On the scaling of the (H-T)-phase diagram of CuGeO${}_3$

The H-T phase diagram of CuGeO${}_3$ has been determined, for different values of the hydrostatic pressure, utilizing optical absorption spectroscopy on the Cu${}^{2+}$ d-d transitions. It is shown that the intensity of the related zero phonon line transition is very sensitive to the local environment of Cu${}^{2+}$, allowing for precise determination of all phase transitions. It is found that the phase diagrams at various pressures do not scale according to the Cross-Fischer theory. An alternative scaling is proposed.