Research Update: Tricritical point and large caloric effect in a hybrid organic-inorganic perovskite

The tricritical point (TCP) and the character of phase transitions are essential for understanding the properties of versatile materials. This has been demonstrated for a hybrid organic-inorganic perovskite, (CH3)2NH2Mg(HCOO)3, undergoing an order-disorder phase transition. Its tricritical point and the p-T phase diagram have been determined. Pressure gradually reduces the 1st-order character of the transition, which finally becomes continuous at 247 K/400 MPa. The structural origin of this tricritical point is associated with the compression of voids hosting the [(CH3)2NH2]+ cations. We show that above the tricritical point, in the continuous region, despite the absence of latent heat, a high transition entropy is retained. Owing to this property, the continuous region of this transition in the vicinity of TCP is well suited for solid-state cooling. The adiabatic temperature change of about 8.1 K at 206.5 MPa is one of the highest reported so far. Owing to the negative pressure coefficient dTc/dp = −45.9 K GPa−1, (CH3)2NH2Mg(HCOO)3 can be combined with another material transforming with dTc/dp > 0 into a highly efficient tandem system, where both the compression and decompression cycles contribute to the cooling process.