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DOE OSTI2020

Materials Data on CsSb(PO4)2 by Materials Project

CsSb(PO4)2 crystallizes in the trigonal P-3 space group. The structure is three-dimensional. Cs1+ is bonded to six equivalent O2- atoms to form distorted CsO6 octahedra that share corners with six equivalent PO4 tetrahedra and edges with six equivalent CsO6 octahedra. All Cs–O bond lengths are 3.18 Å. Sb5+ is bonded to six equivalent O2- atoms to form SbO6 octahedra that share corners with six equivalent PO4 tetrahedra. All Sb–O bond lengths are 1.99 Å. P5+ is bonded to four O2- atoms to form PO4 tetrahedra that share corners with three equivalent CsO6 octahedra and corners with three equivalent SbO6 octahedra. The corner-sharing octahedra tilt angles range from 42–63°. There is one shorter (1.49 Å) and three longer (1.58 Å) P–O bond length. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a single-bond geometry to three equivalent Cs1+ and one P5+ atom. In the second O2- site, O2- is bonded in a distorted bent 150 degrees geometry to one Sb5+ and one P5+ atom.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on CsSb by Materials Project

CsSb is Magnesium tetraboride-like structured and crystallizes in the orthorhombic P2_12_12_1 space group. The structure is three-dimensional. there are two inequivalent Cs1+ sites. In the first Cs1+ site, Cs1+ is bonded in a 6-coordinate geometry to six Sb1- atoms. There are a spread of Cs–Sb bond distances ranging from 3.86–4.18 Å. In the second Cs1+ site, Cs1+ is bonded in a 5-coordinate geometry to five Sb1- atoms. There are a spread of Cs–Sb bond distances ranging from 3.84–4.05 Å. There are two inequivalent Sb1- sites. In the first Sb1- site, Sb1- is bonded in a 7-coordinate geometry to five Cs1+ and two equivalent Sb1- atoms. There are one shorter (2.90 Å) and one longer (2.92 Å) Sb–Sb bond lengths. In the second Sb1- site, Sb1- is bonded in a 8-coordinate geometry to six Cs1+ and two equivalent Sb1- atoms.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Improved robustness of sequentially deposited potassium cesium antimonide photocathodes achieved by increasing the potassium content towards theoretical stoichiometry

Alkali antimonide semiconductor photocathodes are promising candidates for high-brightness electron sources for advanced accelerators, including free-electron lasers (FEL), due to their high quantum efficiency (QE), low emittance, and high temporal resolution. Two challenges with these photocathodes are (1) the lack of a universal deposition recipe to achieve crystal stoichiometries and (2) their high susceptibility to vacuum contamination, which restricts their operation pressure to ultrahigh vacuums and leads to a short lifetime and low extraction charge. To resolve these issues, it is essential to understand the elemental compositions of deposited photocathodes and correlate them to robustness. Here, we report depth profiles for potassium cesium antimonide photocathodes, which were investigated using synchrotron radiation x-ray photoelectron spectroscopy, and the robustness of those photocathodes. We prepared two types of photocathodes with different potassium contents via sequential thermal evaporation. Depth profiles revealed that the photocathodes with a potassium deficit had excess cesium at the surface, while the ratio of potassium and cesium to antimony decreased rapidly within the film. In contrast, the photocathodes with sufficient potassium had close to the theoretical stoichiometry of K 2 CsSb at the surface and maintained that stoichiometry for over half the entire film thickness. Both photocathode types had a similar maximum QE at 532 nm; however, exposure to oxygen revealed that the photocathode with a crystalline stoichiometry of K 2 CsSb maintained QE at one order of magnitude higher pressure compared to its potassium-deficit counterpart. These results highlight the importance of synthesizing potassium cesium antimonide photocathodes with sufficient potassium to achieve the theoretical crystalline stoichiometry for both high QE and improved robustness.

47 OTHER INSTRUMENTATION↗