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The detailed structural change in the charge-discharge process for the 10 and 20 mol % manganese-substituted nickel hydroxide was investigated by using high-energy synchrotron X-ray diffraction ͑XRD͒ and X-ray absorption fine structure ͑XAFS͒. On the charge-discharge process, the 10 and 20 mol % manganese-substituted nickel hydroxide showed the ␤-Ni͑OH͒ 2 /␥-NiOOH and the ␣-Ni͑OH͒ 2 /␥-NiOOH phase transformation, respectively. The manganese ions were inserted on the only nickel sites for the 10 mol % manganese-substituted nickel hydroxide, and on both nickel sites and 18h sites for the 20 mol % manganese-substituted nickel hydroxide. The ␣-Ni͑OH͒ 2 structure could be stabilized by the presence of the manganese ions on the 18h sites. The structural refinement for the manganese-substituted nickel hydroxides has been done successfully on the basis of two phase models of the ideal phases and the fault ones. As compared to ideal phases, the fault phases were characterized by the shift of the nickel atoms, and showed a larger amount of the intercalated potassium ions and H 2 O ͑OH − ͒ molecules in the interlayer. The occupancy sites for the potassium ions and H 2 O molecules were contained for the refinements, bringing about a better agreement between the observed and calculated patterns.A nickel hydroxide ͓Ni͑OH͒ 2 ͔ is used as the active material for the positive electrode of the nickel-metal hydride ͑Ni-MH͒ batteries. It is known that the Ni͑OH͒ 2 has two polymorphs, i.e., the ␤-Ni͑OH͒ 2 and the ␣-Ni͑OH͒ 2 . 1 The theoretical capacity of the ␤-Ni͑OH͒ 2 is 289 mAh/g, corresponding to one electron reaction for the ␤-Ni͑OH͒ 2 /␤-NiOOH redox couple. Larger discharge capacities could be obtained for the ␣-Ni͑OH͒ 2 /␥-NiOOH redox couple because the oxidation state of nickel in the ␥-NiOOH reaches 3.3-3.7. 2 It is known that ␣-Ni͑OH͒ 2 is unstable in the alkaline electrolyte, being easily transformed to the ␤-Ni͑OH͒ 2 . 1 To stabilize the ␣-Ni͑OH͒ 2 structure, several efforts have been made through the partial substitution of the nickel in the Ni͑OH͒ 2 by cobalt, 3 iron, 4 manganese, 5-7 aluminum, 8 and zinc. 9 It was reported that the trivalent manganese ions were incorporated only in the nickel sites, accompanied by the intercalation of the anions such as CO 3 2− , SO 4 2− , and NO 3 2− into the interlayer to compensate the positive charge due to the trivalent manganese ions. [5][6][7] However, those efforts were not successful because the trivalent manganese ions are unstable due to a 3d 4 electronic configuration, and are easily oxidized to the tetravalent ions in the chargedischarge cycling. An excess of the positive charge such as the tetravalent manganese ions leads to the proton deficiencies to maintain the charge balance. The anions are deintercalated from the interlayer because of the electrostatic repulsion between the anions and the unprotonated oxygen ions in the interlayer. 5-7 An ␣-Ni͑OH͒ 2 is transformed to a ␤-Ni͑OH͒ 2 .The structure for ␤-Ni͑OH͒ 2 consists of close-packed oxygen planes with ABABABAB stack...

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