Marta N. Sanz-Ortiz

Photoluminescence properties of Jahn-Teller transition-metal ions

This work investigates the influence of electron-phonon coupling associated with E tensor product of e and T tensor product of e Jahn-Teller (JT) effect in different transition-metal (TM) ions on de-excitation phenomena through nonradiative multiphonon relaxation, i.e., photoluminescence (PL) quenching. We developed a configurational curve model which is able to predict from the absorption spectrum whether a given JT-TM ion is PL or quenched. The prediction is made on the basis of an adapted Dexter-Klick-Russell parameter for JT systems, defined in terms of spectroscopic parameters through Lambda(JT)=alphaDelta(e)(abs)/E(abs), where Delta(e)(abs) refers to the splitting of the parent octahedral E(g) states by the JT distortion in E tensor product of e (alpha=3/4) or T tensor product of e (alpha=1/4), and E(abs) is the energy of the first absorption band involving electronic transition between E(g) and T(2g). We show that PL in any JT-TM ion occurs whenever Lambda(JT)<0.1 or is quenched if Lambda(JT)>0.2. This result is noteworthy since it allows us to establish structural requirements for the JT-TM ion and the host crystal to be PL. Although PL properties of materials containing TM ions depend on a variety of structural factors such as the electronic configuration, the site symmetry, and the crystal field produced by neighboring atoms, the present model achieves this goal through a simple spectroscopic parameter: Lambda(JT). In this work we correlated the PL properties of different sixfold-coordinated JT systems such as Ti(3+), Cu(2+), Mn(3+), Cr(2+), Fe(2+), Co(3+), and Ni(3+) in halides and oxides with Lambda(JT) obtained from their respective absorption spectra. From this analysis we conclude that depending on the nature of the JT coupling and its strength, PL is either strongly favored or quenched in T tensor product of e while it is mostly quenched in E tensor product of e systems due to the larger JT distortion.

Optical and magnetic characterisation of Co3+ and Ni3+ in LaAlO3: interplay between the spin state and Jahn–Teller effect

The coordination, the electronic structures and the spin of the ground state of Ni(3+) (3d(7)) and Co(3+) (3d(6)) introduced as impurities in LaAlO(3) are investigated through optical spectroscopy and magnetic measurements. The unusual trivalent valence state in both transition-metal ions was stabilised via a sol-gel process followed by high oxygen pressure treatments. We show that the crystal-field strength at the nearly O(h) transition-metal site in LaAlO(3) locates Ni(3+) and Co(3+) near the spin state crossover, yielding a low-spin ground state in both cases. We analyse how the interplay between the Jahn-Teller (JT) effect and the spin state affects the magnetic moment of the ion and its temperature dependence. The optical spectra reveal a JT effect associated with a low-spin ground state in Ni(3+) and with a thermally populated high-spin low-lying first excited state in Co(3+). The corresponding JT distortions are derived from structural correlations. We conclude that the JT effect is unable to stabilise the intermediate spin state in Co(3+). A low-spin ground state in thermal equilibrium with a high-spin low-lying first excited state is detected in diluted Co(3+)-doped LaAlO(3). These results are compared with those obtained in the parent pure compounds LaNiO(3) and LaCoO(3).

Synthesis under pressure and characterizations through optical spectroscopy of jahn-teller cations (LS Ni3+, is Co3+) as probes diluted in a perovskite matrix

The objective is to explore through optical spectroscopy and magnetic measurements the coordination and electronic structures of transition-metal ions introduced as impurities with unusual valence states in the oxide perovskite LaAlO3. The selected transition-metal ions Ni3+(3d7) and Co3+(3d6) are characterized by an electronic configuration likely leading to an orbital degenerate E state in Oh symmetry, and thus electron-lattice coupling due to the Jahn-Teller effect may induce low symmetry distortion around the impurity oxygen octahedron. We show that a sol-gel process followed by high oxygen pressure treatments yields stabilization of trivalent state in oxide perovskite. Information about the coordination, electronic structure and aggregation around the magnetic impurity was obtained from X-ray diffraction, FTIR and optical spectroscopy. Finally, evidence on the possible existence of intermediate spin state in Co3+ is under consideration.

Spin transition in Co3+ by optical absorption and time-resolved spectroscopy under pressure: an appraisal of the different spin states

This work investigates the electronic structure of CoF3 by optical absorption and its relationship with the ground-state spin and the Jahn–Teller (JT) effect exhibited by Co3+(d6) in octahedral coordination (CoF6)3−. The results are compared with other Co3+ fluorides and oxides, where Co3+ is high spin (HS) and low spin (LS), respectively. In CoF3 we detect an absorption band with a doublet structure at 1.89 eV (657 nm) and 1.45 eV (855 nm) at 2.5 GPa, which is associated with the T ⊗ e JT effect on the 5T2 HS ground state. Absorption measurements under pressure have been carried out around the HS–LS (5T2 ↔1A1) spin crossover transition in the 0–20 GPa range. We show that the JT effect is stable in a wide pressure range. No sign of intermediate spin (3T1, 2 states) is observed in the explored pressure range.

Time-resolved spectroscopy in LiCaAlF6 doped with Cr3+: dynamical Jahn?Teller effect and thermal shifts associated with the 4T2 excited state

This work investigates the centre distribution of the Cr(3+) impurity, the dynamical Jahn-Teller effect in the first (4)T(2) excited state and the thermal shifts of the absorption and emission peaks in LiCaAlF(6):Cr(3+) by means of time-resolved emission spectroscopy. The electronic and vibrational fine structure observed in both the absorption and emission spectra at low temperature are assigned according to the vibrational modes of the internal (CrF(6))(3-) complex and the lattice modes. Zero-phonon lines associated with (4)T(2) --> (4)A(2) and (2)E --> (4)A(2) transitions were detected and assigned on the basis of available high pressure data in LiCaAlF(6):Cr(3+). We have identified the vibrational coupled modes responsible for the vibrational structure of the low temperature emission spectrum and the reduction of the zero-phonon line (ZPL) splitting caused by the dynamical Jahn-Teller effect in the (4)T(2) excited state (Huang-Rhys factor, S(e) = 0.92). In addition, from the temperature variation of the emission intensity I(T), transition energy E(T) and bandwidth H(T), we obtained the vibrational modes that are coupled to the emitting state. We have evaluated the two main contributions to the photoluminescence thermal shift through thermal expansion and high pressure measurements: the implicit contribution induced by changes of thermal population and the explicit contribution induced by thermal expansion effects--40% and 60% of the total shift, respectively.

Pressure-induced luminescence from broadband to narrow-line emission in Cr3+-doped LiCaAlF6 at room temperature

This work investigates pressure-induced excited-state-crossover phenomena in Cr3+-doped LiCaAlF6 at room temperature. The aim is to find the structural requirements yielding the change in the Cr3+ photoluminescence (PL) behaviour from a single broadband emission, which is located at 1.62 eV (765 nm) at ambient conditions, to a ruby-like narrow-line emission at 1.87 eV (662 nm). For this purpose, emission and excitation spectroscopy as well as lifetime measurements as a function of pressure were performed in the 0–35 GPa range. Interestingly, we have been able to transform the broadband Cr3+ PL at ambient pressure into a ruby-like emission at 28 GPa. This behaviour, together with the variations of the 2E and 4T2 excited-state energies, and the PL lifetime with pressure, can be explained on the basis of the electron–ion coupling associated with the 4T2 and 2E states.