Anomalous electronic properties in layered, disordered ZnVSb

New materials discovery is the driving force for progress in solid state physics and chemistry. Here we solve the crystal structure and comprehensively study physical properties of ZnVSb in the polycrystalline form. Synchrotron x-ray diffraction reveals that the compound attains a layered ZrSiS-type structure ($P4$/nmm, $a$ = 4.09021(2) Å, $c$ = 6.42027(4) Å). The unit cell is composed of a 2D vanadium network separated by Zn-Sb blocks that are slightly distorted from the ideal cubic arrangement. A considerable amount of vacancies were observed on the vanadium and antimony positions; the experimental composition is ${\mathrm{ZnV}}_{0.91}{\mathrm{Sb}}_{0.96}$. Low-temperature x-ray diffraction shows very subtle discontinuity in the lattice parameters around 175 K. Bonding V-V distance is below the critical separation of 2.97 Å known from the literature, which allows for V-V orbital overlap and subsequent metallic conductivity. From ab initio calculations, we found that ZnVSb is a pseudogap material with an expected dominant vanadium contribution to the density of states at the Fermi level. The energy difference between the antiferromagnetic and nonordered magnetic configurations is rather small (0.34 eV/f.u.). X-ray photoelectron spectroscopy fully corroborates the results of the band structure calculations. Magnetic susceptibility uncovered that, in ZnVSb, itinerant charge carriers coexist with a small, localized magnetic moment of ca. 0.25 ${\mu }_{\mathrm{B}}$. The itinerant electrons arise from the ordered part of the vanadium lattice. Fractional localization, in turn, was attributed to V atoms neighboring vacancies, which hinder full V-V orbital overlap. Furthermore, the susceptibility and electrical resistivity showed a large hysteresis between 120 K and 160 K. The effect is not sensitive to magnetic fields up to 9 T. Curie-Weiss fitting revealed that the amount of itinerant charge carriers in ZnVSb drops with decreasing temperature below 160 K, which is accompanied by a concurrent rise in the localized magnetic moment. This observation together with the overall shape of the hysteresis in the resistivity allows for suggesting a plausible origin of the effect as a charge-transfer metal-insulator transition. Ab initio phonon calculations show the formation of a collective phonon mode at 2.8 THz (134 K). The experimental heat capacity reflected this feature by a boson peak with Einstein temperature of 116 K. Analysis of the heat capacity with both an ab initio perspective and Debye-Einstein model revealed a sizable anharmonic contribution to heat capacity, in line with disordered nature of the material. Further investigation of the electron and phonon properties for ZnVSb is likely to provide valuable insight into the relation between structural disorder and the physical properties of transition-metal-bearing compounds.

Figure 1

Ternary compounds containing transition metals with Zn and Sb tend to form in either the (a) ZrSiS or (b) half-Heusler structure types. Here Zn is in gray, Sb in orange, and the transition metal (TM) in red. Below the eightfold coordination polyhedra around the transition metals are highlighted.

Figure 2

(a) X-ray diffraction pattern from 11-BM beamline on ZnVSb powder (red) and Rietveld fit (black) using the ZrSiS structure type; the difference pattern is shown in blue. (b) Laboratory powder x-ray diffraction collected across 90–300 K. Inset shows the temperature-dependence of the (112) reflection from ${\mathrm{K}}_{\alpha 1}$ and ${\mathrm{K}}_{\alpha 2}$ radiation. (c) Rietveld refinement of the patterns presented in (b) yield linear changes in lattice parameters.

Figure 3

Electronic structure of ZnVSb calculated using the Hubbard corrections $U=+3$ eV and $+6$ eV for vanadium and zinc $d$ orbitals, respectively. (a) Electronic band structure of ZnVSb shows a metallic ground state with the appearance of several flat, dispersionless states around the Fermi level. (b) The projected density of states shows that the states around the Fermi level are mostly derived from the V $3d$ orbitals. Exchange splitting of the $3d$ orbitals above and below the Fermi level results in the dip in the total DOS at $E_F=0$ eV, as shown in detail at the zoomed region in (c).

Figure 4

(a) Temperature variation of magnetic susceptibility for ZnVSb in the field of 0.1 T. Red and blue symbols correspond to increasing and decreasing temperature, respectively. Solid curves denote fits of Eq. (1), while dashed lines extend the obtained function beyond fitting range as guide to the eye. The inset displays susceptibility measurement in the regime of thermal hysteresis. (b) Field dependencies of magnetization measured at temperatures 2 K, 5 K, 10 K, 80 K, and 140 K. Empty and filled symbols denote increasing and decreasing field, respectively. (c) Temperature dependence of electrical resistivity for ZnVSb measured in the field of 0 T (filled symbols) and 9 T (open symbols). Red and blue denote measurements performed with increasing and decreasing temperature, respectively.

Figure 5

XPS data of ZnVSb taken at 100 K (blue lines) and 200 K (orange lines): (a) wide energy range scan with the inset displaying the Sb MNN Auger lines; (b) the Sb $3d$ and V $2p$ core levels; (c) the extended valence band with the inset showing an enlarged view on the valence band.

Figure 6

(a) Phonon dispersion relation for ZnVSb, red and black lines correspond to acoustic and optical branches, respectively. (b) Related partial phonon density of states.

Figure 7

Temperature dependence of the specific heat for ZnVSb. (a) Experimental data compared to lattice contribution calculated from ab initio phonon density of states (blue line); see text for the details. Inset displays the temperature regime investigated with smaller step between data points. (b) The low-temperature data in $C_p/T\text{vs}{T}^2$ representation. Solid line is Debye fit; see text. Inset shows the data in the logarithmic axis $x$ for better visualization of the very lowest temperatures. (c) Analysis of the capacity with the Debye-Einstein model corrected for anharmonicity. The Debye, Einstein, and electronic contributions are displayed with black dotted, solid, and dashed lines, respectively. Inset shows data in $C_p/T^3\text{vs}T$ representation.