Figure 3.1.5.14 

Structure of tris-sarcosine calcium chloride, (CH₃NHCH₂COOH)₃CaCl₂. The hydrogen ion (proton) on the COOH group is relocated in the crystal onto the N atom to form a zwitter ion, forming an H—N—H group that hydrogen bonds to adjacent chlorine ions. Each nitrogen forms two such hydrogen bonds, whereas each chlorine has three, forming a very complex network of hydrogen bonding. The phase transition is actually displacive, involving a rather rigid rolling of whole sarcosine molecules, which stretches the N—H bonds; it is not order–disorder of hydrogen ions in a Cl⋯H—N double well. (The Cl⋯H—N wells are apparently too asymmetric for that.)