  Solid-state cooling is an environmentally friendly and highly scalable
technology that may solve most of the problems associated with current
refrigerant methods. Solid-state cooling consists of applying external fields
on caloric materials, which react thermally as a result of induced phase
transformations. From an energy efficiency point of view, mechanocaloric
compounds, in which the phase transitions of interest are driven by mechanical
stresses, probably represent the most encouraging type of caloric materials.
Conventional mechanocaloric materials like shape-memory alloys already display
good cooling performances however in most cases they also present critical
mechanical fatigue and hysteresis problems that limit their applicability.
Finding new mechanocaloric materials and mechanisms able to overcome those
problems while simultaneously rendering large temperature shifts, is necessary
to further advance the field of solid-state cooling. In this article, we review
novel families of mechanocaloric materials that in recent years have been shown
to be specially promising in the aspects that conventional mechanocaloric
materials are not, and which exhibit unconventional but significant caloric
effects. We put an emphasis on elastocaloric materials, in which the targeted
cooling spans are obtained through uniaxial stresses, since from an applied
perspective these appear to be the most accomplished. Two different types of
mechanocaloric materials emerge as particularly hopeful from our analysis,
compounds that exhibit field-induced order disorder phase transitions involving
either ions or molecules (fast-ion conductors and plastic crystals), and
multiferroics in which the structural parameters are strongly coupled with
polar and/or magnetic degrees of freedom (magnetic alloys and oxide
perovskites).
