J. MATER. CHEM., 1991, 1(3), 437-440 Luminescence Properties of A,ReCI, Crystals Marco Bettinelli", Colin D. Flint*band Gianluigi Ingletto" a Dipartimento di Chimica lnorganica, Metallorganica e Analitica, Universita di Padova, Via Loredan 4, 35131 Padova, Italy Laser Laboratory, Department of Chemistrx Birkbeck College, Gordon House, 29 Gordon Square, London WClH OPP, UK lnstituto di Biochimica, Facolta di Medicina Veterinaria Universita di Parma,' Via del Taglio 43100 Parma, Italy At temperatures below 60 K A,ReCI, (A= K, Rb, Cs) crystals emit intense, well resolved luminescence in the 13 900-12 800 cm-' region under blue-green excitation. The emission is strongly temperature dependent and also depends somewhat on the excitation wavelength and crystal quality.It is here assigned as the vibronic structure associated with the r7(,T2J -+Ts(4A,g) transition of centrosymmetric and essentially octahedral ReCIg- ions at up to nine distinct defect sites. The compounds (NR,),ReCI, (R=CH,, C2H5) also luminesce in the same spectral region but more weakly than the alkali-metal salts. [N(n-C,H,),], ReCI, does not luminesce under these conditions. The nature of the defect sites and their excitation processes are discussed. Keywords: Luminescent material; Alkali-metal salt; Defect site The luminescence behaviour of the ReC1;- ion diluted into transparent hosts and K2PtC16 has been studied in some detail.'*2 Emission occurs from the r7(2T2g) and r8(2T1,) states to the ground state T8(,A2,) and any intermediate states.Although numerous vibrational and electronic spectral st~dies,~,~as well as magnetic,'^^ and calori- metric" measurements have been reported for the undiluted K2ReCl, crystals and this compound has been used as a model for a number of related theoretical studies, no lumi- nescence from this class of compounds has been reported. During careful measurements of the absorption spectra of K2ReC1, and Cs2ReC16'l at liquid-helium temperatures, we detected intense luminescence to low energy of the r8(4A2g)+r7(2T2,)electronic origin. In this paper we report a detailed analysis of the vibrational structure and temperature dependence of the emission from these crystals and also from the analogous Rb2ReC1, compound. Interionic coupling in these compounds is rather strong, as evidenced by the intensity of the co-operative absorption bands and the observation of magnon sidebands in absorption below the Nee1 temperature of K2ReC16." Energy migration through the lattice is efficient until the excitation becomes trapped at a defect site.In contrast no co-operative absorptions, magnon sidebands or trap emission is observed for K2ReF612.'3 and the vibrational structure in the absorption spectrum demonstrates a remark- ably complex vibronic behaviour. We observe rather weaker emission from (NR,)2ReC1, (R=CH3, C2H5); in this case the emission is from non-centrosymmetric ReCl; -traps, the weakness of the emission no doubt reflecting the contribution of the C-H vibrational motion to the non-radiative decay rate.[N(n-C4H9),],ReC1, does not emit significantly from the r7(2T2g) state. Structural, Magnetic, Electronic and Vibrational Data Cs2ReC16 and K2ReC16 are cubic at room temperature (space group Fm3m 0h5).9,14The Re4+ ions occupy sites of perfect Oh symmetry. K2ReCl, undergoes several structural phase transitions at low temperatures and becomes antiferromag- netic at 12 K.6 No structural or magnetic phase transitions have been reported for Cs2ReC1, in the 300-4K range but there is some evidence for crystal strain at low temperatures." No details about the crystal structure of Rb2ReCI6 are known, but it is reasonable to assume that it is isostructural with K2ReC16 and Cs2ReC16 at room temperature.[N( CH,),] ReC1, and [N( c2H5)4] ReC1, are cubic (Frn3rn from X-ray powder diffraction data, isostructural with K2ReC16)15 and monoclinic (C2/c from single-crystal data, isostructural with [N(C,H,),], SnCl,),', respectively. The ReC1;-complex ions show only very small deviations from a perfect Oh symmetry in the latter crystal. The reported Weiss constants derived from bulk magnetic susceptibility measurements are higher for K2ReC16 (88 K) than for Cs2ReC1, (45 K).5 It is possible to predict, on the basis of a crude molecular-field model, that the Nkel temperature for Cs2ReC1, will be significantly lower than 12 K. Tetraalkylam- monium hexachlororhenates have been reported to be mag- netically di1~te.I~ The electronic structure of the ReCl2- ion is well under- stood. The t;, strong-field configuration gives rise to the six states Tg(4A2,), T8(2T1,)(7643 cm- l); T8(2E2,) (8906 cm-'); r6(2T1,) (9344cm-'); T7(2T2g)(13 841 cm-') and (1 5 299 cm- ').The energies are given for the excited states for ReC1;- in K2PtC1, at 4 K.2 The corresponding absorption energies in undiluted A2ReC16 are closely similar." Experimental Good-quality crystals of K2ReC16 and Cs2ReC1, were grown as previously described. Rb2ReC16 crystals, also of high optical quality, were grown by the same method as was used for Cs2ReC1,. Large crystals of (NR,),ReC16 (R =CH3, C2H5, n-C,H9) could not be grown owing to their low solubility but these compounds were obtained as microcrystalline powders by the addition of excess of an aqueous solution containing (NR,)' to a solution of K2ReC1, in 2moldrnb3 aqueous HCl.Luminescence spectra were recorded using excitation from an argon laser and a 1 m monochromator as previously described.l8 For most measurements the laser power incident on the sample was <25 mW and since the absorbance of the crystals is low in the 454-514 nm region, no sample heating was detected. For the alkali-metal salts the intensity of the emission under these conditions was such that the mono- chromator slits were closed to their mechanical stop at less than 10 pm. The measured spectral band width of the mono- chromator under these conditions is less than 1 cm-'. Absorp- tion spectra, lifetimes and excitation spectra were measured as previously described." Results The luminescence spectrum of a crystal of Rb,ReC16 at 4 K using 457.9 nm excitation is shown in Fig.1. More than 20 features with half-height widths of ca. 10cm-' are readily resolved together with some broader features. The intensity of these features decreases rapidly as the temperature is raised, with the higher-energy bands showing the strongest tempera- ture dependence. The relative intensity of the lines also depends on the wavelength of line used for excitation. Careful examination of a large number of spectra measured under different conditions enables many of the more prominent lines to be divided into nine pairs where the relative intensity of the two lines is invariant with temperature and excitation wavelength.The separations between the members of these pairs are between 22 and 36 cm-' and the higher-energy member is, generally, 1.5-2 times more intense than the lower member. We propose that these pairs correspond to transitions occurring at nine distinct but closely related sites in the crystal; many of these pairs are identified in the figure. The features in the absorption spectrum of the A2ReC16 crystals in the region of the Ts(4A2g)+r7(2T2g) transition are similar to those in the absorption spectrum of the ReC1;- ion diluted into cubic crystal^,^." but less well resolved. They have a close mirror symmetry with the luminescence from the r7(2T2g)state of the diluted ReCl;-. From the absorption spectrum we estimate, using the ground-state values of v6 and v4 (131 and 165 cm-' respectively),' that the intrinsic elec- tronic origin of the Ts(4Az,)-+r7(2T2,) transition of Rb2ReC16 (unobserved) is close to 13 875 cm-'; this estimate is likely to be correct within a few wavenumbers since the vibrational wavenumbers of the internal motions are expected to be XI J.MATER. CHEM., 1991, VOL. I closely similar in the ground and excited electronic states as they are both derived from the t;, configuration. The luminescence of Rb2ReC16 does not closely resemble that of the ReC1;- ion, diluted into cubic crystals. The highest- energy pair is weak and observed only with 514.5 nm exci- tation at temperatures near 4 K. However the wavenumbers of this pair, 13 759 and 13 724 cm- ',are within a few wave- numbers of the expected position of the v6 and v4 vibronic origins of the intrinsic emission from the r7(,Tzo) state.A difficulty with assigning these bands to the intrinsic emission is that the band that would be assigned as vg is more intense than that due to v4, whereas the reverse is the case for the intrinsic absorption. This intensity distribution matches that of the defect emission to low energy (see below). We cannot rule out the possibility that the intrinsic emission makes at least some contribution to the intensity of the bands, but it seems more likely that the emission is due to a very shallow trap. The prominent features at 13 731 and 13 702 cm-' are assigned as v6 and v4 vibronic origins on an unobserved electronic origin, which is calculated to occur near 13 862 cm- using the ground-state vibrational frequencies. The difference between the wavenumber of this origin and that of the intrinsic origin is too large to be attributed to the sum of experimental error plus possible errors in the excited- state wavenumbers of the vibrational modes.It is likely, therefore, that this prominent emission arises from a shallow trap rather than the intrinsic level. As before, support for this view again comes from the relative vibronic intensities of v6, v4 and v3. Because the r7(2T,,)+r,(4A2g) transition is intra- configurational t;,-+t$, vibronic intensity distributions for the same species are expected to be similar. The v3 vibronic origin is expected to be near 13 555 cm- 'but is not unambigu- ously observed owing to the presence of other weak emissions near this wavenumber, but the intensity of this mode is clearly much lower than that of the other two vibronic origins.In the absorption spectrum v4 is somewhat more intense than v3 whereas v6 is weaker than either v4 or v3. This is similar to the intensity distribution for the r7(2Tz,)-r,(4A2,) emis-7 I I 1 II I I I I Fig. 1 Luminescence spectra of Rb,ReCl, at 4.0 K excited at (a)514.5, (b)457.9 nm. Note the changes in the abscissa scale. The v6, v4 vibronic origins (and vj where observable) on most of the traps are indicated by the bars J. MATER. CHEM., 1991, VOL. 1 sion of the dilute crystals containing ReCl;-.For the emission based on the 13 863 cm-' origin and indeed for all other emissions in the pure crystals, the intensity distribution is v6>v4 whereas v3 is much weaker and difficult to resolve from the other emission in the 13 550 cm-' region. The analysis of the remainder of the spectrum follows similarly; the seven further pairs of features identified to low energy are each readily assigned as v6, v4 vibronic origins of the r7+r8 luminescence transition derived from different traps, and study of the weaker features enables v3 vibronic origins to be identified for most of these. In three cases very weak progressions in v1 can be located based on the v6 and v4 vibronic origins. The resultant analysis (Table 1) accounts for the position and relative intensity of all observed spectral features. For most of the nine traps the relative intensity distribution (v6 >v4 > >v3) is similar, but different from that of the same transition in both the intrinsic absorption spec- trum and the emission from ReClg- in dilute crystals.Where bands are well separated v6 is significantly sharper than v4. All the vibrational frequencies are similar on the eight sites but the small differences appear to be outside experimental error. In no case is the pure electronic origin observed, nor are any splittings of the vibronic origins detected. Thermal cycling of the crystal to 80 K followed by cooling to 4 K resulted in a marked broadening of all spectral features so that the weaker traps could not be resolved and on examination at room temperature the crystal was found to be shattered.Presumably there is a structural phase transition in Rb2ReCl6 in the 4-80 K region analogous to those observed in K2ReC16 at 110 and 113 K.4 A second crystal gave only the broadened spectrum suggesting that it underwent the phase transition during the initial cooling. The luminescence behaviour of Cs2 ReCI, is closely similar to that of the Rb salt, although somewhat less well resolved and with corresponding features at 50-75 cm- ' lower energy. The v6, v4, v3 intensity distributions of the trap emission are comparable to those of the Rb salt although v6 is even more prominent. For K2ReCl6 all features are very much broader so that only the most prominent trap gave a clearly defined maximum but the luminescence behaviour was consistent with that of the Rb salt.Somewhat unexpectedly, both (NR4)2ReC16 (R =CH3, C2H5) luminesce at 4K, although more weakly than the alkali-metal salts. The spectra are different in that the pure electronic origins associated with the various traps are the most intense features, although relatively weak v6 and v4 vibronic origins could be detected. No emission from [N(n- C4H9)4] 2 ReC1, was observed. Attempts were made to elucidate the excitation process by measuring the excitation spectrum and decay curves of the emission. No sharp features were observed in the excitation spectrum between 520 and 470 nm (19 230 and 21 276 cm-') Table 1 Vibrational analysis for the r,(2T2&-*r,(4AzB) trap emission for Rb,ReCl, at 4 K (457.0 and 514.5 nm excitation) no.trap energy/cm-'" '6 v4 v3 "1 13 875 126' 15Ib 13 862 131 160 315, 326 360 13 812 134 157 322 -13 721 135 160 322 -13 640 131 160 ---13 494 133 159 319 13 480 129 161 3 14 343 13 428 129 165 315 352 13 410 133 155 "Derived from the vibronic structure, in no case is the pure electronic origin observed; bobserved only in the 514.5 nm excited spectrum. for any of the emission bands. Each emission had an exponential decay curve with a strongly temperature-depen- dent lifetime in the region of 100 ps at 4 K decreasing to a few ps at 30 K. Discussion The lowest-energy charge-transfer state2' of the ReCl2- ion lies at ca.29 700 cm-'. The 4T2, crystal-field state probably lies in the same spectral region but has not been observed. The highest ti, single-ion crystal-field state is T8(2T1,) at ca. 15400cm-'. Between these regions we have identified a number of co-operative absorptions in K2ReCl6 and Cs2ReC16 involving simultaneous excitation of two adjacent ions to combinations of the five ti, excited state in Oh*symmetry." Similar co-operative absorptions here have also been observed for Rb2ReC16. The resultant 15 pair states are likely to contribute weak absorption bands in the 16 000-31 000 cm-' region. There are even weaker features due to v1 progressions based on these pair states. The highest-energy feature observed in our single-crystal absorption spectra is a very weak feature at 18 602 cm-'.Subsequently, two further pair transitions at 21 570 and 21 920cm-' have been resolved21 [I'8(2T1,)+ I'7(2T2B) plus the v1 progression on this state]. A band due to the 2r8(4A2g)+2r6(2T1g) transition expected at 18 900 cm-' was not observed. The absorbance of the crystals at 19 436 cm-' (the 514 nm argon laser line) is extremely low; at higher energy there is a weak rising background but the absorbance of this does not become comparable to the co- operative absorbance until 21 300 cm- '. Nevertheless, both the 514 and the 457 nm (21 839 cm-') laser lines excite intense and generally similar luminescence (but with some variation in the intensity due to individual traps).We assume that the excitation process in our experiments is into weak, broad, defect-induced absorption bands. These defect levels relax rapidly to the r7(2T2g) band and thence to the observed traps. We emphasize the remarkable intensity of the emission when the excitation process is into such weak absorption bands. Nine different traps up to 365cm-' below the T7(?Tig) band have been identified in Rb2ReC16, and it is likely that a similar range of traps occur in both Cs2ReCl6 and K2ReC16. In each case, trap emission is characterized by a vanishingly weak electronic origin, a vibronic origin intensity distribution of the type v6>v4> >v3, no observable splittings of the vibronic origins, a v6 vibronic origin which is significantly sharper than v4 and v3, and very weak v1 progressions. These features show that the defects are essentially centrosymmetric ReC1;-ions.The trap depth is an approximate measure of the decrease in Re4+ interelectron repulsion at the trap site relative to the intrinsic site. The traps are therefore likely to be due to ReC1;-ions at compressed lattice sites near interstitial species or dislocations. These dislocations may also be the source of the absorption in the 19436cm-' region. As the temperature is raised, the intensity from the shallower traps decreases more rapidly than that from the deeper traps, as expected. The vibronic intensity distribution of the trap emission is of particular interest. It is quite different from that of isolated ReClg- ions.This raises a number of intriguing questions concerning the source of the vibronic intensity.22 We thank G. Sperka for preparing the crystals of Rb2ReC16. We also thank NATO and CNR for the award of fellowships to M.B. References 1 A. M. Black and C. D. Flint, J. Chem. SOC., Faraday Trans. 2, 1977, 73, 877. 440 2 C. D. Flint and A. G. Paulusz, Mol. Phys., 1981,43, 321. 3 P. B. Dorain, in Transition Metal Chemistry, ed. R. L. Cariin, Dekker, New York, 1968,vol. 4,p. 1. 4 G.P. O’Leary and R. G. Wheeler, Phys. Rev. B, 1970, 1,4409. 5 B. N. Figgis, J. Lewis and F. E. Mabbs, J. Chem. SOC., 1961, 3138. 6 R. H.Busey and E. Sonder, J. Chem. Phys., 1962,36, 93. 7 H. G. Smith and G. E. Bacon, J. Appl.Phys., 1966,37, 979. 8 V. J. Minkiewicz, G. Shirane, B. C. Frazer, R. G. Wheeler and P. B. Dorain, J. Phys. Chem. Solids, 1968,29, 881. 9 H. D. Grundy and I. D. Brown, Can. J. Chem., 1970,48,1151. 10 R. H. Busey, H. H. Dearman and R. B. Bevan Jr., J. Phys. Chew., 1962,66, 82. 11 M. Bettinelli and C. D. Flint, J. Phys. C, 1988,21, 5499. 12 M. Bettinelli, L. Di Sipio, G. Ingletto, A. Montenero and C. D. Flint, Mol. Phys., 1985,56, 1033. 13 M. Bettinelli, L. Di Sipio, G. Ingletto and C. D. Flint, Chem. Phys. Lett., 1970,138, 361. J. MATER. CHEM., 1991,VOL. 1 14 G. Sperka and F. A. Mautner, Cryst. Res. Technol., 1988, 23K, 109. 15 K. W. Bagnall, D. Brown and R. Colon, J. Chem. SOC., 1964, 30 17. 16 M. Bettinelli, L. Di Sipio, G. Valle, C. Aschieri and G. Ingletto, 2. Kristallogr., 1989,188, 155. 17 V. Spitzyn, A. 1. Zhirov, M. Yu Subbotin and P. E. Kazin, Russ. J. Inorg. Chem. (Engl. Transl.), 1980,25, 556. 18 G. Sperka, H.P. Fritzer, M. Bettinelli and C. D. Flint, Spectro-chim. Acta, Part A, 1988,44,1377. 19 M. Bettinelli and C. D. Flint, Chem. Phys. Lett., 1990, 167, 45. 20 J. C. Collingwood, S. B. Piepho, R. W. Schwartz, P. A. Dobosh, J. R. Dickinson and P. N. Schatz, Mol. Phys., 1975,29, 793. 21 K.Gatterer, personal communication, 1990. 22 R. Acevedo and C. D. Flint, Theoret. Chim. Acta, in the press. Paper 0/05647B;Received 17th December, 1990