Critical angular displacement

The angular displacements can be employed for calculating from structural data the critical temperature of the phase transition the smaller is the difference between the orientations of the H-donor and H-acceptor groups, the lower is the critical temperature at which these two groups can assume the same... 

It is a well-known property of the KDP-type ferroelectrics that the deuteration of the hydrogen bonds considerably increases the critical temperature of the phase transition. This feature can be attributed to the lengthening of the O- -O distance caused by deuteration [26], but it can be also observed that the deuteration increases the difference between the orientation of the donor and acceptor groups. It occurs that lengthening of short hydrogen bonds increases the angular displacements the displacements are also increased by deuteration... 

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