Work Function of a Room‐Temperature, Stable Electride [Ca24Al28O64]4+(e–)4

the electrical conductivity was measured by the four-probe method. We estimated an electron density from the conductivity and absorption intensity at 2.8 eV [8,9,29]. Photoelectron Measurements: UPS, IPES, and PYS were performed using a homemade apparatus. A fresh surface of the sample was prepared by filing the surface with a diamond file in a preparation chamber attached to the measurement chamber under a vacuum of ca. 5×10 –7 Pa before each photoelectron measurement. The measurements were conducted under a vacuum of ca. 1×10 –7 Pa. UPS spectra were obtained using a VGS Class150 electron analyzer and discharge lamp that emitted excitation lines of He I (21.2 eV) and He II (40.8 eV). The energy resolution was approximately 200 meV. The binding energy was calibrated by the E F of clean polycrystalline Au. The vacuum level E VAC was measured from a secondary electron cut-off energy. In preliminary experiments, we found that external negative biases less than –3 V should be applied to the C12A7:e – samples to guarantee that the vacuum level of the C12A7:e – exceeded that of the spectrometer and that the measured cut-off appropriately reflected the EVAC. IPES spectra were taken in the Bremsstrahlung isochromat spectroscopy mode by monitoring the intensity of 9.5 eV photons emitted from the sample by monochromatic electron-beam irradiation. PYS measurements were performed using monochromatic photons with a resolution of 0.1 eV in the range 2.0–3.3 eV emitted from a xenon lamp placed outside the chamber. The sample was negatively biased from 0 to 50 V during the measurements. HAXPES was performed using a Scienta SES R-4000 electron analyzer and synchrotron light from BL47XU ( h m =7935.2 eV) at SPring-8 in Hyogo, Japan. The energy resolution was less than 100 meV. A data acquisition time of ca. 6 h was required to obtain a clear Fermi edge at the CCB, as shown in Figure 1b.