Synthesis, Crystal and Electronic Structures, and Properties of the New Pnictide Semiconductors A2CdPn2 (A = Ca, Sr, Ba, Eu; Pn = P, As)

A series of ternary Zintl phases, Ca2CdP2, Ca2CdAs2, Sr2CdAs2, Ba2CdAs2, and Eu2CdAs2, have been synthesized through high temperature metal flux reactions, and their structures have been characterized by single-crystal X-ray diffraction. They belong to the Yb2CdSb2 structure type and crystallize in the orthorhombic space group Cmc2(1) (No. 36, Z = 4) with cell dimensions of a = 4.2066(5), 4.3163(5), 4.4459(7), 4.5922(5), 4.4418(9) angstrom; b = 16.120(2), 16.5063(19), 16.904(3), 17.4047(18), 16.847(4) angstrom; c = 7.0639(9), 7.1418(8), 7.5885(11), 8.0526(8), 7.4985(16) angstrom for Ca2CdP2 (R1 = 0.0152, wR2 = 0.0278), Ca2CdAs2 (R1 = 0.0165, wR2 = 0.0290), Sr2CdAs2 (R1 = 0.0238, wR2 = 0.0404), Ba2CdAs2 (R1 = 0.0184, wR2 = 0.0361), and Eu2CdAs2 (R1 = 0.0203, wR2 = 0.0404), respectively. Among these, Ca2CdAs2 was found to form with another closely related structure, depending on the experimental conditions monoclinic space group Cm (No. 8, Z = 10) with lattice constants a = 21.5152(3) A, b = 4.30050(10) angstrom, c = 14.3761(2) A and beta = 110.0170(10)degrees (R1 = 0.0461, wR2 = 0.0747). UV/vis optical absorption spectra for both forms of Ca2CdAs2 show band gaps on the order of 1.0 eV, suggesting semiconducting properties, which have also been confirmed through electronic band structure calculations based on the density-functional theory. Results from differential scanning calorimetry measurements probing the thermal stability and phase transitions in the two Ca2CdAs2 polymorphs are discussed. Magnetic susceptibility measurements for Eu2CdAs2, indicating divalent Eu2+ cations, are presented as well.