Evaluation of the sites occupied by Ti in the TiVFeZr multicomponent alloy with C14-type laves phase structure

Hydrogen is an energy vector and if associated with renewable energy sources becomes a reasonable solution for the next generation and storage of energy. Metal hydrides can improve the density of stored energy, since hydrogen is a low-density gas. Among metal hydrides, multicomponent intermetallic metal hydrides have been intensively studied. The most common intermetallic phase found in metal hydrides is the AB2 C14-type Laves phase. A-type atoms (4f) are larger than B-type atoms (B1: 2a and B2: 6h), so that the structure presents three interstices to store hydrogen: A2B2, AB3 or B4. From a hydrogen store perspective, the A-type elements are frequently hydride-forming elements, such as Ti, Zr and V, while B-type non-hydride forming elements, such as Fe. In the case of the equiatomic TiVFeZr multicomponent alloy previously reported by the candidate, from the structure point of view, Ti and Zr occupies A-type sites and V and Fe B-type sites. C14-type Laves phase alloys typically absorb a maximum capacity of about 1 H/M (hydrogen to metal ratio). However, the TiVFeZr alloy absorbs 1.5 H/M, consequently, to maintain the stoichiometric of the alloy due to the measured composition and the larger capacity, Ti must occupy both A and B sites activating more interstices, which can only be determined by neutron diffraction. In this context, determining whether Ti occupies both sites using GEM line will improve our thermodynamic model for C14-type Laves phase alloys for hydrogen storage.

氢气是一种能量载体（energy vector），若与可再生能源结合，将成为下一代能源存储的合理解决方案。由于氢气本身为低密度气体，金属氢化物（metal hydrides）可有效提升储能密度。在各类金属氢化物中，多组分金属间化合物氢化物（multicomponent intermetallic metal hydrides）已得到广泛研究。金属氢化物中最常见的金属间相为AB₂型C14拉弗斯相（AB2 C14-type Laves phase）。A位原子（4f位点）尺寸大于B位原子（B1对应2a位点、B2对应6h位点），因此该结构存在三种可储氢的间隙位点：A₂B₂、AB₃或B₄。从储氢角度而言，A位元素通常为氢化物形成元素，例如钛（Ti）、锆（Zr）和钒（V），而B位元素则多为非氢化物形成元素，例如铁（Fe）。以本研究团队此前报道的等原子比TiVFeZr多组分合金（equiatomic TiVFeZr multicomponent alloy）为例，从结构层面来看，钛与锆占据A位位点，钒与铁则占据B位位点。C14型拉弗斯相合金的最大储氢容量通常约为1 H/M（氢金属比，hydrogen to metal ratio）。然而TiVFeZr合金的储氢容量可达1.5 H/M，因此结合实测的合金成分与更高的储氢容量，为维持合金的化学计量比（stoichiometric），钛必须同时占据A位与B位位点，从而激活更多的储氢间隙，这一结论仅能通过中子衍射（neutron diffraction）技术得以验证。在此背景下，利用GEM谱线（GEM line）判断钛是否同时占据双位点，将有助于优化我们针对储氢用C14型拉弗斯相合金的热力学模型。