Low-positive, near-zero, and negative thermal expansion materials are relatively rare. Although these unusual thermal expansion properties have been reported in the past for a few phases, systematic studies of the physical mechanisms and crystallography underlying these properties are more recent. This increased interest in this exotic class of materials arose from newly discovered materials showing these properties near room temperature or over a large temperature range.
The existence of such material properties in useful temperature ranges is fundamental for application purposes.
The effort to optimize the use of these materials in bulk form and as fillers in different matrices, for a variety of properties, ranging from high thermal shock resistance to controlled thermal expansion of polymeric, metallic, or ceramic matrices, is a more recent endeavor.

Since research in the area of low-positive, near-zero, and negative thermal expansion materials is still very recent, there are severe gaps of knowledge, principally related to the effective application of these materials. For example, more systematic studies of the important application properties such as hygroscopicity and first-order phase transition are still lacking.

Production of anhydrous phases with tailored coefficients of thermal expansion and synthesis of dispersed powders through soft-chemical routes are some of the most basic questions. In addition, consolidation of such powders by sintering to translucent or transparent highly dense bulk materials, with high mechanical resistance, is still far from systematic.
Although some studies show the potential of these materials for controlling the thermal expansion of polymeric, metallic, or ceramic matrices, we are still far from the full development of technologies that take advantage of these composites.
The proposal of new potential applications also requires exploration of all related properties of these materials.
Yet some more, fundamentals aspects, such as defect chemistry, are completely unknown, as are important crystallographic aspects, such as water molecules locations inside framework structures.