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

TL;DR

This study combines high-pressure experiments and theoretical calculations to analyze how the electronic band structure of AWO4 compounds changes under pressure, revealing phase transitions and band-gap behavior without metallization.

Contribution

It provides a comprehensive pressure-dependent analysis of band structures in AWO4 compounds, integrating experimental data with theoretical insights to interpret electronic and structural changes.

Findings

Band-gap energy varies with pressure, with a notable collapse over 1.2 eV in BaWO4.

PbWO4 shows a large negative pressure coefficient of -62 meV/GPa.

No metallization occurs in any of the studied compounds under high pressure.

Abstract

The optical-absorption edge of single crystals of CaWO4, SrWO4, BaWO4, and PbWO4 has been measured under high pressure up to ~20 GPa at room temperature. From the 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 Eg pressure dependence (-2.1 < dEg/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 dEg/dP. The band-gap collapse is larger than 1.2 eV in BaWO4. We also calculated the electronic-band structures and their pressure evolution. Calculations allow us to interpret experiments considering the different electronic configuration of divalent metals. Changes in the pressure evolution of Eg 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 predicted by calculations.