Giant barocaloric effect in hexagonal Ni 2 In-type Mn-Co- Ge-In compounds around room

The most widespread cooling techniques based on gas compression/expansion encounter environmental problems. Thus, tremendous effort has been dedicated to develop alternative cooling technique and search for solid state materials that show large caloric effects. An application of pressure to a material can cause a change in temperature, which is called the barocaloric effect. Here we report the giant barocaloric effect in a hexagonal Ni 2In-type MnCoGe0.99In0.01 compound involving magnetostructural transformation, Tmstr, which is accompanied with a big difference in the internal energy due to a great negative lattice expansion(ΔV/V ~ 3.9%). High resolution neutron diffraction experiments reveal that the hydrostatic pressure can push the Tmstr to a lower temperature at a rate of 7.7 K/kbar, resulting in a giant barocaloric effect. The entropy change under a moderate pressure of 3 kbar reaches 52 Jkg −1 K −1 , which exceeds that of most materials, including the reported giant magnetocaloric effect driven by 5 T magnetic field that is available only by superconducting magnets. Caloric effects of materials driven by different external fields such as pressure, magnetic field, and electric field are known as barocaloric, magnetocaloric, and electrocaloric effect, respectively. Any change of lattice, spin, electric polarization ordering is accompanied by entropy change, thus the caloric effect can be measured by isothermal entropy change or adiabatic temperature change. It is easily understandable that barocaloric effect is universal noting that an application of pressure on any material can cause a change in lattice ordering and lead to a caloric effect. Actually, the conventional cooling techniques in our daily life or industry applications are based on compression and expansion cycles of gases, but these popular techniques directly or indirectly cause many environmental problems. In the past two decades, the discovery of solid state materials with giant magnetocaloric/electrocaloric/ barocaloric effect has indeed promoted the development of solid state refrigeration techniques 1–6 . Generally, barocaloric effect is small for most of solid state materials 7,8 , such as PrxLa1−xNiO3, Ce3Pd20Ge6, EuNi2(SixGe1−x)2, CeSb, and HoAs, because the applied pressure cannot produce substantial changes in the structure and/or magnetic ordering. The entropy change produced by a moderate pressure is not enough to fulfill the requirement of the practical refrigeration. Here, we report a sizable barocaloric effect in a Mn-Co-Ge-In compound, which originates from a pressure-driven orthorhombic-hexagonal magnetostructural transition. To the best of our knowledge, this is the first time that the giant barocaloric effect has been observed in a system with a hexagonal Ni 2In-type structure. High resolution neutron diffraction experiments reveal that the phase transition is accompanied with a significant re-construction of crystal structure, noting the lattice change can be as large as ΔV/V ~ 3.9%, which exceeds that of the most other caloric materials with a lattice contribution. Such a significant re-construction of