Structural Study of Ordering in the Normal-Commensurate Transition of {N(CH sub( 3)) sub( 4)} sub( 2)MnCl sub( 4) -Models and Adaptation

The ordering process of constituent molecules in phase III of {N(CH3)4}2MnCl4 is discussed on the basis of the results of X-ray structural analyses performed at nine temperatures below the normal-commensurate transition point. The order-disorder and displacive models are analyzed and compared in terms of the temperature dependences of structural parameters. Consequently, it is concluded that both tetramethylammonium and tetrachloromanganese ions behave as order-disorder units, and that their occupation probability, representative of the order parameter, changes monotonically with decreasing temperature. The results of our detailed analysis indicate that the centers of mass of the tetrahedral molecules shift continuously together with occupation ordering. The temperature dependence of the atomic displacement parameters is also investigated to confirm that the transition is of the order-disorder type.