Giant barocaloric effects at low pressure in ferrielectric ammonium sulphate

Abstract

Caloric effects are currently under intense study due to the prospect of environment-friendly cooling applications. Most of the research is centred on large magnetocaloric effects and large electrocaloric effects, but the former require large magnetic fields that are challenging to generate economically and the latter require large electric fields that can only be applied without breakdown in thin samples. Here we use small changes in hydrostatic pressure to drive giant inverse barocaloric effects near the ferrielectric phase transition in ammonium sulphate. We find barocaloric effects and strengths that exceed those previously observed near magnetostructural phase transitions in magnetic materials. Our findings should therefore inspire the discovery of giant barocaloric effects in a wide range of unexplored ferroelectric materials, ultimately leading to barocaloric cooling devices.

ARTICLE

Received 17 Jul 2015 | Accepted 5 Oct 2015 | Published 26 Nov 2015

P. Lloveras1, E. Stern-Taulats2, M. Barrio1, J.-Ll. Tamarit1, S. Crossley3, W. Li3,4, V. Pomjakushin5,A. Planes2, Ll. Manosa2, N.D. Mathur3 & X. Moya2,3

Caloric effects are currently under intense study due to the prospect of environment-friendly cooling applications. Most of the research is centred on large magnetocaloric effects and large electrocaloric effects, but the former require large magnetic elds that are challenging to generate economically and the latter require large electric elds that can only be applied without breakdown in thin samples. Here we use small changes in hydrostatic pressure to drive giant inverse barocaloric effects near the ferrielectric phase transition in ammonium sulphate. We nd barocaloric effects and strengths that exceed those previously observed near magnetostructural phase transitions in magnetic materials. Our ndings should therefore inspire the discovery of giant barocaloric effects in a wide range of unexplored ferroelectric materials, ultimately leading to barocaloric cooling devices.

DOI: 10.1038/ncomms9801 OPEN

Giant barocaloric effects at low pressure in ferrielectric ammonium sulphate

1 Departament de Fsica i Enginyeria Nuclear, ETSEIB, Universitat Politcnica de Catalunya, Diagonal 647, Barcelona, 08028 Catalonia, Spain. 2 Facultat de Fsica, Departament dEstructura i Constituents de la Matria, Universitat de Barcelona, Marti Franqus 1, Barcelona, 08028 Catalonia, Spain. 3 Department of Materials Science, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK. 4 School of Physics and Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan 430074, China. 5 Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institut (PSI), CH-5232 Villigen PSI, Switzerland. Correspondence and requests for materials should be addressed to X.M. (email: mailto:[email protected]

Web End [email protected] ).

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Foodstuffs, beverages, medicine, electronics and populated spaces all require cooling, but existing refrigeration and air-conditioning units rely primarily on the compression and

expansion of environmentally harmful uids. Resurgent interest in solid materials that display magnetically, electrically and mechanically driven phase transitions near room temperature13 has provoked interest in the possibility of environment-friendly cooling applications, but these will only come to fruition if it is possible to develop or discover inexpensive materials that show large reversible thermal changes in response to elds that are small and easy to generate.

Mechanical stress is easy to generate, but large barocaloric (BC) effects driven by hydrostatic pressure near phase transitions have only been observed in a small number of relatively expensive magnetic materials, where changes of magnetization are accompanied by changes in crystal symmetry4,5 or volume alone68 (Table 1). (Large BC effects have also been observed in poly(methyl methacrylate) away from any transition9.) Here we demonstrate giant BC effects near the ferrielectric phase transition1013 in a powder of ammonium sulphate (AS) [(NH4)2SO4], which is made from cheap abundant elements and enjoys widespread agricultural use as a fertilizer. We use calorimetry to identify pressure-driven isothermal entropy changes of |DS|B60 J K 1 kg 1, which exceed the corresponding values that have been found for metallic alloys near rst-order magnetic phase transitions (B1025 J K 1 kg 1;

Table 1), and predicted for PbTiO3 and BaTiO3 near rst-order ferroelectric phase transitions14,15 (B34 J K 1 kg 1). These giant entropy changes are driven using small changes of hydrostatic pressure |Dp| |p patm|B|p|B0.1 GPa, yielding

giant BC strengths1 |DS|/|Dp|, |Q|/|Dp| and |DT|/|Dp| (Table 1) (where Q is the heat, T is the temperature and atmospheric pressure patmB0 GPa). Our giant BC effects may be understood via pressure-driven changes in ionic ordering, whereas the smaller BC effects in magnetic materials48 arise due to pressure-driven changes in the density of electronic states near the Fermi level.

ResultsFerrielectric phase transition in AS at atmospheric pressure. At room temperature, AS adopts a centrosymmetric orthorhombic structure (Pnam) with four formula units per unit cell comprising three ionic groups (Fig. 1a) that are understood to adopt a disordered conguration at any given instant16,17. On cooling, the material is generally considered to undergo a reversible order disorder phase transition to an orthorhombic polar structure (Pna21) that is ferrielectric10,11. Our heat ow dQ/dT measurements conrm that this transition occurs in two steps10

13. First, the symmetry change arises from a non-isochoric rst-order transition at T1B221 K associated with partial ionic ordering (Fig. 1b). Second, further ordering yields additional changes of

volume in a continuous manner down to B160 K (Fig. 1b-d). (Figure 1d was obtained using temperature-dependent lattice parameters (Supplementary Fig. 1) calculated from X-ray diffraction patterns (Supplementary Fig. 2).) The rst-order transition is weakly hysteretic and occurs at T1B224 K on heating.

Its start and nish temperatures on cooling are Tc1B223 K and

Tc2B216 K, respectively, and its start and nish temperatures on heating are Th1B222 K and Th2B229 K, respectively.

Integration of (dQ/dT)/T yields the corresponding entropy change DS(T) (Fig. 1c), with |DSf| 1306 J K 1 kg 1 for the

full transition. Integration of dQ/dT across the full transition yields a corresponding heat of |Qf| 292 kJ kg 1. These values

are in good agreement with previous experimental values12,13 of |DSf|B126133 J K 1 kg 1 and |Qf|B2830 kJ kg 1, and are consistent with the change of entropy |DSf| 3Rln2 130 J K 1 kg 1 expected16 for an order-

disorder transition involving three ionic groups per formula unit (R 8.314 J K 1 mol 1). For the rst-order transition

alone, integration yields |DS1| 654 J K 1 kg 1 and latent

heat |Q1| 14.51.0 kJ kg 1. These values correspond to B50%

of the aforementioned values for the full transition and closely match literature values13 of |DS1| 61 J K 1 kg 1 and

|Q1| 13.6 kJ kg 1 for deuterated AS [(ND4)2SO4], where no

aspect of the transition is modied by the deuteration.

On heating through the ferrielectric transition, X-ray diffraction data conrm the expected changes in crystal structure10,11,13. The unit-cell volume V decreases by B0.9% across the full transition (DVf 4.40.2 3) and by B0.5% across the rst-order

transition alone (DV1 2.50.2 3) (Fig. 1d). Given that BC

effects per unit mass m due to pressure change Dp p2 p1 may be

expressed using the Maxwell relation m 1(qV/qT)p (qS/qp)T

as1 DS(p1-p2) m 1R

p2

p1 (qV/qT)p0dp0, we anticipate inverse BC effects in the transition regime where (qV/qT)p 0o0 and we

anticipate conventional BC effects away from the transition regime where (qV/qT)p 040.

Ferrielectric phase transition in AS under applied pressure. For the rst-order transition, heat ow measurements dQ/dT reveal a strong pressure-induced shift in T1 (Fig. 2a,b), with dT1/dp 574 K GPa 1 on heating and dT1/dp 454 K GPa 1

on cooling. A similar shift of 456 K GPa 1 on heating is

obtained via the Clausius-Clapeyron equation dT1/dp Dv1/DS1,

using DS1 654 J K 1 kg 1 (Fig. 1c) and specic volume

change Dv1 (2.90.2) 10 6 m3 kg 1 (from Fig. 1d).

These large values of dT1/dp are similar to those reported for single-crystal AS18,19 and magnetic alloys (Table 2), and indicate that the narrow rst-order transition of width Tc1 Tc2BTh2 Th1B7 K may be fully driven in either

direction using moderate values of |Dp|B0.15 GPa.

Table 1 | Giant BC effects at rst-order phase transitions.

Giant BC material T |DS| |DT| |Q| |Dp| |DS/Dp| |DT/Dp| |Q/Dp| RC Ref.

K J K 1 kg 1 K kJ kg 1 GPa J K 1 kg 1 GPa 1 KGPa 1 kJ kg 1 GPa 1 J kg 1 Ni49.26Mn36.08In14.66 293 24 [4.5] 7.1 0.26 92.3 17.3 27.3 120 4

Gd5Si2Ge2 270 11 1.1 2.9 0.20 55 5.5 14.5 81 5

LaFe11.33Co0.47Si1.2 237 8.7 2.2 2.0 0.20 43.5 11 10 180 6

Fe49Rh51 308 12.5 [8.1] 3.8 0.11 114 74 34.5 105 7

Mn3GaN 285 21.6 [4.8] 6.2 0.09 232 51.4 66.2 125 8

AS 219 60 [8] 13.2 0.10 600 80 132 276 This work

BC, barocaloric; |Dp|, hydrostatic pressure change; |Q|, isothermal heat; RC, refrigerant capacity; |DS|, isothermal entropy change; T, starting temperature; |DT|, adiabatic temperature change.|DS|, |DT| and |Q| arise at T, due to changes of |Dp|. The corresponding strengths |DS|/|Dp|, |DT|/|Dp| and |Q|/|Dp| were maximized by choosing the smallest values of |Dp| compatible with maximizing |DS|. Bold entries denote data derived from direct measurements. Italicised entries denote data derived from quasi-direct1 measurements. Bracketed entries denote data derived via-cDTCTDS Q using zero-pressure specic heat capacity c. For all entries, Q TDS.

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relation with S replacing S. From this formulation, we

anticipate large values of DS given an AS volumetric

thermal expansion coefcient V 1(qV/qT)p whose magnitude B10 4 K 1 (Supplementary Fig. 5a) exceeds the corresponding values6,2024 of B10 710 5 K 1 for the magnetic

BC materials of Table 1.

To conrm that the fall in |DS1(p)| arises due to additional changes of entropy DS away from T1(p), we evaluated

DS (p) on applying pressure above T1(p) at T 236 K by

assuming (qV/qT)p to be independent of pressure such that DS

(p) [m 1(qV/qT)p

0]p (pressure-dependent data are unavailable due to inaccurate low-pressure control coupled with excessive neutron acquisition times). Choosing T 4T1(p) is convenient,

because it avoids the forbidden possibility of T1(p) falling to T at

high pressure. Using the resulting values of DS (p) at T 236 K

(Supplementary Fig. 5b) to displace at this temperature the nite-pressure plots of DS1(T) (Supplementary Fig. 4a,b for heating and cooling, respectively), we have constructed nite-pressure plots of total entropy change DS(T,p) (Fig. 3a,b) specied with respect to the zero-pressure total entropy below the rst-order transition at 208 K. Whether the calorimetrically accessible value of DS1(T) was

measured on heating (for Fig. 3a) or cooling (for Fig. 3b), the resulting values of DS(208 K, p) match well with predictions of DS (p) that were obtained by setting T to 208 K (Supplementary

Fig. 5b), thus providing quantitative conrmation that the fall in |DS1(p)| arises due to the sign change in BC effects across the rst-order transition.

Our plot of DS(T,p) for data obtained on heating (Fig. 3a) permits us to establish isothermal BC effects on applying pressure (Fig. 3c), as heating and high pressure both tend to favour the high-temperature, high-pressure centrosymmetric phase. Similarly, our plot of DS(T,p) for data obtained on cooling (Fig. 3b)

permits us to establish isothermal BC effects on decreasing pressure (Fig. 3c), as cooling and low pressure both tend to favour the low-temperature ferrielectric phase. Near and above the value of Tc1(p 0) indicated, discrepancies in isothermal entropy

change on applying and removing pressure evidence irreversibility. By contrast, reversible BC effects are apparent a few degrees below Tc1(p 0) and at all lower temperatures studied, consistent

with no signicant thermal hysteresis in the rst-order transition (Fig. 2b). The largest reversible isothermal entropy change |DS|B605 J K 1 kg 1 arises at B219 K and exceeds the giant

BC effects reported for magnetic alloys (Table 1). The sharpness of the transition in DS(T) (Fig. 3a,b) permits this large entropy change to be achieved with a low value of |Dp| 0.1 GPa

(Fig. 3c), yielding giant BC strengths1 |DS|/|Dp| and |Q|/|Dp| (Table 1). Larger pressures extend reversible BC effects to lower temperatures, causing the large refrigerant capacity25 RC |DS| (FWHM of DS(T)) (Table 1) to increase (Fig. 4)

despite the small reduction in |DS1(p)| (Fig. 2d) and therefore |DS(p)|. For any given value of applied pressure, AS outperforms all of the magnetic alloys so well that comparable RC values would require much larger changes of pressure (Fig. 4).

Our largest value of |DS|B605 J K 1 kg 1, arising due to |Dp| 0.1 GPa at B219 K, corresponds to an adiabatic tempera

ture change |DT| (T/c)|DS|B81 K, using a specic heat

capacity c 1,70080 J K 1 kg 1 (Supplementary Fig. 6) that

is assumed to be independent of pressure as usual48. The resulting value of |DT|/|Dp| is seen to be the largest observed for giant BC materials (Table 1).

DiscussionOur observation of giant reversible BC effects in ferrielectric salts made from inexpensive abundant elements should inspire the study of BC effects in similar materials, most immediately bulk

a

Nitrogen

c

b

a Hydrogen

Oxygen

Sulphur

b

6

0

1 kg1 ) d Q /d T (kJ K 1 kg 1 )

0

Th1

Tc2

Th2

Tc1

6

c

120

| Sf|

| S1|

V (3 )S (J K

60

d

500

495

| Vf|

| V1|

Orthorhombic

Pna21

Orthorhombic

Pnam

490 120 160 200 240 280 320

T (K)

Figure 1 | Ferrielectric phase transition in AS at atmospheric pressure. (a) Unit cell of the high-temperature orthorhombic phase (Pnam). (b) Heat ow dQ/dT on cooling (blue) and heating (red) across the full transition. Baselines are black and dQ/dT40 denotes endothermic processes.

(c) Resulting entropy change DS(T) with respect to the low-temperature phase, revealing entropy changes for the rst-order transition (|DS1|) and the entire transition (|DSf|). (d) Unit-cell volume V(T) on heating, revealing volume changes for the rst-order transition (|DV1|) and the entire transition (|DVf|).

The discrepancy in values of T1(p) measured on heating and cooling (Fig. 2b) evidences a thermal hysteresis that is suppressed below the maximum value of pB0.3 GPa for our calorimeter. At even higher pressures, neutron diffraction data for deuterated AS reveal that dT1/dp remains constant (open symbols, Fig. 2b), while |DV1| falls (Fig. 2c), implying via the ClausiusClapeyron equation a pressure-induced suppression of |DS1|. (Figure 2c was obtained using temperature-dependent lattice parameters (Supplementary Fig. 1) calculated from neutron diffraction patterns (Supplementary Fig. 3).) This suppression was conrmed (Fig. 2d) from nite-pressure plots of |DS1(T)| (Supplementary Fig. 4a,b) obtained from the calorimetric data of Fig. 2a, as described in Methods.

BC effects in AS. The fall in |DS1(p)| arises because of additional changes in isothermal entropy DS (p) that are reversible, large

and change sign across the rst-order transition. Above T1(p),

these additional entropy changes correspond to conventional BC effects associated with elastic heat, which arises at all temperatures, except while driving transitions. Near and below T1(p),

these additional entropy changes correspond to inverse BC effects, because the continuous part of the full transition precludes elastic heat. The additional entropy changes would be challenging to detect via the calorimetry of Fig. 2, but they may be expressed1 away from the rst-order transition as DS(p1-p2) m 1R

p2

p1 (qV/qT)p0dp0, using the aforementioned Maxwell

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a c

10

5

0

5

10 200 220 240T (K) T (K)

470 0 100 200 300 400

500

490

480

0.25, 0.2, 0.15, 0.1, 0 GPa

1 kg1 )

Heating

Heating

0 GPa

3 )

0.45 GPa

1 GPa

V (3 )

|S 1|(J K

3

0

|V 1|(

T 1 (K) d Q /d T (kJ K

Cooling

Cooling

0 1

p (GPa)

b

d

1 kg1 )

220

200

60

30

Heating Cooling

0

0.1 0.2

180 0 0.2 0.4 0.6 0.8 1.0p (GPa) p (GPa)

0

Figure 2 | Ferrielectric phase transition in AS under applied pressure. (a) Heat ow dQ/dT on cooling and heating across the transition for different values of increasing pressure p, after baseline subtraction. (b) Transition temperature T1(p) for the rst-order transition, obtained below 0.3 GPa from the calorimetric data of a (closed symbols) and below B1.0 GPa from neutron diffraction of deuterated AS (open symbols). (c) Unit-cell volume V(T) obtained on cooling at selected pressures from neutron diffraction of deuterated AS (closed symbols), with inset showing |DV1(p)|. The X-ray diffraction data of

Fig. 1d are included to demonstrate consistency (open symbols). (d) Entropy change |DS1(p)| for the rst-order transition, obtained from the calorimetric data of a. Lines in b and d represent linear ts.

Table 2 | Properties of rst-order phase transitions in giant BC materials.

Giant BC material |DV1| |DV1|/V1 |dT1/dp| Ref.

3 % KGPa 1Ni49.26Mn36.08In14.66 0.2 0.4 18 4

Gd5Si2Ge2 3.4 0.5 32 5

LaFe11.33Co0.47Si1.2 18 1.2 73 6

Fe49Rh51 0.3 1.2 (ref. 24) 54 7

Mn3GaN 0.6 1.2 (ref. 34) 65 8

AS 2.50.2 0.5 452 This work

AS, ammonium sulphate; BC, barocaloric; |dT /dp|, pressure-driven shift in transition temperature; |DV |, unit-cell volume change; |DV |/V , relative unit-cell volume change. For AS, we give the shift obtained over a wide pressure range on cooling.

a

c

60

60

p (GPa)

p (GPa)

60

30

0

30

60

S(J K1 kg1 )

S(J K1 kg1 )

0.10

0.10

30

30

T+

T+

0.15

0.15

p

0.20

0.20

0

0.25

0.25

0

0

200 220 240

0.00

0.00

b

S(J K1 kg1 )

T p (GPa)

c1

0.10

0.15

0.20

0.25

0

p

200 220 240

T (K)

T (K)

Figure 3 | Giant inverse BC effects in AS. (a,b) Entropy change DS(T,p) with respect to S(T 208 K, p 0) (black dot), deduced using additional entropy

change DS(p) at T 236 K, to offset the pressure-dependent entropy change DS1(T) that arises on (a) heating and (b) cooling through the rst-order

transition. (c) Isothermal entropy change DS for increasing pressure (0-p) as deduced from a and for decreasing pressure (p-0) as deduced from b. Reversibility is apparent up to a few degrees below Tc1(p 0)B223.5 K.

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(NH4)2SO4

RC (J kg1 )

600

400

00 0.1 0.2 0.3

200 Mn3GaN

|p| (GPa)

LaFe11.33Co0.47Si1.2

Fe49Rh51

Ni49.26Mn36.08In14.66

Gd5Si2Ge2

Figure 4 | Refrigerant capacity RC for giant BC materials. For the materials in Table 1, we compare values of RC |DS| (FWHM of DS(T))

for selected pressure changes of magnitude |Dp| |p patm|B|p|, using DS(T) for BC cooling, and constraining T to ensure reversibility (except for

LaFe11.33Co0.47Si1.2, where only BC cooling data are available, and

Ni49.26Mn36.08In14.66, where only BC heating data are available). Solid lines

represent linear ts.

ferroelectrics that display large thermally driven entropy changes associated with displacive and order-disorder phase transitions. In future, it would be attractive to increase transition temperatures by chemical substitution26,27 or using an electric eld28. It would also be attractive to perform direct thermal measurements in the vicinity of room temperature, to conrm the large BC effects predicted using the Maxwell relation (Supplementary Fig. 5b), which are reversible over a wide range of temperatures.

Our ndings should stimulate the development of cooling devices based on BC materials, whose energy efciency29,30 is good with respect to magnetocaloric, electrocaloric and elastocaloric materials3. Unlike elastocaloric materials driven by uniaxial stress, there are no losses or mechanical breakdown associated with plastic deformation. Unlike magnetocaloric materials, there is no need to generate large magnetic elds at great expense. Unlike electrocaloric materials, there is no need to fabricate multilayer devices to exploit giant effects in lms31. Moreover, the phase transitions giving rise to large BC effects can be driven over a wide range of operating temperatures, unlike both magnetocaloric and electrocaloric materials.

Methods

Samples. Powders of AS (Z99.0%) and deuterated AS (Z99.0%) were purchased from Sigma-Aldrich. The typical grain size was o100 mm. AS was used for calorimetry and X-ray diffraction. Deuterated AS was used for neutron diffraction to reduce incoherent scattering.

Calorimetry at atmospheric pressure. Measurements of heat ow dQ/dT were performed at atmospheric pressure using a commercial TA Q2000 differential scanning calorimeter at 10 K min 1. Heat |Qf| |R

TT (dQ/dT0)dT0| and entropy

change |DSf| |R

TT (dQ/dT0)/T0dT0| across the full transition were obtained after subtracting baseline backgrounds32, with Ta chosen above (below) the transition on cooling (heating) and Tb chosen below (above) the transition on cooling (heating). The entropy change on partially driving the transition by heating to temperature T is DS(T) R

TT (dQ/dT0)/T0dT0. The entropy change on partially driving the transition by cooling to temperature T is DS(T) |DSf| R

TT (dQ/dT0)/T0dT0.Zero-eld heat capacity data were obtained using the TA Q2000 on cooling in the modulated differential scanning calorimetry mode, with the constant temperature method33. The temperature step was 1 K, the temperature modulation was 0.5 K and the period was 60 s.

Calorimetry under applied pressure. Measurements of heat ow dQ/dT at constant hydrostatic pressure were performed at 12 K min 1, using a differential thermal analyser constructed in-house, with chromel-alumel thermocouples, a CuBe Bridgman pressure cell operating up to 0.3 GPa and a circulating thermal bath (Lauda Proline RP 1290, 183473 K). AS was mixed with aninert peruorinated liquid and hermetically encapsulated by Sn. DW-Therm (Huber Kaltemaschinenbau GmbH) was used as pressure-transmitting medium.

For more details, see refs 47. Absolute measurements of temperature in the differential thermal analyser and differential scanning calorimeter differ by B1 K.

X-ray diffraction. High-resolution X-ray diffraction was performed in transmission using Cu Ka1 1.5406 radiation in an INEL diffractometer, with a curved

position-sensitive detector (CPS120), a 0.5-mm diameter Lindemann capillary and a 700 series Oxford Cryostream Cooler.

Neutron diffraction. High-resolution neutron diffraction was performed at the Paul Scherrer Institute, using the high-resolution powder diffractometer for thermal neutrons. Deuterated AS was mixed with NaCl powder to determine the applied pressure, and the mixture was encapsulated in a Pb clamp cell operating up to B1 GPa. Temperature was varied using a cryostat operating in 1.4320 K. The neutron wavelength was set to 1.88570 . Lattice parameters were determined by pattern matching using FullProf software.

Data availability. All relevant data are presented via this publication and Supplementary Information.

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Author contributions

X.M. conceived the study and led the project. X.M., Ll.M., A.P., P.Ll., M.B. and J.-Ll.T. planned the experiments. P.Ll., E.S.-T. and X.M. performed the calorimetric measurements at atmospheric pressure. P.Ll. and M.B. performed the calorimetric measurements under pressure. M.B. performed the X-ray diffraction measurements. P.Ll. and X.M. performed the neutron diffraction measurements, with support from V.P. X.M. wrote the manuscript with N.D.M. using substantive feedback from P.Ll., M.B., J.Ll.T., A.P. andLl.M. E.S.-T., S.C., W.L. and V.P. also contributed to the preparation of the manuscript.

27. Ohi, K., Osaka, J. & Uno, H. Ferroelectric phase transition in Rb2SO4-(NH4)2SO4 and Cs2SO4-(NH4)2SO4 mixed crystals. J. Phys. Soc. Jpn 44, 529536 (1978).

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Acknowledgements

This work was supported by the UK EPSRC grant EP/M003752/1, by CICyT (Spain) project numbers MAT2013-40590-P and FIS2011-24439, and by DGU (Catalonia) project 2014SGR00581. We thank D. Sheptyakov for assistance with high-pressure neutron diffraction. P.Ll. acknowledges support from SUR (DEC, Catalonia). E.S.-T. acknowledges support from AGAUR. W.L. acknowledges support from the Key Laboratory of Cryogenics (TIPC, CAS) and the NSFC. X.M. is grateful for support from the Spanish MEC Ramn y Cajal programme and the Royal Society.

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How to cite this article: Lloveras, P. et al. Giant barocaloric effects at low pressure in ferrielectric ammonium sulphate. Nat. Commun. 6:8801 doi: 10.1038/ncomms9801 (2015).

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