Role of high pressure for understanding luminescent phenomena Rafael Valiente a,n , Carlos Renero-Lecuna a , Fernando Rodríguez b , Jesús González b a Departmento Física Aplicada, Facultad de Ciencias, Universidad de Cantabria, 39005 Santander, Spain b DCITIMAC, Facultad de Ciencias, Universidad de Cantabria, 39005 Santander, Spain article info Article history: Received 16 October 2014 Received in revised form 19 November 2014 Accepted 26 November 2014 Keywords: High pressure Luminescescence Upconversion Phase transitions Excited state crossover abstract High-pressure techniques make possible to investigate the changes in the electronic properties induced by modifications of the local or crystal structure of the material without changing the chemical composition. The different sensitivity of excited states to crystal-field strength enables energy tuning of the states, which are eventually responsible for the optical properties. It is possible to induce resonance between levels producing exotic effects like upconversion as well as excited state crossover or high-spin to low-spin transitions. Herein, we present selected examples of high-pressure effect for understanding luminescent phenomena or even inducing new ones. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Spectroscopy deals with the correlation between crystal and local structure, electronic structure and optical properties, upon variation of thermodynamic variables like temperature, pressure, magnetic and electric fields or all at the same time. The advance of high- pressure techniques in spectroscopy labs is related with the design of the diamond anvil cell (DAC). Diamonds present a broad transpar- ency range allowing experiments in an ample wavelength range from short x-ray at synchrotron facilities to far infrared spectroscopy at home labs. High-pressure allows us to study the effect of volume changes (about 10%) without modification of the chemical composition. So, it is possible to explore pressure effects on different material properties: it induces energy level shifts, structural modifications (local or bulk) or phase transitions and their consequences on electronic properties, changes of color (piezochromism), high-spin to low-spin transitions, and excited state crossover. Making use of the different sensitivities of the state to crystal field strength (electron-ion coupling), the applica- tion of an external pressure provides the way to tune the energy of the states responsible of the optical properties (d orbitals in transition metal ions and f in lanthanides) to induce resonance between levels yielding upconversion phenomena, i.e., conversion of low energy radiation in higher energy photons from the emitting ions. Besides, pressure modifies all those processes depending on the interatomic distances like exchange interaction, energy transfer, and cross relaxation, among others. Finally, it allows us to perform in-situ studies of optical properties of new crystallographic phases, which can solely be induced at high pressures; inaccessible at ambient conditions. We illustrate, with selected examples, the effect of pressure on optical properties of materials, which have recently been explored by our group. 2. Investigation of the excited state geometry compared to ground state Ab-initio calculations have predicted a shortening of the Ce–X bond length (X ¼ F, Cl, Br) upon 4f ( 2 F 5/2 )-5d ( 2 T 2g ) excitation, whereas 4f ( 2 F 5/2 )-5d ( 2 E g ) would show the opposite behavior. This prediction has been proved by high-pressure optical spec- troscopy. Ce 3 þ shows parity-allowed electric-dipole f-d transitions in the UV–vis spectral range. In O h symmetry, the Ce 3 þ 4f-5d transitions are mainly split into ( 2 F 5/2 þ 2 F 7/2 )-( 2 E g þ 2 T 2g ). The absorption 4f ( 2 F 5/2 )-5d ( 2 T 2g ) and emissions ( 2 T 2g - 2 F 5/2 þ 2 F 7/2 ) of Cs 2 NaLuCl 6 : Ce 3 þ as a function of pressure are shown in Fig. 1. The 4f 1 ( 2 F 5/2 )-5d 1 ( 2 T 2g ) band shifts to lower energies with pressure showing a linear dependence (Fig. 1) as E exp abs (f-d) ¼ 29100–530P, with P and E abs expressed in GPa and cm –1 , respec- tively. Similarly, the emission bands behave linearly with pressure with shift rates of –416 710 cm –1 /GPa and –415 710 cm –1 /GPa (Fig. 6) [1]. Within a single configurational coordinate model, on the assump- tion that the mode A 1g is mainly responsible of the electron–phonon Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence http://dx.doi.org/10.1016/j.jlumin.2014.11.043 0022-2313/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ34 94220 1846; fax: þ34 942201402. E-mail address: valientr@unican.es (R. Valiente). Please cite this article as: R. Valiente, et al., J. Lumin. (2014), http://dx.doi.org/10.1016/j.jlumin.2014.11.043i Journal of Luminescence ∎ (∎∎∎∎) ∎∎∎–∎∎∎