Phase Competition in High-Quality Epitaxial Antiferroelectric PbZrO₃ Thin Films

Abstract

Antiferroelectric PbZrO 3 has attracted renewed interest in recent years because of its unique properties and wide range of potential applications. However, the nature of antiferroelectricity and its evolution with the electric field and temperature remain controversial, mostly due to the difficulty of obtaining high-quality single-crystal samples. The lack of consensus regarding the phase transition in PbZrO 3 is not only important on a fundamental side but also greatly hinders further applications. Herein, high-quality PbZrO 3 epitaxial thin films are successfully fabricated by pulsed laser deposition. The structural and physical properties of the films are systematically studied via a combination of electric property measurements, X-ray diffraction, scanning transmission electron microscopy imaging, and second-harmonic generation studies. Our studies unveil the noncentrosymmetric nature of PbZrO 3 films at room temperature. Moreover, the Curie temperature increased to 270°, ∼40°higher than that in the bulk, and no intermediate ferroelectric phase was observed. Besides, an incipient ferroelectric with relaxor-like behavior above the Curie temperature due to the existence of a local polar cluster in the high-temperature paraelectric phase is experimentally observed for the first time. Our studies provide a better understanding of PbZrO 3 thin films and pave the way for practical applications of antiferroelectric material in modern electronic devices.

Research status and prospect of lead zirconate-based antiferroelectric films

“…结 构 [49,50] ，常 温下 PZO 的 每 个 正 交晶 胞由 8 个赝 立 方单 胞组 成 ， 晶格 常 数 a≈5.884Å=√2a T ，b≈11.68Å=2√2a T ，c≈8.220Å=2a T [50] 。PZO中氧八面体的不同步 旋转(aac 0 )导致了铅离子在面内沿着正交晶胞的+a或者-a轴方向存在反平行位 移，在室温下铅离子的平均位移为0.25Å，其反平行排列的偶极子极化方向沿赝 立方单胞<110>方向， 在外场诱导下转换为FE后， 其极化方向变为赝立方单胞<111> 方向 [50] 。PZO块体的居里温度≈230 o C，且在居里温度附近较窄温度区域内存在FE 特性 [51] ，然而在薄膜中，居里温度不仅有较大幅度的提升而且介电温谱中未发现 有FE相变行为 [50] 。近些年对AFE的重新探索发现，AFE中存在丰富的亚稳态，首 先是AFE中的弱FE行为在薄膜中被发现，表现出"尺寸效应" [22] ，并一度被认为是 非中心对称的Pba2结构 [21] ，使得人们对于AFE材料中心对称性提出新的质疑，另 外还有人解释这是AFE中共存了FE的结果 [23] 。近期，唐云龙等基于高分辨率透射 电子显微镜确定了PZO相变中的晶格细节，发现了一种新的具有角度和幅度调制 的类FE偶极子结构，通过改变电子束辐照时间追踪了PZO中的相变细节：类FE 到AFE相变、AFE和局部FE共存、最后到FE的完整相变过程，如图3(a)所示，…”

“…8 该发现颠覆了长期以来对PZO相变过程的传统认知 [47] ；此外，Si上面生长的PZO 多晶薄膜研究中也观察到具有不相称的调制特性的FiE结构 [7] ，如图3(b)所示， 这是由于长程FE相和短程AFE相之间的竞争失败，在这种情况下，未补偿的不相 称调制转化为零电场下的净剩余极化；Lane W. Martin等通过改变脉冲激光沉积 过程中的氧压(从而改变原子动能和随后的轰击效应)，发现反铁电PZO薄膜的 极化行为可以受到点缺陷的强烈影响，沉积过程中氧压的降低导致了意想不到的 "类铁电"特性，并且通过电场调节可以观察到从AFE到"类铁电"的有趣演变 [48] ， 如图3(c)所示。到目前为止关于AFE薄膜中弱FE的结构和性能认识都没有达到 共识，由于薄膜易受到界面、应变、缺陷等因素影响，AFE薄膜中观察到的非中 心对称结构和弱FE行为是否为本征特性，尚待进一步考证。 2.2 电学特性 图4 (a) PZO块体的介电温谱图 [3] ；(b) PZO基反铁电薄膜的C-V特性曲线 [52] ；(c) PZO薄膜的P-E和I-E回线 [50] ； (d) PZO薄膜在直流电场E DC 和交流电场E AC 作用下的 电致应变和压电系数 [7] Fig. 4.…”

“…4. (a) Temperature dependent dielectric spectrum of PZO bulk materials, reproduced from [3] ; (b) C-V characteristics of PZO based AFE thin films [52] ; (c) P-E and I-E loops of PZO thin films, reproduced from [50] ; (d) Electromechanical response as strain and effective longitudinal piezoelectric coefficient as a function of E AC and E DC , respectively [7] .…”

“…其中ε(0)，ε(ꝏ)，C和T o 分别是低频介电常数、光频介电常数、居里-外斯常数和 居里-外斯温度。在薄膜中介温特性通常表现出与块体不同的特点，一般而言薄膜 表现出(类似)二级相变特性 [50] ，为了准确描述该种相变的弥散行为，需用修正 的居里-外斯定律 [53][54][55] ：…”

Atomic Insight into the Successive Antiferroelectric–Ferroelectric Phase Transition in Antiferroelectric Oxides

“…PZO has long been recognized as the canonical antiferroelectric, wherein the antiferroelectricity should be mainly ascribed to the antiparallel displacement of Pb 2+ cations along the ⟨011⟩ direction. However, several nontrivial polarization structures observed in PZO-based antiferroelectrics may challenge this recognition, − such as sinusoidal lead atomic shifts and orthogonally aligned and uncompensated dipole arrangements. In addition, first-principles predictions even suggested that the Pbam phase may not be favored either at room temperature or at the ground state .…”