Tuning the Intermediate Valence Behavior in the Zintl Compound Yb₁₄ZnSb₁₁ by Incorporation of RE³⁺ [Yb_14–xRE_xZnSb₁₁ (0.2 ≤ x ≤ 0.7), RE = Sc, Y, La, Lu and Gd]

Abstract

The solid solutions of Yb 14−x RE x ZnSb 11 (RE = Sc, Y, La, Lu, and Gd; 0.2 ≤ x ≤ 0.7) were prepared to probe the intermediate valency of Yb in Yb 14 ZnSb 11 . The substitution of Yb with RE 3+ elements should reduce or remove the intermediate valency of the remaining Yb ions. Large crystals are grown from Sn-flux, and the structure and magnetic susceptibility are presented. All compounds crystallize in the Ca 14 AlSb 11 structure type and the RE 3+ ions show Yb site substitution preferences that correlate with size. Two compositions of Yb 14−x Y x ZnSb 11 were investigated [x = 0.38(3), 0.45(3)] by temperature-dependent magnetic susceptibility and the broad feature in magnetic susceptibility measurements at 85 K in pristine Yb 14 ZnSb 11 attributed to valence fluctuation decreases and is absent for x = 0.45(3). In compounds with nonmagnetic RE 3+ substitutions (Sc, Y, La, and Lu), temperaturedependent magnetic susceptibility shows a transition from intermediate valency fluctuation toward temperature-independent (Y, La, and Lu) or Curie−Weiss behavior and possibly low temperature heavy Fermion behavior (Sc). In the example of the magnetic rare earth substitution, RE = Gd, the Curie−Weiss-dependent magnetic moment of Gd 3+ is consistent with x. Hall resistivity of Yb 14−x Y x ZnSb 11 showed that the carrier concentration decreases with x and the signature of the low-T intermediate valence state seen for x = 0 is suppressed for x = 0.38 and gone for x = 0.45.

Impact of Lu-Substitution in Yb14–xLuxZnSb11: Thermoelectric Properties and Oxidation Studies

“…Because the crystal structure does not have a site available for the single Yb 3+ , intermediate valency of Yb exists, and an average valency of 2.07 for all 14 Yb atoms provides the precise accounting. 6 Studies on the magnetism within this system have shown that Yb 14 ZnSb 11 exhibits a fluxional valency with Yb 2+/3+ , in which the amount of Yb 3+ within the system decreases with temperatures. 7 This deviation from the ideal zintl formulism in its lowest energy state leads to the system being electron deficient and heavily doped p-type (∼1 × 10 21 h + cm –3 ).…”

“…The effect of Lu-substitution in Yb 14 ZnSb 11 is in contrast to that observed for Yb 14 MnSb 11 and is attributed to differences in the Yb valency. 6,10 The introduction of Lu into the system helped stabilize the YbSb phase upon oxidation; however, it did not negate the formation and subsequent melting of YbSb 2 , which proves to be problematic for forming…”

“…The first is a shift of the Fermi level deeper into the valence band (more p-type doping). This is unlikely as Lu is a trivalent cation that replaces either Yb 2+ or Yb 3+ in the system and should result in either no change or a lowering of the carrier concentration . The alternative possibility is that Lu replacing Yb leads to a shift in the relative energies of the light band at gamma and the pocket of degenerate bands between N and P in the electronic band structure.…”

“…This is unlikely as Lu is a trivalent cation that replaces either Yb 2+ or Yb 3+ in the system and should result in either no change or a lowering of the carrier concentration. 6 The alternative possibility is that Lu replacing Yb leads to a shift in the relative energies of the light band at gamma and the pocket of degenerate bands between N and P in the electronic band structure. Crystal orbital Hamiltonian population analysis showed that the top valence bands within this structure type are dominated by Yb–Sb bonding and Sb–Sb states with slight antibonding characters.…”

“…The structure of Yb 14 ZnSb 11 (Figure ) can be described using the Zintl formulizm as consisting of ZnSb 4 10– + Sb 3 7– + 4Sb 3– + 13Yb 2+ + 1Yb 3+ . Because the crystal structure does not have a site available for the single Yb 3+ , intermediate valency of Yb exists, and an average valency of 2.07 for all 14 Yb atoms provides the precise accounting . Studies on the magnetism within this system have shown that Yb 14 ZnSb 11 exhibits a fluxional valency with Yb 2+/3+ , in which the amount of Yb 3+ within the system decreases with temperatures .…”