Usharani Nandhini J, Shringi Rajat, Sanghavi Harshil, Subramanian S, Bhattacharya S S
Nano Functional Materials Technology centre, (NFMTC), Department of Metallurgical and Materials engineering, Indian Institute of Technology Madras, Chennai-600036, India.
Dalton Trans. 2020 Jun 7;49(21):7123-7132. doi: 10.1039/d0dt00958j. Epub 2020 May 14.
A nanocrystalline high entropy oxide with near-equimolar composition consisting of 5 transition metal cations and one alkali cation (Co,Cu,Mg,Na,Ni,Zn)O was synthesised by a reverse co-precipitation (RCP) process and characterised by standard methods of X-ray diffraction (for crystallite size and phases), electron microscopy (for particle morphology and size distribution) and Fourier transform infra-red spectroscopy (for bond identification and bond lengths). Charge compensation in the lattice by the formation of Co and/or Ni (in order to offset the +1 oxidation state of Na) and the creation of oxygen vacancies was quantified from X-ray photoelectron spectroscopy and Raman spectroscopy and further studied using vibrating sample magnetometry (VSM). The influence of different transition metals in being able to accommodate the larger and aliovalent sodium ion in a single phase-pure rocksalt lattice was investigated and the criteria for element selection in such multicomponent systems for single-phase formation examined. Presence of multivalency/non-stoichiometry to accommodate a different-sized cation and maintaining electroneutrality were identified as the critical criteria for single-phase formation in multicomponent systems and further confirmed through synthesis of various lower combination systems (by systematic removal of one transition metal cation) and by addition of bivalent Ca as well as cations of higher valencies. These criteria would aid in designing the compositions of high entropy oxides with aliovalent substitutions.
通过反向共沉淀（RCP）法合成了一种具有近等摩尔组成的纳米晶高熵氧化物，其由5种过渡金属阳离子和一种碱金属阳离子（Co、Cu、Mg、Na、Ni、Zn）O组成，并采用标准方法进行了表征，包括X射线衍射（用于微晶尺寸和物相）、电子显微镜（用于颗粒形态和尺寸分布）以及傅里叶变换红外光谱（用于键识别和键长）。通过X射线光电子能谱和拉曼光谱对晶格中通过形成Co和/或Ni（以抵消Na的 +1氧化态）以及产生氧空位进行的电荷补偿进行了量化，并使用振动样品磁强计（VSM）进行了进一步研究。研究了不同过渡金属在单相纯岩盐晶格中容纳较大的异价钠离子的能力的影响，并研究了此类多组分系统中单相形成的元素选择标准。确定了多价/非化学计量的存在以容纳不同尺寸的阳离子并保持电中性是多组分系统中单相形成的关键标准，并通过合成各种较低组合系统（通过系统地去除一种过渡金属阳离子）以及添加二价Ca以及更高价态的阳离子进一步得到证实。这些标准将有助于设计具有异价取代的高熵氧化物的组成。
