Design, Synthesis, and Optoelectronic Properties of the High-Purity Phase in Layered AETMN₂ (AE = Sr, Ba; TM = Ti, Zr, Hf) Semiconductors

Abstract

We report the synthesis and optoelectronic properties of high phase-purity (>94 mol %) bulk polycrystals of KCoO2-type layered nitrides AETMN2 (AE = Sr, Ba; and TM = Ti, Zr, Hf), which are expected to exhibit unique electron transport properties originating from their natural two-dimensional (2D) electronic structure, but high-purity intrinsic samples have yet been reported. The bulks were synthesized using a solid-state reaction between AENH and TMN precursors with NaN3 to achieve high N chemical potential during the reaction. The AETMN2 bulks are n-type semiconductors with optical band gaps of 1.63 eV for SrTiN2, 1.97 eV for BaZrN2, and 2.17 eV for BaHfN2. SrTiN2 and BaZrN2 bulks show degenerated electron conduction due to the natural high-density electron doping and paramagnetic behavior in all of the temperature ranges examined, while such unintentional carrier generation is largely suppressed in BaHfN2, which exhibits nondegenerated electron conduction. The BaHfN2 sample also exhibits weak ferromagnetic behavior at temperatures lower than 35 K. Density functional theory calculations suggest that the high-density electron carriers in SrTiN2 come from oxygen impurity substitution at the N site (ON) acting as a shallow donor even if the high-N chemical potential synthesis conditions are employed. On the other hand, the formation energy of ON becomes larger in BaHfN2 because of the stronger TM–N chemical bonds. Present results demonstrate that the easiness of impurity incorporation is designed by density functional calculations to produce a more intrinsic semiconductor in wider chemical conditions, opening a way to cultivating novel functional materials that are sensitive to atmospheric impurities and defects.

Control of Hole Density in Russellite Bi2WO6 via Intentional Chemical Doping

“…When n reached a critical value as x was increased, the well-known relationship | S | ∝ 1/| n | occurred. The values of n , which were estimated from ρ and S , with the effective mass of Bi 2 WO 6 , of Bi 2 W 1– x Nb x O 6 and Bi 2 W 1– x Ta x O 6 with x ≥ 0.2 and 0.1, respectively, are shown in Figure c. As expected, n increases monotonically as a function of x for both Bi 2 W 1– x Nb x O 6 and Bi 2 W 1– x Ta x O 6 with x ≥ 0.2 and 0.1, respectively.…”

Mechanistically Guided Materials Chemistry: Synthesis of Ternary Nitrides, CaZrN2 and CaHfN2

“…All of the AB N 2 phases ( A = Mg, Ca, Sr, Ba; B = Ti, Zr, Hf) have been synthesized previously, ,− with the exception of CaZrN 2 and CaHfN 2 . As summarized in Figure , these phases crystallize with several structure types: rocksalt ( F m 3̅ m ) , KCoO 2 -type ( P 4/ nmm ), and α-NaFeO 2 -type ( R 3̅ m ) .…”

Tolerance Factor and Phase Stability of the KCoO2-Type AMN2 Nitrides

“…Recently, there is increasing attention in the KCoO 2 -type AMN 2 nitrides (A and M are usually alkaline earth and IVB group elements, respectively) owing to their unique structural feature (Figure a) and the interesting physical properties including high dielectric permittivity and thermoelectric behavior (from theoretical calculation). – The KCoO 2 -type AMN 2 nitrides consist of alternative one single Bi 2 O 2 -like M 2 N 2 conducting layer (similar to the Bi 2 O 2 covalent network in the Aurivillius families of layered compound) and two consecutive rocksalt-type AN insulating layers (akin to layered superconducting cuprates), and feature uncommon odd coordination geometries for both the A and M cations with 9 and 5-coordinate polyhedral environments (Figure b), respectively . In the 9-coordinate polyhedron, there are five short and four long A–N bond; while in the 5-coordinate polyhedron, there are one short and four long M–N bonds, forming square pyramids which share edges along the M 2 N 2 layers with the terminal N atoms within the closest neighboring MN 5 units antiparallelly aligned along the c axis perpendicular to the layers. – , …”