High-Pressure Study of the Elpasolite Perovskite La2NiMnO6.

Here we report a high-pressure investigation into the structural and magnetic properties of the double perovskite La2NiMnO6 using neutron scattering over a temperature range of 4.2-300 K at ambient pressure and over a temperature range of 120-1177 K up to a maximum pressure of 6.6 GPa. X-ray diffraction was also used up to a maximum pressure of 64 GPa, over a temperature range of 300-720 K. The sample was found to exist in a mixed rhombohedral/monoclinic symmetry at ambient conditions, the balance of which was found to be strongly temperature- and pressure-dependent. Alternating current magnetometry and X-ray absorption near-edge structure measurements were made at ambient pressure to characterize the sample, suggesting that the transition-metal sites exist in a mixed Ni3+/Mn3+ and Ni2+/Mn4+ state at ambient temperature and pressure. Analysis of the magnetic properties of the sample shows that the Curie temperature can be enhanced by ∼12 K with 2 GPa applied pressure, but it is highly stable at pressures beyond this. We report a pressure-volume-temperature equation of state for this material over this combined temperature and pressure range, with an ambient temperature bulk modulus of ∼179(8) GPa. The previously reported transition from monoclinic to rhombohedral symmetry upon heating to 700 K is seen to be encouraged with applied pressure, transforming fully by ∼1.5 GPa. Raman spectroscopy data were collected up to ∼8 GPa and show no clear changes or discontinuities over the reported phase transition to rhombohedral symmetry or any indication of further changes over the range considered. The ambient-pressure Grüneisen parameter γth was determined to be γth = 2.6 with a Debye temperature of 677 K. The individual modal parameters γj at ambient temperature were also determined from the high-pressure Raman data.

Structure and physical properties of SeCo1-x Mn x O3.

We describe the high-pressure (4 GPa) high-temperature (∼1100 K) synthesis of the solid solution series SeCo1-x Mn x O3 (0 < x < 1) using H2SeO4 and transition metal oxide mixtures sealed in Pt capsules. Neutron powder diffraction has been performed to determine progression of the structure across the solution. All samples crystallise with orthorhombic Pnma symmetry, and octahedral tilting is determined to increase with Mn content. SQUID magnetometry measurements were performed, and reveal that the Néel temperature shifts only by approximately 1 K over the series.

Rich Polymorphism of a Metal-Organic Framework in Pressure-Temperature Space.

We present an in situ powder X-ray diffraction study on the phase stability and polymorphism of the metal-organic framework ZIF-4, Zn(imidazolate)2, at simultaneous high pressure and high temperature, up to 8 GPa and 600 °C. The resulting pressure-temperature phase diagram reveals four, previously unknown, high-pressure-high-temperature ZIF phases. The crystal structures of two new phases-ZIF-4-cp-II and ZIF-hPT-II-were solved by powder diffraction methods. The total energy of ZIF-4-cp-II was evaluated using density functional theory calculations and was found to lie in between that of ZIF-4 and the most thermodynamically stable polymorph, ZIF- zni. ZIF-hPT-II was found to possess a doubly interpenetrated diamondoid topology and is isostructural with previously reported Cd(Imidazolate)2 and Hg(Imidazolate)2 phases. This phase exhibited extreme resistance to both temperature and pressure. The other two new phases could be assigned with a unit cell and space group, although their structures remain unknown. The pressure-temperature phase diagram of ZIF-4 is strikingly complicated when compared with that of the previously investigated, closely related ZIF-62 and demonstrates the ability to traverse complex energy landscapes of metal-organic systems using the combined application of pressure and temperature.

Pressure promoted low-temperature melting of metal-organic frameworks.

Metal-organic frameworks (MOFs) are microporous materials with huge potential for chemical processes. Structural collapse at high pressure, and transitions to liquid states at high temperature, have recently been observed in the zeolitic imidazolate framework (ZIF) family of MOFs. Here, we show that simultaneous high-pressure and high-temperature conditions result in complex behaviour in ZIF-62 and ZIF-4, with distinct high- and low-density amorphous phases occurring over different regions of the pressure-temperature phase diagram. In situ powder X-ray diffraction, Raman spectroscopy and optical microscopy reveal that the stability of the liquid MOF state expands substantially towards lower temperatures at intermediate, industrially achievable pressures and first-principles molecular dynamics show that softening of the framework coordination with pressure makes melting thermodynamically easier. Furthermore, the MOF glass formed by melt quenching the high-temperature liquid possesses permanent, accessible porosity. Our results thus imply a route to the synthesis of functional MOF glasses at low temperatures, avoiding decomposition on heating at ambient pressure.