第一原理計算研究稀土鈣鈦礦氧化鎳的電子與聲子結構 (稀土 = 鑭，鐠，釹)

稀土鈣鈦礦氧化鎳，RNiO3 (RNO; R = La-Lu)，是物理學中金屬-絕緣體轉換(MIT) 的典型例子。LaNiO3 (LNO) 塊材在低溫，室溫和高溫下，晶體結構都屬於菱面體 (R-3c) ，並且該材料是沒有磁性的導體。在室溫下，PrNiO3 (PNO) 和 NdNiO3(NNO) 塊材形成 Pbnm 晶體，並在各自的 MIT 溫度以下變為絕緣單斜晶結構 (P21/n)，這同時對應於由無磁性轉變成反鐵磁的溫度（奈爾溫度）。在使用一些原子分析技術下，Lin 的研究報告了對長在 (001) 方向的應變薄膜 LaNiO3/SrTiO3，PrNiO3/SrTiO3 和 NdNiO3/SrTiO3 的特性，其中 LaNiO3 的室溫相仍保持金屬，而 PrNiO3和 NdNiO3 的室溫相則變為絕緣體。對於 PrNiO3 和 NdNiO3，金屬-絕緣體轉換也可以藉由外延應變觸發。此外，LaNiO3，PrNiO3 和 NdNiO3 皆以單斜 C2/c 對稱性結晶，這晶體結構是在塊材中缺失的，卻在應變薄膜中看到，但尚未得到很好的解析。在這項研究中，我們透過第一原理計算在菱面體（R-3c）和單斜（C2/c）的電子結構特性。在菱面體結構（R-3c），我們發現這三種材料都是金屬。接著，在單斜結構（C2/c)，LaNiO3 仍然屬於金屬，這與實驗一致；但對於 PrNiO3和 NdNiO3，材料必須具有 G 型反鐵磁結構才能產生能隙。由於 G 型反鐵磁，PrNiO3 和 NdNiO3 在單斜結構（C2/c) 中的能隙分別為 0.054 eV 和 0.183 eV。隨後，我們研究了材料在菱面體相（R-3c）中的聲子特性。我們在聲子計算中模擬材料在薄膜上的情況，並且計算其聲子能帶。我們的研究結果顯示三種材料的相變路線都是從 R-3c 到 C2/c，這可能被認為是對於早期的研究適當的理論解析。

關鍵字

稀土鈣鈦礦氧化鎳；金屬-絕緣體轉換；奈爾溫度；薄膜；相變； C2/c 對稱性結晶

並列摘要

Rare-earth perovskite nickel oxides, RNiO3 (RNO; R = La–Lu), are the typical example of metal–insulator transitions (MIT) in physics. Bulk LaNiO3 (LNO) is in the rhombohedral crystal structure (R-3c) at all temperatures; the material is metallic and non-magnetic. At room temperature, bulk PrNiO3 (PNO) and NdNiO3 (NNO) crystallize in orthorhombic Pbnm and become insulating monoclinic structure (P21/n) below the MIT temperatures which are identical to their Néel temperatures. Using atomically resolved techniques, Lin’s research reports on a study of strained LaNiO3/SrTiO3, PrNiO3/SrTiO3, and NdNiO3/SrTiO3 films in (001) direction, with the room temperature phase of LaNiO3 staying metallic while the room temperature phases of PrNiO3 and NdNiO3 become insulating. For PrNiO3 and NdNiO3, metal–insulator transitions can also be triggered by epitaxial strain instead of cooling down the temperature. Additionally, the LaNiO3, PrNiO3, and NdNiO3 crystallize in the monoclinic C2/c structure that is not observed in the bulk phase diagram and has been seen in strained films, but it has not been well understood. Therefore in this study, we investigate electronic properties by first-principles calculations in both the rhombohedral (R ̄3c) and monoclinic (C2/c) structures of the three materials. In rhombohedral (R-3c) structure, we find that the three materials are all metals. Further on, in monoclinic (C2/c) structure, LaNiO3 remains in metal, which is in agreement with the experiment; both PrNiO3 and NdNiO3 must have G-type antiferromagnetic structure to induce the gap. PrNiO3 and NdNiO3 in C2/c structure are gapped by 0.054 eV and 0.183 eV respectively, due to the G-type antiferromagnetic structure. Following that, phonon bands of the materials in the rhombohedral structure (R-3c) are investigated. We simulate the materials on the film and do the phonon calculations. Our findings provide a phase transition route from R-3c to C2/c in all three materials, which may be considered proper theoretical analysis for the earlier experimental study.

並列關鍵字

Rare-earth perovskite nickel oxides ； metal–insulator transitions, ； Neel temperature ； thin film ； phase transition ； monoclinic C2/c symmetry

參考文獻

[1] R. J. H. Voorhoeve, J. P. Remeika, D. W. Johnson, and P. K. Gallagher, Rare-earth manganites: catalysts with low ammonia yield in the reduction of nitrogen oxides, Science 180, 62 (1973).

Google Scholar

[2] M. A. Peña and J. L. G. Fierro , Chemical structures and performance of perovskite oxides, Chemical Reviews 101, 1981 (2001).

Google Scholar

[3] Y. Tokura and N. Nagaosa, Orbital physics in transition-metal oxides, Science 288, 462 (2000).

Google Scholar

[4] B. Raveau, The perovskite history: more than 60 years of research from the discovery of ferroelectricity to colossal magnetoresistance via high Tc superconductivity, Prog. Solid State Chem 35, 171 (2007).

Google Scholar

[5] J. Torrance, P. Lacorre, A. Nazzal, E. Ansaldo, and C. Niedermayer, Systematic study of insulator-metal transitions in perovskites RNiO3(R=Pr,Nd,Sm,Eu) due to closing of charge-transfer gap, Phys. Rev. B 45, 8209 (1992).

Google Scholar

主題瀏覽