$\mathrm{NH}\cdots \mathrm{N}$ hydrogen bonded crystals of $[{\mathrm{C}}_6{\mathrm{H}}_7{\mathrm{N}}_2{]}^{+}̇{\mathrm{ClO}}_4^{-}$ and $[{\mathrm{C}}_6{\mathrm{H}}_7{\mathrm{N}}_2{]}^{+}̇{\mathrm{BF}}_4^{-}$ undergo ferroelectric phase transitions at 377.0(5) and 378.1(5) K, respectively. Owing to particularly simple structure, built of linear $\mathrm{NH}\cdots \mathrm{N}$ bonded aggregates, the spontaneous polarization of the crystals can be straightforwardly calculated from ionic displacements. The $\mathrm{NH}\cdots \mathrm{N}$ bonded structures have been compared with transformations of analogous bistable $\mathrm{OH}\cdots \mathrm{O}$ hydrogen bonds in ${\mathrm{H}}_2\mathrm{O}$ ices and ${\mathrm{KH}}_2{\mathrm{PO}}_4$-type ferroelectrics.

Our single-crystal X-ray diffraction study of methylammonium lead triiodide, MAPbI₃, provides the first comprehensive structural information on the tetragonal phase II in the pressure range to 0.35 GPa, on the cubic phase IV stable between 0.35 and 2.5 GPa, and on the isostructural cubic phase V observed above 2.5 GPa, which undergoes a gradual amorphization. The optical absorption study confirms that up to 0.35 GPa, the absorption edge of MAPbI₃ is red-shifted, allowing an extension of spectral absorption. The transitions to phases IV and V are associated with the abrupt blue shifts of the absorption edge. The strong increase of the energy gap in phase V result in a spectacular color change of the crystal from black to red around 3.5 GPa. The optical changes have been correlated with the pressure-induced strain of the MAPbI₃ inorganic framework and its frustration, triggered by methylammonium cations trapped at random orientations in the squeezed voids.

A new $\mathrm{N}\mathrm{H}\mathrm{\cdots }\mathrm{N}$ hydrogen-bonded ferroelectric crystal of $[{\mathrm{C}}_{\mathrm{6}}{\mathrm{H}}_{\mathrm{12}}{\mathrm{N}}_{\mathrm{2}}\mathrm{H}{\mathrm{]}}^{\mathrm{+}}\mathrm{R}\mathrm{e}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{-}}$ ($\mathrm{d}\mathrm{a}\mathrm{b}\mathrm{c}\mathrm{o}\mathrm{H}\mathrm{R}\mathrm{e}{\mathrm{O}}_{\mathrm{4}}$) exhibits exceptional dielectric properties that result from the unique structure where all the bistable $\mathrm{N}\mathrm{H}\mathrm{\cdots }\mathrm{N}$ hydrogen bonds are parallel and directed exactly in the same sense. Consequently, the main structural origin of the spontaneous polarization of the crystal is the identical orientation of the asymmetric $\mathrm{N}{\mathrm{H}}^{\mathrm{+}}\mathrm{\cdots }\mathrm{N}$ hydrogen bonds along [001]. This first observation of a ferroelectric with parallel arrangement of the $\mathrm{N}{\mathrm{H}}^{\mathrm{+}}\mathrm{\cdots }\mathrm{N}$ bonded aggregates, gives temperature-independent and the highest spontaneous polarization ever reported for an organic or water-soluble substance.

Photomagnetic compounds are usually achieved by assembling preorganized individual molecules into rationally designed molecular architectures via the bottom-up approach. Here we show that a magnetic response to light can also be enforced in a nonphotomagnetic compound by applying mechanical stress. The nonphotomagnetic cyano-bridged Fe^II–Nb^IV coordination polymer {[Fe^II(pyrazole)₄]₂[Nb^IV(CN)₈]·4H₂O}_n (FeNb) has been subjected to high-pressure structural, magnetic and photomagnetic studies at low temperature, which revealed a wide spectrum of pressure-related functionalities including the light-induced magnetization. The multifunctionality of FeNb is compared with a simple structural and magnetic pressure response of its analog {[Mn^II(pyrazole)₄]₂[Nb^IV(CN)₈]·4H₂O}_n (MnNb). The FeNb coordination polymer is the first pressure-induced spin-crossover photomagnet.

Since the late 1950's, high-pressure structural studies have become increasingly frequent, following the inception of opposed-anvil cells, development of efficient diffractometric equipment (brighter radiation sources both in laboratories and in synchrotron facilities, highly efficient area detectors) and procedures (for crystal mounting, centring, pressure calibration, collecting and correcting data). Consequently, during the last decades, high-pressure crystallography has evolved into a powerful technique which can be routinely applied in laboratories and dedicated synchrotron and neutron facilities. The variation of pressure adds a new thermodynamic dimension to crystal-structure analyses, and extends the understanding of the solid state and materials in general. New areas of thermodynamic exploration of phase diagrams, polymorphism, transformations between different phases and cohesion forces, structure–property relations, and a deeper understanding of matter at the atomic scale in general are accessible with the high-pressure techniques in hand. A brief history, guidelines and requirements for performing high-pressure structural studies are outlined.