The ground-state properties of an S = 1/2 quasi-one-dimensional quantum spin system (CH3)2NH2-CuCl3 were investigated by X-ray structure analyses and high-field magnetization measurements. X-ray studies confirmed that the structural phase transition from monoclinic to triclinic phases occurs between 300 and 200 K. The magnetic chain realized at low temperatures involves three types of chlorine-bridged pathways that result in bond alternation with n = 4, where n is the period of the spatial structure. High-field magnetization measurements at the lowest temperature of 0.5K revealed a unique magnetization process with an initial rapid increase in a low field and a relatively gradual increase in a high field, separated by the magnetization plateau with half of the saturation moment. On the basis of these experimental facts, a novel quantum chain model, which is characterized by the alternation of an S = 1 triplet dimer and an S = 0 singlet dimer is proposed for the ground state of the present system.
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