[en] Highlights: • Synchrotron X-ray diffraction structural studies of Ca_1_−_xLa_xFeAs_2 are presented. • Crystal twins are found in Ca_1_−_xLa_xFeAs_2 with nominal compositions of x = 0.25. • Contribution of sp hybridization for arsenic zigzag chains are discussed. • Possible structures of novel 112-type iron arsenides are discussed. - Abstract: Synchrotron X-ray diffraction studies of Ca_1_−_xLa_xFeAs_2 with monovalent arsenic zigzag chain layers are presented. While the crystal twins appear in all samples for the nominal composition of x = 0.25, we successfully obtained the samples without crystal twins for the nominal composition of x = 0.17. We present the structural parameters obtained using the synchrotron X-ray diffraction data and physical properties for the samples with x = 0.17. We further discuss the tactics for exploring novel 112-type iron arsenides without arsenic zigzag chains.