DOE OSTI2020
The ternary arsenides Eu 3 InAs 3 and Sr 3 InAs 3 have been obtained by reactions of the elements in In flux at 1373 K. Structure elucidation by single-crystal X-ray diffraction reveals that Eu 3 InAs 3 and Sr 3 InAs 3 adopt the same orthorhombic structure (space group Pnma, Z = 4, Ca 3 AlAs 3 structure type) with unit cell parameters a = 12.9179(9) Å, b = 4.3990(3) Å, c = 13.9337(10) Å and a = 13.0218(11) Å, b = 4.4364(4) Å, c = 14.1339(12) Å, respectively. The structure consists of linear chains of corner-sharing InAs4 tetrahedra, [InAs 2 As 2/2 ] 6– , and Eu 2+ /Sr 2+ cations. Therefore, both Eu 3 InAs 3 and Sr 3 InAs 3 are valence-precise Zintl phases. As expected from the closed-shell electronic configurations, semiconducting behavior is confirmed by resistivity measurements on single crystals for both and by electronic band structure calculations for Sr 3 InAs 3 . The temperature dependence of resistivity and the computational work are in agreement that Eu 3 InAs 3 and Sr 3 InAs 3 are intrinsic semiconductors with narrow band gaps. Thermopower measurement on single-crystalline samples of Eu 3 InAs 3 shows that in the whole measured temperature range, from 300 to 700 K, the values for the Seebeck coefficient are negative. The observation of a negative Seebeck coefficient with very large absolute value (>400–500 μV K –1 at 700 K) is unexpected among the Zintl phases and suggestive that electrons are the majority charge carriers. Such a rare, n-type charge transport in an undoped compound such as Eu 3 InAs 3 , a material that has not been purposely optimized, could indicate native “defect” chemistry, and not extrinsic doping, as a reason for the unusual behavior. Here, a possible explanation involves a mixed-valent Eu 2+ /Eu 3+ state, which might be inferred from the measured effective paramagnetic moment of 7.2 μB per Eu atom, which is lower than the theoretically predicted value for free-ion moment of 7.9 μ B /Eu.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗