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 Cmc2₁ (No. 36, Z = 4) with cell dimensions of a = 4.2066(5), 4.3163(5), 4.4459(7), 4.5922(5), 4.4418(9) Å; b = 16.120(2), 16.5063(19), 16.904(3), 17.4047(18), 16.847(4) Å; c = 7.0639(9), 7.1418(8), 7.5885(11), 8.0526(8), 7.4985(16) Å 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) Å, b = 4.30050(10) Å, c = 14.3761(2) Å and β = 110.0170(10)° (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.