Electronic structure of KD_(2x)H_(2(1-x)) PO_4 studied by soft x-ray absorption and emission spectroscopies

摘要

The surface and bulk electronic structure of tetragonal (at 300 K) and orthorhombic (at 77 K) KD_(2x)H_(2(1-x))PO_4 single crystals (so-called KDP and DKDP), with a deuteration degree x of 0.0, 0.3, and 0.6, is studied by soft x-ray absorption near-edge structure (XANES) and x-ray emission (XES) spectroscopies. High-resolution O K-edge, P L_(2,3)-edge, and K L_(2,3)-edge XANES and XES spectra reveal that the element-specific partial density of states in the conduction and valence bands is essentially independent of deuteration x. We give assignment of XANES and XES peaks based on previous molecular orbital and band-structure calculations. Projected densities of states in the conduction band also appear to be essentially identical for tetragonal (at 300 K) and orthorhombic (at 77 K) phases, consistent with previous band structure calculations. However, a decrease in sample temperature from 300 to 77 K results in measurable changes to the valence band (probed by XES) but not to the conduction band (probed by XANES). The lower limit on the room-temperature band gap of KDP and DKDP is estimated as ～ 7.6 eV. Results also show that high-intensity x-ray irradiation results in decomposition of these hydrogen-bonded materials into water and KPO_3 cyclophosphates and polyphosphates.