(CH3)4NHgBrI2 exhibits at room temperature a polar columnar structure based on edge-sharing anion tetrahedra with a double anion occupancy. Its orthorhombic space group is Pb2//1m. An unusual multiple phase transition sequence revealed by differential scanning calorimetry, dilatometry and permittivity vs. temperature measurements leads to a non-polar high temperature phase with a trigonal mercury coordination and the monoclinic space group P2//1/n.