Abstracts

Persistent luminescent (PersLum) materials are composed of an emitting center, host and supplementary/optional co-dopant which may act as an emitting centre. Research of these materials is still often focused on emission centre, host's properties are neglected while co-dopants’ role is based on speculation. Emission centres’ properties are well-known, mostly the same as for the conventional phosphors studied since 1960s. However, times change, and study of charge compensation (CC) arose from the cost of phosphors. The price of R³⁺ dopants is not anymore the decisive factor due to low amount and recycling of the dear phosphor part(s). Now the host must be as cheap as possible lest, at the same time, jeopardize the performance of the dopant.The R³⁺ dopants are efficient and stable luminescence sources and inexpensive at low concentrations. That said, even the Eu³⁺ content was decreased because of austerity measures. R^2+/3+ hosts are now excluded, cheaper phosphors are based on solid solutions (SS) between M²⁺ (Ca²⁺, Sr²⁺, Ba²⁺) hosts and R^2+/3+ dopants. This evolution requires new emphasis to Materials Science/Engineering. The basic rule on compounds’ neutrality - presently forgotten - must be rigorously respected. The size and charge constrains based on Vegard’s rules – including structures of solid solutions' end members - must be reconsidered. Formation of compounds must be studied in detail to account for their Lewis acid-base behavior - valid in solid state, as well, not only in gas phase or solutions. Brute breaking bonds to achieve compound’s neutrality is not any more a reply – vacancies are forbidden by Thermodynamics. Instead, Crystallography must find means to include, not exclude ions in the host with enough space available.Trivial examples of charge compensation (CC) include cancelling excess positive charge due to doping with inclusion of an anion (F^-), or two as a divalent ion (O^2-) creating/using R³⁺- O^2- - R³⁺ bridges. Smarter solutions are available to circumvent the powerful Vegard’s charge rule: Na⁺ + R³⁺ = Ca²⁺ + Ca²⁺. Changing the oxidation state(s) of host’s species (Ti^IV → Ti³⁺) will be routine in the future to compensate the excess/deficit charge due to doping. Let the (un)successful CC be voluntary or accidental, it creates traps for electrons & holes. Kröger-Vink notation can help understand the processes and species: Ti^′_Ti, Ti^•_Ti, and Ti^×_Ti are the electron (e^-) or hole (h⁺) traps for the Ti²⁺, Ti^IV and Ti³⁺ species, respectively, in regular Ti³⁺ site, to name a few.Finally, after identification of traps and their properties, the ultimate conundrum remains: How to obtain trap depths? Bond valence model (BVM) calculations based on structural data yield the trap depths and information on structural stability Trap energies which matching thermoluminescence curves are obtained easily. Full-scale design of PersLum materials is now possible!