Thanks to the development of experimental high-pressure techniques and methods for crystal-structure prediction based on quantum mechanics, in the past decade, numerous new compounds, mostly binary, with atypical compositions have been predicted, and some have been synthesized. Differing from conventional solid-state materials, many of these new compounds are comprised of various homonuclear chemical species, such as dimers, trimers, pentagonal and heptagonal rings, polymeric chains, atomic layers and 3D networks. Strikingly, it has been shown that pressure can alter the chemistry of an element by activating its (semi)core electrons, unoccupied orbitals and even the non-atom-centred quantum orbitals located on the interstitial sites, leading to many new surprising phenomena. This Review provides a summary of atypical compounds that result from the effects of high pressure on either the chemical bonds or the local orbitals. We describe various unusual chemical species and motifs, show how the chemical properties of the elements are altered under pressure and illustrate how compound formation is favoured even in situations in which chemical bonds are not formed. An extraordinary new picture of chemistry emerges as we piece together these unexpected high-pressure phenomena. In marked contrast to the previously held beliefs regarding the behaviour of solids under pressure, we are learning that the quantum-mechanical features of electrons, such as those that lead to the formation of directional bonds, inhomogeneous distributions of electrons and atoms, as well as variations in symmetry, might be magnified under pressure. We discuss the influence of these phenomena on future studies that will probe chemistry at higher pressures and explore more complex chemical compositions than those that have been studied to date.

由于基于量子力学的实验高压技术和用于晶体结构预测的方法的发展，在过去的十年中，已经预测了许多具有非典型组成的新化合物，大多数是二元化合物，并且已经合成了一些化合物。与传统的固态材料不同，许多新化合物由各种同核化学物质组成，例如二聚体，三聚体，五边形和七边形环，聚合物链，原子层和3D网络。令人惊讶的是，已显示出压力可以通过激活元素的（半）核心电子，未占据的轨道，甚至位于间隙位置的非原子中心的量子轨道来改变元素的化学性质，从而导致许多新的令人惊讶的现象。这篇综述总结了由于化学键或局部轨道受到高压作用而产生的非典型化合物。我们描述了各种不寻常的化学物种和图案，显示了元素在压力下如何发生化学变化，并阐明了即使在没有化学键形成的情况下化合物也更易于形成。当我们将这些意想不到的高压现象拼凑在一起时，出现了一种非同寻常的化学新图景。与先前关于固体在压力下的行为的信念形成鲜明对比的是，我们正在学习电子的量子力学特征，例如那些导致方向键形成，电子和原子的不均匀分布以及对称性变化，可能会在压力下放大。