第一原理研究LuPO4中氧空位的性质

First Principles Study on the Property of O Vacancy in LuPO4 Crystal

磷酸结构的晶体在掺杂二价阳离子后容易形成产生焦磷酸结构(P2O7)^4-,这种含有焦磷酸结构的氧化物材料十分适合做质子导体、燃料电池、气体传感器以及陶瓷膜等。本文利用第一性原理研究了LuPO4晶体中氧空位的结构性质,结果显示当氧空位带二价正电时,会引发氧空位周围原子奇特的畸变,形成焦磷酸结构。为了解释这种结构畸变的机理,本文利用过渡态搜索计算了结构变化过程中势能面的变化情况,正一价氧空位形成焦磷酸结构需要越过2.4eV的势垒,而正二价氧空位形成焦磷酸结构则不需要越过任何势垒,因此很容易形成焦磷酸结构。最后给出氧空位不同带电态的晶格结构、电子态密度以及电荷密度分布等基本物理性质,氧空位处于正二价态结构下,氧空位附近的P原子与O原子成键,又由于O原子有较强的电负性,P的s轨道电子向O的p轨道转移。P的s、p轨道在禁带中出现了与总态密度对应的缺陷能级,结果表明带正二价氧空位的晶体性质发生了明显变化。

Phosphoric acid crystal doped with bivalent cation is easy to form pyrophosphate structure(P2O7)^4-. This kind of oxide material containing pyrophosphate structure is very suitable for proton conductor, fuel cell, gas sensor, and ceramic film, etc. In this research we applied first principles to study the structural behaviour of oxygen vacancy in LuPO4 crystal. The results showed that oxygen vacancies with two positive charges distort structure largely and form pyrophosphate structure. In order to verify the feasibility of this structure transition, the nudged elastic band method is used to find the highest saddle point of potential energy. The calculated results show that transition state energies of oxygen vacancy with +1 and +2 charge forming pyrophosphate structure are about 2.4 and 0 eV, respectively. So pyrophosphate structure easily forms for oxygen vacancy with +2 charge. Finally, the lattice structure, density of states and charge density distribution are obtained. P atom and O atom around the oxygen vacancy with +2 charge can form chemical bond. The electron on the P s will shift to O p orbit for oxygen with strong electronegative, which introduces the defect level in forbidden band. It is indicated that the property of the crystal changes drastically for the existence of the oxygen vacancy with two positive charges.