Observation of an Unexpected n-Type Semiconducting Behavior in the New Ternary Zintl Phase Eu3InAs3

The ternary arsenides Eu3InAs3 and Sr3InAs3 have been obtained by reactions of the elements in In flux at 1373 K. Structure elucidation by single-crystal X-ray diffraction reveals that Eu3InAs3 and Sr3InAs3 adopt the same orthorhombic structure (space group Pnma, Z = 4, Ca3AlAs3 structure type) with unit cell parameters a = 12.9179(9) angstrom, b = 4.3990(3) angstrom, c = 13.9337(10) angstrom and a = 13.0218(11) angstrom, b = 4.4364(4) angstrom, c = 14.1339(12) angstrom, respectively. The structure consists of linear chains of corner-sharing InAs4 tetrahedra, [InAs2As2/2](6-), and Eu2+/Sr2+ cations. Therefore, both Eu3InAs3 and Sr3InAs3 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 Sr3InAs3. The temperature dependence of resistivity and the computational work are in agreement that Eu3InAs3 and Sr3InAs3 are intrinsic semiconductors with narrow band gaps. Thermopower measurement on single-crystalline samples of Eu3InAs3 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 mu V 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 Eu3InAs3, a material that has not been purposely optimized, could indicate native defect chemistry, and not extrinsic doping, as a reason for the unusual behavior. A possible explanation involves a mixed-valent Eu2+/Eu3+ state, which might be inferred from the measured effective paramagnetic moment of 7.2 mu(B) per Eu atom, which is lower than the theoretically predicted value for free-ion moment of 7.9 mu(B)/Eu.