Journal of Alloys and Compounds, 192 (1993) 105-107 105 JALCOM 2160

Phase equilibria in the system In203-M2ZnO4-ZnO at 1350 °C (M=Fe, Ga, AI) and crystal chemical consideration of InMO3 (ZnO)m phases with LuFeO3 (ZnO)m-type structures

Masaki Nakamura, Noboru Kimizuka, Takahiko Mohri and Mitsumasa Isobe National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba-shi, Ibaraki-ken 305 (Japan)

Abstract

Phase equilibria in the system InzO3-M2ZnO4-ZnO (M~Fe, Ga, AI) at 1350 °C determined by a classical quenching method are compared. There are homologous phases InMO3(ZnO),, (m is an integer) with solid solution ranges. The solid solutions between In203(ZnO)m and InMO3(ZnO),, consist of InO~.5 layers, (Znln~_~,lMxl)O2.5 layers (0<xa<l) and ZnO layers. In the ranges between InMOa(ZnO),,, and MzO3(ZnO)m, there exist (In~_x2M~2)O~.5 layers, (ZnM)Oz5 layers (0 <x2 < 1) and ZnO layers. The stabilities and crystal structures of the homologous phases InMO3(ZnO)m (m is an integer) with solid solution ranges are discussed in terms of the ionic radii and the site preference effects of the M cation elements.

1. Introduction

It is interesting to know the relationship between the stabilities and structures of inorganic oxides and their constituent cation elements. A systematic study of the phase relations in the systems R203-MzO3-M'O (R = rare earth element; M = trivalent cation element; M ' = divalent cation element) at elevated temperatures revealed that there are homologous phases (RMO3),, M 'O and RMO3(M'O), , (m and n are integers) with layered structures [4]. Kasper [5] synthesized the new compounds InzO3(ZnO),, (m = 2-5, 7) at 1050-1550 °C. Cannard and Tilley [6] analysed the crystal structures of the homologous compounds by means of an electron diffraction method. Kimizuka et al. [7] synthesized RMO3 (ZnO)m (R-----In, Sc; M = F e , Ga, A1).

In the present paper, we compare the phase relations in the system In203-M2ZnOn-ZnO ( M = F e , Ga, A1) at 1350 °C [1-3], and discuss the stabilities and crystal structures of the homologous compounds InMO3 (ZnO)m which have solid solution ranges and related compounds.

2. Experimental details

In203, Fe203, Ga203, A1203 and ZnO powders were used as the starting compounds. The experimental methods and facilities used are described elsewhere [1] in detail. Each mixture was sealed in a Pt tube and

heated at 1350 °C for a period and rapidly cooled to room temperature. It was concluded that equilibrium was attained when the X-ray powder diffraction pattern of a specimen showed no change with successive heat t reatment of the specimen. Some of the samples obtained were supplied for scanning electron microscopy and high resolution electron microscopy.

3. Results and discussion

The phase relations in the systems In203- MzZnO4-ZnO at 1350 °C are shown in Fig. 1. The solid solution ranges of the homologous phases InMO3 (ZnO)m ( r e = l , 2, 3, 7, 9, 11, 13) and those of the spinel phases InxMz_xZnO4 are listed in Table 1. The homologous phases for M = F e or Ga have ranges of solid solutions on the M-rich side and on the In-rich side. There is FezO3(ZnO)m in the system ZnO-FezZnO4. For M=A1, there exist solid solution ranges on the In-rich side only. The solid solution ranges of the spinel phases were seen for M = F e or Ga but not for M - A l . A solid solution of ZnO with a distorted wurtzite structure and (InGaO3)2ZnO were seen in the system for M-=Ga.

The crystal structural models of LuFeO3(ZnO)m for m = 1 and 4 [8] are shown in Fig. 2. All the ions are considered to be on the two-dimensional triangular lattices, e.g. A, B and C lattices. The O ion triangular

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106 M. Nakamura et aL / In2Oy-MzZnO¢--ZnO and InMO~(ZnO)m phases and structures

In203 In203

ZnO

In203

Ga2ZnO 4 ZnO Fe2ZnO 4 ZnO A12ZnO 4

(o) (0) {c )

Fig. 1. The phase relations in the system (a) In2Oa-Fe2ZnO4-ZnO, (b) In2Oy--Ga2ZnO4-ZnO and (c) In2Oa-AlzZnO4-ZnO at 1350 °C. I, II and III denote the phases InMOa(ZnO), InMO3(ZnO)2 and InMO3(ZnO)s.

TABLE 1. The solid solution ranges of homologous phases and spinel phases

m In2_xFexOa(ZnO)m In2_~Ga~Oa(ZnO)m In2_xAlxOa(ZnO)m

1 0.72 <x < 1.00 0.67 <x < 1.08 Nil. 2 0.31 <x~< 1.15 0.32 <x < 1.32 0.30 <x < 0.68 3 0.00 <x ~< 1.33 0.00 <x < 1.46 0.00 <x < 0.77 7 0.00 <x ~< 1.50 0.00 <x ~< (!,70-1.74) 0.00 <x < 0.92 9 0.00 <x ~ 1.80 0.00 ~<x < (1.56-1.72) 0.00 <x < 1.00

11 0.00 <x ~< (1.60-2.00) 0.00 <x ~ (1.57-1.64) 0.00 <x < 1.00 13 0.00 <x ~< 2.00 0.00 <x ~ (1.49-1.75) 0.00 <x < 1.00

Spinel Fe z _xlnxZnO4 Ga 2 _~In~ZnO4 Al2_xInxZnO 4 0 <x < 0.40(2) 0 <x < 0.128(4) 0 <x < 0.00

O(C) Lu(A)

O(A) M(C) O(C) M(A) O(A) O(B) Lu(C)

O(C) M(B) 0(B) M(C) o(c) O(A) Lu(B) O(B) M(A) O(A) M(B) O(B) Lu(A)

A~BB 2C'~A I I

ACBACBA

O(C) Lu(A) ~ - - ]

I o<) I O(A I O(C) M(A)~ &

M c 9+-o o(c), ~,A,~ O(B) ~ ~ '~C~D~ 8

B 8 O(A) M( ) ( ~ ,~

O(B) M(A)~ I O(A) M ( B ) ( ~ ' ~ ( ~ I

O(A) M(B)(~ O(B) Lu(A) ~ --

ACBACBA

(o) (b)

Fig. 2. The crystal structural models for (a) LuFeO 3 (ZnO) and (b) LuFeO3 (ZnO)4. A, B and C represent three kinds of triangular lattices; M, sites occupied by Fe and/or Zn ion; ® Lu ion; o, Fe and/or Zn ion; ©, O ion.

lattice layers are stacked as ...BABABCACAC... and Lu ions occupy the octahedral sites formed by oxygen layers. Fe and/or Zn ions take tetrahedral or trigonal bipyramidal sites. From a comparison of the X-ray powder diffraction patterns of InMO3(ZnO)m and those of LuFeO3(ZnO)m, the compounds are considered to be isostrnctural with each other.

We can consider that the structures of InMO3(ZnO)m consist of three kinds of layers: InO1.5 layers, (ZnM)O2.5 layers and ZnO layers. They are stacked perpendicularly to the c axis in the hexagonal system. Since InMO3(ZnO)m has z pieces of InO1.5 layers, z pieces of (ZnM)Oz.5 layers and z ( m - 1) pieces of ZnO layers (z is the molecular unit number in a unit cell; z = 2 for even m values; z = 3 for odd m values), the lattice constant c can be calculated as follows:

c=z[p +q + (m-1)r]

where p is the thickness of the InO1.5 layer, q is the thickness of the (ZnM)O2.5 layer and r is the thickness of the ZnO layer. The values p + q = 0 . 8 6 5 8 nm and

M. Nakamura et al. / Ir1203-1~2Zn04 and InMOs(ZnO),, phases and structures 107

r=0.2598 nm for InGaO3 (ZnO),, are obtained from Fig. 3(a).

It should be noted that c/2= 0.2604 nm, where c is the lattice constant for ZnO with wurtzite structure [9]. The lattice constants a and c in the system In203(ZnO)al-Ga203(ZnO)n are shown in Fig. 3(b). The lattice constant a decreases with increasing con- centration of M203(ZnO)m. The lattice constant c de- creases with increasing concentration of M2Oa(ZnO)m in the range from In203(ZnO),,, to InMO3(ZnO),~, and increases in the range from InMO3(ZnO),,, to M203(ZnO)m. The c value has a minimum point at I n M O 3 ( Z n O ) m .

From the fact that the lattice constant c for RFeM'O4 increases with the decrease in the ionic radius of the constituent rare earth element in this type of compound [4], we can conclude that there are two kinds of layers for In and M to be able to occupy in the region In203(ZnO),~-InMO3(ZnO),,-M203(ZnO),,. M ions co- exist with In ions in the (Znlnl_x,Mx002.5 layer in the region between InzO3(ZnO)m and InMO3(ZnO)m, in which there exist InO1.5 layers, (Znlnl _~,M~:)O2.5 layers (0<x~<l) and ZnO layers. In the region between InMO3(ZnO)m and MzO3(ZnO),,, there exist (Inl_xzMxz)Ol.s layers (0 <x 2 < 1), (ZnM)Ozs layers and ZnO layers. In the system InzO3-CrzZnO4-ZnO at 1350 °C, there are neither homologous phases InCrO3(ZnO).,

~.6. . .~2~=~4

0 0 5 10 15 (o/ m-1 InGaO 3(ZnO)l 1

1 0 . 5 0 ~ ' 5o(~ote;/o)' "

1 In203(ZnO) 11 Ga203(znO) 11

(b)

Fig. 3. (a) The relation between c/z and (m - 1) in InGaO3(ZnO)m. (b) The dependence of the hexagonal lattice constants a and c in the system In203(ZnO)irGa203(ZnO)n upon the Ga203 con- centration.

nor solid solutions between In203(ZnO),, and In- CrO3(ZnO),,, [10].

To understand the stabilities of the homologous phases in the system In203-M2ZnO4-ZnO (M-Fe , Ga, AI, Cr), we consider the ionic radii of In 3+ (0.0800 nm), Fe 3÷ (0.0645 nm) (HS), Ga 3÷(0.0620 nm), Cr 3÷ (0.0615 nm) and A13+(0.0535 nm) for an octahedral site [11].

Considering the ionic size, it is natural that Cr 3 ÷ is a possible constituent cation for InMO3(ZnO),,, com- pounds. However, in addition to the effect of the ionic radius of the M ions, we take account of the effects of their site preference. In 3+, Fe 3+, Ga 3 + and AP + can take both tetrahedral or trigonal bipyramidal sites and octahedral sites in oxide compounds. Therefore, in the system In203-M2ZnO4-ZnO (M=Fe, Ga, AI), there exist solid solutions with InMO3(ZnO)m-type structures whose range is that of In203(ZnO),,- InMO3(ZnO),,,-MzO3(ZnO)m (M=Fe, Ga) and that of I n 2 0 3 ( Z n O ) , , ~ - I n A 1 0 3 (ZnO),,~.

Since the 'AP + ion may be too small to occupy the octahedral site of In 3+, there are no solid solutions between InA103(ZnO)m and AI203(ZnO),,,. The site preference effect of Cr 3 +, which usually takes octahedral sites in oxide compounds, prevents the formation of the homologous phases InCrO3(ZnO)m in the system In203-Cr2ZnO4-ZnO.

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