Synergistic doping and stabilization of magnetically tunable LnTi$_3$(Sb,Sn)$_4$ (Ln:Ce--Gd) kagome metals

Abstract

Here we present our synthesis and characterization of the LnTi$_3$(Sb,Sn)$_4$ (Ln: Ce, Pr, Nd, Sm, Gd) family of cleavable kagome metals. While these materials are isostructural to the LnTi$_3$Bi$_4$ family, they only form as (Sb,Sn) solid-solutions with no corresponding LnTi$_3$Sb$_4$ or LnTi$_3$Sn$_4$ phases. We use a combination of first-principles density functional theory (DFT) and Crystal Orbital Hamilton Population (COHP) calculations to show that (Sb,Sn) alloying has a stabilizing effect on the structure by adjusting the Fermi level, filling bonding states, depopulating antibonding states, and adjusting the density-of-states (DOS) towards local minima, an effect we call ``synergistic doping.'' The tunable Fermi level also has a profound effect on the magnetism, which we demonstrate through a detailed characterization of the SmTi$_3$(Sb,Sn)$_4$ series. The series hosts multiple magnetic ground states resulting from competing magnetic interactions that are tunable by the (Sb,Sn) ratio. While the focus of this work is on SmTi$_3$(Sb,Sn)$_4$, we briefly comment on the (Sb,Sn) solubility range and the conferred magnetic tunability in the other rare-earths compounds (Ln: Ce, Pr, Nd, Gd) as well. Our work demonstrates how the (Sb,Sn) synergistic pair can be used to stabilize the LnTi$_3$(Sb,Sn)$_4$ structure while simultaneously providing a means to tune the magnetism, ultimately providing a potential route to develop new intermetallics with chemical, magnetic, and electronic tunability.

Figures (14)

• Figure 1: (a) Abbreviated view of the FmmmLnTi$_3$(Sb,Sn)$_4$ crystal structure. (b) A schematic comparison of the structural motifs in the LnTi$_3$Bi$_4$ and LnTi$_3$(Sb,Sn)$_4$ families. (c) (Sb,Sn) solubility ranges for each of the flux-grown LnTi$_3$(Sb,Sn)$_4$ single crystals. We've also highlighted compositions where competition with Ln$_2$Ti$_9$Sb$_{11}$ phase is observed. (d) Processing diagram showing the incorporation of (Sb,Sn) into the crystal structure depending on the nominal Sb percentage in the flux. (e) Shift in the SmTi$_3$(Sb,Sn)$_4$c-axis lattice parameter as a function of composition, derived from {00L} facet scans.
• Figure 2: (a) Experimental ARPES data for a Sn-rich SmTi$_3$(Sb,Sn)$_4$ crystal, with several bands highlighted in white. Note the concave-down band approximately 0.5 eV below E$_\text{F}$. (b) Analogous ARPES data for a Sb-rich SmTi$_3$(Sb,Sn)$_4$ crystal. The concave-down band is now 0.64 eV below E$_\text{F}$, indicating an effective doping of approximately 270 meV per Sb/Sn atom. (c) Electronic band structure for the hypothetical SmTi$_3$Sn$_4$. Several values of E$_\text{F}$ are shown: the native stannide (purple), the native antimonide (red), and the range approximating the alloying range (green/blue). (d) Density of states (D(E)) and Crystal Orbital Hamilton Population (COHP) calculations for SmTi$_3$Sn$_4$ showing that electron doping (Sb-substitution) in the stannide is favorable through reduction of D(E$_\text{F}$) and population of additional bonding states. (e) Breakdown of integrated (ICOHP) contribution to the total bonding band energy for Ti-Ti and Sn-Sn bonds (shown) relative to all other atom pairs. (f) Dispersion for hypothetical SmTi$_3$Sb$_4$ shown with same set of E$_\text{F}$ as before. (g) Analogous D(E) and COHP calculations for the antimonide. Hole doping (Sn-substitution) in antimonide is favorable through reduction of antibonding states at E$_\text{F}$ while maintaining low D(E$_\text{F}$). (h) Breakdown of integrated ICOHP contribution for antimonide.
• Figure 3: Here we present the basic properties of the Sb-rich (green) and Sn-rich (blue) termini of the SmTi$_3$(Sb,Sn)$_4$ solid-solution. (a,e) Field-cooled magnetization along three principle crystallographic directions, showing easy c-axis magnetization. Both systems show a higher-temperature cusp followed by a lower-temperature magnetization rise and plateau. (b,f) Large-pulse zero-field heat capacity measurements showing the multiple transitions in both compositions. Sn-rich samples a second order peak at 21 K followed by a split first-order peak at 15 K. Sb-rich samples instead show a set of two second-order peaks at 9.7 K and 10.9 K. (c,g) Isothermal magnetization results with H$\parallel$c clearly demonstrate the FM nature of the Sn-rich sample and the more complex A(FM) interactions in the Sb-rich endpoint. (d,h) Magnetoresistance measurements showing the emergence of negative magnetoresistance (MR) in the AFM state, which persists through 4 K in the Sb-rich sample. The onset of stronger FM order in the Sn-rich sample suppresses the AFM and the negative MR, consistent with the first-order nature of the AFM to FM transition Sn-rich heat capacity.
• Figure 4: Here we present a full series of samples traversing the SmTi$_3$(Sb,Sn)$_4$ solid-solution to demonstrate the gradual shift in magnetic and thermodynamic properties. Shown from left to right are: (1) the measured (Sb,Sn) content of the single crystals, (2) a plot showing the temperature-dependent magnetization and zero-field heat capacity results superimposed and scaled in arbitrary units, and (3) the corresponding isothermal magnetization curves at base temperature and an intermediate temperature (gray) intended to demarcate the transition between the two heat capacity features (where possible). For clarity, the intermediate temperature (gray) is also marked on the temperature-dependent magnetization.
• Figure 5: Here we summarize the temperature-composition phase diagrams for the SmTi$_3$(Sb,Sn)$_4$ solid-solution, and further investigate the field-temperature phase diagrams for the Sb-rich and Sn-rich termini of the alloys. Temperature-composition phase diagrams derived from (a) heat capacity and (b) magnetization measurements demonstrate the competition between the higher-temperature AFM order and the onset of FM order at lower temperatures. For Sb-rich compositions, the AFM order and FM interactions intertwine into the A(FM) state. (c,d) Temperature-dependent magnetization curves for the Sn-rich and Sb-rich termini at under various magnetic fields. The AFM-like cusp in the Sn-rich sample is evident and appears gradually suppressed under applied field. The AFM-like cusp in the Sb-rich sample is more subtle at low fields, but is strengthened and more dominant at intermediate fields (highlighted in black for clarity). (e,f) Field-dependent large-pulse heat capacity measurements to help visualize the multiple heat capacity peaks and their interplay with fields. (g,h) Composite plots where the gradient background is the second-derivative of the field- and temperature-dependent magnetization. Solid points are identified from the field- and temperature-dependent heat capacity results.