Abstract: Phase diagrams in the subsolidus area of the systems FeVO4 – CdO and FeVO4 – Cd2V2O7 have been deduced using the results of XRD and DTA analyses. On the basis of these diagrams and some additional verifying research, a projection of the subsolidus area of the CdO – V2O5 – Fe2O3 system onto the plane that comprises the components’ concentration triangle has been presented. The H-type phase is the only phase formed in this system. It co-exists at equilibrium with other phases in six subsidiary subsystems.
Department of Inorganic and Analytical Chemistry, Szczecin University of Technology,
71-065 Szczecin, Poland
Anna Blonska-Tabero
SSC-2008
1. Introduction Research on phase relations in multi-component metal oxide systems frequently leads to obtaining new phases formed with the participation of all components of the systems under investigation. These studies allow the composition of the new phases to be accurately described, and determination of their range of co-existence with the other phases formed in a given system. Research on phase relations over the entire concentration range of components in MO – V2O5 – Fe2O3 (M = Co, Zn) systems [1-5] has shown that within these systems there exist phases belonging to two families. One of them is made up by compounds with the general formula M2FeV3O11, formed through a reaction between FeVO4 and M2V2O7. The other family is represented by phases possessing the structure of howardevansite, i.e. the NaCuFe2V3O12 mineral [6]. Both phases with the howardevansite type structure are products of a reaction between FeVO4 and M3V2O8, but they have different compositions, corresponding to the formula Co2.616Fe4.256V6O24 [1,2] in the first case, and to Zn3Fe4V6O24 in the other [3,4]. Results of recent studies on phase relations in the system CdO – V2O5 – Fe2O3 [7] indicate that also within this system a phase of the howardevansite type structure is formed. An interesting
fact is that this phase is formed in the FeVO4 – Cd4V2O9
cross-section, indicating a significant difference from the phases obtained in the analogous ternary systems [1-4]. Moreover, the phase which is formed in the system CdO – V2O5 – Fe2O3, unlike the Co2.616Fe4.256V6O24 and Zn3Fe4V6O24 phases, exhibits variable composition within a range of homogeneity. Its composition can be expressed in the formula Cd4Fe7+xV9+xO37+4x, where - 0.5 < x < 1.5 [7]. It has also been established that in the system CdO – V2O5 – Fe2O3 no compound belonging to the family M2FeV3O11 (formed in the two analogous ternary systems) is created. A sample corresponding in composition to „Cd2FeV3O11” is a mixture of CdV2O6, Cd2V2O7 and the new phase of the howardevansite type structure [7]. Thus, the CdO – V2O5 – Fe2O3 system is an interesting subject for the extension of studies which have so far been carried out only in a fragmentary way.
The aim of this work was to describe phase relations in the whole subsolidus area of the system CdO – V2O5 – Fe2O3. The results obtained have allowed us to determine the range of co-existence of the new phase with other compounds formed within this system. Moreover, they have enabled us to determine whether any previously unidentified compounds, other than the new phase mentioned above, are formed in the CdO – V2O5 – Fe2O3 system.
2. Experimental ProceduresReagents used in this research were: CdCO3 (p.a., Serva, Germany), V2O5 (p.a., Riedel-de Haën, Germany), Fe2O3 (p.a., POCh, Poland), Cd2V2O7, CdV2O6, FeVO4 and Fe2V4O13. The vanadates were obtained by heating stoichiometric mixtures of CdCO3 with V2O5, or Fe2O3 with V2O5.
The reactions were performed by the conventional sintering method. Appropriate portions of reacting substances were homogenised by grinding, and heated for several stages under air. On each heating stage the samples were cooled down in the furnace to ambient temperature, ground and examined by XRD method. The heating temperatures were chosen on the basis of DTA curves of selected samples. When the composition of the sample did not change after two consecutive heating stages, the process of heating this sample was finished. For the verifying investigations, mixtures of appropriate phases were heated after homogenisation in two 20-hour stages at temperatures fifty to seventy degrees centigrade below their melting points. Next they were cooled down to ambient temperature, ground and examined by XRD.
The DTA measurements were carried out in air using a Paulik-Paulik-Erdey type derivatograph (MOM, Hungary). Samples of 500 mg were heated in quartz crucibles in the temperature range 20 - 1000°C at a rate of 10 °C min-1.
The types of phase present in the samples were deduced from their powder diffraction patterns obtained by means of an X-ray diffractometer DRON-3 (Bourevestnik, Sankt Petersburg, Russia). The source of radiation used was a copper tube equipped with a nickel filter. The identification of the phases was conducted with the aid of XRD characteristics contained in the PDF cards and the data presented in [7].
Phase equilibrium diagrams were constructed using the XRD and DTA results for samples at equilibrium. The melting temperatures of phase mixtures remaining at equilibrium in given areas were read in each case from the corresponding DTA curve, as the onset temperature of the first endothermic effect not due to polymorphic transformation.
3. Results and DiscussionThe first stage of the research involved determining the phase relations in two systems, FeVO4 – CdO and FeVO4 – Cd2V2O7, constituting cross-sections of the CdO – V2O5 – Fe2O3 system. Fig. 1 shows the positions of the systems studied in the component concentration triangle of the ternary system. The position of the new phase, i.e. Cd4Fe7+xV9+xO37+4x, where -0.5 < x < 1.5 (hereafter the H-type phase) is also shown. In the samples (whose compositions are marked as ○), the phases identified besides the H-type phase were FeVO4
Figure 1. The position of the systems studied and the H-type phase
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Subsolidus area of the system CdO – V2O5 – Fe2O3
or Cd2V2O7 and Fe2O3 respectively. Their presence was evidenced by the XRD method, but their quantities corresponded to the minimum limit of detection [7].
22 mixtures were prepared for the tests. Their compositions, their heating regimes, and the analysis results obtained by XRD after the last heating stage are shown in Tables 1 and 2. The compositions of samples 4, 5, 17 and 18 were selected in such a way that they were situated on a line joining Fe2O3 or CdV2O6 with the H-type phase. This line corresponds to the compositions of the final samples in which only the H-type phase was identified. On the basis of the XRD and DTA results presented for all samples having achieved the state of equilibrium, phase equilibrium diagrams of the FeVO4 – CdO and FeVO4 – Cd2V2O7 systems in the subsolidus area have been constructed (Figs. 2 and 3). Dashed lines in these figures separate diphase fields from triphase fields.
It can be seen from the diagram of the FeVO4 – CdO (Fig. 2) system that it is a cross-section of the system CdO – V2O5 – Fe2O3. This cross-section goes through six subsidiary subsystems of the latter system. One
of these subsidiary subsystems constitutes a diphase field, whereas the remaining five are triphase fields. The H-type phase co-exists at equilibrium with other compounds in three subsidiary subsystems, specifically: [H-type phase – FeVO4 – Fe2O3], [H-type phase – Fe2O3] and [H-type phase – Fe2O3 – Cd2V2O7]. The positions of the dashed lines correspond to compositions with 27.58 mol% CdO and 32.00 mol% CdO. They are situated on lines that join Fe2O3 with the H-type phase. They mark the boundaries of composition regions where FeVO4 or Cd2V2O7 were identified in the samples along with the H-type phase and Fe2O3.
The FeVO4 – Cd2V2O7 (Fig. 3) system is also a cross-section of CdO – V2O5 – Fe2O3 which crosses its three subsidiary subsystems. The H-phase co-exists at equilibrium with other compounds in all these subsidiary subsystems. Moreover, two of these subsystems are triphase fields, whereas the third is a diphase field. A line of polymorphic transformation of CdV2O6 [8,9] is also marked in the diagram. Positions of dashed lines correspond to the contents: 26.09 mol% Cd2V2O7 and 31.58 mol% Cd2V2O7.
Table 1. The FeVO4-CdO system. Compositions of initial mixtures, their heating regimes and XRD analysis results for samples after the final heatingstage.
Table 2. The FeVO4-Cd2V2O7 system. Compositions of initial mixtures, their heating regimes and XRD analysis results for samples after the finalheating stage.
Figure 2. Phase diagram of the FeVO4 – CdO system in the subsolidus region
Figure 3. Phase diagram of the FeVO4 – Cd2V2O7 system in the subsolidus region
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Subsolidus area of the system CdO – V2O5 – Fe2O3
They are situated on lines that join CdV2O6 with the H-type phase. They mark the boundaries of composition regions where FeVO4 or Cd2V2O7 were identified in the samples along with the H-type phase and CdV2O6.
The information contained in both phase equilibrium diagrams permitted a division of a part of the subsolidus area of the CdO – V2O5 – Fe2O3 system into several subsidiary subsystems. The suggested division failed to take into account the area circumscribed by the compounds CdV2O6, FeVO4 Fe2V4O13, V2O5. Phase relations in this area were studied through heating appropriate mixtures of ready-made phases mixed in suitable proportions. The aim of the test was to determine which phases co-exist with cadmium metavanadate(V) at equilibrium. For the test two mixtures: CdV2O6 with FeVO4 (sample 1) and CdV2O6 with Fe2V4O13 (sample 2) were prepared. The compositions of both mixtures are provided in Table 3. Sample 1 was heated in two 20-hour stages at 690°C, while sample 2 underwent a procedure of two 20-hour heating stages at 580°C. The
XRD patterns of both samples did not change, which proves that the composition of initial mixtures comprised only phases co-existing at equilibrium. These results led to the conclusion that there exist two subsidiary subsystems: [CdV2O6 – FeVO4 – Fe2V4O13] and [CdV2O6 – Fe2V4O13 – V2O5] in this area of the concentration triangle. With intent to confirm these assumptions two mixtures of these phases, presumed to co-exist at equilibrium, were prepared. Their composition is shown in Table 3 (samples 3 and 4). The mixtures were heated in two stages, each of them lasting 20 h, at the temperature of 580°C. The XRD patterns of the mixtures did not undergo any changes, which confirms that the composition of the initial mixtures comprised only phases co-existing at equilibrium.
The results of all of the investigations performed were combined to construct a phase equilibrium diagram of the CdO – V2O5 – Fe2O3 system for the whole subsolidus region. A projection of the solidus surface onto the plane of the components’ concentration triangle
Table 3. Compositions of the verifying mixtures
No. Composition of verifying mixtures
Composition of verifying mixtures in terms of components of CdO-V2O5-Fe2O3 system/mol%
CdO V2O5 Fe2O3
1 CdV2O6, FeVO4 33.33 50.00 16.67
2 CdV2O6, Fe2V4O13 20.00 60.00 20.00
3 CdV2O6, Fe2V4O13, FeVO4 14.29 57.14 28.57
4 CdV2O6, Fe2V4O13, V2O5 14.29 71.42 14.29
Figure 4. Projection of the solidus surface onto the plane of the component concentration triangle for the CdO –V2O5 – Fe2O3 system.
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of this system is shown in Fig. 4. The lines connecting the H-type phase with other compounds are drawn with a dashed line. This diagram indicates that in the CdO – V2O5 – Fe2O3 system eleven subsidiary subsystems can be distinguished where the following phases co-exist at equilibrium (Table 4).
Fig. 5 presents fragments of DTA curves for those samples which contain the H-type phase at equilibrium. The first effects recorded on these curves are due to
melting of mixtures of appropriate phases co-existing at equilibrium. Further effects are related to phase transitions occurring above the solidus line.
The results obtained lead to the conclusion that in the CdO – V2O5 – Fe2O3 system, no phase other than the H-type phase is formed in equilibrium mixtures, which is made up of all three components of this system.
4. ConclusionsThe only phase that is formed in the CdO – V- 2O5 – Fe2O3
system with an involvement of all its components is the H-type phase.
In the CdO – V- 2O5 – Fe2O3 system 11 subsidiary subsystems can be distinguished. In two of them two solid phases are present at equilibrium, while the remaining 9 subsystems each have three solid phases co-existing at equilibrium.
The H-type phase co-exists at equilibrium with other - phases in six of the subsidiary subsystems.
Figure 5. Fragments of DTA curves from which the melting temperatures of the respective subsidiary subsystems were determined (Roman
numerals denote the numbers of the corresponding subsidiary subsystems)
Table 4. Melting temperatures of phase mixtures remaining at equilibrium
