Pressure induced metal to semiconductor transition in 2D electride - Dr. Huiyang Gou

SEPTEMBER 4, 2018

New work from a team led by Dr. Huiyang Gou from HPSTAR reveals the metal-to-semiconductor transition in compressed Ca₂N a two-dimensional (2D) electride as found in sodium. This work is published in Advanced Science.

Electrides are stoichiometric ionic compounds in which the trapped electrons serve as the anions. The electrons are very loosely bound in the lattices, not belonging to any particular atoms or molecular. This gives electrides very unique electrical properties, such as high electrical mobility and rapid electrical transport.

Dicalcium nitride (Ca₂N) is the first 2D electride material. The potential applications especially in advanced batteries excited material scientists to deeply probe this 2D electride. “We know pressure will significantly change materials bonding characters, leading to new structures and even creating some unexpected phenomenon”, said Dr. Huiyang Gou, a staff scientist of HPSTAR. “For 2D electride, we are especially interested in the final fate of the high-mobile anionic electrons under extreme pressures”.

At ambient pressure, Ca₂N shows metal character because of the loosely confined electrons in the interlayer voids. Calculations predicted that it will turn to semiconductor under compression while yet to be experimentally proved. The research team led by Dr. Huiyang Gou used in situ synchrotron X-ray diffraction, electrical resistance measurements combined with theoretical calculations to probe the crystal structural and electrical properties under high pressures.

They found that Ca₂N has a significant resistance increase at ~14 GPa and completely transforms to semiconductor above 20 GPa. “The huge electrical-resistance increase of Ca₂N under compression can be attributed to the reductions of electronic dimensionality from 2D, 1D to 0D from out in-situ synchrotron x-ray diffraction and density functional theory (DFT) calculations”, Drs. Hu Tang and Bo Gao explained. “Under high pressure, the anionic electrons are confined in Ca8 cages at high pressure, leading to the semiconductor character”.

“Our results demonstrate unexplored experimental evidence for a pressure-induced metal-to-semiconductor switching in Ca₂N and offer possible strategy for producing new electrides under moderate pressure. The reported work is important for the deep understanding of the phase evolutions of electride materials.” added by Dr. Gou.

Caption: Metal-to-insulator transition and electronic dimensionality variations under compression.

压力诱导的材料从金属态向半导体态或者绝缘体态发生转变一直以来都是高压凝聚态物理研究的热门课题。北京高压科学研究中心的缑慧阳研究员与日本东京工业大学的细野秀雄教授合作，在电子化合物的高压研究方面取得新进展。通过高压下同步辐射x射线和电阻测量发现Ca₂N这种二维电子化合物在较低的压力下（14GPa）就会发生结构转变，并且伴随着金属态到半导体态的转变特征。计算研究揭示了这一独特的电学性能的转变是由于压力诱导的Ca₂N中阴离子电子维度发生了变化，从常压下层间的二维宽松分布逐步转变到高压下零维Ca8笼子中紧密束缚。该研究成果发表在近期的Advanced Science。