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The successive structural phase transitions in the layer compound have been studied using single-crystal x-ray diffraction. The intermediate phase between 194 K and 214 K is found to be incommensurate; the satellite maps show that the modulation mode which characterizes the intermediate phase has . It is shown that the previously reported satellite reflections at are ascribable to twins. A structure model is presented for the incommensurate phase. In the low-temperature phase below 197 K, the satellite reflections are shifted to a commensurate position, .

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