Manganese oxide-zirconium oxide solid solutions. An X-ray diffraction, Raman spectroscopy, thermogravimetry and magnetic study Mario Valigi,*" Delia Gazzoli,' Roberto Dragone,G Alessandra Marucci' and Giorgio Matteib Tentro CNR SACSO, c/o Chemistry Department, University of Rome, 'La Sapienza', P.Ee A. Moro 5, 1-00185 Rome, Italy 'IMAI, CNR-Area della Ricerca di Roma, PO Box 10, I-0001 6 Monterotondo Stazione, Italy Manganese oxide-doped zirconium oxide samples, prepared by heating mixtures of coprecipitated hydroxides at 1073 K in a hydrogen stream (water content 0.2% by volume), were analysed to obtain information on the solid solution formation. The state and the thermal stability of the incorporated species were also investigated. The samples (manganese content up to 14.74 mass%), were studied 'as-prepared' and after subsequent thermal treatments in oxygen up to 753 K.The results of several techniques [X-ray diffraction (XRD), Raman spectroscopy, thermogravimetry (TG) and magnetic susceptibility measurements] show that in the 'as-prepared' samples (1073 K, H2) a high fraction of manganese is incorporated in the zirconia structure, only a small fraction being present as an MnO separate phase. Most of the manganese in solid solution is present in the +2 oxidation state, the remainder as +3 and +4.TG experiments and magnetic susceptibility measurements reveal that the Mn3+ and/or Mn4+ are formed both during the cooling in hydrogen by reaction with water present as an impurity in the gas phase, and during the exposure to the atmosphere.As the amount of manganese in solid solution increases, the volume of the zirconia unit cell slightly decreases. The solid-solution formation favours the tetragonal and the cubic modifications at the expense of the thermodynamically stable monoclinic phase. When the samples are heated up to 753 K in oxygen, the Mn2+ in solid solution is partially oxidized to Mn3+ and/or Mn4+. TG and XRD experiments show that the oxidation starts at low temperature and takes place in solid solution without appreciable manganese oxide segregation. Zirconia solid solutions containing metal ions have been widely investigated because of their scientific and technological inter- est. Usually high temperatures are required for their prep- aration, a procedure that results in highly sintered materials.The preparation of metastable phases by low-temperature processing techniques has been developed, leading to materials with large surface areas and of potential interest as hetero- geneous On the other hand, manganese oxides have long been known as catalytic materials. They have been studied mainly as supported systems, in particular on al~mina,~ while very few papers on manganese oxide dispersed on zirconium oxide have been This system is of great interest, as demonstrated by recent reports on a new class of catalysts, based on superacid sulfated zirconia and containing manganese and iron as promoter^,'^-'^ which are active for hydrocarbon isomerization even at room temperature.Some attention has also been devoted to the manganese oxide- zirconium oxide solid ~olutions.'~-'~ These systems have several points of interest, for example their redox properties since manganese may be present in several oxidation states and the oxygen ions in the zirconia structure are known to have a rather high mobility. Moreover, the detailed investigation of this system may provide information on the effect of the incorporated ions on the host structure, a relevant aspect of zirconia as a material. All these considerations, and specifically the possibility of obtaining information on the conditions of the solid-solution formation, on the extent of dispersion of the incorporated ions and on the features of the oxidic matrix, have prompted us to undertake the present research.The study was carried out mainly by X-ray diffraction (XRD), thermogravimetry (TG), Raman spectroscopy , magnetic susceptibility measurements and chemical analysis. Experimental Sample preparation The samples were prepared by a coprecipitation method by dissolving 9 g of ZrOC12.8H20 (Erba RP) and a given amount of manganese nitrate, Mn(N0,)2-4H20, (Erba RP) in 100 ml of water. Ammonia was added to the stirred solution till the pH was raised to 8.2. The obtained precipitate was separated from the supernatant solution and thoroughly washed with ammonia-containing water to remove C1- ions (negative test in the solid). The solid was then dried at 363 K for 24 h in air. After grinding in an agate mortar, the powder was placed in a platinum vessel and heated at 1073 K for 10 h in a hydrogen stream (from a cylinder, water content cu.0.2% by volume). Such a temperature was high enough to obtain XRD spectra of good quality in the back-reflection region without causing the formation of monoclinic zirconia, the stable phase in the temperature range of preparation. The hydrogen atmosphere was adopted to have manganese in a reduced state. At the end of the thermal treatment the sample was cooled to room temperature, still in a hydrogen atmosphere, and finally exposed to the atmosphere. ZrO, was obtained by a similar procedure except for the addition of manganese nitrate. The samples were designated ZMnx where x stands for the Mn content (in mass%).The samples were studied (i) 'as-prepared', (ii) after rinsing with a HC1 solution and drying under vacuum at room temperature, and (iii) as in (ii) but with subsequent thermal treatments at a given temperature in oxygen for 5 h. The rinsing was carried out by mixing a fraction of each ZMnx sample at room temperature for 12 h with a concentrated HCl solution under stirring. It was checked that such a treatment was able to dissolve MnO, Mn203, Mn304 and MnO,, but did not affect ZrO,. The solid was then separated from the liquid fraction by centrifugation, thoroughly washed in water, and dried at room temperature under vacuum. The liquid fractions were collected and analysed for the manganese content (see later).To clarify the state of manganese in samples heated in hydrogen, the sample ZMnll.50, after rinsing with HC1 and heated at 753 K for 5 h in oxygen, was also studied after a further thermal treatment at 1073 K for 0.5 h in a hydrogen flow. For this treatment a silica reactor connected to a vacuum line and having a lateral tube suitable for the magnetic J. Muter. Chem., 1996, 6(3), 403-408 403 Table 1 Analytical data and ZrOz unit-cell constants for manganese-containing zirconia samples heated at 1073 K for 10 h in a hydrogen stream ZMn4 18 0 10 4 09 5 090 1004 m-ZrO,, t-ZrO, ZMn6 70 0 27 6 50 5 094 1002 m-ZrO,, t-ZrO, ZMn7 79 0 32 7 64 5 093 1000 c-ZrO, ZMnll50 100 1064 5 084 1000 c-ZrO, MnO ZMnl4 74 2 50 12 57 5 085 1000 c-ZrO,, MnO For designation of samples see text Amount of Mn remaining in solid residue after HCI attack ' m=monochnic, t=tetragonal, c=cubic measurement was used At the end of the heating, the sample was cooled in dry hydrogen (liquid-nitrogen trap) After evacu- ation, the sample was transferred to the lateral tube, sealed off under vacuum and submitted to the magnetic measurement without exposure to the atmosphere Chemical analysis The manganese content was determined by atomic absorption spectroscopy (AAS) A known amount of each ZMnx sample was dissolved in 2 5 ml of concentrated (40%) HF solution After dilution, the solution was analysed using manganese standard solutions containing ZrO, dissolved in HF in a concentration similar to that of the sample The amount of manganese rinsed away with HC1 [Mnrlns (%)I was similarly determined using standard solutions with appropriate HC1 concentrations The analytical results are collected in Table 1 X-Ray diffraction measurements A Philips diffractometer (PW 1725) and Cu-Ka (Ni-filtered) radiation were employed The 28 angular region from 10 to 150" was scanned To determine changes in the unit-cell parameters of tetragonal or cubic zirconia, the following nine reflections were used (620), 28= 146", (006), 125", (153), 124", (044), 116", (511), 103", (422), 95", (042), 84", (133), 81", (400), 74" The graphical method of extrapolation against the Nelson and Riley function was adopted For each sample the position of the diffraction peak was measured several times and showed an angular uncertainty of +_O 05" The axial ratio c/a= C was determined by minimizing the deviations of the values of the zirconia unit-cell parameter a following a least-squares pro- cedure At least two determinations were made for each specimeq It was observed that the errors in a and C are +O 002 A and &O 001, respectively This means th%t the unit- cell parameter c could be measuredo to +O 007 A and the zirconia unit-cell volume, V,to k0 3 A3 Raman spectroscopy Raman spectra were obtained in back-scattering geometry from self-supporting disks using a SPEX 1877 TRIPLEMATE spectrograph equipped with a liquid-nitrogen cooled EG & G PAR CCD multichannel detector and an Ar ion-laser source (line 514 5 nm) The laser power at the sample position was 5 mW to avoid sample modification due to irradiation The experimental Raman spectra were treated by GRAMS/386 software (by GALACTIC) in order to remove the baseline and to fit the curve Magnetic susceptibility determination The magnetic determinations were carned out by the Gouy method at 4000, 6000 and 8000G in the temperature range 98-298 K A Mettler balance reading to &O 01 mg was employed The instrument was calibrated with Hg[Co(CNS),] l9 The magnetic susceptibility of ZrO,, treated as the manganese-containing samples, was determined with the same apparatus 404 J Muter Chem , 1996,6( 3), 403-408 Thermogravimetry The thermogravimetric apparatus consists of an electrobalance (Cahn RG) reading to 0 01 mg connected to a vacuum line to perform experiments in controlled atmosphere During the experiment the sample was kept in a constant flow (20 ml min-l) of gas (oxygen, nitrogen or hydrogen) The temperature was changed by a linear programmer at a rate of 2 K min-' XPS measurements X-Ray photoelectron (XP) spectra were obtained with a Leybold-Hereaus LHS 10 spectrometer interfaced with a Hewlett-Packard 2113 B computer, using Al-Ka (1486 6 eV) radiation The sample, in the form of a fine powder, was pressed onto a tantalum plate attached to the sample rod The spectra were recorded in the sequence Mn 2p, 0 Is, C Is, Zr 3d Zr 3d,,, binding energy (EB)at 1825eV was taken as reference Results X-Ray diffraction Fig 1 shows the XRD patterns of ZMnx samples, and the results of the phase analysis are given in Table 1 Undoped ZrO, showed essentially the monoclinic form The addition of manganese caused a sharp decrease in the fraction of the monoclinic modification and a parallel increase in the fraction of the higher-symmetry polymorphs (tetragonal and/or cubic) The XRD patterns of the two more copcentrated samples shoy the most intense MnO lines (d=2 57 A, 28 = 34 91 ",d =2 22 A, 28=40 63", Fig 1) These lines are no longer observed on the diffraction patterns of the same samples submitted to the HCl treatment (not shown) The zirconia unit-cell parameter a and the axial ratio C =c/a, are also given in Table 1 Their values were not affected by the HCl rinsing The effect of the manganese addition on 10 I5 20 25 30 35 40 45 50 55 60 2Ndegrees Fig.1 XRD patterns for 'as-prepared' ZMnx samples a, ZrO, (monoclinic modification), b, ZMn4 18, c, ZMn7 79, d, ZMnll 50, e, ZMnl4 74 *, MnO reflections 5.120 \ \ 5.110 2 5.100 Ip,P Fig. 2 Zirconia unit-cell constants for ZMnx samples as a function of the manganese content in the solid residue. X, a; 0,c. the zirconia unit-cell constants is shown more clearly in Fig. 2 where a and c are plotted against the manganese content (atom%) in the solid residue. The latter quantity was obtained from Mnsol res(atom%)= 100{A/[A+( 100-70.94A)/123.22]} 129.0 " 200 300 400 5IIo 600 700 800 900 TIK Fig. 4 Zirconia unit-cell volume, V, for ZMnx samples rinsed with HCl and heated at the specified temperatures for 2 h, in oxygen.A, ZMn7.79; 0,ZMnll.50. Table 2 Wavenumbers (in cm-') of the bands in the Raman spectra for manganese-doped zirconium oxide samples (for designation of samples see text) ZMn7.79 ZMn4.18 ZMnll.50 ZMn6.70 ZMn14.30 m-Zr0220,21 t-Zr0222 c-Z~O,,~ where A =,,,(Mnsol %)/54.94, and 54.94, 70.94 and 123.22 are 200w 192vsthe Mn, MnO and Zr02 molar masses, respectively. As Fig. 2 268s 264w 272s 275wshows, while a exhibits only a small contraction, c is found to 224w 309wdecrease strongly up to a manganese content of about 16 atom% where it reaches the value of a. For higher manganese 325m 319m content an axial ratio c/a= 1 is observed.The variation of the zirconia unit-cell volume, V,is shown in Fig. 3. The volume V exhibits a small but significant contraction, mostly due to the decrease in c. The zirconia unit-cell volume, V, for ZMnx samples heated at 753 K for 5 h in a oxygen stream is also plotted in Fig. 3. The heating in oxygen causes an additional contraction of V. For comparison, data from ref. 17 are also shown. The change of V as function of heating at a given temperature (2 h, in oxygen) is shown in Fig. 4 for the samples ZMn7.79 and ZMnl1.50 rinsed with HCl, as typical examples. For thermal treatments up to 673 K a shrinkage of I/ was found. For higher temperatures (up to 823 K, the highest temperature studied), V remains constant. No effect on the axial ratio c/a was observed, and no extra lines, due to formation of separate phases, were found. Raman spectroscopy The frequencies of the bands present in the Raman spectra are collected in Table 2 and compared with data from literature relating to monoclinic,20*21 tetragona122 and zirconia.The Raman analysis suggests that the samples may be divided into two sets according to the manganese content. Sample ZMn4.18 and ZMn6.70 showed seven bands while ZMn7.79, ZMnll.50 and ZMn14.30 have simpler spectra characterized 133.0 132.0 130.0 335m 355w 349w 380w 383m 440w 420w 463m 464m 476s 503w 539w 565m 561w 597m 590m 604w 617mw 639m 644s 647s Peak intensity: vs, very strong; s, strong; m, medium; mw, medium weak; w, weak.by broad and weak bands. As shown in Table2, the spectra of samples of the first group match the Raman bands of tetragonal zirconia,22 those of the second one show the Raman spectrum typical of cubic Zr02.23 Magnetic measurements The specific magnetic susceptibility values measured for Zr02, xgZr02,were positive with a small temperature dependence, but were independent of the treatment to which the zirconia was submitted. At 120 and at 292K the measured values were 6 x and 4 x erg GP2g-l, respectively. The specific magnetic susceptibility for undoped Zr02 is reported to be -1.1 x erg G-2 g-1.24 The positive values measured in our case may be accounted for in terms of small deviations from stoichiometry and/or in terms of paramagnetic contami- nations.The second hypothesis is more probable. In fact electron paramagnetic resonance (EPR) spectra showed a line +at g = 4.2 characteristic of Fe3 .25 However, other paramag- 129.0 t+'+,c netic species not detectable by EPR (e.g. Fe2+) cannot be excluded. The molar susceptibility of manganese, xMn,was obtained by subtracting the measured contribution of the matrix from the specific susceptibility of the sample, xgsample, using: Fig. 3 Zirconia unit-cell volume, V, as a function of the manganese XMn = 54.94{[XTrnPle-Zr02(%)x xgzro2]/Mn("/~)}content in the solid residue. 0, ZMnx samples; 0, ZMnx samples rinsed with HC1 and heated at 753 K, for 5 h in oxygen; 0,from ref.17. where 54.94 is the Mn molar mass and Zr02(%) and Mn(%) J. Muter. Chem., 1996, 6(3), 403-408 405 are the percentages of ZrO, and of manganese, respectively The molar susceptibility of manganese was found to be field- independent and to obey the Curie-Weiss law x=C/(T+@), where C is the Curie constant and 0 the Weiss temperature The constant C is related to the effective magnetic moment, Peff > by Peff =(8CP2 Table 3 shows the results of the magnetic measurements for samples before and after rinsing with HCl and for samples heated in oxygen at 753 K after HCl treatment For each sample the rinsing with HC1 had virtually no effect on the magnetic moment, while the heating in oxygen caused a decrease in this value The values of 0 were low and were not affected appreciably by the treatments For the sample ZMnll 50 reheated in hydrogen at 1073 K in the silica reactor, then cooled to room temperature in dry hydrogen and submit- ted to the magnetic determinations without exposure to the atmosphere, the values of peffand 0 were 5 93 pB (pB=Bohr magneton =9 274 x J T-'), and -30 K, respectively Thermogravimetry and XPS measurements Fig 5 shows the results of the thermogravimetry experiment for the ZMnll 50 sample, rinsed with HCl and heated in oxygen at 753 K for 5 h, taken as a typical example No change in mass was observed for ZrO, submitted to a similar thermal 00 05 10 i15 O2 02 Fig.5 TG curves for the ZMnll 50 sample, rinsed with HC1 heated at 753 K for 5 h, in oxygen (a) Heating and cooling in H, containing 0 2% water, followed by heating in N,, (b)heating in H, (2% water) and cooling in dry H, (liquid-nitrogen trap), (c) heating and cooling in 0, subsequent to (b) B, mass in dry H, treatment Fig 5(u) refers to the experiment in a hydrogen atmosphere, a decrease in mass was observed up to 873 K but for higher temperatures no mass variation was found The sample was then cooled to room temperature in the same apparatus and an increase in mass from 473 to 373 K was observed To test whether this behaviour was due to adsorption of water present in the gas, a subsequent heating in a nitrogen stream was performed no change in mass was observed up to 460 K The thermogravimetric behaviour of a second portion of the same sample is shown in Fig 5(b) The experimental conditions were similar to those for Fig 5(u), the only exception being the cooling carried out in a dry hydrogen stream (liquid- nitrogen trap) no mass increase was observed in the cooling range Fig 5(c) shows a thermogravimetry experiment in oxygen subsequent to that just described Note that an increase in mass was recorded at room temperature by changing the atmosphere from dry hydrogen to oxygen After equilibration, the temperature was raised to 753 K The mass increased, reaching a maximum at ca 573 K From 573 to 753 K the mass decreased and remained constant during the heating at this temperature for 5 h In the subsequent cooling to room temperature an increase in mass was found For some ZMn samples rinsed with HCl and heated in oxygen at 753 K for 5 h, Table 4 collects the mass variations in the thermogravimetry experiments in hydrogen (from room temperature to 1073 K), A%", An XPS analysis performed on sample ZMnl4 74, heated in hydrogen at 1073 K, rinsed with HCl, dried under vacuum and exposed to the atmosphere at room temperature, detects only the signal of Mn4+, the Mn 2p3,, EB value being 642 2t-0 2 eV 26 Discussion Samples prepared in H, The treatment in a hydrogen stream at 1073 K causes the incorporation of most of the manganese in the zirconia struc- ture The small fraction not in solid solution is present essen- tially as MnO The separate phase, directly revealed by XRD in the two more concentrated samples (Table 1)was quantitat- ively evaluated by rinsing the samples with HC1 The solid- solution formation is deduced from the variation of the zirconia unit-cell volume, V,as the manganese content increases (Fig 3) The incorporation of a high fraction of manganese into the zirconia structure explains the low values of the Weiss tempera- ture, @ (Table 3) In fact, the Mn2+-0-Mn2+ superexchange interactions, responsible for the an tiferromagne tic properties of MnO (0 610K),27 are markedly decreased in the solid solution in diamagnetic ZrO, The manganese incorporation opposes the formation of the otherwise thermodynamically stable monoclinic phase of zir- conia, and favours the tetragonal and the cubic modifications (Fig 1) As the manganese content increases cubic zirconia is formed at the expense of the tetragonal phase In fact, as Fig 2 Table 3 Magnetic data for manganese-containing zirconia samples subjected to different treatments treatments ZMn6 70 -7 5 78 6 50 -5 4 06 5 70 6 45 -5 4 94 ZMn7 79 -b -b 7 64 -10 3 89 5 58 7 58 -10 4 95 ZMnll50 -22 5 70 10 64 -20 4 03 5 68 10 57 -20 4 73 ZMnl4 74 -43 5 52 12 57 -35 408 5 71 12 41 -30 4 89 From Mnsol taking into account the mass increase during the heating in oxygen up to 1073 K (from TG) Not determined 406 J Muter Chem , 1996, 6(3), 403-408 Table 4 Composition of ZMn samples after HCl rinsing and heating at 753 K for 5 h in oxygen samples Co,/erg G-' mol-' K ZMn6.70 6.45 3.05 ZMnll.50 10.57 2.80 ZMn14.30 12.41 2.99 shows, starting from ca.16% atomic (ZMn7.79) the value of the zirconia unit-cell constant c equals that of the a parameter, whereas for lower manganese concentrations c is higher than a. Srinivasan et a[.,' have recently questioned the assignment of the cubic and tetragonal zirconia structures from XRD analyses performed on a restricted 28 range, since the two polymorphs, having similar lattice parameters, exhibit very similar front-reflection XRD patterns. In the present work, however, a firm assignment was reached since it was carried out by extending the analysis to the back-reflection region, and it was confirmed by Raman spectroscopy. The application of the latter technique is of particular help since tefragonal2, and monoclinic20.21 ZrO, have different spectra.As Table 2 shows, the Raman spectra of ZMn4.18 and ZMn6.70 match that of the tetragonal zirconia, while for samples with higher manganese content, starting from ZMn7.79 (manganese content = 15.97 atom%), the spectrum of the cubic modification is found. The stabilization of tetragonal and cubic structures by aliovalent ions was recently It was suggested that the aliovalent dopants stabilize the tetragonal and cubic structures because their effective charge alters the interatomic force constant. From magnetic measurements (Table 3) we conclude that in the 'as-prepared' samples most of the manganese is present as Mn2+, while a small fraction is in higher oxidized states (Mn3+ and/or Mn4+).In fact, for these samples the values of the magnetic moment, peff, are lower than 5.92 pB, the value expected for Mn2+. Moreover, since after rinsing with HCl, the magnetic moment remains lower than that expected for Mn2+, the oxidized manganese ions (Mn3+ and/or Mn4+) must be in solid solution. Their formation occurs during the cooling to room temperature at the end of the thermal treat- ment in a hydrogen stream and/or during exposure of the sample to air at room temperature. The presence of Mn4+ on the upper layers of these samples was indeed detected by XPS. On the other hand, direct evidence of the oxidation during the cooling in hydrogen is obtained from thermogravimetric experiments (Fig. 5). An increase in mass is observed from 473 to 373 K when the sample is cooled in the thermogravimetric apparatus [Fig.5(a)].This phenomenon cannot be ascribed to adsorption of water present in the gas stream since no loss in mass is recorded in the subsequent heating in nitrogen. Indeed, the mass increase is a consequence of the Mn2+ oxidation to Mn3+ and/or to Mn4+ according to the reactions: Zr, -,MnX2+0,-, +-Y 2 H20+ Zr, -xMnx-y2+Mny3+0, +-Y H,-x+y,2 2 Zr, -xMnx2+0,-+yH,O + Zr, -xMnx-y2+Mn,4+0,-+ +yH, as demonstrated by the fact that no increase in mass is observed if water is removed in the cooling step [Fig. 5(b)]. The value of the magnetic moment (5.93 pB)measured for the ZMnll.50 sample heated at 1073 K in hydrogen in a silica reactor, cooled in dry hydrogen and submitted to the magnetic measurement without exposure to air, confirms that if water is absent the thermal treatment in hydrogen is able to keep all the manganese in the +2 oxidation state.On the other hand, Fig. 5(c)shows that when this sample is exposed to air at room temperature, an increase in mass amounting to 0.41% is A%Hz Mn2+(%) Mn3+(%) ~n~+(%) 0.96 1.8 2.7 1.9 1.86 1.2 5.9 3.5 1.92 2.7 6.2 3.5 observed. Since no mass increase was found for undoped zirconia and the ZMnll.50 sample (cooled in dry hydrogen) contains solely Mn2+, the increase in mass corresponds to the oxidation of a limited amount of the latter ions. Therefore, an estimation of the solid-solution composition may be carried out, indicating with x,y,z the amount of Mn2+, Mn3+ and Mn4+ present in 100 g of sample after the air contact: x+y +z =Mnsolres (YO) (1) 16/54.94 (0.5y+~)=A% (2) 4.38~+3y + 1.892=CMn,,, res(Y~) (3) where 16 and 54.94 are the oxygen and manganese molar masses, respectively, C (=4.03) is the Curie constant for the sample ZMnll.50 rinsed with HC1 (Table 3), and 4.38, 3 and 1.89 are the Curie constant expected for Mn2+, Mn3+ and Mn4+, respectively.The solution of the system gives x= 8.4, y= 1.6 and z=O.6. This result points out that in our preparation conditions a large fraction of the incorporated manganese (almost 80%) remains in the +2 oxidation state. Since the other ZMnx samples exhibit values of the Curie constant similar to that found for ZMnll.50 (Table 3), the presence of a large fraction of Mn2+ in solid solution must be expected for them also.Therefore, on the !asis of the ionic radii of the involved spec@ (YZr4+(CN 8,=O.84 4, TMnZ+(CN ,,=0.96 A, YMn3+(CN 6)=0.645 A, rMn4+(CN 6)=0.53 A; CN =coordination number)30 an expan-sion of the zirconia unit-cell volume should be foreseen since Mn2+ is markedly present, and larger values for the ionic radii of Mn3+ and Mn4+ should be taken when eight-coordination is considered. In fact, for the transition-metal ions of the first series in the usual oxidation states, an increase in the ionic radius of around 15% is found passing from six- to eight- co~rdination.~~In contrast, Fig. 2 shows that a small but significant decrease in I/ is measured.This result shows that the incorporation of aliovalent ions into the zirconia structure cannot be described on the basis of a simple ionic picture. In fact, the incorporation of ions with a positive charge lower than +4 takes place with formation of anionic vacancies. Their number and their extent of association control the coordination number of cations and thus the value of their ionic radii. Therefore for a correlation between the zirconia unit-cell volume and the solid-solution composition it is neces- sary to have a deeper insight into the cation local structures and their variation with the solute concentration. It was recently sho~n,~',~~ that Vegard's law may be applied to the zirconia solid solution by considering an ion-packing model based on defect clusters.By extrapolating data concerning zirconium oxide solid solutions of several metal oxides, taken from the literature, to zero dopant content it was shown that the ynit-cell volume for undoped cubic zirconia is I/= 133.1 A3.33 This value is in goo{ agreement with the value extrapolated in our case, V= 133.2A3 (Fig. 3). Samples heated at 480 "Cin oxygen for 5 h As already shown, in the rinsed samples most of the manganese is present as Mn2+ and only a small fraction is present in higher oxidation states. When these samples are heated in oxygen, the fraction of oxidized manganese increases as demon- strated by the thermogravimetric behaviour exemplified in Fig. 5(c) for the ZMn11.50 sample.The small decrease in mass J. Mater. Chew., 1996, 6(3), 403-408 407 in the range 573-753 K suggests that for temperatures higher than ca 573 K, Mn4+ (formed during the heating and/or present at room temperature) tends to transform to Mn3+, according to the chemistry of manganese oxides 33 in particular, MnO, is reported to decompose at higher temperatures giving Mn203 at ca 873 K We note that in the present work the transformation takes place at a lower temperature This may be related to the higher mobility of oxygen ions in the zirconia structure When kept at constant temperature the sample composition remains fixed, at least on a 5 h timescale [Fig 5(c)], while an oxidation occurs when the sample is cooled to room temperature Therefore the composition at room temperature is different from that at 753 K As Table 3 shows, magnetic moment values close to 4 9 pB are measured for samples after the heating in oxygen, thus confirming that the fraction of oxidized manganese has increased On the other hand, the redox process takes place in solid solution In fact, while no separate phase is observed on the XRD patterns of these samples, Fig 4 shows that the zirconia unit-cell volume V decreases as the temperature of the heating in oxygen increases As already noted, it is not possible to correlate quantitatively V to the composition, nevertheless the volume contraction is a clear indication that Mn2+ is converted to ionic species of smaller ionic radius (Mn3+ and/or Mn4+) Fig 4 also shows that for samples heated in the range from ca 600 to 823 K, V (measured at room temperature) is constant This behaviour indicates that the sample composition at room temperature does not depend significantly on the actual tem- perature of the thermal treatment We believe that this is a consequence of the oxidation taking place during the cooling from the higher temperature range [Fig 5(c)] The composition of the samples may be evaluated by combining magnetic, thermogravimetry and analytical data Since the manganese is completely reduced to Mn2+ by heating in hydrogen at 1000 K, the mass variation in the thermogravi- metry experiment in hydrogen up to this temperature is related to the conversion to Mn2+ of the oxidized manganese (Mn3+ and Mn4+) present in the sample at room temperature Therefore, indicating with x,y,z the amount of Mn2+, Mn3+ and Mn4+ in 100 g of sample after the heating at 753 K for 5 h in oxygen, eqns (l), (2) and (3) may be considered, substituting A% with A%H2(the mass change in the thermo- gravimetnc experiment in hydrogen) in eqn (2), and C with Co2 (the experimental Curie constant for samples heated in oxygen at 753 K) in eqn (3) The results are listed in Table 4 The simultaneous presence of Mn2+, Mn3+ and Mn4+ is apparent, in spite of the experimental errors in the various determinations In this context, note the results of a recent study on the manganese oxide-zirconium oxide system l7 The samples, pre- pared by heating in air a mixture of co-precipitated zirconium and manganese hydroxides in the range 773-1073K were studied, by XRD among other techniques The values of the zirconia unit-cell volume as a function of the manganese content are shown in Fig 3 and agree with our data, but in ref 17 the incorporation of manganese, essentially as Mn4+, was claimed Conclusions By heating in hydrogen at 1073K, manganese is incorporated into the zirconia structure as Mn2+ However, the presence of water in the gas stream during the cooling step and the exposure of the sample to the atmosphere (at room tempera- ture) cause the oxidation of a limited fraction of Mn2+ to Mn3+ and/or Mn4+ The incorporation of the foreign species in the solid solution favours the tetragonal modification of zirconia, which becomes cubic for a manganese content higher than ca 16 atom% In spite of the value of the Mn2+ ionic 408 J Muter Chew, 1996, 6(3), 403-408 radius [rM,,Z+(CN 8)=0 96 h]being larger than that of the host cation [rZr4+(CN 8) =0 84 A], the incorporation of manganese causes a decrease of the zirconia unit-cell volume This phenom- enon indicates that the system cannot be treated on a simple ionic basis Heating in oxygen up to 753 K induces an oxidation of Mn2+, partially transformed to Mn3+ and/or Mn4+ The oxidation takes place in solid solution and causes a contraction of the zirconia unit-cell volume On the basis of magnetic susceptibility determinations, thermogravimetry and analytical data, the composition of the oxidized samples was deduced Financial support from the Italian MURST (Finanziamenti progetti di ricerca di interesse nazionale-quota 40%) is gratefully acknowledged References 1 M Valigi and D Gazzoli, Reactivity of Solids, ed P Barret and L-C Dufour, Elsevier, Amsterdam, 1985, p 1081 2 P D L Mercera, J G Van Ommen, E B M Doesburg, A J Burggraaf and J R H Ross, Appl Catal, 1990,57,127 3 P D L Mercera, J G Van Ommen, E B M Doesburg, A J Burggraaf and J R H Ross, Appl Catal, 1991,71,363 4 A Cimino, D Cordischi, S De Rossi, G Ferraris, D 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