When KDP crystals are exposed to intense ultraviolet or X-ray irradiation at room temperature undesired transient optical absorption bands are produced in the 300-650 nm spectral regions, which in turn limit the device performance. The laser-induced optical absorption was attributed to point defects created during crystal growth or generated by irradiation. Namely, at high intensities, two-photon interband absorption generates electron-hole (e-h) pairs which quickly relax and eventually evolve into electronic defect states that often lie within the band gap.
Davis et al hypothesized that proton transport is a major component of the mechanism responsible in the production of the electron center; namely, a H+ ion occupying a normal lattice site traps a free electron created by the laser beam, thus forming a neutral H atom which in turn easily moves away from its original site, while the oxygen atom closest to the vacancy traps the hole and forms the [HPO4]- radical. Subsequent electron paramagnetic resonance (EPR) spectroscopy and resonance Raman scattering experiments identified two types of H defects: (a) the [HPO4]-hole center which consists of a hole trapped on an oxygen ion adjacent to a hydrogen vacancy, and (b) the H0 electron center based on a hydrogen interstitial atom.
Thus, the experimental results suggest that Hydrogen point defects play a major role in controlling both the ferroelectric transition and the transient absorption properties of KDP. On the other hand, theoretical studies are lacking and hence the underlying atomic process and in particular the effect of the defect charged state on the relevant atomic configurations and reaction processes remain uncertain and unresolved.
The purpose of this work is to present an ab initio study of two type of Hydrogen point defects (interstitial and vacancy) in KDP, and to investigate the effect of charged state on bonding and defect reactions. We find that the H neutral interstitial remains intact and does not interact with the host. On the other hand, in the positively charged state, the H interstitial forms an extra O-H bond, while in the negatively charged state, the H interstitial forms a H2 molecule with the simultaneous creation of a H vacancy. Thus, H interstitial in both charged states catalyzes the severing of the HB network. For the H vacancy, the most remarkable result is that the addition of a hole leads to a dramatic decrease of the O-O bond length between the oxygen atoms close to the vacancy and the formation of a peroxyl bridge.
Figure 1 shows the change of the relaxed atomic structure induced by an interstitial Hydrogen (Hi) atom with the two nearest-neighbor PO4 tetrahedral units for the neutral (Fig. 1a), negative (Fig. 1b) and positive (Fig. 1c) charged states.
In the neutral state, the hydrogen defect remains intact in the interstitial position and does not form any bonds with the host atoms. We will refer to this configuration as the ``isolated Hi". The addition of an electron yields a dramatic change in the defect configuration. A host H atom is attracted to the interstitial Hi and gets displaced from its normal lattice site. This results in the formation of an interstitial H2 molecule leaving behind a hydrogen vacancy, which will be referred to as the ``H2 + Hv" configuration. The Hh-Hi bond length is 0.76 Å, close to the values of 0.77 Å for the free H2 and of 0.74 Å for molecular hydrogen in silica, respectively. The formation of the vacancy leads to an increase of the O-O separation to 3.36 Å from the corresponding value of 2.48 Å in pure KDP. In sharp contrast, in the positively charged state, the hydrogen interstitial bonds with its nearest-neighbor O atom creating an extra O-H bond (0.98 Å), which in turn greatly weakens the neighboring O-H bond of the host atoms. This configuration will be referred to as the ``hydroxyl" configuration. Thus, H interstitial in both charged states catalyzes the scission of the HB network.
In Fig. 1 we also show the local atomic structural changes induced by a Hydrogen vacancy with its two nearest-neighbor PO4 tetrahedral units for the neutral (Fig. 1d), negative (Fig. 1e) and positive (Fig. 1f) charged states. The most remarkable feature in the case of the vacancy is that the addition of a hole leads to a dramatic decrease of the O-O bond length between the oxygen atoms close to the vacancy from 2.73 Å in the neutral case to 1.48 Å, thus forming a peroxyl bridge. This value is close to the values of 1.19 Å in free molecular oxygen and of 1.49 Å for the peroxyl bridge in vitreous silica.
In order to explore the energetics associated with the reactions of the interstitial, we have calculated the energy of a series of intermediate configurations in which all the degrees of freedom but the Hi-Hh or the Hi-Oh distance are relaxed. Figure 2 shows the relative total energy as a function of the Hi-Hh distance for the neutral (Fig. 2a) and the negative charged state (Fig. 2b), and as a function of the Hi-Oh distance for the positive charged state (Fig. 2c), respectively.
In the neutral state (Fig. 2a), we find two energy minima at 1.9 Å and 0.75 Å corresponding to the ``isolated Hi" configuration and to the ``H2 + Hv" configuration, respectively. We find that the first configuration has the lowest energy and that an energy barrier of 1.2 eV needs to be overcome to move from the``isolated Hi" to the ``H2 + Hv" configuration. In sharp contrast, the addition of an electron (Fig. 2b) removes the energy barrier and yields a single stable structure at 0.75 Å corresponding to the ``H2 + Hv" configuration. Thus, the additional electron greatly assists the formation of an interstitial H2 molecule. In the positively charged state (Fig. 2c), we find that an energy barrier of 1.4 eV needs to be overcome to move from the ``isolated Hi" to the stable O-H ``hydroxyl" configuration.