先兆型铁电体CaTiO3的第一性原理研究

在广义梯度近似(GGA)下利用全电势线性化的缀加平面波法(FPLAPW)计算了钛酸钙(CaTiO3)的电子结构.将实验测得的晶胞体积记为V0,计算中所用的晶胞体积记为V.当V/V0=1.0时, Ti离子位移为零相应于总能量低能态,钛酸钙不会发生铁电相变.但如果其体积膨胀10%,则Ti离子的位移将导致能量极小值.这意味着在立方钛酸钙中有发生铁电相变的趋势,表明在钛酸钙中存在着体积诱发的铁电相变,即钛酸钙为先兆型铁电体.态密度在V/V0=1.1时, Ti d电子和O(2) p电子之间存在强烈的轨道杂化,这种杂化是出现铁电性的必要条件.电场梯度的结果也表明了这一点.

Electronic Structure of Incipient Ferroelectric CaTiO3

CaTiO3is widely used in electronic ceramic materials, it is also a key component of Synroc, a synthetic rock form used to immobilize nuclear waste. There has been considerable interest in the structure phase transitions of this typical perovskite oxide. As reported by Kennedyet al., CaTiO3is orthorhombic with space group Pbnm below 1380 K and belongs to another orthorhombic space group Cmcm between 1380 K and 1500K. At 1500 K, it transforms into tetragonal with space group I4/mcm. Above 1580 K, it becomes cubic with space group Pm3m. Lemanovet al.measured the dielectric properties of CaTiO3at low temperature, and classified it as an incipient ferroelectric or a quantum paraelectric. CaTiO3is called a“higher”quantum paraelectric because its dielectric constant saturates at higher temperature than that for other quantum paraelectrics such as SrTiO3and KTaO3. To understand the physics underlying these properties, first-principles calculation of the electronic structure is desirable. In order to understand the tendency of the CaTiO3cubic perovskite to suffer a ferroelectric transition, we calculated the electronic properties of CaTiO3for different displacement of Ti in the 001direction. At the experiment equilibrium volume (V/ V0=1.0), CaTiO3is not energetically favored with the proposed ferroelectric distortion. However, with a 10% volume expansion(V/ V0=1.1), the same displacement results in a shallow, but well defined double well. It implies that there is a tendency to ferroelectricity in cubic CaTiO3crystal. This is consistent with the experimental results that point out that CaTiO3is an incipient ferroelectric. From our calculation, the ferroelectric state can be induced by a negative pressure.The DOS of Ti d and O p atΔz=0.03 for V=V0shows there is a weak hybridization between Ti d and O p. While for a 10% volume expansion, this hybridization becomes stronger. Like other perovskites, it is this hybridization between Ti 3d and O 2p that weakens the short range repulsions and stabilizes the ferroelectricity. The strong hybridization implies that the interaction between Ti and O is highly covalent. This is consistent with the analysis of the total energy. From the calculated total energy, it is shown that CaTiO3has a tendency to a ferroelectric state in which the atom Ti displaces in the 001 direction. An analysis of the DOS and electric field gradients reveals that there is a hybridization between Ti 3d and O 2p. This hybridization is essential leading to the incipient ferroelectricity.