| KDP-type crystals are a class of optical materials with excellent comprehensive properties,and have important applications in electro-optic,piezoelectric,non-linear optics fields.Water and impurity are two important factors that can affect the growth habit and quality of KDP-type crystals.The existence of water not only ensures the crystal growth and surface machining,but also adversely affects the quality of crystals,leading to the deliquescence,corrosion pits,and so on.In addition,there are various impurities in KDP-type crystals,including metal ions,organic impurities,and various inorganic substances.These impurities can be adsorbed on the crystal surfaces,thus changing the crystal morphology and optical quality.For the influences of water molecules and impurities on KDP-type crystals,a lot of experimental studies have been carried out.However,there are few theoretical studies.In view of the above situation,we studied the adsorption behaviors of water molecules and impurities on the ADP and KDP surfaces by density functional theory(DFT)calculations.This study not only can help people to understand the complex interactions between them,but also can provide potent guidances for practices.The main contents and conclusions are as follows:(1)The adsorption behaviors of 1-3 water molecules on the ADP(100)and(101)surfaces were investigated.It was found that the adsorption energies are all negative values,which means that the adsorption processes are spontaneous and exothermic.In addition,the adsorption of water molecules on the(100)surface is more stable than that on(101)surface.With increasing number of water molecules,the adsorption energies for both surfaces also increase.There are two bonding modes between water molecules and surfaces.Firstly,water molecules can form hydrogen bonds with H and O atoms on the surfaces.In addition,when two or three water molecules adsorbed on the(100)surface,the interaction between water and the surface H atom is strong enough to form the covalent bonds.The chemical bonds of water molecules and ADP surfaces are generally elongated after adsorption,what's more,one H-O covalent bond on the ADP(100)surface are breaken.(2)The adsorption of ethanol molecule on the KDP(100)and(101)surfaces was systematically investigated.The negative adsorption energies indicate that ethanol can be spontaneously adsorbed on the KDP surfaces.The adsorption behavior is caused by the interaction netween hydroxyl group and the surface H,O,K atoms.Hydroxyl group can form two hydrogen bonds and one ionic bond with(100)surface,while there are only one hydrogen bond and one ionic bond formed between ethanol and(101)surface.Therefore,the adsorption of ethanol on the(100)surface is more stable.In addition,chemical bonds of the ethanol molecule and KDP surfaces are generally elongated after adsorption,binding forces between atoms become weak and the structures are unstable.(3)The adsorption behaviors of metal ions(K+,Na+,Ca2+,Cu2+,Al3+,Cr3+)on the KDP(100)surface were systematically studied.All adsorption energies are negative,meaning that adsorption processes are spontaneous and exothermic.Adsorption stability icreases in the order of Na+<K+<Cu2+<Ca2+<Al3+<Cr3+,and shows a consistent trend as the ion valence(monovalent<divalent<trivalent).In addition,bonding mechanisms between metal ions and KDP(100)surface have a certain relationship with ion radius.Metal ions with large radius(Na+,K+,Ca2+)interact with the surface by more ion bonding,while small ions(Cu2+,Al3+,and Cr3+)usually combine with the surface by covalent bonding.All metal ions can affect the surface structure,and the bonds on the surface are elongated after adsorption.At the same time,the structures of hydrogen bonds also are obviously changed.Compared with other metal ions,trivalent ions have the greatest influences on the KDP(100)surface.Al3+ and Cr3+ can attract one H atom to their surroundings.making the surface H-O bond broken. |
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