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Structure Tuning, Strong Second Harmonic Generation Response, and High Optical Stability of the Polar Semiconductors Na<sub>1–<i>x</i></sub>K<sub><i>x</i></sub>As<i>Q</i><sub>2</sub>

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31

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39

References

2021

Year

Abishek K. Iyer, Jeong Bin Cho, Hye Ryung Byun, Michael J. Waters, Shiqiang Hao, Benjamin M. Oxley, Venkat Gopalan, Christopher Wolverton, James M. Rondinelli, Joon I. Jang,
Journal of the American Chemical Society

Abstract

The mixed cation compounds Na<sub>1-<i>x</i></sub>K<sub><i>x</i></sub>AsSe<sub>2</sub> (<i>x</i> = 0.8, 0.65, 0.5) and Na<sub>0.1</sub>K<sub>0.9</sub>AsS<sub>2</sub> crystallize in the polar noncentrosymmetric space group <i>Cc</i>. The <i>A</i>As<i>Q</i><sub>2</sub> (<i>A</i> = alkali metals, <i>Q</i> = S, Se) family features one-dimensional (1D) <sup>1</sup>/<sub>∞</sub>[<i>AQ</i><sub>2</sub><sup>-</sup>] chains comprising corner-sharing pyramidal <i>AQ</i><sub>3</sub> units in which the packing of these chains is dependent on the alkali metals. The parallel <sup>1</sup>/<sub>∞</sub>[<i>AQ</i><sub>2</sub><sup>-</sup>] chains interact via short As···Se contacts, which increase in length when the fraction of K atoms is increased. The increase in the As···Se interchain distance increases the band gap from 1.75 eV in γ-NaAsSe<sub>2</sub> to 2.01 eV in Na<sub>0.35</sub>K<sub>0.65</sub>AsSe<sub>2</sub>, 2.07 eV in Na<sub>0.2</sub>K<sub>0.8</sub>AsSe<sub>2</sub>, and 2.18 eV in Na<sub>0.1</sub>K<sub>0.9</sub>AsS<sub>2</sub>. The Na<sub>1-<i>x</i></sub>K<sub><i>x</i></sub>AsSe<sub>2</sub> (<i>x</i> = 0.8, 0.65) compounds melt congruently at approximately 316 °C. Wavelength-dependent second harmonic generation (SHG) measurements on powder samples of Na<sub>1-<i>x</i></sub>K<sub><i>x</i></sub>AsSe<sub>2</sub> (<i>x</i> = 0.8, 0.65, 0.5) and Na<sub>0.1</sub>K<sub>0.9</sub>AsS<sub>2</sub> suggest that Na<sub>0.2</sub>K<sub>0.8</sub>AsSe<sub>2</sub> and Na<sub>0.1</sub>K<sub>0.9</sub>AsS<sub>2</sub> have the highest SHG response and exhibit significantly higher laser-induced damage thresholds (LIDTs). Theoretical SHG calculations on Na<sub>0.5</sub>K<sub>0.5</sub>AsSe<sub>2</sub> confirm its SHG response with the highest value of <i>d</i><sub>33</sub> = 22.5 pm/V (χ<sub>333</sub><sup>(2)</sup> = 45.0 pm/V). The effective nonlinearity for a randomly oriented powder is calculated to be <i>d</i><sub>eff</sub> = 18.9 pm/V (χ<sub>eff</sub><sup>(2)</sup> = 37.8 pm/V), which is consistent with the experimentally obtained value of <i>d</i><sub>eff</sub> = 16.5 pm/V (χ<sub>eff</sub><sup>(2)</sup> = 33.0 pm/V). Three-photon absorption is the dominant mechanism for the optical breakdown of the compounds under intense excitation at 1580 nm, with Na<sub>0.2</sub>K<sub>0.8</sub>AsSe<sub>2</sub> exhibiting the highest stability.

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