Giant barocaloric effect in hexagonal Ni2In-type Mn-Co-Ge-In compounds around room temperature

来源：admin 发布时间：2016-05-16

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. Generally, barocaloric effect is small for most of solid state materials, such as Pr_xLa₁_−xNiO3, Ce3Pd20Ge6, EuNi2(Si_xGe₁_−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 Ni2In-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 crystal structure brings about a big difference in the internal energy. Careful refinements on the structure reveal that the interlayer Mn-Mn distance behaves critically sensitive to pressure, indicating the origin of the pressure-driven magnetostructural transition and giant barocaloric effect.

Currently, magnetic refrigeration is being considered to be a competitive solid state refrigeration technique around room temperature due to the discovery of giant magnetocaloric materials, such as Gd5(Si,Ge)4, MnFeP₁_−xAs_x, MnAs, La(Fe,Si)13 and NiMn-based Heusler alloys. A common feature of these materials is the concurrent change of crystallographic and magnetic properties during phase transitions. In other words, magnetic phase transition always takes place along with a discontinuous change in lattice parameters and/or crystal symmetry. For most of them, both magnetic field and pressure can drive the first-order phase transition. Thus these materials should also display a barocaloric effect, as predicted by some theoretical investigations. Manosa et al.4,5 observed a considerable barocaloric effect near room temperature in La-Fe-Co-Si and Ni-Mn-In systems, in which the entropy changes (ΔS) are 8.6 Jkg⁻¹K⁻¹ under 2.1 kbar and 24.4 Jkg⁻¹K⁻¹ under 2.6 kbar, respectively, reaching 75% and 90% of the total (11.4 Jkg⁻¹K⁻¹ and 27.0 Jkg⁻¹K⁻¹). Such ΔS magnitude has exceeded the elastic heating effect of most materials and is also larger than the reported magnetocaloric effect (MCE) induced by magnetic fields that are available with permanent magnets4. Recently, Matsunami et al. reported giant barocaloric effect enhanced by the frustration in the antiferromagnetic Mn3GaN. The entropy change reaches 22.3 Jkg⁻¹K⁻¹ under a hydrostatic pressure change of 1.39 kbar.

Ternary compounds MM’X with hexagonal Ni2In-type structure have recently attracted much attention due to their magnetic shape memory effect and possible large magnetocaloric effect related to the magnetostructural coupling. As a member of MM’X family, the stoichiometric MnCoGe alloy does not show magnetostructural coupling. It undergoes a diffusionless martensitic structural transition, Tstru, from a Ni2In-type hexagonal structure (space group P6₃/mmc) to a TiNiSi-type orthorhombic structure (space group Pnma) at Tstru ~ 420 K and a separated ferromagnetic ordering transition at a lower temperature Tc ~ 345K. Fortunately, both magnetic interaction and crystallographic stability are sensitive to chemical pressures, such as substitution, doping, or interstitials. Introducing atoms with different radii and valence electrons can simultaneously tune magnetic and crystallographic transitions and make the two separated transitions to overlap with each other. As a result, magnetostructural coupling can be created. For the MM’X family, the austenitic hexagonal phase has a smaller unit cell volume than the martensitic orthorhombic phase. This fact indicates that introducing smaller atoms or vacancies may probably stabilize hexagonal phases and shift Tstru to a lower temperature. Indeed, magnetostructural transition has been experimentally realized through introducing atom vacancies or smaller atoms, such as MnCo₁_−xGe, Mn₁_−xCoGe, Mn₁_−xCr_xCoGe. However, the change of local environments is not the sole route to affect Tstru. Valence electron concentration (e/a) may also play an important role. We found that introducing larger atoms with fewer valence electrons can also lower Tstru and create the magnetostructural transition. Indium (In) atom (2.00 Å, 5s²5p¹) has a larger atomic radius but fewer valence electrons than Mn(1.79 Å, 3d⁵4s²), Co(1.67 Å, 3d⁷4s²), or Ge(1.52 Å, 4s²4p²). We found that the replacement of Mn, Co, or Ge by a little amount of In can create magnetostructural coupling. Particularly, the magnetostructural transition temperature shows a monotonous decrease with increasing In doping for MnCoGe₁_−xIn_x.
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 Ni2In-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⁻¹K⁻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.