Pressure-Induced Two-Color Photoluminescence in MnF 2 at Room Temperature

A novel two-color photoluminescence (PL) is found in MnF2 at room temperature under high pressure. Contrary to low-temperature PL, PL at room temperature is unusual in transition-metal concentrated materials like MnF2, since the deexcitation process at room temperature is fully governed by energy transfer to nonradiative centers. We show that room-temperature PL in MnF2 originates from two distinct Mn2 emissions in the high-pressure cotunnite phase. The electronic structure and the excited-state dynamics are investigated by time-resolved emission and excitation spectroscopy at high pressure.

Transition-metal ions are often used as efficient photoluminescence (PL) centers in compounds like Ca; MgF 2 : Mn 2 [1-3], Al 2 MgO 4 : Co 2 [4], KMgF 3 : Ni 2 [5,6], MgF 2 : Co 2 [3,7], Al 2 O 3 : Cr 3 [8,9], Al 2 O 3 : Ti 3 [10,11], or YAlO 3 : Mn 3 [12,13].Hence, transition-metal-doped materials have received considerable attention for their capability as scintillators, optical sensors and memories, laser media, etc. [14,15].Despite the intense research aimed to improve PL in materials, the microscopic processes governing excited-state deexcitation yielding PL are still not understood fully.Therefore, prediction of whether a given transition-metalrelated material shows PL at ambient conditions or under pressure and temperature is in many cases uncertain.A puzzling behavior concerns the loss of PL with increasing transition-metal concentration, which is quenched in most concentrated materials at room temperature (RT).Transition-metal fluorides and oxides such as MnF 2 , CoF 2 , NiF 2 , Cr 2 O 3 , Ti 2 O 3 , Mn 2 O 3 illustrate this behavior [8,15,16].The unlikely nonradiative deexcitation observed in concentrated materials is explained in terms of exciton migration and subsequent transfer to nonradiative traps (exciton capture centers), which are eventually responsible for the absence of RT PL [8,16] (Fig. 1).Today, the knowledge of the excited-state dynamics in exchange coupled systems still remains a challenge [17].The activation energies associated with exciton migration and transfer to nonradiative traps depend on the crystal structure, and thus can be modified through structural modifications induced by changes of chemical composition or phase transformation prompted by temperature or pressure [18].Usual strategies aimed at finding out new methods for the control of exciton migration and trapping are mainly focused on the passivation of nonradiative traps through purification and doping with efficient PL centers [19,20].Nevertheless, structural changes yielding modifications of the transitionmetal local structure and exchange pathway yet remain unexplored.
MnF 2 [16] has been a model material for investigating exciton migration and trapping through selective doping with alkaline-earth-metal and rare-earth-metal atom traps acting as centers for exciton capture and selective emission [20 -22].As a result, the excited-state dynamics related to Mn 2 4 T 1 excitons and their capture by PL and nonradiative traps is well understood in terms of the MnF 2 rutile structure following the scheme of Fig. 1.The temperature dependence of the red emission band indicates that PL is quenched for T > 120 K in the purest MnF 2 single crystal [23] and T > 200 K for MnF 2 nanocrystals [24].
Here we report a novel procedure to obtain PL in MnF 2 at RT by applying high pressure.The idea consists of inducing structural transformations in MnF 2 , as long as (i) they provide more efficient PL sites for Mn 2 [1,2], and (ii) they reduce exciton migration and transfer through changes in the Mn-F-Mn exchange pathway [8].MnF 2 exhibits at RT an interesting phase transition sequence: it transforms with increasing pressure from rutile (TiO 2 ) to fluorite (CaF 2 ), cotunnite (-PbCl 2 ) and scrutyinite (-PbO 2 ), each providing different Mn 2 coordination and Mn-F-Mn paths [25].The high-pressure phases fulfill structural requirements to reduce the nonradiative probability by multiphonon relaxation within Mn 2 , and also to inhibit exciton migration within the lattice.Spectroscopy at high pressure has revealed that radiative and nonradiative processes governing PL in Mn 2 -doped fluorite crystals (Ca 1ÿx Sr x F 2 ) mainly depend on the host site volume [1].As a result, we have induced PL at RT in the nonphotoluminescent Mn 2 -doped SrF 2 and BaF 2 by applying high pressure [2].
The PL properties of MnF 2 at high pressure are investigated to elucidate whether high-pressure phases enable PL at RT, and to identify the PL centers and their excited-state dynamics by means of time-resolved emission and excitation spectroscopy under pressure.The high-purity floatzone grown MnF 2 single crystals and spectroscopic setups employed here are described elsewhere [2,23,24,26].
The low-temperature MnF 2 PL at 2.12 eV (585 nm) corresponds to the 4 T 1 ! 6A 1 transition (Fig. 1) within the nearly O h -symmetry MnF 6 4ÿ unit of the rutile structure.As Fig. 2 shows, the red emission is unlike the green emission observed in both eightfold (O h ) and fourfold (T d ) coordinations in Mn 2 halides, which is explained by their weaker crystal field as compared to sixfold coordination (O h ) [1,27].The absorption (or excitation) spectrum associated with Mn 2 reflects its local symmetry and crystal field in fluorides with different coordination numbers (Fig. 2) [27].
At ambient pressure, the MnF 2 emission shifts towards lower energies with increasing temperature due to population of deeper Mn 2 centers, and is quenched for T > 120 K (Fig. 1).Although the nature of nonradiative centers remains unknown yet, Mn 3 traces (<1 ppm) can partially account for the PL quenching at RT in MnF 2 [23,28].The variation of the MnF 2 emission spectrum with pressure, which is shown in Fig. 3, indicates that PL remains quenched at RT up to 13 GPa.However, a new pressureinduced two-color PL appears at RT for pressures above 14 GPa in the cotunnite phase.The PL spectrum consists of two emission bands at 2.34 and 1.87 eV, which we associate with two different Mn 2 centers: a regular Mn 2 site (S) and a strongly perturbed Mn 2 site (D), respectively.The assignment is based on the variation of PL and its associated excitation spectra with pressure for different Mn 2 coordination (Fig. 2).Both emissions correspond to the same excitation spectrum, and thus indicate that red PL mainly occurs through energy transfer from nearby ninefold-coordinated Mn 2 in the high-pressure cotunnite phase.Actually, their excitation spectrum at 16.5 GPa consists of an unresolved band around 2.8 eV (450 nm), and a sharp excitation near 3.0 eV (410 nm).Except for wavelength limitations imposed by the excitation source, the spectrum resembles the one measured for MnF 8 6ÿ formed in Mn 2 -doped CaF 2 [1], whose associated green emission is located at 2.6 eV at ambient pressure.Interestingly, the emission and corresponding excitation spectra for Mn 2 -doped CaF 2 and MnF 2 in their highpressure cotunnite phase are similar, thus indicating that the green S-band emission in MnF 2 is characteristic of ninefold coordinated Mn 2 .
In the case of Mn 2 -doped CaF 2 , where no exciton transfer is permitted, this is the unique RT PL emission.Accordingly, we associate the red D-band emission (1.87 eV) observed in MnF 2 at RT and 16.5 GPa with axially perturbed Mn 2 centers, which become populated by energy transfer from regular nearby Mn 2 S centers (hereafter named RN): RN !D. Time-resolved spectros- copy performed in the S and D bands confirms the proposed model (Fig. 4).The intensity time dependence for the red emission indicates that the associated lifetime ( long D 0:9 ms) is shorter than the green PL lifetime ( S 5 ms) despite RN Mn 2 transfer to D centers.Although this behavior seems to be contradictory with the long-lived decay time of the green emission, it is consistent with an exciton dynamics involving RN !D excitation transfer and largely suppressed exciton migration within regular ninefold coordinated Mn 2 (S !S) in the cotunnite phase.In fact, the D band is the dominant feature of the time-resolved PL spectrum obtained a few microseconds after the excitation pulse, whereas the S band is enhanced if PL detection is delayed 2 ms [Fig.4(a)].The energy transfer involved in the red emission is revealed by the sharp increase (0:2 s) of the time-dependent emission at 1.6 eV before decay [Fig.4(b)].Furthermore, the D-Mn 2 PL exhibits two different decay processes, which confirm that the energytransfer S !RN !D mechanism governs the red PL.In the short-time regime, it decays with short D 2:5 s [Fig.4(b), inset], basically corresponding to the radiative lifetime of the axially perturbed Mn 2 centers (D).This unexpectedly short lifetime for Mn 2 , which usually takes values of several milliseconds for oxides and halides [1][2][3]8,29], is consistent with the increase of the electricdipole transition probability observed in highly noncentrosymmetric Mn 2 in comparison to more centrosymmetric Mn 2 centers.In the long-time regime, the D-emission decay ( long D 0:9 ms) is related to the deexcitation of D-Mn 2 centers, which are populated as a result of exciton diffusion from remote S-Mn 2 [30,31].
The lifetime for the S band at 16.5 GPa ( S 5 ms) decreases as compared with the measured lifetime for isolated MnF 9 7ÿ centers formed in Mn 2 -doped Sr; BaF 2 ( 13 ms) at the same pressure in the cotunnite phase [1,2].This fact reflects additional deexcitation channels for S-Mn 2 via S !RN !D and transfer to nonradiative centers.In our model, the rate equations describing the exciton dynamics for the Mn 2 sites are given by (1) where N D , N S , and N RN refer to the number of excited axially perturbed (D), regular (S), and regular nearby (RN) Mn 2 centers, respectively.The three coupled equations provide the time-dependent intensity for the two emission bands taking values of D short D 2:5 s and S 5 ms for lifetimes, and 5 s ÿ1 and 0:1 ms ÿ1 0:0001 s ÿ1 ( ) for S ! S and RN !D transfer rates, respectively [Fig.4(b)].0 is the Mn 2 lifetime in absence of migration; 0 13 ms in CaF 2 : Mn 2 in the cotunnite phase [1,2].ÿ1 K accounts for transfer to nonradiative centers and ft for the time-dependence of the population N RN due to diffusion-limited energy-transfer processes S !RN [31].The model also explains why through the different exchange coupling between regular S-S Mn 2 pairs and D-RN Mn 2 pairs.In fact, the energy-transfer rate between two coupled Mn 2 centers, named A and B, is given by the Fermi golden rule, H represents the exchange coupling Hamiltonian responsible for energy transfer between A and B (* stands   [32].This equation justifies not only why RN !D S !S, but also why strongly decreases on passing from the fluorite and rutile phases to the cotunnite phase in MnF 2 yielding RT PL.Except for the exchange interaction, the main difference between and arises from the big overlap associated with D-ground and RN-excited states, since both RN emission and D excitation, g emis RN E and g exc D E, appear in the same spectral range.For S-S, however, the overlap integral involves g emis S E and g exc S E and is therefore strongly reduced due to the large Stokes shift of 0.4 eV between S-emission and S-excitation bands (Fig. 2) [32].The reduction of S ! S in the cotunnite phase, which is crucial for the appearance of RT PL in MnF 2 , is associated with the decrease of the Mn-F-Mn exchange angle on passing from rutile (128 ) to fluorite (110 ) and cotunnite (90 ) phases [9,25,33].Pressure-induced phase transitions result in a reduction of the exchange interaction and hence the S ! S transfer rate at high pressure, thus supporting our present findings.
Although the origin of the axially perturbed centers is uncertain, oxygen-bound (or OH-bound) Mn 2 [23] is likely responsible for the D-band emission.The presence of one O 2ÿ (or OH ÿ ) ligand in MnOF 8 8ÿ splits the 4 T 1 excited state due to the axial crystal field, shifting the low-lying Mn 2 emitting state to lower energies with respect to the MnF 9 7ÿ S band.The energy difference of 0.47 eV between D and S agrees with expectations based on the axial crystal field produced by an oxygen ligand at Mn 2 , which leads to a splitting of the 4 T 1 level of about 0.5 eV [24,27].
It is worth noting that rutile and fluorite phases are not PL in MnF 2 at RT. Pressure-induced Mn 2 emission appears just above the fluorite-to-cotunnite phase transition at 13 GPa, and thus neither MnF 6 4ÿ nor MnF 8 6ÿ seems to be efficient for PL in concentrated materials in contrast to MnF 9 7ÿ .Upon pressure release, the Mn 2 PL disappears in the scrutyinite phase.This phase is known to enhance exciton migration and subsequent transfer to nonradiative traps after findings in milled MnF 2 [24].
In conclusion, high-pressure phases of MnF 2 are efficient to reduce exciton migration among Mn 2 yielding increased PL efficiency.Accordingly, a two-color PL was induced in MnF 2 at RT in the cotunnite phase above 13 GPa.Although PL is quenched upon pressure release in the scrutyinite phase for P < 13 GPa, the present findings highlight the role of high-pressure phases to obtain PL in concentrated materials at RT and provide new insights to improve photoluminescent materials.
Support from the Spanish MEC (No. MAT2005-00099) and the UC (Program I3) is acknowledged.

FIG. 2 (FIG. 1 (
FIG. 2 (color).(a)Tanabe-Sugano diagram for d5 ions showing the energy level diagram as a function of the O h crystal-field strength =B, where is the crystal-field splitting energy and B the interelectronic Racah parameter.The diagram explains the different excitation and emission spectra obtained for different Mn 2 coordination MnF 6 4ÿ (red) and MnF 8 6ÿ (green).(b) Variation of the photoluminescence (in gray) and corresponding excitation spectra of Mn 2 in different crystal phases and local structures.The spectra correspond to the optical absorption of MnF 2 at ambient conditions and its emission at T 10 K (asterisk); excitation and emission of CaF 2 : Mn 2 at ambient conditions; CaF 2 : Mn 2 at T 290 K and 10 GPa; and MnF 2 at 290 K and 16.5 GPa.The crystal cell and coordination polyhedron at the cation site are shown for each phase.Peak labeling corresponds to the O h symmetry notation.Note the similarity between the excitation spectra of MnF 2 at 16.5 GPa and CaF 2 : Mn 2 at 10 GPa in the cotunnite phase MnF 9 7ÿ .

FIG. 4 (
FIG. 4 (color).(a) Time-resolved emission spectra of MnF 2 at 300 K and 16.5 GPa.The emission spectra were obtained by photon-counting techniques measuring for 1 ms immediately after the excitation pulse (red) and 2 ms delayed after the excitation pulse (blue).(b) Time dependence of the PL intensity for the D band at 1.8 eV.The long-decay photoluminescence lifetime is 0.9 ms.The inset shows a magnification of It around 2 s after the laser pulse.The rise of the photoluminescence intensity for t > 0:1 s reveals transfer phenomena from regular nearby Mn 2 sites.The measured transfer rate and the associated short-decay lifetime of the D band are 5 s ÿ1 and 2:5 s, respectively.The S band shows a decay with an associated lifetime, S 5 ms.
FIG.3.Variation of the photoluminescence spectra of MnF 2 with pressure at room temperature in the 0-20 GPa range.The MnF 2 was excited into the Mn 2 excited states by pumping with a 407 nm laser beam.The two-color photoluminescence bands, D and S, at 1.87 eV (red) and 2.34 eV (green), respectively, correspond to the 4 T 1 ! 6A 1 (O h symmetry notation) transitions from two different Mn 2 centers of MnF 2 in the cotunnite phase.EdE is the density-of-states factor of the excitation and emission centers, respectively, ensuring energy conservation λ