Structural phase transitions at high temperatures have been investigated in members of a new group of germanate analogues of (KTP) with general formula , , and , using high-resolution neutron powder diffractometry at the pulsed neutron source ISIS, Rutherford Appleton Laboratory. The temperature-induced ferroelectric - paraelectric phase transition in the KTP structure type, accompanied by a change of space group from to Pnan, is continuous second order and is both displacive and order - disorder in nature, in contrast to the purely displacive phase transition in produced by high pressure at ambient temperature. The regularity of the increasing values of found in a series of KTP analogues is in agreement with Abrahams - Jamieson - Kurtz (AJK) criteria. The behaviour of the A cations determines the ferroelectric properties; M octahedra together with the A cations make a considerable contribution to the nonlinear optical properties. The redistribution of A cations in the structural holes formed by the framework must be polar, and the cations must not suppress the electronic state of the titanyl bridge oxygens in the chain of octahedra, which is favourable for the existence of a charge transfer excited state delocalized along the chains. The alternating short and long interatomic distances in crystals with good second-harmonic generation (SHG) is typical not only for the M - O bridge, but also for the A - O bridge.