Order-disorder transitions possible explanations

Among the thiocyanates another order-disorder phase transition is possible. This involves a transition from an ordered structure similar to potassium thiocyanate to a disordered structure isostructural with potassium cyanate. This has been observed in potassium thiocyanate at 145 °C but has not been observed up to 300 °C in the nearly isostructural sodium salt. Iqbd (41) has suggested an explanation for this behaviour in terms of the greater anisotropy of the inter-ionic interaction potential in potassium thiocyanate compared with the sodium salt. 

The scheme in Figure 2 illustrates a possible alternative explanation for the observation that bond lengths in m-enol systems are intermediate between single and double bonds. If the molecules have statistically disordered enol systems, the hydrogen atoms of the hydrogen bond will be distributed over two positions in the crystal structure. Indeed this was the case for the C polymorph of naphtazarin above 110 K at this temperature there is a second-order transition to a state with an ordered enol hydrogen [2],... 

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