A combined high-pressure experimental and theoretical study of the electronic band-structure of scheelite-type AWO4 (A = Ca, Sr, Ba, Pb) compounds

The optical-absorption edge of single crystals of CaWO4, SrWO4, BaWO4, and PbWO4 has been measured under high pressure up to similar to 20 GPa at room temperature. From these measurements, we have obtained the evolution of the band-gap energy with pressure. We found a low-pressure range (up to 7-10 GPa) where alkaline-earth tungstates present a very small E-g pressure dependence (-2.1 < dE(g)/dP < 8.9 meV/GPa). In contrast, in the same pressure range, PbWO4 has a pressure coefficient of -62 meV/GPa. The high-pressure range is characterized in the four compounds by an abrupt decrease of Eg followed by changes in dE(g)/dP. The band-gap collapse is larger than 1.2 eV in BaWO4. We also calculated the electronic-band structures and their pressure evolution. The calculations allow us to interpret experiments considering the different electronic configurations of divalent metals. Changes in the pressure evolution of E-g are correlated with the occurrence of pressure-induced phase transitions. The band structures for the low-and high-pressure phases are also reported. No metallization of any of the compounds is detected in experiments nor is it predicted by the calculations. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3622322]