Incipient geometric lattice instability of cubic fluoroperovskites,Physical Review B

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Incipient geometric lattice instability of cubic fluoroperovskites
Physical Review B ( IF 3.2 ) Pub Date : 2021-10-25 , DOI: 10.1103/physrevb.104.144304
R. M. Dubrovin ₁ , A. C. Garcia-Castro ₂ , N. V. Siverin ₁ , N. N. Novikova ₃ , K. N. Boldyrev ₃ , Aldo H. Romero ₄ , R. V. Pisarev ₁

Affiliation

Inorganic metal halide perovskites are promising materials for next-generation technologies due to a plethora of unique physical properties, many of which cannot be observed in the oxide perovskites. On the other hand, the search for ferroelectricity and multiferroicity in lead-free inorganic halide perovskites remains a challenging research topic. Here, we experimentally show that cubic fluoroperovskites exhibit proximity to incipient ferroelectrics, which manifested in the softening of the low-frequency polar phonons in the Brillouin zone center at cooling. Furthermore, we reveal the coupling between harmonic and anharmonic force constants of the softening phonons and their correlation with the perovskite tolerance factor. Next, using first-principles calculations, we examine the lattice dynamics of the cubic fluoroperovskites and disclose the incipient lattice instability at which the harmonic force constants of low-lying phonons tend to decrease with a reduction of tolerance factor at all high-symmetry points of the Brillouin zone. The correlations with the tolerance factor indicate the geometric origin of observed incipient lattice instability in the cubic fluoroperovskites caused by the steric effect due to the volume filling of the unit cell by different radius ions. These results provide insights into the lattice dynamics and potential ferroelectric properties of inorganic lead-free metal halide perovskites, relevant to further design and synthesis of new multifunctional materials.

中文翻译：

立方氟钙钛矿的初始几何晶格不稳定性

无机金属卤化物钙钛矿具有多种独特的物理特性，是用于下一代技术的有前途的材料，其中许多在氧化物钙钛矿中无法观察到。另一方面，在无铅无机卤化物钙钛矿中寻找铁电性和多铁性仍然是一个具有挑战性的研究课题。在这里，我们通过实验表明立方氟钙钛矿表现出接近初始铁电体，这表现为冷却时布里渊区中心的低频极性声子软化。此外，我们揭示了软化声子的谐波和非谐波力常数之间的耦合及其与钙钛矿耐受因子的相关性。接下来，使用第一性原理计算，我们检查了立方氟钙钛矿的晶格动力学，并揭示了初始晶格不稳定性，在该不稳定状态下，低位声子的谐波力常数往往随着布里渊区所有高对称点的容差因子的降低而降低。与容差因子的相关性表明观察到立方氟钙钛矿中初始晶格不稳定性的几何起源，这是由于不同半径离子对晶胞进行体积填充而引起的空间效应。这些结果提供了对无机无铅金属卤化物钙钛矿的晶格动力学和潜在铁电性质的见解，与新型多功能材料的进一步设计和合成有关。

更新日期：2021-10-25

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