressure.