It often happens that a substance possessing a smectic liquid crystalline phase just above the melting point passes into a nematic liquid crystalline phase at a higher temperature. Account for this type of behavior.

Liquid crystals possess properties of both liquids and crystals. They have a fluidity like liquids but maintain order over longer molecular ranges like crystals. There are several types of liquid crystalline phases. The two phases we're focusing on are: 1. Smectic phase: In the smectic phase, molecules are arranged in well-ordered layers which can slide past each other, allowing for fluidity. Within each layer, the molecules are less ordered and possess random orientations. 2. Nematic phase: In the nematic phase, molecules have no long-range order in their position but are directionally aligned. The nematic phase is less ordered than the smectic phase.

Temperature plays a significant role in the transition between different liquid crystalline phases. As the temperature increases, molecules gain more thermal energy and their motion becomes more random. This causes a loss of order in the molecular structure.

Now that we understand the differences between smectic and nematic phases, let's discuss their behavior with increasing temperature. A substance in the smectic phase has more order than one in the nematic phase. When the material is just above its melting point and in the smectic phase, the temperature is relatively low, allowing for the formation of layer-like structures. When the temperature increases, molecular motion becomes more random due to the increased thermal energy. This added energy compromises the long-range positional order of molecules, causing the layers to break down and transition into the less ordered nematic phase. In the nematic phase, molecular alignment is maintained to some extent, but there's no significant positional order in the layers. In conclusion, a substance can exhibit a smectic liquid crystalline phase just above the melting point due to the relatively low temperature allowing for the formation of ordered layers in the structure. As the temperature increases, molecular motion becomes more random, breaking the ordered layers and causing a transition into the less ordered nematic liquid crystalline phase.