Phase relations in the yttria – neodymia system at 1500 ° C

The phase equilibria in the binary system Nd2O3−Y2O3 at 1500 °C were studied in the overall concentration range by means of X-ray analysis and petrography. The samples of various compositions were derived from nitrate acid solutions (prepared by dissolving neodymia and yttria powders in hot diluted nitric acid) by evaporation, drying and heat treatment at 1100 and 1500 °C. Solid solutions based on different crystalline modifications of the components were obtained. The boundaries of solubility and concentration dependence of the lattice parameters for the phases formed in the system were defined.


I. Introduction
Oxides of rare earth elements (REE) have unique structural and functional properties (fire resistance, resistance to aggressive environments, high thermomechanical properties) [1][2][3].Materials based on them are widely used in electronics, optoelectronics, mechanical engineering, laser technology, chemical industry, metallurgy, medicine, etc. [4,5].The Nd 2 O 3 −Y 2 O 3 system is of special interest and the basis for new materials.
Phase equilibria in binary Ln 2 O 3 −Ln ′ 2 O 3 systems were studied and characterized by the formation of limited solid solutions based on different polymorphs of initial components.From two to five polymorphs are known for REE oxides (Ln 2 O 3 ): low temperature hexagonal (A, space group P63/mmm), monoclinic (B, space group C2/m), low temperature cubic (C, space group Ia3), high temperature hexagonal (H) and high temperature cubic (X) [6][7][8][9][10].The stability of the low temperature crystalline modifications of REE oxides in the temperature range of 1000-1800 °C depends on the size of the ionic radius of Ln 3+ .Polymorphic transitions in the oxides La. . .Gd may be reversible (such as C − −− ⇀ ↽ −− − H) and irreversible (C − −− → A, C − −− → B), which are studied in details by Glushkova [11,12].
Phase relations and structure of the phase formed in the Nd 2 O 3 −Y 2 O 3 system were studied by different authors [13][14][15][16][17][18].Liquidus is characterized by peritectic type reaction between liquid and solid phases (L + H − −− ⇀ ↽ −− − X) at 2370 °C for 84 mol% Y 2 O 3 , and the point of minimum melting temperature is at ∼2250 °C for 30 mol% Y 2 O 3 .In the Nd 2 O 3 −Y 2 O 3 system, the continuous series of solid solutions of H-type as well as limited solid solutions based on X-, A-, B-and C-REE oxide polymorphs are formed, which react between each other in solids by eutectoid types of transformations.Phase equilibria had been most thoroughly investigated at high temperatures (2000-2400 °C).Only a limited number of publications [13,14] can be found at temperatures below 1900 °C, but data for both variants of phase diagram were shown hypothetically with dashed lines and defined with low accuracy.Phase reactions in the Nd 2 O 3 −Y 2 O 3 system at temperatures 1300-1600 °C have been studied experimentally and thermodynamic assessment was done as well [15].The boundary of the phase field was defined for C-Y 2 O 3 solid solution with 35 mol% Nd 2 O 3 and at 1300-1600 °C and two-phase field (C + B) with 50 mol% Nd 2 O 3 with low accuracy [15], where the concentration step was taken from 10 to 20 mol%.The homogeneity field extension for B-Nd 2 O 3 was not defined experimentally, but was calculated through the Thermo-Calc [10].These data were in a contradiction with previously published results [13,14], thus both have to be verified before developing ternary phase diagrams grounded on proper binary system knowledge.Thus, the study of phase equilibria in the binary Nd 2 O 3 −Y 2 O 3 system is urgent and requires further research.
In the present paper, the interaction between yttria and neodymia at 1500 °C in the overall concentration range has been studied.

II. Experimental procedure
Yttrium oxide, Y 2 O 3 , neodymium oxide, Nd 2 O 3 (all 99.99% produced by Merck Corp.) and analytical grade nitric acid were used as the starting materials.The neodymia and yttria powders were preliminary dried at 200 °C for 20 h followed by dissolving in hot diluted nitric acid (1:1).The Nd 2 O 3 −Y 2 O 3 samples were prepared in concentration step of 1-5 mol% from nitrate solutions with their subsequent evaporation and decomposition at 800 °C for 2 h.The prepared powders were uniaxially pressed at 10 MPa into pellets of 5 mm in diameter and 4 mm in height.To study phase relationships at 1500 °C the as-prepared samples were thermally treated in two stages: at 1100 °C (for 1626 h in air) and then at 1500 °C (for 60 h in air) in the furnaces with heating elements based on Fecral (H23U5T) and Superkanthal (MoSi 2 ), respectively.The heating rate was 3.5 °C/min.The two-step annealing allows removing residuals of nitrogen oxides from the samples, but it is also positive for preliminary densification and following homogenization at 1500 °C.Annealing of the samples was continuous.Cooling was carried out within the furnace.
The X-ray analysis of the samples was performed by powder method using DRON-3 at room temperature (CuKα radiation) with step size of 0.05-0.1 degrees in the range 2θ = 15-90°.Lattice parameters were refined by the least squares fitting using the LATTIC code.The algorithm of the Lattic code is grounded on regression analysis of diffraction data.The accuracy in the lattice parameter of cubic phases was within 0.0002 nm.Phase composition has been determined in accordance with the International powder standards (JSPDS International Center for Diffraction Data 1999).
The composition of the samples was monitored by spectral and chemical analysis selectively.The petrographic studies of annealed samples were carried in polarized light.The optical characteristics were specified in polarizing microscope MIN-8 with the aid of highly refractive immersion liquids.
Microstructures were examined on polished sections and rough fractured surfaces of annealed samples in backscattered electron (COMPO) and secondary electron (SE) modes by electron-probe X-ray microanalysis (EPXMA).
Stoichiometric composition was controlled selectively by chemical and X-ray fluorescence spectrum analysis.Key difference from the literature data concerns homogeneity field size for the B-phase.In the present research, the boundary compositions have been defined Thus, the homogeneity field is substantially narrower than measured [13,14] and calculated [15] in the literature.It is suggested that the too wide concentration step of 10 to 20 mol% in the referred papers was the reason for low accuracy.The diffraction patterns of the samples characterizing the phase region of the solid solutions in the Nd 2 O 3 −Y 2 O 3 system at 1500 °C are shown in Fig. 3.The X-ray diffraction and petrography data have been  2).The second phase is darker in colour and has been identified as a cubic (C) modification of Y 2 O 3 (Table 2).With the decrease of neodymium oxide concentration, the content of the isotropic C-phase increases.In the samples containing 30 to 40 mol% Nd 2 O 3 , the cubic modification of yttrium oxide forms the matrix phase (Fig. 4b).The EPMA results for the two phase (A + B) sample with 10 mol% Y 2 O 3 helped to define equilibrium compositions for A-and B-solid solutions as 4. bility of neodymia in yttria was found to be 28 mol% at 1500 °C using combination of petrography, XRD and EPMA data.One should note that the petrography helped to recognize the traces of anisotropic B-phase embedded into the C-matrix in the sample containing 30 mol% Nd 2 O 3 , otherwise undefined with the XRD below its sensitivity limit.Newly found solubility is different as compared with the experimental data of Coutures et al. [13] and Adylov et al. [14], where the boundary was defined at ∼35 mol% Nd 2 O 3 by XRD analysis only using quite big concentration step of 10 mol%.

IV. Conclusions
The phase equilibria in the Nd 2 O 3 −Y 2 O 3 system were studied at 1500 °C in the whole concentration range using XRD, petrography and microscopy observations.It has been established that the solubility boundary (studied system is characterized by three types) is localized: for hexagonal (A-Nd Studies on solid phase interaction of Nd 2 O 3 (hexagonal modification, A) and Y 2 O 3 (cubic modification, C) at temperature of 1500 °C has shown that the Nd 2 O 3 −Y 2 O 3 system consists of three types of solid solutions, based on hexagonal modification (A-Nd 2 O 3 ), monoclinic modification (B-Nd 2 O 3 ), cubic modification (C-Y 2 O 3 ), separated by the two-phase fields (A + B) and (B + C) (Fig. 1).

Figure 1 .
Figure 1.Phase equilibria in the Nd 2 O 3 −Y 2 O 3 system at 1500 °C ( -single-phase samples; -two-phase samples) at 1500 °C (60 hours).The lattice parameter increases from a = 1.0604 nm for the pure Y 2 O 3 to a = 1.0711 nm for the boundary solid solution.The solubility of Y 2 O 3 in the hexagonal A-Nd 2 O 3 modification is ∼4 mol% Y 2 O 3 .In accordance with the XRD phase analysis, in the samples containing from 80 to 100 mol% of Nd 2 O 3 (1500 °C), instead of the hexagonal modification of A-Nd 2 O 3 , the hexagonal neodymium hydroxide A-Nd(OH) 3 was observed as soon as neodymia absorbs water from wet air and transforms into hydroxide.The lattice parameters are changed from a = 0.6418 nm, c = 0.3743 nm for the pure A-Nd(OH) 3 to a = 0.6447 nm, c = 0.3722 nm in the two-phase sample (A + B) containing 95 mol% Nd 2 O 3 .

Figure 2 .Figure 3 .
Figure 2. Concentration dependences of the lattice parameters for solid solutions based on C-Y 2 O 3 (a) and B-Nd 2 O 3 (b, c) in the Nd 2 O 3 −Y 2 O 3 system heat-treated at 1500 °C

Figure 4 .
Figure 4. SEM microstructures of the samples from the Nd 2 O 3 −Y 2 O 3 system heat-treated at 1500 °C: a) 55 mol% Y 2 O 3 -45 mol% Nd 2 O 3 and b) 60 mol% Y 2 O 3 -40 mol% Nd 2 O 3 (light phase -<B-Nd 2 O 3 >, gray phase -<C-Y 2 O 3 >, black -pores) completed by the electron microscopy results (Fig. 4).It was established that in the concentration range of 50 to 70 mol% Y 2 O 3 , the isotropic phase of C-Y 2 O 3 was clearly identified in the anisotropic B-Nd 2 O 3 phase.The amount of anisotropic phase B-Nd 2 O 3 is significantly reduced with increasing of yttria concentration.The microstructures of the two-phase (B + C) boundary samples are shown in Fig. 4. In the samples containing 55 mol% Y 2 O 3 -45 mol% Nd 2 O 3 and 60 mol% Y 2 O 3 -40 mol% Nd 2 O 3 , two structural components appear which differ markedly by contrast.The light phase is a matrix, and according to the local X-ray analysis, it belongs to the monoclinic (B) modification of neodymium oxide (Fig. 4a and Table2).The second phase is darker in colour and has been identified as a cubic (C) modification of Y 2 O 3 (Table2).With the decrease of neodymium oxide concentration, the content of the isotropic C-phase increases.In the samples containing 30 to 40 mol% Nd 2 O 3 , the cubic modification of yttrium oxide forms the matrix phase (Fig.4b).The EPMA results for the two phase (A + B) sample with 10 mol% Y 2 O 3 helped to define equilibrium compositions for A-and B-solid solutions as 4.2 mol% Y 2 O 3 -95.8mol% Nd 2 O 3 for A phase and 22.4 mol% Y 2 O 3 -77.6 mol% Nd 2 O 3 for B phase, respectively (Table 2).For the sample containing 20 mol% Nd 2 O 3 only one isotropic phase C-Y 2 O 3 has been found.The solu- 2 mol% Y 2 O 3 -95.8mol% Nd 2 O 3 for A phase and 22.4 mol% Y 2 O 3 -77.6 mol% Nd 2 O 3 for B phase, respectively (Table 2).For the sample containing 20 mol% Nd 2 O 3 only one isotropic phase C-Y 2 O 3 has been found.The solu- 2 O 3 ) at 0-4 mol% Y 2 O 3 , for monoclinic solid solutions (B-Nd 2 O 3 ) between 20 and 45 mol% Y 2 O 3 , and for the cubic solid solutions (C-Y 2 O 3 ) in the concentration range of 72-100 mol% Y 2 O 3 , which are separated by wide two phase fields (A + B) and (B + C).In accordance with Vegard's law the lattice parameter linearly increases from a = 1.0604 nm for pure Y 2 O 3 to

Table 1 .
The chemical and phase compositions, volume of the phases, lattice parameters of solid solutions, in the Nd 2 O 3 −Y 2 O 3 system at 1500 °C, annealed for 60 h (XRD and petrography data) * Under given conditions (T = 1500 °C, 60 h, in air) the hexagonal A-Nd 2 O 3 cannot be fixed, the hexagonal phase of A-Nd(OH) 3 is formed instead: <A> -solid solution based on hexagonal modification of Nd 2 O 3 ; <C> -solid solution based on cubic modification of Y 2 O 3 ; <B> -solid solution based on monoclinic modification of Nd 2 O 3 ; tr -traces; ↑, ↓ -increase and decrease of the phase amount.

Table 2 .
Phase composition in the samples from the Y 2 O 3 −Nd 2 O 3 system, annealed at 1500 °C for 60 h in air (EPMA data were obtained from SEM images shown in Fig. 5) <A-Nd 2 O 3 > O.V. Chudinovych et al. / Processing and Application of Ceramics 11 [1] (2017) 1-6