Effect of pressure on the optical-absorption edges of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">CsGeBr</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">CsGeCl</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

We have investigated the effect of hydrostatic pressure on the optical-absorption edges of the ferroelectric compounds ${\mathrm{CsGeBr}}_{3}$ and ${\mathrm{CsGeCl}}_{3}$. The absorption edge energies of the ambient pressure modifications (trigonally distorted cubic perovskite) are ${\mathit{E}}_{\mathit{G}}$ = 2.32(5) eV and 3.67(5) eV, respectively. We find extraordinary large redshifts of the absorption edges under pressure for the distorted modifications (e.g., -0.61 eV/GPa for the bromine compound) as well as high-pressure cubic phases. Experimental results are interpreted on the basis of Hartree-Fock band structure calculations for ${\mathrm{CsGeBr}}_{3}$ in the cubic primitive structure. The absorption edges are attributed to the lowest direct gap transition at the R point of the Brillouin zone. \textcopyright{} 1996 The American Physical Society.

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