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Mixed anion compounds afford considerable compositional diversity and tunability of function. A challenging aspect of anion control of properties of periodic solids is to preserve the crystal lattice while substituting for different anions of widely varying size and hardness. Post-synthetic modification routes that place cations or anions in non-equilibrium configurations represent a promising means of decoupling composition and crystal structure and for accessing metastable solids; however, such methods remain relatively unexplored for anion placement. Here, we report the synthesis of LaOI nanocrystals by a non-hydrolytic sol-gel condensation reaction and their transformation into LaOBr, LaOCl, and LaOF nanocrystals along hard-soft-acid-base principles using post-synthetic metathesis reactions with ammonium halides. Anion displacement proceeds along halide planes preserving the tetragonal matlockite structure. Energy-variant X-ray excited optical luminesce signatures of alloyed Tb3+-ions serves as a sensitive quantum reporter of the preservation of the cation sublattice and hardening of the crystal structure upon anion replacement.

Ionic compounds wherein lanthanide cations are arrayed alongside anions adopt a wide range of crystal structures as a result of the variation of ionic radii and electronic configurations across the lanthanide series. Owing to the constricted nature of 4f orbitals, local coordination environments and structural preferences in such compounds are primarily dictated by electrostatic interactions and steric considerations with the primary driving force being the minimization of the crystal lattice energy. In this review, we examine a broad class of dianionic rare-earth compounds, lanthanide oxyhalides that present a multidimensional design space for tuning of functional properties by dint of the possibilities for extensive alloying on cationic and anionic sublattices; the considerable span of ionic radii and hardness across the lanthanide series and down the halide group, respectively; a large tolerance window for point defects such as oxide and halide vacancies; as well as multiple accessible polymorphs. In addition to their structural versatility, control over functional properties is accessible based on alteration of microstructure and surface chemistry. Synthetic strategies for accessing atomistic, nanoscale, and mesoscale control are discussed using illustrative examples placing particular emphasis on non-hydrolytic sol—gel processes that provide access to well-defined solid-solution nanocrystals. The reactivity and post-synthetic modification of these compounds is further delineated. Methods for defining color centers within these compounds, cooperativity of the optical response of the color centers with the host lattice (determined by overlap integrals specific to each structure type and the polarization and ligand field effects of different halide ions) and adjacent color centers (reliant on energy transfer schema), and their practical application in phosphors are further detailed. Mechanisms facilitating down- and up-conversion of energy absorbed from incident electromagnetic radiation or high-energy particles are delineated with particular emphasis on X-ray and electron-beam excitation. The accessible energy conversion mechanisms, efficacious radiative recombination channels, and opportunities for systematically tuning color through compositional modulation have led to the emergence of these materials as potential candidates for phosphors. These materials have potential applications in solid-state lighting, radiation detection, and as the active elements of imaging devices. The tolerance towards large concentrations of anion vacancies and the facile diffusivity of anions in these compounds further underpins the function of lanthanide oxyhalides as ion conductors, gas sensors, and heterogeneous catalysts. Increasing interest has focused on utilization of well-defined color centers for quantum information science and multiplexed sensing in biological systems. Such applications require additional control of the positioning of dopant atoms and understanding of their interactions with the host lattice and other dopant atoms.

The significant heat loss and severe thermal fluctuations inherent in steam-assisted gravity drainage (SAGD) and cyclic steam stimulation (CSS) impose considerable constraints on well cementing. In order to obtain better energy efficiency and mechanical robustness, there is considerable interest in the development of low-thermal-conductivity cement that can provide a combination of enhanced thermal insulation and mechanical resilience upon thermal cycling. However, the current palette of thermal cements is exceedingly sparse. In this article, we illustrate a method for decreasing the thermal conductivity of cement by inclusion of hydroxyethylcellulose-functionalized halloysite nanotubes. Halloysite/hydroxyethylcellulose inclusions offer an abundance of disparate interfaces and void space that can effectively scatter phonons, thereby bringing about a pronounced reduction of thermal conductivity. The microstructure of the nanocomposite cementitious matrix is strongly modified even as the compositional profile remains essentially unaltered. Modified cement nanocomposites incorporating halloysite nanotubes along with hydroxyethylcellulose in a 8:1 ratio with an overall loading of 2 wt.% exhibit the lowest measured thermal conductivity of 0.212 ± 0.003 W/m.K, which is substantially reduced from the thermal conductivity of unmodified cement (1.252 W/m.K). The ability to substantially decrease thermal conductivity without deleterious modification of mechanical properties through alteration of microstructure, inclusion of encapsulated void spaces, and introduction of multiple phonon-scattering interfaces suggests an entirely new approach to oilwell cementing based on the design of tailored nanocomposites.