Growth, phase transitions and properties of Li2Ti3O7 crystals

Journal of Crystal Growth 80 (1987) 403—407 North-Holland, Amsterdam

403

GROWTH, PHASE TRANSITIONS AND PROPERTIES OF Li2Ti3O7 CRYSTALS JIANG Yan-dao, ZHANG Yun-zhi, SHU Qi-mao and YAN Xiu-li Institute of Physics, Chinese Academy of Sciences, Be(jing, People’s Rep. of China

Received 20 March 1986; manuscript received in final form 28 October 1986

Metastable crystals of Lj2Tj3O7 of dimensions up to a diameter of 20 mm and a length of 20 mm have been grown by the Czochralski method from oriented seeds. The effects of seed orientation, growth atmosphere and dopant on crystal growth have been studied. Phase transition processes in the crystals and the H phase structure have been measured. Optical and acoustic properties of the crystals have also been determined.

1. Introduction Li 2Ti 307 (crystal R) has a ramsdellite structure and belongs to the orthorombic system, space group Pbnm. It was first grown using the Bridgman method by Mikkelsen [1] and the structure, phase diagram and ionic conductivity are given in refs. [2-4]. Crystal R is only stable above 960°C and although it can be obtained from the melt, it possibly undergoes two destructive phase transitions during cooling, which are the eutectoid phase decomposition and the phase transition from R to H, a hexagonal phase (after Mikkelsen). Further study of the crystal growth and the phase transitions of this material are necessary to characterize the system and to produce the large crystals which are required in order to investigate their physical properties. In this paper the Czochralski method has been used to grow large oriented single crystals of Li2Ti3O7.

2. Experimental Polycrystalline starting material with high-purity reagents was mixed and synthesized according to the formula 74.5 mol% Ti02 + 25.5 mol% Li20. A platinum crucible of 50 mm diameter acted as both a container for the material to be synthesized and a susceptor for the RF coil. The crystals were

pulled at a rate of 3 mm/h with a rotation rate of 20 rpm. The growth atmosphere was either air, oxygen or nitrogen. Seed orientations along the a-, b- and c-axes were used for growth. In order to avoid the two destructive phase transitions mentioned above, the temperature in the region 3 cm above the melt was maintained above 1000°C by using an after-heater and the crystal was cooled rapidly from 1000 to 300°C within 2—4 mm by air quenching. The phase transitions were determined by DTA and powder X-ray diffraction at both room temperature and high temperature. For convenience, the products of the eutectoid phase decompositions, Li 4Ti5 012 + Ti02, are abbreviated to “eutectoid phases”.

3. Crystal growth 3.1. The influence of ambient atmosphere Both the transparency and color of the crystals are not affected by the growth atmosphere for either pure oxygen or air at 1 atm pressure. All of the crystals have a pale yellow color. But at the top surface of every boule, there was an opaque layer with thickness of 0.5 mm composed of two phases (R-Li2Ti3O7 and Li4Ti5O12). A multilayer platinum reflective shield was assembled over the growing crystal, reducing the radiation heat loss

0022-0248/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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Growth, phase transitions and properties of Li,Ti

307 crystals

from the top surface of the boule and suppressing the opaque layer resulting from the phase transitions. A transparent boule can be obtained so long as the cooling rate is not less than 250°C/mm. If it is less than 60°C/mm,a transparent boule will become entirely opaque. Crystals grown in an ambient atmosphere of high-purity nitrogen at 1 atm pressure are black and have no phase transition layer at the top surface of the boule. 3.2. The influence of seed orientation

Seed orientations along the a-, b- and c-axes were used and fig. 1 shows schematic cross-sections of crystals grown along the three crystallographic axes. The cross sections of both the a-axis and the b-axis crystals were approximately ellipses in which the major axis was the c-axis. The (010) and (100) faces appeared on the cylindrical surface of the a-axis and b-axis boules respectively. The cross-section of the c-axis boule was nearly a square with a pair of (100) faces and a pair of (010) faces on the “cylindrical” surface. From this it might be considered that the growth rate of the a-axis crystals equals that of the b-axis crystals and is less than that for the c-axis. Usually there are “cloudy layers” within the crystal perpendicular to the c-axis in both the a-axis and b-axis boules. The “cloudy layer” in the b-axis boules is more opaque than that in the a-axis ones and it forms a cruciform shape in the c-axis boule. In most cases, the cooling is so rapid that the crystals usually crack into two parts, or sometimes into fragments. The c-axis crystal is more liable to crack than the b-axis or a-axis crystals. The larger the boules grown, the more easily they crack.

Splitting rarely happens for a boule with a diameter of 20 mm and a length of less than 15 mm. 3.3. The influences of dopants

In order to prevent the two-phase transformations, crystals R were grown from a melt containing dopants and were quenched in air from the growth zone (usually at a temperature above 1000°C). Crystals doped with Na2Ti3O7 by adding amounts of 1 and 5 at% to the melt are both transparent. When the doping level is increased up to 15 at%, a less transparent crystal is obtained. When K2Ti307 is used as a dopant, the crystal formed by adding amounts of 5 at% dopant to the melt is transparent. Increasing amounts added up to 10 at% produces a less transparent crystal. When the crystal is doped with Cs2Ti3O7 at a dopant level of 5 at% in the melt, the top of the boule (~10 mm long) is transparent but the bottorn appears opalescent. We can expect that as the ionic radii of the dopants increase, crystal growth becomes more difficult and the dopant level should therefore be decreased. Crystals doped with 7 at% MgO are trailsparent. When the amount of MgO dopant rises to 12.5 at%, a white opaque boule is obtained. When 1 wt% Al203 and 1 wt% Sc203 are added to the remaining melt, a wholly transparent boule can be grown again. In order to obtain corresponding phase transition temperatures, DTA curves of these crystals were recorded using an 1090 Thermal Analyzer (Du Pont Instrument) at a heating (cooling) rate of 10°C/mm. The instrument was calibrated by using a quartz sample. The phase transition tern(100)

“CLOUDY LAYER”

~

~O) (010)

a

(100) b

“CLOUDY LAYER”

Fig. 1. Cross-sections of a crystal with different orientations of growth.

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Growth, phase transitions and properties of Li,Ti

Table 1 Influence of dopants on phase transition temperature (in Phase transition

Eutectoid phase R phase

—~

O

C) of crystals

Dopant (at%) Without dopant N 2 Air 02

Metastable R phase

H phase

R phase

eutectoid phase + H phase

405

10, crystals

Na

K

Cs

5

15

5

15

5

Mg

Mg 75 12.5 A1 Sc 2.0

578

588

600

741

797

583

595

598

623

661

963

964

968

968

991

962

962

965

994

932

768

788

800

—

—

—

—

peratures shown in table 1 are much higher than the true transition temperatures given by X-ray diffraction. These differences of temperature measurement are presumed to be a DTA hysteresis effect. However, table 1 shows that the dopants influence the phase transition temperatures, especially for the samples doped with Na.

4. Phase transition of the crystals The phase transition processes of the crystals have been examined by DTA and powder X-ray diffraction. The DTA curves of crystal R and of phase H are shown in fig. 2. On heating crystal R, the first peak occurs at about 600°C(heat release), the second is at about 970°C (heat absorption) and the third is at about 1313°C (melting). During cooling, a fourth peak appears at about 1310°C (solidification) and on decreasing the temperature

/

—

——

further, a fifth peak appears at about 800°C(heat release). As mentioned above, these temperatures differ from the true transition temperatures. The metastable crystal R, after annealing in air at 560 ±10°Cfor 20 h, transforms into phase H. The DTA curve of phase H, except for the disappearance of the first peak, is the same as that of crystal R. We can thus expect that the first peak corresponds to the phase transition from R to H. The metastable crystal R, after annealing at 880 ± 10°Cfor 20 h, transforms into the eutectoid phase. The DTA curve of the eutectoid phase sample is the same as that of the H phase sample. We can thus anticipate that the second peak corresponds to the phase transition from eutectoid phase to (high temperature) R. No apparent heat release or absorption from H to the eutectoid phase has been observed in our experiments. The eutectoid phase sample, after annealing at 560 ±10°C (the stable temperature of the H phase) for 20 h, is still a eutectoid phase. This means that the phase transition from H to eutectoid On cooling phasethe is irreversible. (high temperature) R phase to

—H It PH&SE

room temperature in the thermal analyzerphase, at a rate of 10°C/mm, a mixture of eutectoid H and R phase resulted. As the eutectoid phase

/

AT

cannot convert to the H phase, twoduring kinds of phase transitions occur simultaneously cooling,

E~DO

_____________________________________ 800

1000 1200

~COOLNG

860

Fig. 2. DTA curves of a metastable crystal R and of phase H.

namely the transition from R to the eutectoid and that from R to H These phase transitions corre spond to the fifth peak. From a study of high temperature Guinier X-ray powder photographs, the phase transition temperature from metastable R to H is in the range of

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Jiang Yan-dao et at.

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400—450°c

600—650°c

float releass P4etastablo R

Growth, phase transitions and properties of Li,Ti

307 crystals

900—950°c

No heat—effect

—H phase

1301—1318c

Heat absorption

—Eutectoid pha8e

—

R phase

I

-

—

Melt

1309-1317°c

At 760—825°c Heat release Cooling rate ~ 60°c/.in

Cooling rate ~250°c/utin Fig. I The phase transition process of crystal R.

400—450°C. The H phase decomposes into the eutectoid phase in the range of 600—650°C,and the eutectoid phase transforms into the high ternperature R phase in the range of 900—950°C. The data agree with the results given by refs. [3] and [1]. The phase transition process of crystal R is summarized in fig. 3. All of the data for phase transition temperatures were obtained by X-ray diffraction except the melting and freezing ternperatures which are given by DTA. The powder X-ray diffraction pattern of the H phase has not been indexed to date. We have investigated the structure of the H phase using a high resolution electron microscope [5], and have shown that the H phase consists of two phases, H1 and H2. This explains why it has not yet proved possible to index the powder pattern. Both of the phases, H1 and H2, belong the rhombohedral systems. The lattice constants and space groups are determined as follows: H1 phase, a 5.14 A, c 70.2 A, space group =

=

9.36

A, space

group P3,

Fig. 5 shows the absorption coefficient of crystal R from ultraviolet to near-infrared wavelengths. In fig. Sa, curves 1 and 2 correspond to non-annealed and annealed (1000°C, 10 h) crystal specimens respectively. It is clear from fig. 5a that the absorption coefficient of annealed specimens is apparently decreased. The absorption coefficient of crystals in the near-infrared region is given by fig. Sb. A weak peak arises from 0—H bond near 0.4 eV (A 3 ~tm). Fig. S indicates that the transmittance for crystal R covers a wide range, i.e., from 0.35 to 7.5 ~tm. The velocities of sound in crystals (longitudinal wave, 10 MHz) were determined by the ultrasonic echo pulse method, the specimen sizes along a-, band c-axes being 9.34, 14.29 and 13.85 mm, respectively. The velocities of sound in crystal R shown in table 2 are rather high, especially for

~2.6000

~ ~2.3000

~

~a

5. Some physical properties of crystal R 2.2O0(~ Graphs of the refractive index versus wavelength for the a-, b- and c-axial directions for crystal R, labelled ~a’ ~h and n~respectively, are shown in fig. 4. Like most titanates, the magnitude of the refractive index is high.

4000 4375 4750

5125 5500 5875 WAVELENGTH

62506625

7000

(~)

Fig. 4. Refractive index versus wavelength of incident light for crystal R.

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Growth, phase transitions and properties of Li,Ti~O

24.0

-

16

“21.3

-

~14

~18.7

18 T

12 ~10

~16.0

-

‘

1

L)

~

2

~13.3 1.)

8-

~10.7

6-

~8.0-

4

407

7crystals

53. .

20 a 1.0 1.5 2.0 2.5 PHOTON

ENERGY

I

I

3,0 3.5 4.0

(Ev)

2.7 ~ b I I~ I 0.10 0.17 0,23 0.30 0.37 0,43 0.50 PHOTON ENERGY (Ev 1

Fig. 5. Absorption coefficient versus photon energy for crystal R in the wavelength range of (a) ultraviolet to near-infrared and (b) near-infrared.

Table 2 Velocity of sound wave in the crystal along a-, b- and Crystallographic axis

Velocity of sound wave (m/s)

a b

6880

c

~

6970 9280

consists of three parts with different slopes, the inflection points are at about —50 and 90°C, —

respectively. This implies that abrupt variations of activation energy for ionic transition take place near the inflection points. These results will be published in detail later.

References that along the c-axis; it is higher than that in Y3A15012. In addition, the ionic conductivity in the crystal

was measured along the three principal crystallographic directions. The results at room temperature in our experiments approximately agree with those of Boyce and Mikkelsen. At low temperatures, we found that the plot of ln a versus 1/T

[1] J.C. Mikkelson, J. Crystal Growth 47 (1979) 659. [2] B. Morosin and J.C. Mikkelsen, Acta Cryst. B35 (1979)

798. [3] G. Izquierdo and AR. West, Mater. Res. Bull. 15 (1980) 1655. [4] J.B. Boyce and J.C. Mikkelsen, Solid State Commun. 31

(1979) 741. [5] Zou Jin et al., to be published.