Anomalous High-Pressure Jahn-Teller Behavior in CuWO4

High-pressure optical-absorption measurements performed in CuWO4 up to 20 GPa provide experimental evidence of the persistence of the Jahn-Teller (JT) distortion in the whole pressure range both in the low-pressure triclinic and in the high-pressure monoclinic phase. The electron-lattice couplings associated with the eg (E circle times e) and t(2g)(T circle times e) orbitals of Cu2+ in CuWO4 are obtained from correlations between the JT distortion of the CuO6 octahedron and the associated structure of Cu2+ d-electronic levels. This distortion and its associated JT energy (E-JT) decrease upon compression in both phases. However, both the distortion and associated E-JT increase sharply at the phase-transition pressure (P-PT 9.9 GPa), and we estimate that the JT distortion persists for a wide pressure range not being suppressed up to 37 GPa. These results shed light on the transition mechanism of multiferroic CuWO4, suggesting that the pressure-induced structural phase transition is a way to minimize the distortive effects associated with the toughness of the JT distortion.