Pressure effect on iron-based superconductor LaFeAsO1−xHx: Peculiar response of 1111-type structure

Abstract

A systematic study of the crystal structure of a layered iron oxypnictide LaFeAsO1−xHx as a function of pressure was performed using synchrotron X-ray diffraction. This compound exhibits a unique phase diagram of two superconducting phases and two parent phases. We established that the As–Fe–As angle of the FeAs4 tetrahedron widens on the application of pressure due to the interspace between the layers being nearly infilled by the large La and As atoms. Such rarely observed behaviour in iron pnictides implies that the FeAs4 coordination deviates from the regular tetrahedron in the present systems. This breaks a widely accepted structural guide that the superconductivity favours the regular tetrahedron, albeit the superconducting transition temperature (Tc) increases from 18 K at ambient pressure to 52 K at 6 GPa for x = 0.2. In the phase diagram, the second parent phase at x ~ 0.5 is suppressed by pressure as low as ~1.5 GPa in contrast to the first parent phase at x ~ 0, which is robust against pressure. We suggest that certain spin-fluctuation from the second parent phase is strongly related to high-Tc under pressure.

Ab initio study on structural and electronic properties of ReOFeAs (Re: La, Sm, Nd, Ce, Gd) under hydrostatic pressure

“…Figures 3(a), (b) illustrates that, with application of external pressure, As height from Fe layer follows and increasing trend and As-Fe-A angle follows a decreasing trend respectively. LaOFeAs demonstrates an exception to this pattern, fact which is also verified by x-ray diffraction measurements [15].…”

“…Table 2 lists the values of As-Fe-As angle and As height for Fe layer in '1111' compounds for which superconductivity has been established. The ambient TbFeAsO 0.7 so (108.5°/1.38 Ang) [16] and the 15 GPa compressed LaFeAs (114°/1.28) [15,31] constitute the marginal superconducting configurations. It is concluded that, for compression 10 GPa, As-Fe-As angle and As height from Fe layer in SmOFeAs fall well within the range of 1111 superconductors.…”

“…and as the As-Fe-As bond angle decreases towards that of regular FeAs 4 tetrahedral shape, i.e. 109.47°(figure 1) [15][16][17][18]. The distortion in ReO layer is largely involved in the formulation of Fe-As configuration [19].…”

“…Variance with pressure of the ReOFeAs lattice constants along a-axis (a) and c-axis (b). Comparison with diffraction measurements (x)[15,26,29] …”

Three superconducting phases with different categories of pairing in hole- and electron-doped LaFeAs1−xPxO

“…Heavily electron doping cannot be achieved by F-substitution but possible by H-substitution [21][22][23][24]. In order to synthesize H-substituted samples, it is necessary to use a high pressure furnace.…”