Silver aggregates in photoluminescent phosphate glasses of the `Ag2O–ZnO–P2O5' system

Silver aggregates in photoluminescent phosphate glasses of theÀg2O±ZnO±P2O5' system

I. Belharouak a, C. Parent a, B. Tanguy a, G. Le Flem a,*, M. Couzi b

a I.C.M.C.B, Chateau Brivazac, Avenue du Docteur Schweitzer, UPR CNRS 9048, 33608 Pessac, cedex, Franceb L.P.C.M, UMR 5803 CNRS, Universit�e de Bordeaux I, 351 cours de la liberation, 33405 Talence, France

Received 27 May 1998; received in revised form 26 October 1998

Abstract

Photoluminescent properties of glasses of the Ag2O±ZnO±P2O5 system are reported. The compositions of the glasses

are limited by the polyphosphate and diphosphate domain of this system. The phosphate structural units of the network

former are deduced from Raman spectra. A comparison with the luminescent properties of crystallized silver phos-

phates of similar composition allows identi®cation of two di�erent emitting centers: isolated Ag�, Ag�±Ag� pairs, and a

third center resulting from an Ag�±Zn2� interaction. The introduction of 1 mol% SiO2 or Al2O3 in these glasses leads to

precipitation of silver particles, whose dimensions range from nanometers to micrometers, and to the appearance of

luminescent centers (Ag2)� detected around the micrometer particles by optical microscopy. We assume that these

centers are an intermediate stage during the process of silver precipitation. Ó 1999 Elsevier Science B.V. All rights

reserved.

1. Introduction

The investigation of silver (I) luminescence inglasses was the subject of extensive research inrelation to the use of these materials as c-radiationand X-ray dosimeters [1±3]. More recently thepossibility of white light generation was investi-gated in phosphate and borate glasses [4,5]. Thee�ect of the conditions of Na�/Ag� exchange onthe luminescent properties of soda lime glasseshave been reported [6]. In addition silver particlesembedded in glass matrices induce non-linear op-tical properties depending on the size, density anddistribution of these particles [7].

The ¯uorescence of Ag� is due to the transition4d95s ® 4d10, strictly forbidden for the free ionbut partially allowed in crystals or glasses bycoupling with lattice vibrations of odd parity. Inglasses the active center can occupy a variety ofsites which thereby results in an increase in thewidth of the ¯uorescent bands [5]. Moreoverthe identi®cation of the silver species involved inthe luminescence process is complicated by thetendency of Ag� to aggregate and/or to be pho-toreduced [6]. The aggregation and photoreduc-tion can occur simultaneously which creates theconditions for silver particles precipitation.

This tendency of Ag� to aggregate was discus-sed in Ref. [8]. In most cases cationic and anioniccomponents of the structures can be distinguished,this description giving evidence of extended silveraggregates. The Ag�±Ag� distance can be less than

Journal of Non-Crystalline Solids 244 (1999) 238±249

* Corresponding author: Tel.: +33-5 56 84 62 94; fax: +33-5 56

84 27 61; e-mail: le¯[email protected].

0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 0 1 4 - 9

the interatomic distance in silver metal: 0.289 nm.Silver ion pairs are also observed in coordinationcompounds. For example a recent review of thecrystal structure of silver (I) carboxylates reportsthe existence of Ag±Ag distances between 0.278and 0.307 nm [9]. Such silver±silver distances arealso found in phosphates. In AgPO3 the Ag� ionsare in isolated polyhedra with ®ve oxygen neigh-bors [10] and the shortest Ag�±Ag� distance is0.315 nm. Similar arrangement is found in Ag3PO4

(dAg±Ag� 0.3013 nm), in AgCuPO4 (dAg±Ag� 0.3067nm) [11].

In the context of a systematic investigation ofthe silver luminescence in glasses the spectro-scopic properties of phosphate glasses in theNa2O±Ag2O±P2O5 system was reported [4] andtwo emitting centers were identi®ed: isolated Ag�

with an emission in the near UV region (labeledA) and Ag�±Ag� pairs with an emission band atabout 600 nm (labeled C). A third center wasfound in the glasses with the largest amounts ofAg with an emission in the visible range (labeledB), whose decay was faster than the (A) center(labeled B). This center was tentatively attributedto Ag�±Ag0 pairs [4]. This attribution raises thequestion of the reduction process and indicatesthat control of the particle production procedureis essential.

The purpose of the present work was to explorethe conditions for producing (B) centers in glassesof the Ag2O±ZnO±P2O5 system and at the sametime the conditions for formation of silver parti-cles. Two variable chemical parameters were con-sidered:1. the e�ects of the phosphate chains of the glass

former by controling chain lengths in two com-positions between the metaphosphate and thediphosphate lines of the phase diagram (Fig. 1);

2. the e�ect of addition of SiO2 or Al2O3 (1mol%), to consider the possibility of precipita-tion of silver metal by increasing the opticalbasicity as suggested by the concept introducedby Du�y and Ingram [12,13].Accordingly two types of samples were pre-

pared:1. glasses of compositions corresponding to the a,

b and c lines of Fig. 1 and representative of re-spectively silver poor and rich glasses;

2. glasses of the same base composition to whichwere added 1 mol% of SiO2 or Al2O3.This paper reports the experimental procedure

including the elaboration of the glasses, a struc-tural investigation of the former network by Ra-man spectroscopy, the luminescent properties ofthe two types of glasses and a microscopic inves-tigation of the composite glasses containing thesilver particles. Finally the results will be discussedin the context of the luminescence of crystallizedsilver phosphates and of the development ofchemical route to the formation of silver colloids.

2. Experimental procedures

The compositions are listed in Table 1. Thestarting materials were the crystalline forms ofZn(PO3)2, Zn2P2O7, AgPO3 and Ag4P2O7. Ap-propriate mixtures of these phosphates were cru-shed and then introduced in a Pt crucible to bemelted in air at 950°C for 4 h. The resulting liquidswere cooled to room temperature by casting in onepiece on a brass plate. The absence of mass losswas determined during the melting process. Thesamples were annealed for 12 h at temperaturesjust below the glass transition temperatures listedin Table 1.

Fig. 1. Composition of the investigated glasses in the ternary

diagram Àg2O±ZnO±P2O5'. The labels (a1, a2, a3); (b4, b5,

b6) and (c7, c8) correspond to the glass compositions located in

the a, b, and c lines.

I. Belharouak et al. / Journal of Non-Crystalline Solids 244 (1999) 238±249 239

The pure phosphate samples were transparentand colorless. They were stable in air and were notphotoreduced under illumination. When dopedwith silica or alumina, samples of b compositionswere yellow whereas those of a compositions werecolorless and slightly opalescent. Depending ontheir composition the c samples can be less yellowand opalescent.

Chemical analysis was performed on somesamples at the CNRS analysis center (Vernaison,France). The compositions were within 5% of thebatch compositions. The glass transition temper-atures were measured using di�erential thermalanalysis. The densities were obtained by immersingthe samples in calibrated diethyl orthophtalate.The accuracy was �1%.

The Raman spectra have been recorded on atriple monochromator instrument (Dilor Z24).The 514.5 emission line of an argon ion laser(Spectra Physics model 2030) was used for exci-tation with incident power around 200 mW. Ab-sorption, excitation and emission spectra wererecorded at room temperature. The absorptionspectra were recorded using an Ultra Violet (UV)-Visible-near infrared (NIR) spectrometer (VarianCary 2415). The emission spectra were recordedwith a ¯uorometer (Spex Fluorolog F212I).

The microstructure of the composite glasses wasobserved with a transmission electron microscope

(JEOL F ´ 2OOO) using 200 keV in voltage andcoupled with an energy dispersive analyze X(EDAX) system. In addition some glasses wereobserved by optical microscopy using the set-upillustrated in Fig. 2. The samples were located atthe focus of the objective of a microscope and wereilluminated by UV or visible light in a directionperpendicular to the microscope axis. The ob-tained images were scanned by a charge coupleddevice (CCD) camera.

3. Results

3.1. Raman spectroscopy

The assignment of the band spectra was madeby referring to Martin [14]. Fig. 3(a) compares thespectra of polycrystalline powders of AgZn(PO3)3

and Ag2ZnP2O7 and those of the glass samples ofthe same composition. The spectra of the poly-phosphates are typical of long phosphate chains:the two main bands located at 1212 cmÿ1 (crystal)and 1178 cmÿ1 (glass) are assigned to the ms(PO2)mode of the O2=2PO2 units and the bands at 705cmÿ1 (crystal) and 697 cmÿ1 (glass) to the ms(P±O±P) stretch of quasi-in®nite metaphosphate chains.Similarly the spectra of the diphosphate are typicalof the P2O7 group with the ms(PO3) mode at 1056(crystal) or 1035 cmÿ1 (glass) and the ms(P±O±P)stretching mode at 729 (crystal) or 730 (glass) cmÿ1.

The bands in the spectra of glasses with inter-mediate compositions along the b line (Fig. 3(b))

Fig. 2. Experimental set-up for illuminating the opalescent

glasses.

Table 1

Molar compositions and glass transition temperatures Tg of the

investigated samples along the a, b and c lines of the compo-

sition diagram

Samples Molar percentage Tg (�5°C)

% P2O5 % Ag2O % ZnO

aa1 50 03 47 380

a2 40 05 55 375

a3 33 07 60 365

bAgZn(PO3)3 50 17 33 270

b4 46 21 33 268

b5 42 25 33 265

b6 37 30 33 250

Ag2ZnP2O7 33.33 33.33 33.33 240

cc7 33 27 40 295

c8 33 17 50 340

240 I. Belharouak et al. / Journal of Non-Crystalline Solids 244 (1999) 238±249

increase in width and the ms(PO2) mode shift tolower frequency (1178±1035 cmÿ1). This shift isindicative of a `depolymerization' of the phosphatenetwork, resulting in shorter chains. Also, theappearance of a band at 1035 cmÿ1, which is the

dominant one in the spectrum of sample withcomposition Ag2ZnP2O7, is due to diphosphategroups. However, the spectra of all diphosphateglasses (Fig. 3(c)) have shoulders on both sidesof the main band at 1035 cmÿ1, around 1100, and

Fig. 3. (a) Raman spectra of the crystalline and vitreous forms of AgZn(PO3)3 and Ag2ZnP2O7; (b) Raman spectra for glasses of the bline between Ag2ZnP2O7 and AgZn(PO3)3 compositions; (c) evolution of the Raman spectra of glasses of the diphosphate line (c)

composition as the zinc rate increases.


970 cmÿ1, whose amplitudes increase with in-creasing zinc content. Indeed from 31P MAS-NMR, infrared and Raman scattering experimentsperformed on glassy zinc polyphosphate [15,16], ithas been shown that close to the Zn2P2O7 com-position, the diphosphate groups are present to-gether with metaphosphate and orthophosphateunits, according to the scheme

P2O4ÿ7 $ PO3ÿ

4 � ±O±�PO2�ÿ:Moreover, it was suggested that the metaphos-

phate groups appear as cyclic-like (P3O9)3ÿ entities[16]. Hence, we assign the shoulder at �1100 cmÿ1

to ms(PO2) of metaphosphate groups and that at�970 cmÿ1 to ms(PO4) of orthophosphate ones [15].Thus, it appears from Fig. 3(c) that the aboveèquilibrium' would lie to the left (i.e. P2O4ÿ

7

groups are dominant) for the Ag2ZnP2O7 glasscomposition, but would be displaced to the right(i.e. the concentrations of PO3ÿ

4 and ±O±(PO2)ÿ

groups progressively increase) towards theZn2P2O7 glass composition. A similar e�ect waspreviously described by Quinn et al. [17].

No change of Raman spectra could be detectedwhen adding SiO2 or Al2O3 to the composition ofthe investigated sample compositions, in agree-ment with the amounts of these oxides in thematerials (1 mol%).

3.2. Absorption spectra

Fig. 4 compares the UV-VIS absorption spectratypical of the colorless-transparent, yellow andcolorless-opalescent glasses as well as the absorp-tion of the reference glass matrix NaZn(PO3)3. Theintroduction of silver is responsible for the redshift of the absorption edge in the near UV range.In the case of the silica and alumina-doped glasses,opalescent glasses have an absorption band at330 nm whereas yellow glasses have an absorptionband at 425 nm.

3.3. Transmission electron microscopy and opticalmicroscopy

Fig. 5 shows typical transmission electron mi-croscopy (TEM) micrographs of yellow glasses.Spherical silver colloidal particles, identi®ed by

selected area electron di�raction, are distributedover the volume of the sample. The size of thesilver particles is between 5 and 7 nm.

From the (TEM) micrograph and the (EDAX)analysis silver particles in the opalescent glasseswere detected (Fig. 6). The size of these particlesvaries between 1 and 10 lm. Their spherical shapeis evident in the optical microscopy image whensamples are illuminated by white light (Fig. 7(a)).

Fig. 4. Comparison between typical absorption spectra of

glasses doped or not with alumina or silica. The spectrum of

NaZn(PO3)3 glass is given as reference. In inset hhhhh:

Undoped b5 glass composition, nnnnnn: Silica-doped a2

glass composition, ssssss: Silica-doped b5 glass compo-

sition.

Fig. 5. Comparison between experimental and calculated mic-

ro-di�raction pattern in yellow glasses. On the right, the shown

pattern is based on the cubic lattice parameter of the metallic

silver (a� 0.4086 nm).


If the sample is illuminated with UV light, a whitelight emission is observed at the border of thespherical silver particles i.e. probably at the inter-face between these metallic particles and the sur-rounding glasses (Fig. 7(b)).

3.4. Emission and excitation spectra

The luminescence experiments were carried outat 300 K. No luminescence was detected fromsamples with compositions Zn(PO3)2 andZn2P2O7. Therefore the emission of the investi-gated glasses results from the presence of silver.

(i) The case of the pure phosphate glasses:Whatever their compositions, the alumina and

silica free phosphate samples have two emissionbands ± labeled (A) and (C) ± (Fig. 8) and noconcentration quenching was observed for eitheremitting centers as the silver concentration in-creases.

The (A) emission (kexc:� 260 nm) is a bandappearing in the near UV-visible range between280 and 500 nm with a maximum at 380 nm. Theexcitation spectrum of this emission (kem:� 380 nm)is a band located between 230 and 300 nmwith a maximum at 260 nm. The (C) emission(kexc:� 300 nm) extends over a larger wavelengthrange between 400 and 750 nm with a maximum at550 nm. The excitation spectrum of this emissionrecorded at kem:� 600 nm is a band at about300 nm.

(ii) The case of the doped phosphate glasses: Theaddition of SiO2 or Al2O3 leads:· to the appearance of an additional emission

band labeled (B) and simultaneously an in-crease of the (A) and (C) emission intensities

Fig. 6. The microprobe analysis of the vitreous matrix and of

the metallic part of the opalescent glasses.

Fig. 7. (a) Silver aggregates illuminated by white light ± Sample:

a2 doped with silica, (b) Silver aggregates illuminated by UV

light at 320 nm ± Sample: a2 doped with silica.


in the a samples (Fig. 9). The (B) emission cov-ers the entire visible range with a maximum at450 nm;

· to a yellow coloration of the b glasses (Fig. 4);· to the appearance of (A)±(C) emissions with

smaller amplitudes, in the c samples in so faras these emissions are not absorbed by the sam-ples which are colored (Fig. 9).A typical emission and excitation spectra of the

a2 sample doped with SiO2 are given in Fig. 9.Three emission bands are observed at 370 nm (A),450 nm (B) and 570 nm (C). The excitationspectrum of (A) emission is a band with a maxi-mum at about 260 nm and a small band at 325 nm.The excitation spectra of (B) and (C) consist of twooverlapping bands with maxima, respectively, at 360and 300 nm. The overlap of the (A) emission bandwith the excitation bands of (B) and (C) is evidenceof a (A) ® (B),(C) transfer. Similarly the overlapbetween the excitation and the emission bands of (B)and (C) indicates the occurrence of energy transferbetween these two centers. Fig. 9(c) illustrates the(B) ® (C) transfer. Incidently the excitation spec-trum of the 450 nm emission (Fig. 9(b)) merely in-dicates the overlap of the (B) and (C) bands at thiswavelength.

4. Discussion

The following assignements are proposed. TheUV emission (A) and the corresponding excitationspectrum which coincides with the absorption be-ginning at 280 nm is typical of isolated Ag�. The(C) emission is found exclusively in glasses withthe largest Ag amounts and the excitation bandwhich overlaps the absorption band peaking at330 nm are attributed to Ag�±Ag� or Ag�±Zn2�

pairs. The excitation spectrum of the (B) center islocated between 320 and 400 nm with a maximumat 380 nm i.e. at lower energy than the absorptionedge. This center is assigned to (Ag2)� centers al-ready reported for radiophotoluminescent glasses (2).

Finally it is of interest to compare Fig. 7(a) and(b). As the glass is illuminated by a UV light(kexc:� 320 nm) the white light emission occurringaround the silver particles is the signature of the(B) and (C) centers i.e. the centers involving silverpairs. When same sample is subjected to a visiblelight, these centers are not excited and the blackcolor of the silver particles is observed in contrastto the transparency of the glass phase illuminatedby a visible light.

(i) Structure of the glass former: According totheir structures and the zinc content the samplescan be classi®ed into:· polyphosphate glasses with the compositions

AgZn(PO3)3 and a1;· diphosphate glasses with compositions

Ag2ZnP2O7, c7, c8 and a3;· mixed poly-diphosphate glasses a2, b4, b5, b6.

The densities increase with the concentration ofAg2O and/or ZnO (Table 2).

The glass transition temperatures, Tg, ofdiphosphate or polyphosphate glasses increasewith the concentration of ZnO. In the mixedglasses Tg is approximately constant in the a or bsamples but their respective order of magnitude isalso governed by the amount of ZnO.

These evolutions can be explained by the pro-gressive introduction of ZnO as a network former.This interpretation is consistent with the crystalchemistry of polyphosphate or diphosphate com-pounds containing a large electropositive cationsuch as Ag� and a small divalent cation such asZn2�: for instance, in the structure of AgZn(PO3)3

Fig. 8. Emission (kexc:� 250 and 300 nm) and excitation

(kem:� 375 and 600 nm) spectra of both (A) and (C) lumines-

cences for a2 glass composition at room temperature.


Fig. 9. Emission and excitation spectra for sample a2 doped with silica (1 mol%) at room temperature.


[18], the [ZnO6] octahedra share their corners withphosphoric chains to create a three dimensionalframework in which are located the silver cations.Moreover the highest Tg is observed for the samplea1 the composition of which is close to Zn(PO3)2.

(ii) Colour of the silver salts: As a general trendin silver salts the band gap decreases with the in-creasing strength of the silver±oxygen bond, i.e.with a decreasing Ag±O bond length [19]. The topof the valence bond is made up of the 4d or hy-bridized 4d5s orbitals. The shorter the silver oxy-gen distance, the greater the hybridization and thesmaller is the gap between 4d (mixed with 5s) and5s (mixed with 4d) levels. This model explains thecolorless AgPO3 and AgZn(PO3)3 in which thesilver±oxygen distances are relatively large (theshortest Ag±O distances are 0.235 and 0.237 nm)and consequently the silver±oxygen bond is ionic.In our samples, silver ions are distributed in afamily of sites and the decreasing energy of theabsorption edge observed as the silver concentra-tion increases is attributed to a decrease of thesmallest Ag±O distances.

The yellow colour of the b line samples iscaused by the surface plasmon absorption bandpeaking at 425 nm and results from the colloidalsilver particles, as detected by microdi�raction[20].

(iii) Luminescence of the silver(I) centers incrystals: The luminescence of Ag� with the 4d10

electronic con®guration has been studied in alkalihalide crystals [21] and in alkali-earth halides [22].Sites in which Ag� are located have octahedralsymmetry which determines the basic properties ofsilver-activated insulators. The excitation andemission spectra involve the 4d10 M 4d95s transi-tions. The energies of the 4d95s states are depen-dent on the spin±orbit coupling and the crystal®eld e�ects. In glasses the interpretation of ab-sorption and emission spectra is a�ected by vari-

ation in site symmetries from site to site of theluminescent centers. Nevertheless, by comparingthe results with the luminescence of Ag� reportedin crystals or glassy systems some of the centerscan be identi®ed.

The phosphate compound AgZr2(PO4)3 belongsto the Nasicon type structure and it is colorless[23]. Silver atoms are located at the center of anoxygen antiprism in which the silver oxygen dis-tance is 0.256 nm. The closest silver±silver distanceis 0.636 nm. No luminescence was detected in thisphosphate in which the silver environment is cen-trosymmetrical. AgBaP3O9 is isomorphic withNaBaP3O9. In this structure the sodium atoms arelocated in distorted octahedral sites which areisolated from each other by a P3O9 ring. The Na±Odistances are included between 0.231 and 0.259 nmand the closest Na±Na distance is 0.579 nm [24]. Inthe solid solution Na1ÿxAgxBaP3O9 a unique lu-minescence of the (A) type is observed whateverthe silver concentration [25]. Based on these ob-servations we assign, unambiguously, the (A) lu-minescence to isolated Ag�. In contrast, thepairing e�ect is clearly related to the (C) emission:this relation is demonstrated by the singularemission of this type observed in AgPO3 since, inthe structure of this phosphate, all the silver ionsbelong to the Ag�±Ag� pairs mentioned above[10].

Intermediate properties occur in the phosphatesof composition AgM(PO3)3 (M�Mg, Zn) whichcrystallize in the same structure type [18]. In thesephosphates the emission of the magnesium com-pound is only the typical (A) whereas the zinccompound emits both (A) and (C) (Fig. 10). Thecrystal structure of the silver zinc coumpound wascompletely described [18]. The main structuralfeature is the formation of chains of face sharing[ZnO6] and [AgO6] octahedra where the silver ox-ygen distances are included between 0.246 and

Table 2

Densities of the investigated glasses

Domains

a b c

Samples a1 a2 a3 AgZn(PO3)3 b4 b5 b6 Ag2ZnP2O7 c7 c8

Densities (g/cm3) 3.06 3.69 4.22 3.51 3.92 4.07 4.43 4.68 4.53 4.38


0.271 nm. These chains are connected by thephosphate groups. The closest silver±silver dis-tances within and between the chains are 0.645 and0.665 nm, respectively. Therefore no pairing e�ectbetween silver ions are expected from such astructure and the origin of (C) is presently un-known. Based on the Ag±Zn distance in the chains(0.319 and 0.329 nm) we propose the existence of anew d10±d10 luminescent center resulting from aAg�±Zn2� association. This hypothesis is sup-ported by the absence of such centers in the is-omorphic magnesium compound (Fig. 10). The

di�erence between the UV maximum emission in-dicates a modi®cation of the silver environment.

(iv) Luminescence of the pure phosphate glasses:Table 3 summarizes the results for the crystals andglass samples. The silver luminescence of the glasssamples are identical, within accuracy of ourmeasurements, to the luminescence of the crystalsamples: the (A) and (C) centers are detected andtheir excitation and emission spectra are located,respectively, at the same energies. Therefore thesecenters can be assigned to the isolated Ag� for(A) and Ag�±Ag� (or Ag�±Zn2�) pairs for (C).This e�ect of silver pairing was previously ob-served in the spectra of glasses of compositionNa1ÿxAgxPO3 by appearance of the (C) emissionat the expense of the (A) emission as x increased[4]. This result illustrates again the tendency ofsilver to aggregate.

(v) Optical properties of the doped phosphateglasses: The introduction of (1 mol%) of Al2O3 orSiO2 leads to the precipitation of silver particles ofvarious sizes, the creation of the new luminescentcenter (B) detected only in samples of the a com-positions and the increase of the (A) and (C)emission intensities of the same samples in com-parison with the intensity observed in the purephosphate samples.

The reduction of silver ions could be the resultof the oxidation±reduction process:

2O2ÿ � 4Ag� ! 4Ag0 �O"2:

Consequently oxygen is released during the meltstage. As observed by Wang et al. [26] the disso-ciation of Ag±O to form Si±O (or Al±O) and

Fig. 10. (1) Room temperature emission spectrum of crystal-

lized AgMg(PO3)3(kexc:� 230 nm). (2) Room temperature

emission spectrum of crystallized AgZn(PO3)3(kexc:� 290 nm).

Table 3

Location of the maxima of the emission and excitation bands in crystallized phosphates and in selected investigated glasses

(A) Emission (C) Emission

Emission k (nm)

�0.1 nm

Excitation k (nm)

�0.1 nm

Emission k (nm)

�0.1 nm

Excitation k (nm)

�0.1 nm

Reference

Crystallized materials

AgBaP3O9 365 250 ÿ ÿ [25]

AgMg(PO3)3 320 230 ÿ ÿ This work

AgZn(PO3)3 370 255 550 300 [27]

AgPO3 380 250 580 300 [4]

Glass materials

Composition b5 370 260 570 300 This work

Composition a2 375 260 570 300 This work


Ag±Ag bonds results in a loss of the system energy.From a structural point of view the e�ect of in-troducing [SiO4] (or [AlO4]) tetrahedra in phos-phate glasses is to cross-link the phosphate chains.For example considering the pyrophosphate glassof composition Ag2ZnP2O7 an hypotheticalmechanism of silver (I) reduction due to the ad-dition of small amount of SiO2 can be written as

4�P2O7�4ÿ � SiO2 � 8Ag� � 4Zn2�

! �SiP8O28�12ÿ � 4Ag0 � 4Ag� � 4Zn2� �O"2:

A similar reaction can be written for poly-phosphate compositions e.g:

2�PO3�3ÿ3 � SiO2 � 2Zn2� � 2Ag�

! �P6SiO19�4ÿ � 2Zn2� � 2Ag0 � 1

2O"2:

Moreover the introduction of SiO2 or Al2O3 in theglasses increases the optical basicity which is in-dicative of the electron donor power of oxygen [13]and ultimately allows the reduction of Ag� to Ag.

The evolving of oxygen needed in this processmust occur in the liquid state and the precipitationof silver particles probably occurs during quench-ing of the liquid. Actually the larger the particles,the smaller the silver concentration, and the higherthe annealing temperature of the glass.

The (B) centers detected in other glass compo-sitions have shorter lifetimes of emission (s � 1 ns)[27]. We suggest that this lifetime is due to theallowed 4d105s M 4d105p transition related to theformation of (Ag2)� centers already reported insilver activated radiophotoluminescent phosphateglasses [2]. In these a glasses the increase of the (C)emission is related to the e�ect of the (B) ® (C)energy transfer. On the other hand the precipita-tion of silver particles with dimensions �1 lm re-duces the concentration of silver ions in the glassphase. Therefore, possible concentration quench-ing of the (A) emission is suppressed.

The absorption band observed at 425 nm in theyellow samples is typical of surface plasmon ab-sorption which can be described by the classicalMIE theory [20,28,29]. This band may prevent thedetection of the (B) center.

In these glasses the introduction of SiO2 orAl2O3 reduces of Ag� by an intermediate stage in

which the creation of Ag�±Ag° ((Ag2)�) centersoccurs. These centers are close to the particles ofsilver in the opalescent glasses (Fig. 7(b)) andpossibly at the interface between the glass and themetal.

5. Conclusions

In our samples of Ag2O±ZnO±P2O5 system, theglass transition temperature increases with theconcentration of ZnO while the densities aremainly controlled by the amount of Ag2O. De-pending on the glass compositions, the networkformers are polyphosphate long chains and/ordiphosphate structural units.

In pure phosphate glasses, two luminescentcenters were identi®ed and attributed respectivelyto isolated Ag� and Ag�±Ag� (Zn2�) pairs. Theintroduction of small amounts of SiO2 or Al2O3

leads to the formation of metallic silver particleswith dimension ranging from nanometers to mi-crometers.

Finally the concept of optical glass basicity issuccessfully applied to obtain dispersed silver nu-clei of various sizes and the (B) center appears asan intermediate stage occuring before the precipi-tation of the metallic particles.

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Documents

Silver aggregates in photoluminescent phosphate glasses of the `Ag2O–ZnO–P2O5' system