Maria Barrio

Giant solid-state barocaloric effect in the Ni–Mn–In magnetic shape-memory alloy

The search for materials showing large caloric effects close to room temperature has become a challenge in modern materials physics and it is expected that such a class of materials will provide a way to renew present cooling devices that are based on the vapour compression of hazardous gases. Up to now, the most promising materials are giant magnetocaloric materials. The discovery of materials showing a giant magnetocaloric effect at temperatures close to ambient has opened up the possibility of using them for refrigeration. As caloric effects refer to the isothermal entropy change achieved by application of an external field, several caloric effects can take place on tuning different external parameters such as pressure and electric field. Indeed the occurrence of large electrocaloric and elastocaloric effects has recently been reported. Here we show that the application of a moderate hydrostatic pressure to a magnetic shape-memory alloy gives rise to a caloric effect with a magnitude that is comparable to the giant magnetocaloric effect reported in this class of materials. We anticipate that similar barocaloric effects will occur in many giant-magnetocaloric materials undergoing magnetostructural transitions involving a volume change.

Barocaloric effect in the magnetocaloric prototype Gd5Si2Ge2

We report on calorimetric measurements under hydrostatic pressure that enabled us to determine the barocaloric effect in Gd5Si2Ge2. The values for the entropy change for moderate pressures compare favourably to those corresponding to the magnetocaloric effect in this compound. Entropy data are complemented with direct measurements of the adiabatic pressure-induced temperature change.

Barocaloric and magnetocaloric effects in Fe49Rh51

We report on calorimetry under applied hydrostatic pressure and magnetic field at the antiferromagnetic-ferromagnetic (AFM/FM) transition of Fe49Rh51. Results demonstrate the existence of a giant barocaloric effect in this alloy, a functional property that adds to the magnetocaloric and elastocaloric effects previously reported for this alloy. All caloric effects originate from the AFM/FM transition which encompasses changes in volume, magnetization, and entropy. The strong sensitivity of the transition temperatures to both hydrostatic pressure and magnetic field confers to this alloy outstanding values for the barocaloric and magnetocaloric strengths (|?S|/?p ~ 12 J kg-1K-1kbar-1 and |?S|/µ0?H~ 12 J kg-1K-1T-1). Both barocaloric and magnetocaloric effects have been found to be reproducible upon pressure and magnetic field cycling. Such a good reproducibility and the large caloric strengths make Fe-Rh alloys particularly appealing for solid-state cooling technologies at weak external stimuli.

A new hexagonal phase of fullerene C60

Abstract A new phase of fullerene C 60 with a simple hexagonal unit cell with 6/mmm symmetry was grown by slowly evaporating solutions of C 60 in dichloromethane. X-ray measurements reveal that the c / a ratio is 1.616 at 298 K and increases as temperature decreases. At low temperature, it seems to extrapolate to 1.633, the ideal ratio for a close-packed hexagonal lattice. The unit cell volume which is higher than that of the usual cubic C 60 phases at high temperature decreases near 90 K. A structural model is proposed according to which a 3-fold molecular axis is parallel to the 6-fold crystallographic z -axis. This suggests that the molecules could reorient around this axis at high temperature.

Binary mixtures of nCB and nOCB liquid crystals. Two experimental evidences for a smectic A–nematic tricritical point

Modulated differential scanning calorimetry (MDSC) has been used to obtain specific heat measurements on octyloxycyanobiphenyl (8OCB), octylcyanobiphenyl (8CB) and nonyloxycyanobiphenyl (9OCB) liquid crystals and on several binary mixtures of 8OCB + 9OCB and 8CB + 9OCB. The order of the mesophase transitions, smectic A(SmA) to nematic(N) and N to isotropic (I) on pure components and on binary mixtures has been studied and, in addition, both binary mesophase diagrams have been built. For each set of mixtures, there exists a tricritical point (TCP) composition at the SmA–N transition. The TCP compositions have been found to be X9OCB = 0.63 for the system 8OCB + 9OCB and X9OCB = 0.42 for the system 8CB + 9OCB. In addition, the specific heat critical exponents (α) through second order SmA to N transition have been obtained between 8OCB(or 8CB) and the TCP composition for the two sets of binary mixtures. When these α-values were plotted against the normalised nematic range, a common and uniform crossover trend was found. Indeed, this fact suggests a uniform behaviour in terms of the McMillan ratio for the nCB and nOCB series of liquid crystals. Moreover, for these compounds there exists a common value of (TAN/TNI)TCP of 0.99 and also a value of (TAN/TNI)3D-XY of about 0.96.

Reversible adiabatic temperature changes at the magnetocaloric and barocaloric effects in Fe49Rh51

We report on the adiabatic temperature changes (ΔT) associated with the magnetocaloric and barocaloric effects in a Fe49Rh51 alloy. For the magnetocaloric effect, data derived from entropy curves are compared to direct thermometry measurements. The agreement between the two sets of data provides support to the estimation of ΔT for the barocaloric effect, which are indirectly determined from entropy curves. Large ΔT values are obtained at relatively low values of magnetic field (2 T) and hydrostatic pressure (2.5 kbar). It is also shown that both magnetocaloric and barocaloric effects exhibit good reproducibility upon magnetic field and hydrostatic pressure cycling, over a considerable temperature range.

Benzocaine polymorphism: Pressure-temperature phase diagram involving forms II and III

Understanding the phase behavior of an active pharmaceutical ingredient in a drug formulation is required to avoid the occurrence of sudden phase changes resulting in decrease of bioavailability in a marketed product. Benzocaine is known to possess three crystalline polymorphs, but their stability hierarchy has so far not been determined. A topological method and direct calorimetric measurements under pressure have been used to construct the topological pressure-temperature diagram of the phase relationships between the solid phases II and III, the liquid, and the vapor phase. In the process, the transition temperature between solid phases III and II and its enthalpy change have been determined. Solid phase II, which has the highest melting point, is the more stable phase under ambient conditions in this phase diagram. Surprisingly, solid phase I has not been observed during the study, even though the scarce literature data on its thermal behavior appear to indicate that it might be the most stable one of the three solid phases.

Enantiomer resolution by pressure increase: inferences from experimental and topological results for the binary enantiomer system (R)- and (S)-mandelic acid.

In pharmacy, racemic compounds are often problematic, because generally only one of the enantiomers possesses therapeutic activity and it is often difficult to separate them. Even though this problem is likely as old as the pharmaceutical industry, one thermodynamically obvious way of separating racemic crystals has never been studied experimentally, which is by using pressure. Data have been obtained on the equilibria of the (R)- and (S)-mandelic acid system as a function of pressure and temperature. With the use of thermodynamic arguments including the Clapeyron, Schröder, and Prigogine-Defay equations, it has been demonstrated that the conglomerate (crystals of separated enantiomers) becomes more stable than the racemic compound at approximately 0.64 GPa and 460 K. Even though this pressure is still higher than at the bottom of the Mariana Trench, there are no technical obstacles to produce such conditions, making pressure a viable option for separating enantiomers.

Dimorphism of the prodrug l‐tyrosine ethyl ester: Pressure–temperature state diagram and crystal structure of phase II

Polymorphism is important in the field of solid-state behavior of drug molecules because of the continuous drive for complete control over drug properties. By comparing different structures of a series of L-tyrosine alkyl esters, it became apparent that the ethyl ester possesses dimorphism. Its structure was determined by powder diffraction and verified by density functional theory calculations; it is orthorhombic, P2(1) 2(1) 2(1) with a = 12.8679(8) Å, b = 14.7345(7) Å, c = 5.8333 (4) Å, V = 1106.01(11) Å, and Z = 4. The density of phase II is in line with other tyrosine alkyl esters and its conformation is similar to that of l-tyrosine methyl ester. The hydrogen bonds exhibit similar geometries for phase I and phase II, but the H-bonds in phase I are stronger. The solid II-solid I transition temperature is heating-rate dependent; it levels off at heating rates below 0.5 K min(-1), leading to a transition temperature of 306 ± 4 K. Application of the Clapeyron equation in combination with calorimetric and X-ray data has led to a topological diagram providing the relative stabilities of the two solid phases as a function of pressure and temperature; phase II is stable under ambient conditions.

Two-component systems of isomorphous orientationally disordered crystals. Part 1: Packing of the mixed crystals

Five experimental two-component phase diagrams between orientationally disordered crystals (ODICs) have been established from the low-temperature phase to the liquid state using thermal analysis and X-ray powder diffraction techniques. The high-temperature orientationally disordered phases for the pure components, which belong to the series (CH 3 ) 4–n1 C(CH 2 OH) n1 (n 1 =1,2,3) and (NO 2 )(CH 3 ) 3–n2 C(CH 2 OH) n2 (n 2 =0,1) are all face centered cubic. Continuous disordered mixed crystals in the whole range of concentration have been found, which indicates the existence of an isomorphism relationship. The intermolecular interactions in the ODIC state of these systems and other related two-component systems are discussed using the evolution of the packing coefficient as a function of the composition.