Theoretical study of the ordered-vacancy semiconducting compound CdAl2Se4

We report results from first-principles density functional calculations of the structural, electronic, and vibrational properties of the semiconductin g CdAl2Se4 compound. The defective chalcopyrite-like I (4) over bar phase a nd the spinel-like Fd (3) over barm phases are considered. We find within t he local density approximation (LDA) that the total energies of the I (4) o ver bar and the Fd (3) over barm phases are virtually identical. Within the generalized gradient approximation, however, the ground-state structure is found to be I (4) over bar (in agreement with experiment), while the Fd (3 ) over barm phase is a high-pressure phase that theoretically becomes stabl e at 3 GPa (4.5 GPa experimentally). The I (4) over bar phase is found to h ave a direct electronic band gap while the Fd (3) over barm phase has an in direct band gap. A correction of the LDA band gap for I (4) over bar using a generalized density functional theory is in excellent agreement with meas urements. The vibrational spectrum of the I (4) over bar structure at zero pressure and as a function of pressure is determined. Up to 23 GPa, no mode s become soft.