Circular diffuse scattering of akermanite studied by the optical diffraction method

Circular diffuse scattering appears when the incommensurate phase of akermanite transforms partly to either the normal phase or the commensurate phase by exchanging Ca atoms for larger alkali-earth atoms such as Sr or by exchanging Mg atoms for larger atoms such as Fe and/or by increasing the temperature. The scattering effect has been investigated by using an optical diffraction method. To induce the appearance of circular satellite reflections around the Bragg reflections, the modulation waves must repeat with a phase difference in one direction, x or y, together with a wavelength change. If the modulation waves repeat with phase differences in both x and y directions, circular satellite reflection will be induced without varying the modulation wavelength. Experimental reproduction of circular diffuse scattering shows that this phenomenon is ascribed to modulation waves repeating with a layer-to-layer phase difference and with a constant phase difference within each layer of the akermanite structure.