The Smectic Q Phase, a Crystal of Twist Grain Boundaries with Smectic Order Anne-Marie Levelut, Emmanuelle Hallouin, Dirk Bennemann, Gerd Heppke et Detlef Lötzsch J. Phys. II France, 7 7 (1997) 981-1000 DOI: 10.1051/jp2:1997165