High-pressure Structural Evolution Of Two-Dimensional Ca₂N And Y₂C Electrides

Electride is a unique ionic compound in which extra electrons exist in the crystal voids as anions.Ca2N and Y2C are two-dimensional electrides,in which excess electrons are confined between the cation layers.Theory predicts that Ca2N and Y2C will form high-pressure phases under pressure,however,the study of electrides lacks systematic experimental verification.Due to the unique structural characteristics of electronic compounds,it is particularly important to understand the structure evolution of electrides.Noble gas atoms are often used as the pressure-transmitting media for high-pressure experiments.However,noble gas atoms can be inserted into materials with large structural voids and be trapped by the voids under pressure.Electrides usually have large lattice voids and exhibit strong affinity for light atoms.Although a series of studies have been carried out on electrides,there are no studies of the interaction between noble gas atoms and two-dimensional electrides.In this paper,the two-dimensional electrides Ca2N and Y2C are selected as the research object.We mainly study the effect of noble gases as pressure media on the high-pressure phase transitions of electrides in DAC,and explore the interaction of Ca2N and Y2C with noble gas atoms under pressure.At the same time,the calculations are helpful to study the interaction between electride and noble gas atoms,so that we can have the understanding of the structural transitions of two-dimensional electrides in different pressure-transmitting media.Through the analysis of the in situ synchrotron HP-XRD measurements and crystal structure prediction of Ca2N(helium,neon and argon)and Y2C(neon),the following conclusions are obtained:(1)Within 50GPa,noble gases are chemically inert to electrides in different pressure-transmitting media,and Ca2N-He?Ca2N-Ar?Ca2N-Ne and Y2C-Ne systems are difficult to form.(2)The hydrostatic pressure conditions caused by noble gases,in comparison with non-hydrostatic pressure of Ca2N,change the transition paths with different pressure media and phase transition pressure is reduced significantly.(3)Y2C maintains the R3m structure(within 50GPa)with/without pressure-transmitting media.(4)Theoretical results show that when the anion electrons in the Ca2N act as real atoms,the anion electrons will prevent the noble gas atoms from entering the electride.(5)In the predicted metastable Ca2N-He?Ca2N-Ar?Ca2N-Ne systems,the neutral features of noble gas atoms lead to the reservation of intrinsic electride character of Ca2N.Our results show that although Y2C and Ca2N are both two-dimensional electrides,Y2C and Ca2N have different high-pressure structural performances in different media.The presence of noble gas affects the transition paths and pressure of the Ca2N high-pressure phase.Y2C has no structural phase transition under high pressure,which provides a model for studying the localization of electrons in electrides,the evolution of anion electrons,and the interaction of d electrons of Y with anion electrons under high pressure.However,noble gas atoms cannot form stable compounds with Ca2N and Y2C,and they have a strong repulsive effect with anion electrons,so noble gases can be used as protective gases for electrides under ambient conditions.