We present results on the hydrothermal growth of ()OHFeSe single crystals using floating-zone-grown (A = K, Rb, and Cs) as precursors. The growth proceeds by the hydrothermal ion exchange of Li/Fe–O–H for K, Rb, and Cs, resulting in a stacking layer of ()OH sandwiched between the FeSe layers. Optimal growth parameters are achieved using high quality A 0.80Fe1.81Se2 single crystals added to the mixtures of LiOH, H2O, Fe and C(NH2)2Se in an autoclave and subsequently heated to 120 °C for 2 d, to obtain highest quality single crystals. The obtained crystals have lateral dimensions up to centimeters, with the final size related to that of the precursor crystal used. All ()OHFeSe single crystals show a superconducting transition temperature T c > 42 K, regardless of the phase of the precursor such as superconducting K0.80Fe1.81Se2 (T c = 29.31 K) or non-superconducting Rb0.80Fe1.81Se2 or poor-superconducting Cs0.80Fe1.81Se2 (T c = 28.67 K). The T c and transition width ΔT vary in the obtained single crystals, due to the inhomogeneity of the ionic exchange. X-ray diffraction analysis demonstrates that the 245 insulating phase is absent in the ion-exchanged single crystals, while it is observed in the precursors. Comparative studies of the structure, magnetization, and superconductivity on the parent A 0.80Fe1.81Se2 and the ion-exchanged ()OHFeSe crystals are discussed. A phase diagram including antiferromagnetic spin density wave and superconducting phases is also proposed
single crystal; OHFeSe;
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