Structural transition and possible pressure-induced superconductivity in a suboxide La$_5$Pb$_3$O

TL;DR

This study reveals a temperature-driven structural transition in La$_5$Pb$_3$O from I4/mcm to a dimerized P4/ncc phase around $T_t \approx 225\,\mathrm{K}$, evidenced by diffraction and a lambda-like heat-capacity anomaly. Density functional theory shows only minor changes in the DOS at $E_F$ across the transition and no gap opening, indicating the transition is bonding-driven rather than Fermi-surface driven. High-pressure measurements suppress the transition and reveal a resistive feature consistent with pressure-induced superconductivity, with $T_c$ approaching $\sim 10\,\mathrm{K}$ at $8\,\mathrm{GPa}$, though zero resistance is not observed in the present crystals. Together, the results establish La$_5$Pb$_3$O as a platform to explore the interplay between weak structural instabilities and superconductivity in suboxides, and highlight the role of oxygen stoichiometry and bonding in driving structural transitions.

Abstract

Here we report a structural phase transition and its possible competition with superconductivity in the suboxide La$_5$Pb$_3$O. Upon cooling through $T_t$ = 225 K, La$_5$Pb$_3$O transforms from a high-temperature I4/mcm to a low-temperature P4/ncc structure in which La - Pb dimerization along the c-axis occurs. This transition is accompanied by anomalies in the temperature dependence of electrical resistivity and specific heat. High-pressure electrical transport measurements reveal that hydrostatic pressure suppresses the structural transition and possibly induces superconductivity with a maximum superconducting temperature of 10 K. Density functional theory calculations show minimal changes in the electronic density of states and no gap opening at $E_F$ across $T_t$, suggesting that the transition is driven by bonding effects rather than Fermi surface instability. These findings establish La$_5$Pb$_3$O as a promising platform for exploring the interplay between weak structural transitions and superconductivity.

Figures (9)

• Figure 1: (color online) Evolution with time of X-ray powder diffraction patterns of pulverized La$_5$Pb$_3$O crystals exposed in air at room temperature. The patterns are shifted vertically for clarity.
• Figure 2: (color online) Structure of La$_5$Pb$_3$O. (HOL) cut at (a) 273K and (b) 173K. (c) View along c-axis to highlight that oxygen stays in the center of La tetrahedra. (d) High temperature structure (I4/mcm) with uniform La-Pb bond of 3.64$\AA$ along the c-axis. (e) Below the structure transition, one La shifts toward Pb along the c-axis leading to La-Pb dimerization in the low temperature structure (space group P4/ncc). The shorter La–Pb bond measures 3.52$\AA$, compared to 3.76$\AA$ for the longer one.
• Figure 3: (color online) Synchrotron x-ray study of phase transition in La$_5$Pb$_3$O. (a) Diffraction intensity of the (1, 0, 4) reflection that is forbidden in the high temperature phase shows strong temperature dependence below T$_t$. For 0.35 T$_t<$ T $<$ T$_t$, the evolution could be fit to a power law I=(T$_t$-T)$^2\beta$ (solid red) with $\beta$ = 0.337(11). (inset) Close to T$_t$, the intensity evolution shows a weak first order discontinuity. The data was measured upon warming. (b) The rocking curves of the sample at various temperatures show the presence of both a sharp mosaic peak and a broad diffusive intensity. Both profiles could be fit with a Lorentzian shape with different widths (black line). Only the 217, 220, and 222K data require fitting to a summation of both profiles. (c, d) The measured lattice wave vector as a function of temperature. We notice for (4, 0, 0) order, there is no anomaly across T$_t$, while for (1, 0, 4) order, the wave vector shows a singularity at the phase boundary.
• Figure 4: (color online) Temperature dependence of specific heat. The upper inset shows the linear fitting of Cp/T vs T$^2$ in the low temperature regime. The lower inset highlights the weak anomaly around the structure transition. The solid curve is a guide to the eyes.
• Figure 5: (color online) Temperature dependence of electrical resistivity measured with current parallel to the crystallographic c-axis. The dashed line (green) shows the linear extrapolation of the high temperature resistivity. The residual resistivity (red, open circle) is obtained by subtracting the linear extrapolation from the low temperature resistivity.