Single crystal growth, structure refinement, ferroelastic domains and phase transitions of the hausmannite CuCr₂O₄

Article

Reference

Single crystal growth, structure refinement, ferroelastic domains and phase transitions of the hausmannite CuCr₂O₄

YE, Zuo-Guang, et al.

Abstract

With a view to studying the optical, structural and magnetoelec. properties, single crystals of the hausmannite CuCr2O4 were grown by a high temp. soln. method, based on the thermal decompn. of K2Cr2O7 in the presence of CuO. X-ray powder refinements at room temp.

indicate that the crystal structure of CuCr2O4 can be accurately described by the centrosym.

space group I41/amd. Crystal cuts were examd. by polarized light microscopy in reflection, revealing ferroelastic domain structures consistent with the tetragonal symmetry. The phase transitions in the Cu chromite were studied both by in situ optical domain observations and by DTA and DSC measurements. The ferroelastic phase transition from the tetragonal to the cubic phase was found to take place at 853 K and to be of 1st order. The magnetic phase transition was revealed by a DSC peak at 157 K with a small enthalpy of 0.2 J/g. Crystallog.

data and at. coordinates are given.

YE, Zuo-Guang, et al. Single crystal growth, structure refinement, ferroelastic domains and phase transitions of the hausmannite CuCr₂O₄. Ferroelectrics, 1994, vol. 162, no. 1-4, p.

103-118

Available at:

http://archive-ouverte.unige.ch/unige:31332

Disclaimer: layout of this document may differ from the published version.

1 / 1

Fermeleclrics, 1994, VoL 162, pp. 103-118 Reprints availahle directly from the publisher Photocopying permitted hy license only

© 1994 OPA (Overseas Publishers Association) Amsterdam B.V.

Published under license by Gordon and Breach Science Publishers SA Printed in the United St3tes of America

SINGLE CRYSTAL GROWTH, STRUCTURE REFINEMENT, FERRO ELASTIC DOMAINS

AND PHASE TRANSITIONS OF THE HAUSMANNITE CuCr

₂

0

₄

Z.-G. YE, 0. CROITAZ, F. VAUDANO, F. KUBEL, P. TISSOT and H. SCHMID

Department of Mineral, Analytical and Applied Chemistry, University of Geneva, CH-1211 Genva 4, Switzerland

(Received October 25, 1993; in final form December 21, 1993)

With a view to studying the optical, structural and magnetoelectric properties, single crystals of the hausmannite CuCr204 have been grown by a high temperature solution method, based on the thermal decomposition of K₂Cr₂0₇ in the presence of CuO. X-ray powder refinements at room temperature indicate that the crystal structure of CuCr,04 can be accurately described by the centrosymmetric space group 14/amd. Crystal cuts have been examined by means of polarized light microscopy in reflection, revealing ferroelastic domain structures consistent with the tetragonal symmetry. The phase transitions in the copper chromite have been studied both by in situ optical domain observations and by DT A and DSC measurements. The ferroelastic phase transition from the tetragonal to the cubic phase has been found to take place at 853 K and to be of first order. The magnetic phase transition has been revealed by a DSC peak at 157 K with a small enthalpy of 0.2 J/g.

Keywords: Copper chromite, single crystal growth, crystal structure, ferroelastic domains, structural and magnetic phase transition.

1. INTRODUCTION

The crystal structure of the copper chromite (CuCr₂0 _{4 )} is of the hausmannite type, a tetragonally distorted spinel structure (space group 14₁/amd or I42d at 300 K) due to the Jahn-Teller effect of Cu²⁺ ions.¹-6 It was suggested that the copper chromite becomes magnetically ordered below 135 K with the orthorhombic mag- netic space group Fd'd2' ,⁷ which allows a spontaneous magnetization perpendicular to the mirror d. In this magnetic phase the magnetoelectric (ME) effect was ex- pected to occur with the tensor components n₂₃ and n₃₂ of the linear ME suscep- tibility. ⁷

The magnetic structure was investigated by means of neutron diffraction.¹•6 The copper ions, on 4(a) tetrahedral sites, were found to have parallel magnetic mo- ments while the magnetic moments of the chromium ions, on 8( d) octahedral sites, can be divided into two sublattices. The magnetic interaction mechanism was supposed^1•6 to have the Yafet-Kittellike type with a triangle between the magnetic moments of the two sublattices of the same ions (A-A, B-B) and the spins of the ions of the other sites, consistent with the experimental result of the magnetic moment.⁸ It was also shown,⁶ by the thermal evolution of the magnetic structure, that the A sites become paramagnetic at 126 K whereas the B sites are paramagnetic at 156 K. Between these two temperatures the copper chromite would be weakly

[451]/103

1041(452] Z.-G. YE et at.

ferromagnetic with a small magnetic moment due to a canting angle lower than 180". The magnetic susceptibility is, as a result, magnetic field dependent below 156 K.⁶

In chemical terms, the copper chromite is widely used as a catalyst for hydro- genation and dehydrogenation reactions of organic compounds.⁹ Consequently this material has been extensively studied in polycrystalline form. Various techniques such as solid state reaction, coprecipitation, precursor thermal decomposition, etc. ^{9- 11} were used to synthesize powder samples of the copper chromite. However, no systematic studies of single crystals of CuCr₂0 4 have been reported so far, probably due to the refractory character of the starting components and the unstable valence state of the copper ions.

In the present work, with a view to characterizing the optical, structural and subsequently the magnetoelectric properties, single crystals of the copper chromite have been grown by a high temperature solution method. Structure refinements by X-ray diffraction on powder samples have been performed. Ferroelastic domains have been observed by polarized light microscopy (PLM) in reflection. The fer- roelastic phase transition between the tetragonal and cubic phase and the low temperature magnetic phase transition have been studied both by PLM and by DTA and DSC.

2. GROWTH OF SINGLE CRYSTALS

Since the various syntheses methods have been developed so far for the preparation of CuCr₂0₄ catalysts aimed at the polycrystalline form only, no information was available with regard to the growth conditions of single crystals, except for crys- tallites of CuCr₂0 ₄ and Cu₂Cr₂0 ₄ which were found accidentally as a by-product from a synthesis of Cr0₂ using Cu₂0 flux at a pressure of 65 kbar. ¹²

2.1 Choice of the Flux

Attempts at crystal growth by using either CuO or PbO-PbF₂ as flux were performed at first but remained unsuccessful. In the former case no liquidus was reached up to l400°C with 40 wt% of excess in CuO as flux (note that CuO and Cr20 ³ melt at 1326°C and 2266°C, respectively). In the latter one only platelets of CuO can be formed, whereas the same flux (PbO-PbF₂₎ was demonstrated to be efficient for growing single crystals of other compounds of the hausmannite family, e.g., NiCr₂0_{4 ,} CoCr₂0_{4 ,} etc.¹³

On the basis of these observations, K2Cr²0₇ was chosen as a reactive flux for the crystal growth. Three considerations justified this choice. First, it was reported¹⁴·15 that K2Cr207 undergoes a thermal decomposition at temperatures below 900"C, giving rise to chromium sesquioxide which, freshly formed, will be a reactive reagent for the formation of CuCr₂0 _{4 •} Second, the thermal decomposition process of the potassium dichromate permitted to obtain single crystals of Cr20 3 with the form of hexagonal platelets/^4•15 indicating some solubility of Cr20 3 in the molten po- tassium dichromate, as is the case in the system

v

^205-

vo2

for the growth of

vo2

single crystals. 16 Therefore in the presence of CuO the molten K₂Cr₂0₇ may serve

CRYSTAL GROW1H AND CHARACTERIZATION OF CuCr204 (453)/105

as flux for the single crystal growth of CuCr₂0 _{4 •} Third, the thermal decomposition of K₂Cr₂0₇ involving a reduction of Cr⁶+ to Cr³+:

3/2 0 ²-

+

Cr6+ - Cr³+

+

3/4 0₂

f ,

gives rise to an oxidizing milieu, favouring the stabilization of the valence Cu²+ and hence the formation of CuCr20 4 .

2.2 Growth Conditions

Starting compounds of CuO (Fluka, 99%) and K₂Cr₂0₇ (Fluka, 99.5%) were mixed according to the following formula:

(1 - x)(CuO

+

Cr₂0 ₃₎

+

xK₂Cr₂07,

where x stands for the mole percentage of potassium dichromate in the flux. Note that Cr₂0₃ will be formed by the thermal decomposition of K₂Cr₂0 _{7 .} For x ⁼ 0.2 and 0.3 the starting mixtures were prepared with the weight proportion of (17.8%

CuO

+

82.2% K₂Cr₂0 ₇₎ and (15.9% CuO

+

84.1% K₂Cr₂0 _7), respectively. In both cases 1 wt% of B₂0 ₃ was added to the mixture in order to increase the homogeneity of the solution and hence to improve the quality of the crystals. The reagents ( =20 g per batch) were ground and introduced into a gold or a platinum crucible (V = 10 cm³) covered by a foil (non-sealed) in order to prevent the sublimation of CuO. Thermal treatments were performed at the atmospheric pres- sure in a muffle furnace equipped with 4 thermocouples for the control and reg- ulation of temperature. Single crystals were obtained according to the following optimal conditions for x

=

0.2: heating at 100°C/h from room temperature up to

Tmax = 800-815°C, soaking at Tmax for 24 hand cooling at a rate of 300C/h to 2o•c. After the thermal runs the residues were washed with boiling water and the single crystals were removed from the solidified flux and from the bottom and the walls of the crucible. They were identified by X-ray diffraction using a Guinier camera.

2.3 Growth Results and Discussion

The grown crystals of CuCr₂0 ₄ exhibit the form of octahedra with black semi- metallic lustre, as shown in Figure l(a). Some degenerated octahedra were also found (Figure l(b)), probably grown below the phase transition temperature of 853 K (see Section 5). Figure 2(~) gives the scanned Guinier spectra of crushed crystals, confirming the single phase state of CuCr₂0 _{4 •}

It was found that the maximum soaking temperature and the oxygen pressure are two parameters which influence dramatically the growth result. In fact, the thermal runs with higher maximum temperature (T max > 850°C) or with longer soaking time (>70 h) gave rise to the formation of the crystals of Cu₂Cr₂0_{4 ,} in form of dark green hexagonal platelets or degenerated rhombic crystals with re- markable growth steps as shown in Figure 3. The crystal morphology is consistent with the trigonal R3m symmetry of Cu₂Cr ₂04 , as was confirmed by X-ray diffraction (Figure 2(b)). It is evident that the appearance of Cu₂Cr₂0 ₄ resulted from a re- duction of CuO to Cu₂0 during the syntheses at high temperature.

Longer heating time at high temperatures can also cause the reduction of Cu²+

106/(454] Z.-G. YE eta/.

(a)

(b) ^1mm

FIGURE 1 Habit of single crystals of CuCr204 grown from the flux of K₂Cr₂0_{7 :} (a) octaheral form; (b) degenerated octahedral form.

in the crystals of CuCr₂0 4 , giving rise to the "topotaxy" crystals with the simul- taneous presence of Cu₂Cr₂0₄ and CuCr₂0_{4 ,} distinguishable by optical observation.

This transformation of CuCr₂0 ₄ to Cu₂Cr₂0 ₄ takes place according to the following reaction:

CuCr₂0₄

+

CuO ~ Cu₂Cr₂0 ₄

+

!0² ?',

at temperatures between 830 and 880°C in air due to the presence of the very reactive copper oxide. ¹⁷

Higher oxygen partial pressure should a priori favour the growth of CuCr₂0₄ by preventing the formation of Cu₂0 and the decomposition of CuCr₂0 ₄. However, experiments undertaken at oxygen pressure of 1 atm. (by injecting a constant flow of 0₂ to the liquid surface during the synthesis) or higher than 1 atm. (by sealing the platinum crucible at room temperature) showed that no crystals of CuCr₂0 ₄ can grow because most of K₂Cr₂0 ₇ was hindered to decompose and the equilibrium between CuCr₂0 ₄ and the flux was not reached. Only platelets of CuO and poly-

CRYSTAL GROWTH AND CHARACTERIZATION OF CuCr204 (455]/107

N _N

..

(o) CuCr₂ 0₄

10 ^{- N} ₀ N

- ⁰

"' -

0 8 ^{0 ;;;}

0 ^{- N}

..

0 ^N N

~

'

^{~ 6 -}

~ ^{~ N}

.D 0 0

..

~ 4 ^{- 0 N} _N ^N -

;;; _~ "'

0 ~ ^~

N 0 0

0 - ^{N N}

2 ^-

..

0 "' ⁰

I

^{"' ..;}

J I

^"'

J

^ON

....

^{" "' 0}

^..

0

....

^{o I}

10 20 ^?:() 40 50 60 70 80

18 - ^N 0 (b) Cu₂Cr₂ 04

16 ^- 14 -

0 0 12

0 ^-

~ 10

' ^-

"' 8

.D 0

~ 6 4 2 0

-

..

₀

- ⁰ §

..

₀ ^g _~I

-

~1

^{<D - N 0}

-

^I

⁰

1 I ^~

10 20 40 50 60 70 80

2 Thelo (deg.)

FIGURE 2 X-ray powder diffraction spectra of CuCr204 crystals (a) and of Cu2Cr204 crystals (b) (J\

= 1.54056 A. T = 293 K).

crystalline clusters of CuCr₂0₄ formed under these conditions. It can be seen that the control of the optimal growth conditions with regard to the oxygen pressure appears to be delicate, but the natural thermal decomposition of K₂Cr₂0₇ in air gives appropriate atmospheric conditions for the growth of single crystals of CuCr₂0_{4 .}

3. X-RAY STRUCTURE REFINEMENT

Two crystal structures were so far proposed for the tetragonal phase of CuCr₂0 ₄ with the centrosymmetric space group 141/amd (141)^2-4•6 and the non-centrosym- metric space group I42d (122).¹ In order to elucidate the true nature of the tetrag- onal phase, X-ray structure refinements were performed on two samples: i) CuCr₂0 ₄ powder obtained by coprecipitation of (NH4)Cu(Cr04

h-

2NH₃ prepared according to the Briggs's method¹¹ and by thermal decomposition at 800°C for 24 h, as suggested in Reference 18; ii) on crushed single crystals of CuCr₂0 _{4 .}

The coprecipitated CuCr₂0₄ powder has been analyzed on a Philips powder diffractometer with Co K .. radiation (>..

=

1. 79026

A,

>..₁

=

1. 7889

A,

>..₂

=

1. 7928 A, 1₂/1₁ = 0.5). The step size was 0.04 deg. at a counting rate of 30"/step. The refinement was made using the DBW Rietveld program.¹⁹

The results obtained for the space group 141/amd are shown in Table I. The very low RP, Rwp and Rexp values are in reality due to the fact that, as a result of the artefact of the refinement program, ¹⁹ the unusually high background was not sub-

108/[456] Z.-G. YE eta/.

FIGURE 3 Single crystals of Cu2Cr204 exhibiting growth steps and pronounced trigonal morphology (symmetry R3m).

TABLE I

Structural data of CuCr₂0₄ (14₁/amd)

Atomic Positional and Isotropic Displacement Parameters

x/a y/b zlc U*iso

Cu l/2 3/4 3/8 0.0110(4)

Cr 0 0 1/2 0.0138(6)

0 0 0.4738(6) 0.2449(6) 0.019(1)

Anisotropic Atomic Displacement Parameters

Ull U22 U33 U12 U13 U23

Cu .0088(6) .0088(6) .0154(8) 0 0 0

Cr .012(1) .021(1) .0079(6) 0 0 .0031(7)

0 ^{.024(3) .020(2) .014(2)} 0 0 -.002(1)

Cell Parameters

a= 6.03052(14) b = 6.03052'14) c = 7.78230(21

Rp (%) I ^{Rwp (%)} I RBragg (%) I Rexp (%) I

s

I ^n.var.

0.54 I ^0.7 I ^6.04 I ^0.51 I ^1.50 I ²⁰

stracted during the calculations of these factors. The ESD calculations were per- formed by using the Raragg value (6.04% ), which was calculated on the basis of the structure model by taking into account the background. The number of parameters varied was 20 [2 atomic positions, 1 scale factor, 2 lattice constants, 3 halfwidth and 2 mixing parameters (pseudo-Voigt profile) and 10 anisotropic displacement parameters]. The observed, calculated and difference diagrams are shown in Figure 4. The drawing of the copper chromite structure with Cu-Q tetrahedral and Cr-Q octahedral bonds is shown in Figure 5.

The values of of RP, Rwp and Raragg do not change for the space group I42d. The atomic positions, cell parameters and anisotropic atomic displacement parameters in this symmetry are shown in Table II. It should be noted that the x position for chromium is !, andy for oxygen is zero, within four cr (variables written in bold type in Table II). Based on these refinements the space group 14₁/amd was retained due to the fact that the hkO reflections with h

+

k = 2n or h (and k) = 2n

+

1 (i.e. the 130, 150, ... , reflections) which are authorized in the I42d space group have no intensity and due to the higher symmetry of 14₁/amd.

-

CRYSTAL GROWTH AND CHARACTERIZATION OF CuCr20, [457)/109

0+----,~---r----.----,----,---r----.-.---.----,---

20 40 60 80 100

2 Theta (deg.)

FIGURE 4 Observed, calculated and difference diffraction diagrams of CuCr20, (X ⁼ 1.79026 A, T

= 293 K).

In the I4₁/amd symmetry an inversion of about 10% between the copper ions on A tetrahedral sites and the chromium ions on B octahedral sites was re- ported.3-6 However, refinements carried out in the present work with an increasing inversion percentage showed anincreaseofthe Bragg factor (Figure 6), thus indicating rather a "normal" spinel type structure for the copper chromite, as proposed in Reference 2.

The X-ray diffraction on crushed crystals was performed on a Huber powder diffractometer with Cu Ka₁ radiation (ll. = 1.54056

A).

The scan was made for 26 values between 30° and 100° in steps of 0.02°. The refinement results confirmed those given above.

It is worth noting that the indexation data given in Reference 22 and registered in ASTM files under 26-508 for the tetragonal phase of CuCr₂0 ₄ are compatible neither with the 14₁/amd group nor with the I42d group, the symmetry of the space group having not been specified. The temperature indicated in the ASTM file should moreover be 293 K instead of 239 K.

4. FERROELASTIC DOMAINS

4.1 Domain Structure

Ferroelastic domains of CuCr204 with bireflectance contrast were observed by means of PLM on the polished (lOO)cub and (lll)cub (natural facets) planes. The

110/(458) Z.-G. YE eta/.

• Cr

oa

@ ^Cu

FIGURE 5 Clinographic projection of the structure model of CuCr204 with space group 14₁/amd, formed by the Cu--0 tetrahedra and the Cr-0 octahedra. The hatched circles represent Cu atoms, the solid ones Cr atoms and the open ones 0 atoms. The apparent difference between a and b axes is due to projection.

TABLE II

Structural data of CuCr₂0₄ (142d) Atomic Positional and Isotropic Displacement Parameters

xla ylb lie U*iso

Cu 0 0 0 0.0107141

Cr 0 .. 507(2) 1/4 1/8 0.0153(7)

0 0.2782(7) ·Cl-016(4) 0.1214(7) 0.016(1)

Anisotrooic Atomic Disolacement Parameters

Ull U22 U33 Ul2 Ul3 U23

Cu .0082(6) .0082(6) .0155(8) 0 0 0

Cr .011(2) .0262(9) .0091(6) 0 0 .0064l6)

0 ^.064(6) .023(4) .0 16(2) 0 0 -.024(5)

Cell Parameters

a= 6.03052(14) b- 6.03052 14) c -7.78230(21)

Ro(%) Rwp(%) RBra22 (%)I Rexp (%)

s

n.var.

0.54 0.7 6.02 I 0.51 1.37 23

-

CRYSTAL GROWTH AND CHARACTERIZATION OF CuCr204 [459]/111

10

...

f!

0 0

e

8

CD I

C¥

a

20 40 60 80 100

Inversion percentage

FIGURE 6 Variation of the Bragg factor R8 , . . . of CuCr204 as a function of the inversion percentage with respect to the "normal" spinel model.

crystals of CuCr204 were found to be brittle, so that care had to be taken during the orientation and polishing process. The platelets of CuCr₂0 ₄ remain opaque with a thickness down to about 10 ,...m, whereas those of Cu₂Cr₂0 ₄ appear trans- parent and birefringent (except for the direction of the 3 fold axis) with green absorption colour for a thickness below 50 jl.m. Therefore optical examinations of the CuCr₂0₄ crystals were restricted to the reflection mode.

Figures 7(a) and 7(b) give the ferroelastic domain structure and its schematic representation of a (lOO)cub plane under slightly uncrossed Nicols. Three types of domain orientation can be distinguished according to the reflection contrast. One of them shows an extinction at any angle of the microscope stage under crossed polars, indicating an optical axis perpendicular to the (lOO)cub plate. The two other domain states exhibit bireflectance, the extinction directions of which are oriented parallel to (tOO)cub and the optical axis lying within the crystal plane (// (lOO)cub), as indicated in Figure 7(b). Only (llO)cub-type domain walls have been observed.

They appear as narrow lines along (llO)cul> when oriented perpendicular to the surface of (lOO)cub plates and joining domains having their optical axis in the plane of the plate. The walls joining one domain having the optical axis perpendicular and another one having it parallel to the surface, are inclined by about 45° to the surface with traces along approximately (lOO)cub directions. This domain structure

112/[460] _{Z.-G. YE et al.}

(a)

(b)

FIGURE 7 Structure of the ferroelastic domains of a single crystal of CuCr₂0 4 observed on a (lOO)cub plate by polarized light microscopy in reflection: (a) photograph; (b) schematic. (Due to the crystal cut not being perfectly parallel to (lOO)cub• the domain wall traces intersect at an angle slightly different from 90°.)

is consistent with the tetragonal symmetry of the copper chromite. In fact the Aizu species²⁰ m3ml 'F4/mmml' allows three fully ferroelastic domain states in the tetragonal phase with the 4-fold axis of the ferroic group parallel to the 4-fold axis of the prototype group.

4.2 Domain Contrast

With perfectly crossed Nicols extinction appears in the parallel position, whereas in the 45° ("diagonal") position domains with mutually perpendicular c- and a-axes have the same grey tint. Contrast between these two domains sets in only upon uncrossing by 2-3° the polars by rotating the analyzer at fixed polarizer (0°) position (Figure 8). The formation of this contrast is due to the fact that the impinging linearly polarized light becomes slightly elliptical upon reflection and with the long

CRYSTAL GROWTH AND CHARACfERIZATION OF CuCr₂04 ^(461]/113

(a) (b)

(100)c

FIGURE 8 Interchange of the contrast between two ferroelastic domains with mutually perpendicular c(a)-axis on a (lOO)cub of CuCr₂0₄ upon uncrossing polars in opposite sense (a, b) and the schematic explanation of the formation of the contrast (c).

axis of the ellipse always rotated in the direction of the axis of higher reflectivity ,2¹ as explained by Figure 8(c). The contrast between these two domains on a (lOO)cub plates can be interchanged by alternating the sense of rotation of the analyzer when uncrossing the polars (Figures 8( a) and 8(b)).

It should be noted that domains with the optical axis ( c-axis) perpendicular to the (lOO)cub plane are usually absent in most of cases. They are probably mechan- ically unfavourable with respect to the other kinds of domains under the pressure of polishing since ctet. > atet.· The domain structure on a (lll)cub plate shows alternating large and fine strips with the traces of the domain walls perpendicular to (llO)cub· On a not well polished surface or on a surface with tiny domains, the use of a compensator (de Senarmont, Berek or Laves-Ernst) and of immersion oil are helpful for revealing the domain pattern with better contrast.

5. PHASE TRANSITIONS

5.1 Ferroelastic Phase Transition

Upon heating (using a Leitz 1350 heating stage) the contrast between two domains with perpendicular c( a )-axis (with fixed uncrossed polars) and the bireflectance were found to decrease, indicating a decrease of the optical anisotropy. The on~et

114/[462] Z.-G. YE et al.

of the transition from the tetragonal to the cubic phase was revealed at 853 K by the movement of the ferroelastic domain walls, which resulted in the cracking of the crystals. During this phase transition some domains appears to be "flickering"

with alternating extinction while a milky film was observed to propagate through the crystal surface. The mean ferroelastic Curie temperature was found to be Tc

=

853 + 5 K.

Figure 9 gives the domain patterns of a (lOO)cub platelet undergoing the phase transition. After the passage from the tetragonal to the cubic phase, the tetragonal domain walls disappeared and an isotropic state appeared. However, some traces of domain walls persisted at some areas under uncrossed polars, as can be attributed to the "phantom (or fossil) domains" resulting from the mechanical shearing of the surfaces polished in the tetragonal phase. Cooling through the cubic~ tetrag-

300JJm (a)

(b)

FIGURE 9 Domain patterns of a (lOO)cub platelet of CuCr₂0₄ from PLM in reflection showing the ferroelastic phase transition at Tc = 853 K. (a) T > Tc, in the cubic phase; (b) T < Tc, in the tetragonal

phase. ·

CRYSTAL GROWTH AND CHARACTERIZATION OF CuCr₂0₄ (463]/115

onal transition gave rise to similar domain wall and phase boundary movements and to new breaking of the crystals. Domain structure was also modified with the appearance of new sub-domains with inclined (llO)cub walls and their traces parallel to (lOO)cub (see Figure 9(b)). A thermal hysteresis of 5 degrees was observed at a heating/cooling rate of 3°C/min.

Spectacular breaking of the crystals is a peculiarity of the ferroelastic phase transition in CuCr20_{4 .} The platelets usually jumped off and broke into smaller pieces after several thermal cycles around Tc, as shown in Figure 10. These phe- nomena indicate that the tetragonal ~ cubic phase transition takes place under strong internal stress, resulting from the mechanical incompatibility between the ferroic and the prototype phase. In fact, according to the available data of the lattice parameters at 840 K (nearly below Tc) ,2² the spontaneous strain is calculated to bees ⁼

I

(c, - a,)!( a,

+

c,)

I

⁼ 0.03. The sharp vanishing of this strain gives rise to an enormous internal stress at the phase transition. The related spontaneous strains having different orientation for the ferroelastic domains of different ori- entation, the polydomain platelets tend to break in order to release the mechanical energy.

Thermal analysis by DT A was undertaken with 100 mg of single crystals. The ferroelastic phase transition was detected by an onset of DTA anomaly at 854 K, as shown in Figure 11, confirming the results of the optical domain observation.

FIGURE 10 Disintegration of a platelet of CuCr₂0₄ having undergone the ferroelastic phase transition at T'" = 853 K, due to strong mechanical stress developed during that 1st order transition.

~T

580 590 &X)

Temperature

(·c )

FIGURE 11 Recorded DA T curve of CuCr₂0₄ single crystals (100 mg) showing the ferroelastic phase transition at Tc.

116/[464) Z.-G. YE et al.

-1900

(b) ⁵⁰

0 -2000

-50-=

146.5K (a

l

_... ^E

-2050

---

^31:

31:

"'

' ^{' '} ' ^0.2mJ/mg -100 1il

"'

u Ul

--

⁰

0 c

-2100

156K -150

-2150

-200 -2200

140 145 150 155 160 165 170

TEMP.

K

^(Heating)

FIGURE 12 (a) DSC thermogram (on 48.25 mg of coprecipitated powder of CuCr₂0 _{4 )} showing a maximum peaking at 156 K with a very small enthalpy of 0.2 J/g, resulting from a magnetic ordering.

(b) Time-derivative of the DSC curve recorded upon heating at 5 K/min. The analysis on single crystal samples showed exactly the same result but with a relatively high "noise" due to the multitude of crystals required for the measurement.

A slight weight loss was measured by a thermal balance at temperatures above 1033 K, corresponding to the reaction:

CuCr₂0 ₄ -+ !Cu₂Cr₂0 4

+

!Cr₂0 ₃

+

!02 / ' ,

with a theoretical weight loss of 3.46%.

5.2 Magnetic Phase Transition

With a view to studying the magnetic phase transition reported to occur in CuCr204 , 1

•6 domain examination was performed at low temperatures by means of a horizontal He-flow cryostat adapted to a polarized light microscope in reflection.

It is found that the ferroelastic domain structure does not undergo any modification down to 10 K. Magnetic domains were not observable even under the action of a magnet of 400 Gauss. However, thermal analyses performed on single crystal samples (38.41 mg) and on coprecipitated powder ( 48.25 mg) by the DSC method using SII-Thermo Lab-Dl apparatus, showed both a peak at 156(1) K with a very small enthalpy of 0.2 J/g (Figure 12). This thermal event can be attributed to the magnetic phase transition at 156 K, below which the onset of a weakly ferromagnetic ordering was suggested.⁶ The small thermal anomaly is consistent with a magnetic phase transition, usually of second order like character.

-

CRYSTAL GROWTH AND CHARACTERIZATION OF CuCr₂0₄ [465)/ll7

6. CONCLUDING REMARKS

1) Single crystals of the hausmannite CuCr₂0₄ have been synthesized from the high temperature solution method, based on the thermal decomposition of K₂Cr₂0 ₇ in air between 800 and 815°C in the presence of CuO and 8₂0 _{3 •} The grown crystals of CuCr₂0 ₄ showed regular or degenerated octahedral forms appropriate for op- tical, structural and phase transition studies. The heating temperature and the oxygen partial pressure are two parameters to be mastered for the crystal growth of CuCr₂0_{4 ,} since higher temperature or lower oxygen pressure led to the formation of the more stable Cu₂Cr₂0₄ phase with monovalent copper ions.

2) The crystal structure of CuCr₂0 ₄ refined by X-ray diffraction belongs to the centrosymmetric group 14₁/amd, consistent with Reference 2, 4 and 6 but contrary to the result reported in Reference 1 with the non-centrosymmetric space group I42d. Because of the ferroelastic twinning, refinement on a single crystal of CuCr₂0 ₄ has not been possible so far.

3) The ferroelastic domains have been studied by means of PLM in reflection.

The domain structure is consistent with the tetragonal symmetry with three domain states linked by (llO)cub walls. The extinction directions due to the weak bireflec- tance are parallel to (lOO)cub·

4) The tetragonal- cubic phase transition in CuCr₂0₄ revealed at Tc = 853 ⁺ 5 K both by optical and by DT A measurements shows first order character. Crystals of CuCr₂0 ₄ are found to break due to strong internal mechanical stress developed during the ferroelastic-paraelastic phase transition. The transition to a magnetically ordered phase has been detected at 156 K by DSC measurements. However, as to be expected, it was not observable by ordinary PLM and would have required, e.g., Kerr effect microscopy for detection. In the weakly ferromagnetic phase the magnetoelectric effect can a fortiori be expected to be present.

5) Highly twinned states are usually present in the ferroelastic crystals of CuCr₂0 _4, as indicated by PLM and by X-ray diffraction. Detwinning by a mechanical stress encounters some difficulties owing to the relatively high Curie temperature and the low hardness of the crystals. Therefore synthesis at a temperature below the ferroelastic Curie temperature Tc = 853 K appears to be a promising method to obtain single crystals of CuCr₂0₄ with single domain state. Such a method was recently used for preparing the single crystals of BiFe0_{3 •}²³ Only with single domain crystals of appropriate size, the optical, structural and magnetoelectric properties of CuCr₂0₄ could be studied quantitatively in detail.

ACKNOWLEDGEMENTS

The authors wish to express their gratitude to R. Cros, E. Burkhardt, R. Boutellier and H. Lartigue for their technical help, to Lab Plus Ltd, Bubikon, for DSC measurements, and to the Fonds National Suisse de Ia Recherche Scientifique for financial support.

REFERENCES

1. E. Prince, Acta Crystallogr., 10, 554 (1957).

2. F. Bertaut and C. Delorme, C. R. Acad Sci. (Paris), 239, 505 (1954).

3. H. Ohnishi and T. Teranishi, J. Phys. Soc. Japan, 16, 35 (1961 ).

118/[4661 Z.-G. YE et al.

4. R. Kohlmuller and J. Omali, Bull. Soc. Chim. France, 4383 (1968).

5. J. Arsene, "These de Doctoral d'Etat," Rouen (F), (1979).

6. B. Fricou and M. Perrin, "Tr. Mezhudnar Konf. Magn.", Meeting Date 1973,5,241-245, "Nauka,"

Moscow, USSR.

7. D. E. Cox, Int. J. Magnetism, 6, 67 (1974).

8. T. R. McGuire, L. N. Howard and J. D. Smart, Ceramic Age, 60, 22 (1952).

9. H. Adkins and R. Connor, J. Arner. Chern. Soc., 53, 1091 (1931).

10. H. Charcosset, P. Turlier andY. Trambouze, J. Chirn. Phys., fil, 1249 (1964).

11. S. H. C. Briggs, Trans. Chern. Soc., 83, 391 (1903).

12. B. L. Chamberland, Mat. Res. Bull., 2, 827 (1967).

13. B. M. Wanklyn, F. R. Wondre and W. Davidson, J. Mater. Sci., 11, 1607 (1976).

14. T. Sekiya and H. Okuda, J. Cyrst. Growth, 47, 551 (1976). · 15. Y.-F. Yu Yao, J. Phys. Chern., 69, 3930 (1965).

16. J. B. McChesney, J. F. Potter and H. J. Guggenheim, J. Electrochern. Soc., 115,52 (1968).

17. L. Walter-Levy and M. Goreaud, Bull. Soc. Chim. France, 3, 830 (1973).

18. E. Whipple and A. Wold, J. lnorg. Nucl. Chern., 24, 23 (1962).

19. D. B. Wiles and R. A. Young, J. Appl. Cryst., 14, 149 (1981).

20. K. Aizu, Phys. Rev. B., 2, 754 (1970).

21. H. Schmid, E. Burkhardt, E. Walker, W. Brixel, M. Clio, J.-P. Rivera, J.-L. Jorda, M. Fran.;ois and K. Yvon, Z. Phys. B-Condensed Matter, 72, 305 (1988).

22. V. M. Ust'yantsev and P.M. Mar'evich, Inorganic Materials, 9, 306 (1973) (Izv. Akad. Nauk SSSR, Neorg. Mater., 9, 336 (1973)].

23. F. Kubel and H. Schmid, J. Cryst. Growth, 129, 515 (1993).