BaGa2Pn2 (Pn = P, As): New Semiconducting Phosphides and Arsenides with Layered Structures

Exploration of Multi-Component Vanadium and Titanium Pnictides Using Flux Growth and Conventional High-Temperature Methods

The flux growth method was successfully employed to synthesize millimeter-sized single crystals of the ternary barium vanadium pnictides Ba₅V₁₂As_19+x (x ≈ 0.02) and Ba₅V₁₂Sb_19+x (x ≈ 0.36), using molten Pb and Sb, respectively. Both compositions crystallize in space group P 4 ¯ 3m and adopt a structure similar to those of the barium titanium pnictides Ba₅Ti₁₂ Pn _19+x (Pn = Sb, Bi), yet with a subtly different disorder, involving the pnictogen and barium atoms. Attempts to obtain an arsenide analog of Ba₅Ti₁₂ Pn _19+x using a Pb flux technique yielded binary arsenides. High-temperature treatment of the elements Ba, Ti, and As in Nb or Ta tubes resulted in side reactions with the crucible materials and produced two isostructural compositions Ba₈Ti_13-x M _x As₂₁ (M = Nb, Ta; x ≈ 4), representing a new structure type. The latter structure displays fcc-type metal clusters comprised of statistically distributed Ti and M atoms (M = Nb, Ta) with multi-center and two-center bonding within the clusters, as suggested by our first-principle calculations.

NaSn2As2: An Exfoliatable Layered van der Waals Zintl Phase

The discovery of new families of exfoliatable 2D crystals that have diverse sets of electronic, optical, and spin-orbit coupling properties enables the realization of unique physical phenomena in these few-atom-thick building blocks and in proximity to other materials. Herein, using NaSn₂As₂ as a model system, we demonstrate that layered Zintl phases having the stoichiometry ATt₂Pn₂ (A = group 1 or 2 element, Tt = group 14 tetrel element, and Pn = group 15 pnictogen element) and feature networks separated by van der Waals gaps can be readily exfoliated with both mechanical and liquid-phase methods. We identified the symmetries of the Raman-active modes of the bulk crystals via polarized Raman spectroscopy. The bulk and mechanically exfoliated NaSn₂As₂ samples are resistant toward oxidation, with only the top surface oxidizing in ambient conditions over a couple of days, while the liquid-exfoliated samples oxidize much more quickly in ambient conditions. Employing angle-resolved photoemission spectroscopy, density functional theory, and transport on bulk and exfoliated samples, we show that NaSn₂As₂ is a highly conducting 2D semimetal, with resistivities on the order of 10^-6 Ω·m. Due to peculiarities in the band structure, the dominating p-type carriers at low temperature are nearly compensated by the opening of n-type conduction channels as temperature increases. This work further expands the family of exfoliatable 2D materials to layered van der Waals Zintl phases, opening up opportunities in electronics and spintronics.