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Abstract The high-pressure and high-temperature structural, mechanical, and dinamical stability of rhodium has been investigated via synchrotron X-ray diffraction using a resistively heated diamond anvil cell and density functional theory. The isothermal compression data have been fitted with a Rydberg-Vinet equation of state (EoS) with best-fitting parameters $$V_0$$ V 0 =55.046(16) Å $$^3$$ 3 , $$K_0$$ K 0 = 251(3) GPa, and $$K'_0$$ K 0 ′ = 5.7(2). The thermal equation of state has been determined based upon the data collected following four different isotherms and has been fitted to a Holland and Powell thermal equation-of-state model with $$\alpha _0=$$ α 0 = 3.36(7)x10 $$^{-5}$$ - 5 K $$^{-1}$$ - 1 . The measured equation of state and structural parameters have been compared to the results of ab initio simulations. The agreement between theory and experiments is generally quite good. The present results solve controversies between previous studies which reported values of the bulk modulus from 240 to 300 GPa.

Abstract Disorder–disorder phase transitions are rare in nature. Here, we present a comprehensive low-temperature experimental and theoretical study of the heat capacity and vibrational density of states of 1-fluoro-adamantane (C10H15F), an intriguing molecular crystal that presents a continuous disorder–disorder phase transition at T = 180 K and a low-temperature tetragonal phase that exhibits fractional fluorine occupancy. It is shown that fluorine occupancy disorder in the low-T phase of 1-fluoro-adamantane gives rise to the appearance of low-temperature glassy features in the corresponding specific heat (i.e., “boson peak” -BP-) and vibrational density of states. We identify the inflation of low-energy optical modes as the main responsible for the appearance of such glassy heat-capacity features and propose a straightforward correlation between the first localized optical mode and maximum BP temperature for disordered molecular crystals (either occupational or orientational). Thus, the present study provides new physical insights into the possible origins of the BP appearing in disordered materials and expands the set of molecular crystals in which “glassy-like” heat-capacity features have been observed.

Abstract Epitaxially grown $$\hbox {SrTiO}_{{3}}$$ SrTiO 3 (STO) thin films are material enablers for a number of critical energy-conversion and information-storage technologies like electrochemical electrode coatings, solid oxide fuel cells and random access memories. Oxygen vacancies ( $${\mathrm{V}_{{\mathrm{O}}}}$$ V O ), on the other hand, are key defects to understand and tailor many of the unique functionalities realized in oxide perovskite thin films. Here, we present a comprehensive and technically sound ab initio description of $${\mathrm{V}_{{\mathrm{O}}}}$$ V O in epitaxially strained (001) STO thin films. The novelty of our first-principles study lies in the incorporation of lattice thermal excitations on the formation energy and diffusion properties of $${\mathrm{V}_{{\mathrm{O}}}}$$ V O over wide epitaxial strain conditions ( $$-4 \le \eta \le +4$$ - 4 ≤ η ≤ + 4 %). We found that thermal lattice excitations are necessary to obtain a satisfactory agreement between first-principles calculations and the available experimental data for the formation energy of $${\mathrm{V}_{{\mathrm{O}}}}$$ V O . Furthermore, it is shown that thermal lattice excitations noticeably affect the energy barriers for oxygen ion diffusion, which strongly depend on $$\eta $$ η and are significantly reduced (increased) under tensile (compressive) strain. The present work demonstrates that for a realistic theoretical description of oxygen vacancies in STO thin films is necessary to consider lattice thermal excitations, thus going beyond standard zero-temperature ab initio approaches.

Abstract Traditional refrigeration technologies based on compression cycles of greenhouse gases pose serious threats to the environment and cannot be downscaled to electronic device dimensions. Solid-state cooling exploits the thermal response of caloric materials to changes in the applied external fields (i.e., magnetic, electric and/or mechanical stress) and represents a promising alternative to current refrigeration methods. However, most of the caloric materials known to date present relatively small adiabatic temperature changes ( $$|\Delta T| \sim 1$$ | Δ T | ∼ 1 to 10 K) and/or limiting irreversibility issues resulting from significant phase-transition hysteresis. Here, we predict by using molecular dynamics simulations the existence of colossal barocaloric effects induced by pressure (isothermal entropy changes of $$|\Delta S| \sim 100$$ | Δ S | ∼ 100 J K $$^{-1}$$ - 1 kg $$^{-1}$$ - 1 ) in the energy material Li $$_{2}$$ 2 B $$_{12}$$ 12 H $$_{12}$$ 12 . Specifically, we estimate $$|\Delta S| = 367$$ | Δ S | = 367 J K $$^{-1}$$ - 1 kg $$^{-1}$$ - 1 and $$|\Delta T| = 43$$ | Δ T | = 43 K for a small pressure shift of P = 0.1 GPa at $$T = 480$$ T = 480 K. The disclosed colossal barocaloric effects are originated by a fairly reversible order–disorder phase transformation involving coexistence of Li $$^{+}$$ + diffusion and (BH) $$_{12}^{-2}$$ 12 - 2 reorientational motion at high temperatures.